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The Main Belt Comets and Ice in the Solar System

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We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies.
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Astron Astrophys Rev (2017) 25:5
https://doi.org/10.1007/s00159-017-0104-7
REVIEW ARTICLE
The Main Belt Comets and ice in the Solar System
Colin Snodgrass1·Jessica Agarwal2·Michael Combi3·
Alan Fitzsimmons4·Aurelie Guilbert-Lepoutre5·Henry H. Hsieh6,7·
Man-To Hui8·Emmanuel Jehin9·Michael S. P. Kelley10 ·
Matthew M. Knight10 ·Cyrielle Opitom11 ·Roberto Orosei12 ·
Miguel de Val-Borro13 ·Bin Yang11
Received: 19 June 2017 / Published online: 14 November 2017
© The Author(s) 2017. This article is an open access publication
Abstract We review the evidence for buried ice in the asteroid belt; specifically
the questions around the so-called Main Belt Comets (MBCs). We summarise the
evidence for water throughout the Solar System, and describe the various methods for
detecting it, including remote sensing from ultraviolet to radio wavelengths. We review
progress in the first decade of study of MBCs, including observations, modelling of
ice survival, and discussion on their origins. We then look at which methods will likely
be most effective for further progress, including the key challenge of direct detection
of (escaping) water in these bodies.
BColin Snodgrass
colin.snodgrass@open.ac.uk
1School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
2Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany
3University of Michigan, Ann Arbor, USA
4Queen’s University Belfast, Belfast, UK
5CNRS/UTINAM-UMR 6213 UBFC, Besançon, France
6Planetary Science Institute, Tucson, USA
7Academia Sinica, Taipei, Taiwan
8University of California Los Angeles, Los Angeles, USA
9Universite de Liege, Liège, Belgium
10 University of Maryland, College Park, USA
11 European Southern Observatory, Santiago, Chile
12 Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Bologna, Italy
13 NASA Goddard Space Flight Center, Greenbelt, USA
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5Page 2 of 59 C. Snodgrass et al.
Keywords Comets: general ·Minor planets ·Asteroids: general ·Methods:
observational
1 Introduction
The traditional view of our Solar System neatly divides it into the inner part, home of
the terrestrial planets and rocky asteroids, and the outer region of the gas giants and
icy small bodies. These are separated by the ‘snow line’, which marks the distance
from the Sun where the ambient temperature allows icy bodies to form and survive.
In this picture, the terrestrial planets formed ‘dry’, as only rocky material condensed
from the solar nebula in the inner regions, while the outer planets became giants due to
the fast formation of icy cores, followed by the runaway accretion of the abundant gas
further from the young Sun. Earth’s water was then delivered by occasional impacts
of comets, whose eccentric orbits brought ice from their distant parent regions to the
terrestrial planet region.
Despite the pleasing simplicity of this model, there are a number of awkward prob-
lems: the samples of asteroids that we have on Earth, meteorites, show a variety of
compositions, including aqueously altered minerals which must have formed in the
presence of water, incompatible with asteroids being entirely dry rocks (e.g., Brearley
and Jones 1998). On the other hand, results from the Stardust mission showed that
comet dust contains minerals formed at high temperature, presumably near to the Sun
(Brownlee et al. 2006). Meanwhile, many extrasolar systems with gas giant planets
close to their stars have been discovered, presenting a challenge to formation models
(e.g., Mayor and Queloz 1995;Winn and Fabrycky 2015). Finally, recent observa-
tions have uncovered evidence of ice in unexpected places in the inner Solar System,
including the population of ‘Main-Belt Comets’ (MBCs) which have stable asteroid-
like orbits, inside the snow line, but which demonstrate comet-like activity (Hsieh and
Jewitt 2006).
The solution comes from the recognition that planetary systems are dynamic places,
with the orbits of even the largest planets able to evolve and migrate, especially in
the early period when interaction between forming planets and the protoplanetary
disc is strong. Numerical models that trace how planetary orbits interact and change
can explain the migration of planets to Hot Jupiter orbits (e.g., Trilling et al. 1998).
More importantly, for studies of our own Solar System, these models also reveal the
way that such planetary migrations stir up the population of small bodies. They can
therefore be tested, by comparing their outcomes with the observed architecture of the
Solar System—not only the present-day orbits of the planets, but also the orbits and
different properties of various populations of comets and asteroids. For example, the
‘Nice model’ (Gomes et al. 2005) is able to reconstruct the architecture of the trans-
Neptunian region, possibly implant trans-Neptunian objects in the main asteroid belt
(Levison et al. 2009), and explain the increased impact rate on the Moon during the
Late Heavy Bombardment period, via the gravitational effect of Uranus and Neptune
interacting. The latest family of models, known as the ‘Grand Tack’ (Walsh et al.
2011), uses Jupiter and Saturn migrating first inwards and then ‘tacking’ outwards to
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The Main Belt Comets and ice in the Solar System Page 3 of 59 5
explain the relatively small size of Mars and also to scatter both rocky and icy small
bodies throughout the Solar System.
A common criticism of these dynamical models is that they are highly tunable
and therefore lack predictive power—the input parameters can be arbitrarily adjusted
until they produce a simulated Solar System that looks like our own. Increasing the
number of independent constraints on these models, thus reducing the amount of
available free parameter space, is therefore an important way to refine and improve
them. For example, a recent analysis of meteoritic evidence (Doyle et al. 2015)
appears to rule out the possibility that the parent bodies of carbonaceous chon-
drite meteorites (i.e., C-type asteroids) were formed beyond Jupiter, as suggested
in the Grand Tack model, indicating that at least this feature of the model, if not
the whole model itself, is inconsistent with the physical evidence. Study of the
present-day distribution of icy bodies, particularly those containing water ice, is
another way to provide such constraints on these models, which generally predict
that icy bodies will be found everywhere, but differ in details such as abundance,
distribution, and ratios relative to other materials (e.g., rocky components or other
volatiles).
In this review, we look at the MBCs as a population of icy bodies. We consider
them in the context of water and ice detections throughout the Solar System (Sect. 2),
reviewing what is known about this population in general (Sect. 3) before considering
the specific problems of modelling ice survival in their interiors (Sect. 4) and of
directly studying this ice observationally (Sect. 5), including predictions for activity
levels (Sect. 6). We also consider what the lessons learned about comets in general
from Rosetta tell us about MBCs (Sect. 7) before discussing what future observations
and missions will further advance this field (Sect. 8).
Previous reviews on the subject of MBCs cover the more general topic of ‘active
asteroids’, including considering how non-icy bodies can eject dust and therefore
exhibit comet-like appearances (Bertini 2011;Jewitt et al. 2015c). We briefly discuss
this below, but concentrate on questions related to MBCs and ice in the Solar System.
The topic of water in small bodies, and in the Solar System more generally, is also the
subject of earlier reviews by Jewitt et al. (2007) and Encrenaz (2008), while Hartmann
et al. (2017) consider the topic of water in extrasolar protoplanetary discs. Dones et al.
(2015) give a recent review of the various cometary reservoirs in our Solar System
from a dynamical point of view.
2 Ice in the Solar System
Water, usually in the form of ice, is found throughout the Solar System. Beyond Earth,
it has long been recognised in the outer planets and comets, and is now also observed
throughout the terrestrial planet region. Evidence for water is found with now almost
monotonous regularity on Mars, but more surprisingly, ice has also been identified
in permanently shadowed craters of Mercury (Lawrence et al. 2013) and the Moon
(Colaprete et al. 2010). While these deposits could plausibly have been delivered by
comet impacts in the geologically recent past, evidence for ice in smaller bodies is
more difficult to attribute to an exogenous source.
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5Page 4 of 59 C. Snodgrass et al.
2.1 Water in the planets
Radar mapping of Mercury suggested the presence of polar ice in 1991 (Slade et al.
1992;Harmon and Slade 1992). Thermal models show that in permanently shadowed
regions of high-latitude craters, water ice covered by a regolith layer can be stable to
evaporation over billions of years (Paige et al. 1992;Vasavada et al. 1999). The ice
is thought to have been implanted by either constant micrometeoritic, asteroidal and
cometary influx (Killen et al. 1997), or to stem from a few large impacts by comets
and/or asteroids (Moses et al. 1999;Barlow et al. 1999). The MESSENGER spacecraft
observed areas of high and low near-infrared (NIR) reflectivity, which is interpreted as
surface ice and ice buried under a layer of organic material (Neumann et al. 2013;Paige
et al. 2013). The total amount of polar water ice on Mercury is estimated to 3 ×1015 kg
(Eke et al. 2017), equivalent to 300 comets the size of 67P/Churyumov-Gerasimenko1
(Pätzold et al. 2016).
Water ice has also been hypothesised to exist on the Moon in permanently shadowed
craters near the poles (Spudis et al. 2013;Hayne et al. 2015), although a debate about
alternative interpretations of data is on-going (Eke et al. 2014;Haruyama et al. 2013).
Hydroxyl- and/or water-bearing materials are widely spread across the lunar surface
(Pieters et al. 2009).
On Venus, water has been found only in the form of atmospheric vapour in spurious
quantities of the order of a few parts per million in the nitrogen- and CO2-dominated
atmosphere (Encrenaz 2008). The high deuterium-to-hydrogen (D/H) ratio (de Bergh
et al. 1991) in the Venusian atmosphere is interpreted as an indication for an earlier
escape of water to space from the upper atmosphere, which would be more efficient
for the lighter isotope. The absence of water from the Venusian atmosphere has been
connected to the strong greenhouse effect that makes the existence of ice or liquid water
on Venus unlikely (Ingersoll 1969;Mueller 1970). The water vapour present in the
current atmosphere may be re-supplied by chemical interaction with water-containing
rocks (Mueller 1970) or could be provided by meteoritic and cometary infall (Lewis
1974).
The present-day Earth hosts abundant water in all three states of matter—solid (ice),
liquid, and gas—due to a fortuitous combination of surface temperature and pressure.
The origin and evolution of water on Earth (and on the other terrestrial planets) is a
subject of ongoing research. Two key questions are (1) whether Earth incorporated
a sufficient amount of water at the time of accretion to explain its present day water
content (‘wet accretion’) or if the accreted material was depleted in volatiles due to the
high temperature in the inner solar nebula (‘dry accretion’), and (2) what fraction of
the Earth’s water was delivered later by exogenous sources, e.g., comet and asteroid
impacts (Drake and Righter 2002). Comparing isotope ratios of volatiles in Earth,
comets, asteroids, and meteorites, especially of the D/H ratio, can give us clues to
answering these questions. However, it is not clear to what extent the D/H ratio in
Vienna Standard Mean Ocean Water (VSMOW; the most commonly used standard
isotopic reference for ‘Earth water’—Balsiger et al. 1995) represents that of the early
1Hereafter 67P. We will give the name of comets only when they are first mentioned.
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The Main Belt Comets and ice in the Solar System Page 5 of 59 5
Earth. An increase of D/H by a factor 2–9 over the lifetime of the Earth due to mass
fractionation during atmospheric loss is possible (Genda and Ikoma 2008), and the
Earth’s lower mantle (supposedly least affected by atmospheric processes) has a D/H
ratio lower by up to 20% than VSMOW (Hallis et al. 2015). This places the Earth’s
D/H ratio between that of the protosolar nebula and that of comets from the outer
Solar System (see, e.g., Saal et al. 2013;Altwegg et al. 2015 and references therein,
and Sect. 4.1 below).
On Mars, water currently is present in the form of ice at the polar caps (Bibring et al.
2004;Langevin et al. 2005) and in craters (Armstrong et al. 2005;Brown et al. 2008),
in a small amount of vapour in the atmosphere (Clancy et al. 1992), and embedded
in hydrated minerals (Bibring et al. 2006). Liquid water may exist under specific
conditions (Malin et al. 2006;Martín-Torres et al. 2015). There is strong geologic and
mineralogic evidence that liquid water was more abundant on Mars in the past, when
the atmosphere was thicker (Sagan et al. 1973;Lasue et al. 2013). Like on Venus,
an elevated D/H ratio indicates that a significant amount of water vapour escaped to
space from the atmosphere, affecting H2O more than D2O(Encrenaz et al. 2016). The
original water content of Mars may have been sufficient to cover the planet with a
layer of up to 1 km depth (Lasue et al. 2013). There is currently no evidence for the
existence of water on the Martian moons, Phobos and Deimos (Rivkin et al. 2002).
Saturn and Jupiter contain water in liquid and solid form in their lower cloud layers
(Niemann et al. 1998;Baines et al. 2009), but its abundance in these gas giants is not
well known (Atreya and Wong 2005). Uranus and Neptune are thought to contain a
large layer of ices, including H2O, above a rocky core. Also their atmospheres contain
H2O(Podolak et al. 1995). The rings of the giant planets contain water ice at various
fractions. While Saturn’s rings consist mainly of water ice with a small admixture of
organics and other contaminants (Nicholson et al. 2008), the rings of Jupiter, Uranus
and Neptune contain at best a small fraction of water ice (Lane et al. 1989;Wong et al.
2006;de Kleer et al. 2013). Many of the moons of the outer planets and Pluto contain
a significant fraction of water, with Tethys possibly consisting almost entirely of water
ice (Thomas et al. 2007). Some, such as Jupiter’s Europa and Saturn’s Enceladus, are
thought to contain a tidally heated global or local subsurface ocean of liquid water
beneath the outer shell of ice (Carr et al. 1998;Manga and Wang 2007;Nimmo et al.
2007;Hansen et al. 2008).
2.2 Kuiper Belt objects, Centaurs and comets
Compared to the major planets in the Solar System, small icy bodies have experienced
much less thermal evolution and their physical properties are well preserved. The
Kuiper Belt objects (KBOs) are numerous small icy bodies beyond Neptune, which
are thought to be the most primitive remnants from the early Solar System. As these
objects were formed far beyond the ‘snow line’, volatile ices (especially water ice) are
believed to be a principle constituent in KBOs. Optical and near-infrared spectroscopy
is a widely used method to investigate surface properties as well as the chemical com-
positions of small bodies, although it is technically challenging to obtain spectral
data on KBOs because of their great distance from the Sun and thus faintness. For
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5Page 6 of 59 C. Snodgrass et al.
atmosphere-less bodies, evidence for water comes from solid-state absorption features
in the spectrum of sunlight reflected from their surfaces (see Sect. 5). So far, spectra of
KBOs fall into three categories, namely water-rich, methane-dominated and feature-
less ones (Trujillo et al. 2011). For example, water ice was detected on KBOs Quaoar
and Haumea (including its satellites) with two strong absorption bands at 1.5 and 2.0
µm, respectively (Jewitt et al. 2004;Trujillo et al. 2007;Schaller and Brown 2008).
Other large KBOs exhibit these bands as well as the absorption feature of crystalline
ice at 1.65 µm, for example: Charon (Brown and Calvin 2000;Grundy et al. 2016),
and Orcus (Barucci et al. 2008). Haumea’s satellite Hi’iaka and collisional family
members are also known to be covered by crystalline ice (Dumas et al. 2011;Schaller
and Brown 2008;Snodgrass et al. 2010a;Carry et al. 2012). The spectra of Pluto,
Eris and Makemake show a series of distinct bands of methane in the NIR (Grundy
et al. 2016;Brown et al. 2005;Licandro et al. 2006). Some trace elements, i.e., ammo-
nia and methanol, have been detected on mid-sized KBOs (Brown and Calvin 2000;
Barucci et al. 2008,2011). Compared to spectroscopic observations, Trujillo et al.
(2011) showed that a new near-infrared photometric system is sensitive to water ice
and methane ice while reducing telescope observing time by a factor of 3. This
system is particularly useful for surveying a large number of objects with moderate
amount of telescope time.
According to Jewitt and Kalas (1998), Centaurs are defined as objects with perihelia
q>aJ(aJ=5.2 au) and semimajor axes a<aN(aN=30 au). These objects are
widely believed to be ‘refugees’ from the Kuiper belt (Levison and Duncan 1997),
located on unstable orbits between Jupiter and Neptune with short dynamical lifetimes
of about 106–107years (Dones et al. 1999;Horner et al. 2004). Both optically blue
and red members have been found among Centaurs (Tegler et al. 2008;Peixinho et al.
2012;Fraser and Brown 2012). (5145) Pholus is found to be one of the reddest objects
observed to date in the Solar System (Fink et al. 1992;Davies et al. 1993), whose
spectrum shows not only strong water ice bands, but also an absorption complex at
2.27 µm(Cruikshank et al. 1998). Dalle Ore et al. (2015) studied seven KBOs and
three Centaurs that are among the reddest known. They conclude that these ‘ultra-
red’ objects in general might contain methanol/hydrocarbon ices and their organic
materials could have been produced by irradiation of the volatile ices. Jewitt et al.
(2009) reported observations of a sample of 23 Centaurs and found nine to be active.
They found that ‘active Centaurs’ in their sample have perihelia systematically smaller
than the inactive ones. Centaurs have their perihelia beyond the water ice sublimation
critical distance of 5 au, which suggests that the comet-like activity of Centaurs is
driven by a mechanism different from water ice sublimation. Therefore, the cometary
activity might be powered by conversion of amorphous ice into the crystalline phase
and the subsequent release of trapped gases, such as carbon monoxide and carbon
dioxide (Jewitt et al. 2009). Thermal evolution models have been used to study the
occurrence of crystallisation, the depth that the front would reach, and how it can
contribute to Centaurs’ activity for various orbits and obliquities (Fig. 1;Guilbert-
Lepoutre 2012).
Comets generally have small, irregularly shaped nuclei that are composed of refrac-
tory materials (such as carbon, silicates, etc.) and icy grains (e.g., water ice, carbon
dioxide ice and methanol ice). These objects may have experienced very little alter-
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The Main Belt Comets and ice in the Solar System Page 7 of 59 5
Fig. 1 Left: The depths where the crystallisation front stops for different (ac= equivalent semimajor axis,
obliquity) configurations, after 105years. Note that crystallisation may not be complete above, in the sub-
surface layer. The rotation period is very small compared to all other time scales involved. Right: How the
heat wave and crystallisation front progress in the subsurface layer for three cases (at 8 au and from top to
bottom, under the equator at 0obliquity, under the equator at 90obliquity and under the pole at 90obliq-
uity), during the time the objects may be active (few 102years after an orbital change labelled time = 0).
Black line shows the crystallisation fronts. Image reproduced with permission from Guilbert-Lepoutre
(2012), copyright by AAS
ation since their formation in the coldest regions of the young Solar System. As such,
comets serve as the best probe for studying the physical conditions (e.g., tempera-
ture, pressure and composition) of the outer Solar nebula. However, direct detection
of water ice in comets is surprisingly scarce. To date, solid ice features have only
been observed in a handful of comets via ground-based facilities (Davies et al. 1997;
Kawakita et al. 2004;Yang et al. 2009,2014) and in situ observations (Sunshine et al.
2006;A’Hearn et al. 2012;Protopapa et al. 2014;De Sanctis et al. 2015). Knowledge
of ices in comets mainly comes from spectroscopic observations of emission lines and
bands from the gas coma, which are discussed in more detail in Sect. 5.
2.3 Evidence for water in asteroids
Although the asteroid belt could be an important source for Earth’s water (Morbidelli
et al. 2000), thermal models have shown that surfaces of most asteroids are too hot for
water ice to remain stable against sublimation (see Schorghofer 2008 and Sect. 4). To
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5Page 8 of 59 C. Snodgrass et al.
date, only a few detections of surface ice in asteroids have been reported. On the other
hand, hydrated minerals have been widely observed in asteroids (Rivkin et al. 2015).
The largest body in the asteroid belt, (1) Ceres, contains significant water ice.
Although its surface does not show signatures of water ice, the shape of Ceres was
explained with a significant ice mantle (Thomas et al. 2005). Observations in the
UV hinting at escaping water (A’Hearn and Feldman 1992) were confirmed in the
sub-mm with detections with the Herschel space telescope (Küppers et al. 2014).
Using nuclear spectroscopy data acquired by NASA’s Dawn mission, Prettyman et al.
(2017) analysed the elemental abundances of hydrogen, iron, and potassium on Ceres.
They found that the regolith, at mid-to-high latitudes, contains high concentrations
of hydrogen, consistent with broad expanses of water ice. The presence of ice in the
largest asteroid has inspired debate about its origin, including suggestions that Ceres
could have formed in the KBO region (McKinnon 2012), but there is growing evidence
that it is not the only icy asteroid, and that smaller bodies also have ice.
In the 100 km diameter range, asteroid (24) Themis bears some significance
in the debate of whether ice can originate from and survive in the Main Belt (MB),
as water ice may have been detected at its surface (Campins et al. 2010;Rivkin and
Emery 2010) via an absorption feature near 3 µm. We note that Themis was not found
to be active by Jewitt and Guilbert-Lepoutre (2012), who suggested that if indeed
present at its surface, water ice should be relatively clean and confined to a limited
spatial extent, which was then confirmed by McKay et al. (2017). Jewitt and Guilbert-
Lepoutre (2012) observed Themis far from perihelion, however, which as discussed
in Sect. 6.1, may mean that the sublimation strength at the time was several orders
of magnitude weaker than might be expected for Themis closer to perihelion. Many
members of the Themis family show evidence for hydration (Florczak et al. 1999;
Takir and Emery 2012), and the second largest member of the family after Themis
itself—(90) Antiope—might have some surface water ice (Hargrove et al. 2015).
In a wider survey of large asteroids, Takir and Emery (2012) identified four 3 µm
spectral and orbital groups, each of which is presumably linked to distinct surface
mineralogy. Searches for water and OH features in near-Earth objects (NEOs) were
upended by the discovery of OH in the lunar regolith by three spacecraft (Sunshine
et al. 2009;Pieters et al. 2009;Clark 2009). Detections of water and/or hydroxyl on
large asteroids have been reported for (175706) 1996 FG3 (Rivkin et al. 2015), (16)
Psyche (Takir et al. 2017), and (433) Eros and (1036) Ganymede (Rivkin et al. 2017).
Although smaller (km-scale) asteroids are generally too faint for spectroscopy in the
3µm region, the activity of MBCs offers evidence that water is also present in at least
some smaller asteroids.
3 The Main Belt Comets
3.1 Definitions: active asteroids and Main Belt Comets
An ‘active’ asteroid can be thought of as any body in an asteroidal (rather than
cometary) orbit which is observed to lose mass, normally through the observation
of a dust tail or trail. ‘Asteroidal’ orbits have Tisserand parameters (Kresak 1982;
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The Main Belt Comets and ice in the Solar System Page 9 of 59 5
Kosai 1992) with respect to Jupiter,
TJ=aJ
a+2a(1e2)
aJcos(i)3.05,(1)
and semimajor axes aless than that of Jupiter (aJ=5.2 au). Here a,e, and iare
the semimajor axes (in au), eccentricity, and inclination (in degrees) of the orbit,
respectively. Such orbits are distinct from comets (which have TJ<3.0; cf. Levison
1996). We use TJ=3.05 instead of TJ=3.00 because small drifts in TJbeyond
3.0 are possible due to non-gravitational forces or terrestrial planet interactions (e.g.,
Levison et al. 2006;Hsieh and Haghighipour 2016). Other authors (e.g., Jewitt et al.
2015c)haveusedTJ=3.08 as the boundary for similar reasons; the resulting list of
MBC candidates is the same.
For the purposes of this paper, we use the term MBC to refer to an active asteroid that
exhibits activity determined to likely be due to sublimation (e.g., from dust modelling
results, or confirmation of recurrent activity near perihelion with intervening periods
of inactivity). Various processes can be the source of mass loss for active asteroids
in general, but repeated mass loss over several perihelion passages is most plausibly
explained by sublimation. In addition, mass loss sustained over a prolonged period,
although not unique to sublimation, can also be suggestive of ice sublimation since it
is difficult to explain by a single impact, for example.
We are specifically not considering objects in the main asteroid belt whose
cometary appearance has been shown to be due to mechanisms other than sublima-
tion. These include debris released by impacts (either cratering events or catastrophic
disruption)—e.g., (596) Scheila (Bodewits et al. 2011;Jewitt et al. 2011;Ishiguro et al.
2011a,b;Moreno et al. 2011b) and P/2012 F5 (Gibbs) (Stevenson et al. 2012;Moreno
et al. 2012)—and rotational disruption—e.g., 311P (Jewitt et al. 2015b)—thought to
be an outcome of YORP spin-up for small asteroids (Scheeres 2015). There are other
hypothesised effects that have yet to be conclusively demonstrated to explain observed
‘activity’, such as thermal cracking, electrostatic levitation of dust, or radiation pres-
sure accelerating dust away from the surface (Jewitt et al. 2015c). We also exclude
dynamically asteroidal objects (i.e., with TJ>3.05) outside of the asteroid belt for
which activity has been detected, e.g., (3200) Phaethon (Jewitt and Li 2010;Li and
Jewitt 2013;Hui and Li 2017) and 107P/(4015) Wilson-Harrington (Fernandez et al.
1997). See Jewitt et al. (2015c) and references therein for more extensive discussion of
active asteroids beyond our definition of ‘MBC’. MBCs are of special interest within
the broader class of active asteroids as they indicate the possible presence of water
in bodies of a size that are very common in the asteroid belt, implying that there is
potentially a large population of icy bodies there.
3.2 Discovery of MBCs
Since the discovery of 133P/Elst–Pizarro in 1996, the first recognised MBC, as of
September 2017 ten more members have been detected. We summarise discovery cir-
cumstances of the currently known MBCs in Table 1. The discovery rate has increased
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5Page 10 of 59 C. Snodgrass et al.
Tab le 1 MBC discovery circumstances
Object Discovery date Tel.amVbνc()
133P/Elst–Pizarro 1996 Jul 14 ESO 1.0 m 18.3 21.6
238P/Read 2005 Oct 24 SW 0.9 m 20.2 26.4
176P/LINEAR 2005 Nov 26 GN 8.1 m 19.5 10.1
259P/Garradd 2008 Sep 2 SS 0.5 m 18.5 18.5
324P/La Sagra 2010 Sep 14 LS 0.45 m 18.3 20.1
288P/2006 VW139 2011 Nov 5 PS1 1.8 m 18.7 30.7
P/2012 T1 (PANSTARRS) 2012 Oct 6 PS1 1.8 m 19.6 7.4
P/2013 R3 (Catalina-PANSTARRS) 2013 Sep 15 PS1 1.8 m 20.5 14.0
313P/Gibbs 2014 Sep 24 CSS 0.68 m 19.3 8.0
P/2015 X6 (PANSTARRS) 2015 Dec 7 PS1 1.8 m 20.7 328.9
P/2016 J1-A/B (PANSTARRS) 2016 May 5 PS1 1.8 m 21.4 345.9
aDiscovery telescope: CSS 0.68 m: Catalina Sky Survey 0.68 m; ESO 1.0 m: European Southern Obser-
vatory 1.0 m; GN 8.1 m: Gemini-North 8.1 m; LS 0.45 m: La Sagra 0.45 m; PS1: Pan-STARRS1 1.8 m;
SS 0.5 m: Siding Spring 0.5 m; SW 0.9 m: Spacewatch 0.9 m
bApproximate mean reported V-band magnitude at time of discovery
cTrue anomaly in degrees at time of discovery
significantly since 133P was discovered, thanks to wide-field sky surveys, especially
those designed to find transients or NEOs. Two new MBCs have been added to the list
since the review by Jewitt et al. (2015c): P/2015 X6 (PANSTARRS), and P/2016 J1
(PANSTARRS), both discovered by the Pan-STARRS survey at Haleakala, Hawaii.
P/2016 J1 is particularly interesting as it was observed to split into two pieces (Hui
et al. 2017;Moreno et al. 2017), and its current activity perhaps was triggered by a
small impact, whereby formerly buried ice started sublimating and the torque rapidly
drove the parent to fragment (Hui et al. 2017). It reinforces the idea that rotational
instability is one of the important fates that comets may suffer, especially for those
sub-kilometre sized. In addition to sky surveys such as Pan-STARRS, there have
been attempts to search for MBCs with targeted observations of known asteroids in
main-belt orbits to search for activity (Hsieh 2009), and in archival data from the
Canada–France–Hawaii Telescope (Gilbert and Wiegert 2009,2010;Sonnett et al.
2011) and the Palomar Transient Factory (Waszczak et al. 2013). Recently, a citizen
science project2to crowd source the visual identification of MBCs has been employed
in the search (Hsieh et al. 2016;Schwamb et al. 2017). To date none of these targeted
searches have discovered any MBCs.
3.3 Population estimates
Based on Pan-STARRS1 discovery statistics, on the order of 50–150 currently active
MBCs comparable in brightness to the known MBCs are expected to exist (Hsieh et al.
2http://www.comethunters.org.
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The Main Belt Comets and ice in the Solar System Page 11 of 59 5
2015a), where their respective individual activity strengths will depend on their orbit
positions at the time of observation. We have not yet found them all as they need to be
both active (which they are for only a few months of a 5–6 years orbit) and well placed
for observations by surveys at the same time. While the pace of MBC discoveries has
accelerated somewhat relative to the past to roughly one per year since the start of the
Pan-STARRS1 survey (Table 1), an even higher rate of discoveries will be necessary to
increase the known population to the point at which it can be meaningfully statistically
analysed.
Assuming that we have mostly been discovering the brightest members of the MBC
population, increasing the discovery rate will most likely require increasing the sensi-
tivity of search efforts, to find similar MBCs at less optimal observing geometries, or to
sample the population of intrinsically fainter targets with lower activity levels, which
are presumably more numerous. To date, nearly all of the known MBCs have been dis-
covered by telescopes with apertures smaller than 2 m. The use of larger telescopes for
conducting surveys (e.g., the Dark Energy Survey’s 4 m Blanco telescope or the 8.4 m
Large Synoptic Survey Telescope) immediately allows for more sensitive searches for
MBCs, but their respective observing efforts must also be coupled with at least reason-
ably capable search algorithms like that used by Pan-STARRS1 (cf. Hsieh et al. 2012b).
Other than the direct confirmation of sublimation products from a MBC, the next
most critical priority for advancing MBC research is the discovery of more objects.
A larger population of known objects (perhaps an order of magnitude larger than the
currently known population) will allow us to achieve a more meaningful understanding
of the abundance and distribution of MBCs in the asteroid belt as well as their typical
physical and dynamical properties.
3.4 Outgassing from MBCs
The gas comae of comets are generally observed through fluorescence emission bands
of various species, across a wide range of wavelengths from the UV to radio. The CN
radical has strong emissions in the optical range, especially between 3590 and 4220
Å. The (0–0) band at 3883 Å is one of the most prominent feature of comet optical
spectra. It has been detected at up to almost 10 au for comet Hale–Bopp (Rauer et al.
2003) and it is usually one of the first gas emissions detected in comets from the
ground. The CN radical can also be used as a proxy of water production. The ratio of
water production rates to CN production rates in the coma of a typical Jupiter-family
comet is around Q(H2O)/ Q(CN) = 350, even though variations of the ratio have been
observed along the orbit of a comet as well as from one comet to another (A’Hearn
et al. 1995).
Upper limits of the CN production rate have been determined for several MBCs
using spectroscopic observations on large telescopes: the Very Large Telescope (VLT),
Gemini, the Gran Telescopio Canarias (GTC), and Keck. Hsieh et al. (2012b) and
Licandro et al. (2013), respectively, determined upper limits of Q(CN) = 1.3×1024
and Q(CN) = 3.76 ×1023 mols1for 288P, Hsieh et al. (2013) report an upper
limit of Q(CN) = 1.5×1023 mols1for P/2012 T1 (PANSTARRS), and Licandro
et al. (2011) measure an upper limit of Q(CN) = 1.3×1021 mols1for 133P. For
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5Page 12 of 59 C. Snodgrass et al.
Tab le 2 Upper limits on MBC gas production Adapted from Snodgrass et al. 2017b
MBC Tel. r(au) Q(CN) mols1Q(H2O)amol s1References
133P VLT 2.64 1.3 ×1021 1.5 ×1024 Licandro et al. (2011)
176PbHerschel 2.58 4 ×1025 de Val-Borro et al. (2012)
324P Keck 2.66 3 ×1023 1×1026 Hsieh et al. (2012c)
259P Keck 1.86 1.4 ×1023 5×1025 Jewitt et al. (2009)
288P Gemini 2.52 1.3 ×1024 1×1026 Hsieh et al. (2012b)
GTC 2.52 1.1 ×1024 Licandro et al. (2013)
596cKeck 3.10 9 ×1023 1×1027 Hsieh et al. (2012a)
P/2013 R3 Keck 2.23 1.2 ×23 4.3 ×1025 Jewitt et al. (2014a)
313P Keck 2.41 1.8 ×1023 6×1025 Jewitt et al. (2015a)
P/2012 T1 Keck 2.42 1.5 ×1023 5×1025 Hsieh et al. (2013)
Herschel 2.50 7.6 ×1025 O’Rourke et al. (2013)
VLT 2.47 8 ×1025 Snodgrass et al. (2017b)
aWhere both Q(CN) and Q(H2O) are given, the latter is derived from the former. Values quoted in the
original reference are given
b176P was not visibly active (no dust release) at the time of the Herschel observations
cThe dust ejected from (596) Scheila was almost certainly due to a collision, rather than cometary activity
(e.g., Bodewits et al. 2011;Ishiguro et al. 2011a,b;Yang and Hsieh 2011)
comparison, we note that these upper limits are in line with the lowest recorded pro-
duction rates for ‘normal’ comets near 1 au; one of the lowest successfully measured
was 209P/LINEAR with Q(CN) = 5.8×1022 mol s1(Schleicher and Knight 2016),
while more commonly measured CN production rates are at least a few times 1023
(A’Hearn et al. 1995). The derived water production rates from these measurements,
assuming a ‘typical’ water/CN ratio for comets, are around Q(H2O) = 1025 1026
mols1, which would be low for a typical Jupiter-family comet (JFC) at MB distances
(e.g., Rosetta measurements for 67P inbound agree at Q(H2O) 1026 mol s1at 3
au, and a few times this at around 2.5 au, where most of the MBC measurements were
made; Hansen et al. 2016). Attempts to detect CN in MBCs to date are summarised
by Snodgrass et al. (2017b) and listed in Table 2.
These upper limits on water production all rely on the assumption that MBCs
have a similar water/CN ratio to other comets, which is unlikely to be the case (see
detailed discussion in following sections). Attempts have also been made to directly
detect outgassing water using the Herschel space telescope (de Val-Borro et al. 2012;
O’Rourke et al. 2013), and to detect the photo-dissociation products of water (OH
and O) with the VLT (Snodgrass et al. 2017b;Jehin 2015). These are discussed in
more detail in Sect. 5. A second implicit assumption in any upper limit measurement
is that gas is still present at the time of the observation (i.e., that activity is ongoing).
In most cases these observations were performed when there was dust visible around
the MBC, and ongoing activity is a reasonable assumption, but it is also possible that
an initial period of activity lifted dust and then shut off, leaving slow moving dust
coma and tail visible after faster moving gas already dispersed. Models of dust coma
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The Main Belt Comets and ice in the Solar System Page 13 of 59 5
morphology can be used to differentiate between ongoing activity and remnant dust
from short impulsive events (e.g., Moreno et al. 2011a).
4 MBC origins and survival of ice
Constraining the origin of MBCs is particularly important in the context of under-
standing whether these objects can be representative of ice-rich asteroids native to the
MB, or have been implanted from outer regions of the Solar System during its complex
dynamical evolution (cf. Hsieh 2014a). MBCs could be representative of the source
of the terrestrial planets’ volatiles and of the Earth’s oceans in particular, and can
therefore be extremely astrobiologically significant. It is also important to understand
to what degree MBCs can be expected to have compositions which are comparable to
traditional comets, e.g., their H2O/CN ratio.
4.1 Isotopic ratios
Isotopic ratios, and particularly the D/H ratio in water, are used to trace the formation
location of Solar System ices. In the protoplanetary disc, reactions in the vapour phase
meant that D/H varied with temperature, but when the ice froze out, the ratio became
fixed, meaning that the D/H ratio observed now records, in some way, the temperature
and therefore location at which the ice originally formed. D/H is expected to increase
with the heliocentric distance at which a body formed (Robert et al. 2000;Robert
2006). As mentioned in Sect. 2, the Earth’s oceans have a mean D/H value that is
lower than that measured in most comets, including the most recent measurement by
Rosetta for 67P (Mumma and Charnley 2011;Altwegg et al. 2015). As the D/H values
measured in meteorites are closer to VSMOW, it is possible that asteroids are instead
the dominant source of Earth’s water. Therefore, ice-rich asteroids such as MBCs are
potentially remnants of the population that supplied our oceans. However, no D/H
ratio has yet been measured for an MBC, and a space mission to visit one would be
required to do this (see Sect. 8). This is seen as a priority for future measurements,
both to confirm the match between ice in MBCs and Earth’s water, but also to better
understand the original source region where MBCs formed. In addition to D/H, isotopic
ratios in other volatile elements (O, C, N, S) are measured in comets (Jehin et al. 2009;
Bockelée-Morvan et al. 2015). Combining information from different elements can
place stronger constraints on source location within the protoplanetary disc, but these
measurements are even more challenging than D/H, given the relatively low abundance
of other volatile species relative to water.
4.2 Dynamical evolution
Studying the dynamical stability and evolution of MBCs is a key component in efforts
to identify their likely source regions in the Solar System. Most MBCs have been found
to be mostly stable over 108years or more (Jewitt et al. 2009;Haghighipour 2009;
Hsieh et al. 2012b,c,2013), suggesting that they formed in situ where we see them
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5Page 14 of 59 C. Snodgrass et al.
today in the asteroid belt. These results were corroborated by Hsieh and Haghighipour
(2016) who studied synthetic test particles rather than real objects, concluding that
objects on orbits with both low eccentricities and low inclinations are unlikely to have
been recently implanted outer Solar System objects.
Despite the fact that most MBCs appear to occupy long-term stable orbits, some
MBCs like 238P/Read and 259P/Garradd are rather unstable on timescales of the
order of 107years (Jewitt et al. 2009;Haghighipour 2009), suggesting that they may
be recently emplaced. Though the architecture of the modern Solar System appears to
largely lack reliable pathways by which outer Solar System objects can evolve onto
MBC-like orbits (e.g., Fernández et al. 2002), such interlopers cannot be completely
excluded. Considering only the dynamical influence of major planets and the Sun,
Hsieh and Haghighipour (2016) showed that pathways exist that are capable of tem-
porarily implanting JFCs in the MB, largely shaped by the effects of close encounters
with the terrestrial planets. However, they also showed that such implanted objects
would not be stable for more than 100 Myr. These dynamical pathways appear to work
both ways, with a non-negligible possibility that some JFCs could originate from the
MB (Fernández and Sosa 2015).
A final scenario to consider is whether planet migrations during the early stages of
Solar System evolution, as described by the Grand Tack and Nice models, could have
resulted in the emplacement of icy outer Solar System objects in the MB (Levison
et al. 2009;Walsh et al. 2011). In this case, ice formed in the outer regions of the
Solar System could have been implanted in the MB, and would have experienced
a different thermal history from ice that remained in the outer Solar System, thus
allowing us to probe processes related to the thermophysical evolution of icy bodies in
general. As discussed earlier in this paper though, much work remains in developing
and performing observational tests for these models as well as refining them to the
point where predictions (e.g., the distribution in orbital element space of implanted
objects in the MB) can be made and tested.
Hui and Jewitt (2017) is the first work which systematically examines nongravi-
tational effects of the MBCs. The authors report statistically significant detections of
nongravitational effects for 313P/Gibbs and 324P/La Sagra. Intriguingly, the nongrav-
itational effect on 324P is found to be large (composite nongravitational parameter
107au day2;Hui and Jewitt 2017), which may be correlated with the fact that
it is one of the most active MBCs (Hsieh 2014b), and may support the argument that
some MBCs may have originated as JFCs (Hsieh and Haghighipour 2016). For the
rest of the MBCs, Hui and Jewitt (2017) fail to obtain meaningful detections on non-
gravitational effects, which is consistent with the fact that the activity of the MBCs
is normally orders-of-magnitude weaker than typical comets. Given this, conclusions
about dynamical evolutionary paths of the MBCs by previous authors without consid-
eration of the nongravitational effects are likely unaffected.
4.3 Family origins
In addition to the dynamical considerations discussed above, we need to account for
the formation of asteroid families by catastrophic disruption of large parent bodies
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The Main Belt Comets and ice in the Solar System Page 15 of 59 5
when trying to constrain the origins of MBCs. MBCs P/2012 T1 and 313P have
for example been linked to the Lixiaohua family (Hsieh et al. 2013,2015b), while
133P, 176P/LINEAR, 238P and 288P/2006 VW139 have been linked to the Themis
family (Toth 2000;Hsieh 2009;Hsieh et al. 2009b,2012b). MBC 324P has also been
dynamically linked with a small cluster of six objects (including 324P itself), but due
to the small number of cluster members identified to date, this grouping does not yet
formally satisfy the criteria for being classified as a family or clump as defined by
Novakovi´cetal.(2011).
The link between several MBCs and the Themis family is interesting, not only
because of the water ice detection on Themis itself, but because spectra that have been
obtained for Themis asteroids are best matched by carbonaceous chondrites meteorites
with different degrees of aqueous alteration (Fornasier et al. 2016;Marsset et al. 2016).
These observations are consistent with a large parent body made of significant amounts
of water ice, which differentiated (hence the hydration of minerals) but maintained lay-
ers of pristine/unheated material (Castillo-Rogez and Schmidt 2010). The layering of
composition, or internal heterogeneity, could in this case explain the spectral variabil-
ity observed among Themis family members, which cannot be completely explained
by space weathering processes (Fornasier et al. 2016). Among the Themis family lie
much younger sub-families: the Beagle family, aged less than 10 Myr (Nesvorný et al.
2008), and the 288P cluster, estimated to be 7.5 Myr old (Novakovi´c et al. 2012). It
is possible that MBCs 133P and 288P may be members of those young sub-families
rather than primordial members of the Themis family, increasing the possibility that
water ice—if initially present in the various precursors—could have survived in both
objects until today.
We note that while young families similar to those found for 133P and 288P have
not yet been formally identified for the other known MBCs, the small sizes of other
MBC nuclei suggests that they too may be fragments of recent catastrophic disruption
events. Smaller objects are collisionally destroyed on statistically shorter timescales
relative to larger objects (Cheng 2004;Bottke et al. 2005), meaning that currently
existing smaller objects are statistically more likely to have been recently formed
(e.g., in the fragmentation of a larger parent body) than larger bodies. The young
families resulting from those fragmentation events may perhaps simply have not yet
been identified because an insufficient number of members have been discovered to
date, preventing the identification of the families by standard clustering analyses. As
more asteroids are discovered by ongoing and future surveys, it will therefore be
useful to periodically re-run clustering analyses for all MBCs to check if any new
young families can be identified.
4.4 Thermal processing and ice survival
Hsieh et al. (2015a) noted that MBC activity patterns are predominantly modulated
by variations in heliocentric distance. This observation implies that such activity is
the result of ice being present on a global scale, buried under a slowly growing dust
mantle, allowing for activity to be sustained over multiple perihelion passages, rather
than the result of isolated local active sites containing exposed ice being seasonally
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5Page 16 of 59 C. Snodgrass et al.
illuminated (which would tend to produce activity more randomly distributed along
object orbits). However, given the small number of MBCs and the different individual
cases, both scenarios have to be taken into account. Schorghofer (2008) studied the
survival of water ice inside asteroids, assumed to be spherical icy objects on orbits
ranging from 2 to 3.3 au from the Sun. They introduced the concept of the ‘buried
snowline’, i.e., the limit beyond which subsurface water ice can be sustained for the
age of the Solar System inside asteroids. Due to the very low thermal conductivities
observed for such objects, they found that ice could have survived for billions of years
in the top few metres from the surface, provided the mean temperature of the surface
remains below 145 K: this is achievable in the polar regions of most objects in the
outer MB with low obliquity.
Individual studies have mainly focused on 133P, since it has been active for four
consecutive perihelion passages. Prialnik and Rosenberg (2009) studied two aspects
of the survival of ices in 133P, both during the long-term thermophysical evolution of
the object since its formation, and after an impact in the recent history of the object.
For the 4.6 Gyr evolution, they used a one-dimensional thermophysical model able
to compute both the thermal history of the object and the retreat of various volatiles,
in particular, water, CO, and CO2. They assumed that 133P was formed in the outer
Solar System and then implanted in the MB, where the larger equilibrium temperature
would have caused ices to sublimate, causing the sublimation front to penetrate deeper
below the surface. They found that only water ice would have been able to survive
inside 133P, 50–150 m below its surface. Other minor species would have all been
lost, and would not have been sustained even in the case of an extremely low thermal
conductivity. From these results, they infer that in order for 133P to be active today,
an impact must have occurred, by which material was removed from the surface to
expose ice-rich layers. This idea is consistent with the study of Capria et al. (2012)
who found that impact rates in the MB are consistent with MBC activity being the
result of impacts able to expose water ice at the surface of asteroids.
With this scenario in mind, Prialnik and Rosenberg (2009) then studied the thermal
processing of 133P with a fully three-dimensional model which allows the assessment
of the latitudinal variations of sublimation and dust mantling. Their results show that
a dust mantle is being slowly built after repeated perihelion passages, with a thickness
that is not uniform across the surface due to latitudinal variations of ice evolution. It is
interesting to note that their analysis includes results consistent with the main features
observed in 133P’s secular lightcurve as studied by Ferrín (2006), in particular an
increase in brightness of 2 mag above the bare nucleus at peak activity, and a time
lag of 150 days after perihelion passage for peak activity. Prialnik and Rosenberg
(2009) were able to reproduce these features when considering large tilt angles in their
simulations. A large obliquity was suggested for 133P (Toth 2006), but has not been
confirmed (Hsieh et al. 2010). This would indicate that the behaviour expected from
water ice evolving in the MB can reproduce the activity pattern of this MBC: this
implies that although no signatures of gaseous species have been detected, ice can be
the origin of MBC activity, at least for 133P.
Using a different modelling approach, namely an asynchronous model coupling the
simulation of accurate diurnal variations of the temperature with the simulation of ice
sublimation over long timescales, Schorghofer (2016) revisited the thermo-physical
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The Main Belt Comets and ice in the Solar System Page 17 of 59 5
evolution of 133P. In particular, Prialnik and Rosenberg (2009) focused on 133P having
spent billions of years in the MB, when we know from dynamical studies that it belongs
to the Themis family (aged 2.5 Gyr) and perhaps to the Beagle family (aged 10 Myr)
and thus has only been a discrete object since breaking up from the parent body at
those times. Assuming that 133P is a direct member of the Themis family, Schorghofer
(2016) found that water ice could indeed be found at depths consistent with those of
Prialnik and Rosenberg (2009), but only for large grain sizes in the polar regions of
133P, i.e., ice can be found 2.4 to 39 m below the surface for 1 cm grains. When
considering smaller grain sizes, Schorghofer (2016)’s results suggest that icecould be
expected to be found much closer to the surface, and as close as 0.2 m for 100 µm
grains. Assuming that 133P is a member of the much younger Beagle family, they
suggest that ice could be found as close as a few cm below the surface (0.07–0.16 m
for 100 µm grains) and within the top few metres in all cases (above 3.9 m for 1 cm
grains).
Such different results between the two studies can in part be due to the different
modelling approaches, or the initial thermo-physical properties assumed in the models.
A better understanding of the internal structure of MBCs is crucial, however, for
assessing the expected production rates, whether minor volatiles could have survived,
and therefore to prepare for ground or space-based observations which might confirm
the presence of ice on MBCs. Although an in-depth study of the survival of water
ice and other volatiles is beyond the scope of this paper, we choose to provide an
additional input given the discrepancy between the main studies of 133P’s internal
structure and evolution.
To further explore the stability of water ice in the interior of MBCs, we used
the method described in Guilbert-Lepoutre (2014) who studied the survival of water
ice on Jupiter Trojans. Below the orbital skin depth (a few to tens of metres), the
material is not sensitive to variations of the surface temperature, but to the mean annual
temperature. We therefore assumed equivalent circular orbits which receive the same
amount of energy per orbit as asteroids, including MBCs, using ac=a(1e2)as
orbital input in a numerical model of three-dimensional heat transport to constrain the
temperature distribution over the range of heliocentric distances spanned by MBCs
(see Table 3). We have also considered a tilt angle between the rotation axis and the
MBC orbit plane of 0,45
and 90to obtain a limit on the dust layer thickness required
for ice to survive underneath. For the main thermo-physical parameters, we have used
a 2% albedo and a low thermal inertia of 3 J K1m2s1/2, which is consistent with
values measured for comets, though among the lowest in the range of possible inertias
(further details can be found in Guilbert-Lepoutre 2014 and Guilbert-Lepoutre et al.
2011). In Fig. 2, we show the depth at which water ice can be found after 1 Myr for
the different orbital configurations considered, as preliminary results for a dedicated
study of ice survival in the MB. The water ice depth ranges from a few centimetres
to about 30 m for objects as close to the Sun as ac=2.4 au. It is worth noting that
already after 1 Myr, highly volatile species such as CO or CH4would have already
been lost. However, less volatile species such as CO2, HCN or CH3OH could still be
present within the top 30–100 m, especially at large heliocentric distances. Although
these results cannot be directly compared with existing works in meaningful way,
given the different timescales simulated by the different approaches, they appear more
123
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5Page 18 of 59 C. Snodgrass et al.
Fig. 2 Thickness of the porous dusty crust at the surface of MBCs required for water ice to be thermody-
namically stable underneath after 1 Myr on equivalent orbits ac=a(1e2)displayed as columns. Each
row corresponds to a tilt angle between the object’s rotation axis and its orbital plan. The Bond albedo is
2% and the thermal inertia at the surface is 3 J K1m2s1/2
Tab le 3 MBC properties relevant for thermal models
Object aaebaccEdTqe
133P 3.166 0.159 3.08 4.65 ×1015 160
238P 3.165 0.252 2.96 4.74 ×1015 163
176P 3.196 0.193 3.07 4.66 ×1015 161
259P 2.726 0.342 2.40 5.27 ×1015 175
324P 3.096 0.154 3.02 4.69 ×1015 162
288P 3.049 0.201 2.92 4.78 ×1015 164
P/2012 T1 3.154 0.236 2.98 4.73 ×1015 163
P/2013 R3 3.033 0.273 2.80 4.88 ×1015 167
313P 3.156 0.242 2.97 4.74 ×1015 163
P/2015 X6 2.755 0.170 2.67 4.99 ×1015 170
P/2016 J1 3.172 0.228 3.00 4.71 ×1015 162
aSemimajor axis, in au
bEccentricity
cSemimajor axis, in au, of the equivalent circular orbit, i.e., the circular orbit receiving the same amount
of energy per orbit
dInsolation per orbit, in W m2
eTheoretical equilibrium surface temperature at semimajor axis of equivalent circular orbit, assuming a
sublimating isothermal grey-body (see Sect. 6.1 for description of calculations)
consistent with the results obtained by Schorghofer (2008) on the survival of water
ice, and suggest that a detailed analysis needs to be performed for constraining the
survival of minor volatile species.
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The Main Belt Comets and ice in the Solar System Page 19 of 59 5
5 Detecting water
As discussed in Sect. 3.4 above, CN has been used to search for outgassing around
MBCs, but without success. To use MBCs as probes of the water content of the MB, we
need to directly detect water (or its daughter/grand-daughter species). In this section
we discuss the various means by which this is achieved in comets, and the prospects
for applying these techniques to MBCs.
There are several techniques and wavelength ranges where water production rates in
comets are determined. Water molecules themselves can be detected in their vibrational
fluorescence transition in the NIR at 2.7 µm; however, the main band emission cannot
be detected from the ground because of strong absorption by telluric water in the
atmosphere. There are some weaker hot bands in the IR that can be detected and are
routinely observed in medium-to-bright comets and at heliocentric distances much
smaller than those of MBCs (Bockelée-Morvan et al. 2004). Rotational lines of water
for the same reason also cannot be observed from the ground, but have been observed
from space-borne platforms like ISO (Crovisier et al. 1997) and SWAS (Chiu et al.
2001). This is also how the most sensitive detection of water was made on approach by
the Rosetta spacecraft to comet 67P by the MIRO instrument, a small radio telescope
(Biver et al. 2015).
Otherwise water production is normally determined with greater sensitivity by
observing water dissociation fragments such as OH, H and O(1D). The OH radical is
the primary product of the photodissociation of water at the level of about 82%. Its UV
emission in the (0–0) band at 3080 Å is observed both from ground-based and space-
based telescopes (Feldman et al. 2004). The fluorescence of the UV emissions produces
a population inversion of rotational levels that are observed with radio telescopes at
18 cm (Despois et al. 1981;Schleicher and A’Hearn 1988). Space-based observations
of atomic hydrogen have also been used as a proxy for determining water production
rates. The chain of H2O and OH photodissociation produces two H atoms per water
molecule with nascent velocities of 18 and 8kms
1, respectively (Keller 1976);
measurements of the Lyman-αemission at 1216 Å can be analysed to determine water
production. Oxygen can be used via the forbidden lines of O(1D) at 6300 Å (Fink 2009)
in the inner coma, but the lowest production rates determined this way have been only
a little lower than those from H Lyman-α, on the order of a few ×1027 mol s1
(Fink 2009), also for comets near a heliocentric distance of about 1 au and very small
geocentric distances.
In Table 4we list the various emission features of water (and selected other key
cometary species, for reference), along with example observations of these species in
comets. We have tried to select the weakest detection, i.e., lowest Q(H2O), to show
the limits of current technology, but of course the possibilities with very different
facilities vary over orders of magnitude. Further examples of observations in different
wavelength regimes are given in more detail in the following sub-sections. We give
the diameter of the telescope used and the signal-to-noise per second achieved in
order to allow some comparison to be made—we use this and the geometry of each
observation to approximately scale the different capabilities to the expected MBC
case in Sect. 6. It is worth noting that comet 67P is often the example chosen, as
this otherwise faint comet was observed with all possible techniques as part of the
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5Page 20 of 59 C. Snodgrass et al.
Tab le 4 Examples of detection of water and other key species in cometary comae
Wavelength Species, line Comet Tel./inst. Ø (m) SNR/s Q(mol s1)r(au) (au) References
1216 Å H, Ly-α67P/C-G PROCYON/LAICA 0.0415 7.07 1.24E+27 1.298 1.836 [1]
1304 Å [OI] note (a)
1356 Å [OI] note (a)
3080 Å OH 73P-R/S-W 3 R Lowell 1.1m/Phot. 1.1 0.46 1.30E+25 1.029 0.074 [2]
3080 Å OH 88P/Howell TRAPPIST 0.6 0.14 2.20E+27 2 1.44 [3]
5577 Å [OI] 67P/C-G VLT/UVES 8.2 0.07 5.00E+27 1.36 1.94 [4]
6300 Å [OI] 67P/C-G VLT/UVES 8.2 0.47 5.00E+27 1.36 1.94 [4]
6364 Å [OI] 67P/C-G VLT/UVES 8.2 1.41 5.00E+27 1.36 1.94 [4]
6563 Å HαC/1996 B2 (Hyakutake) McDonald 2.7 0.24 3.00E+29 0.9 0.2 [5]
2.66 µmH
2O, ν322P/Kopff Akari/IRC 0.69 2.48 1.20E+27 2.4 2.4 [6]
2.9 µmH
2O, hot bands C/2014 Q2 (Lovejoy) Keck/NIRSPEC 10 0.75 5.90E+29 1.29 0.83 [7]
6.3 µmH
2O, ν2C/2004 B1 (LINEAR) Spitzer/IRS 0.85 3.17 1.00E+28 2.2 2.0 [8]
1665 GHz H2O, 212–101 C/1995 O1 (Hale–Bopp) ISO/LWS 0.60 0.17a3.30E+29 2.8 3.0 [9]
1113 GHz H2O 81P/Wild 2 Herschel/HIFI 3.5 2.03 8.60E+27 1.61 0.93 [10]
557 GHz H2O 81P/Wild 2 Herschel/HIFI 3.5 4.75 1.13E+28 1.61 0.93 [10]
183 GHz H2O 103P/Hartley 2 IRAM 30 0.10 1.70E+25 1.06 0.15 [11]
18 cm OH 103P/Hartley 2 Nançay 100 0.10 1.70E+28 1.1 0.15 [12]
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The Main Belt Comets and ice in the Solar System Page 21 of 59 5
Tab le 4 continued
Wavelength Species, line Comet Tel./inst. Ø (m) SNR/s Q(mols1)r(au) (au) References
1400–1640 Å CO, 4th+ (A-X) C/2000 WM1 (LINEAR) HST/STIS 2.4 0.25 3.56E+26 1.084 0.358 [13]
1850–2300 Å CO, Cameron 103P/Hartley 2 HST/FOS 2.4 0.06 2.60E+27 0.96 0.92 [14]
3883 Å CN 67P/C-G VLT/FORS2 8.2 0.12 1.40E+24 2.9 2.2 [15]
3883 Å CN C/2012 S1 (ISON) Lowell 1.1m/Phot. 1.1 0.07 1.29E+24 4.554 4.039 [16]
4.26 µmCO
2,ν322P/Kopff Akari/IRC 0.69 2.32 1.30E+26 2.4 2.4 [6]
4.67 µmCO,01 29P/S-W 1 VLT/CRIRES 8.2 0.09 2.64E+28 6.3 5.5 [17]
265 GHz HCN 10P/Tempel 2 JCMT 15 0.08 9.00E+24 1.5 0.98 [18]
aEstimated, no SNR or exposure time quoted in reference
(a) [OI] lines in the UV were detected in C/1995 O1 (Hale–Bopp) via sounding rocket observations (McPhate et al. 1999) and in 67P via Rosetta/Alice (Feldman et al. 2015),
but production rates are not given as the parent(s) and excitation process is model dependent (see Sect. 7)
References: [1] = Shinnaka et al. (2017); [2] = Schleicher and Bair (2011) ; [3] = Opitom et al. 2017, in prep.; [4] = Jehin (2015); [5] = Combi et al. (1999); [6] = Ootsubo
et al. (2012); [7] = Paganini et al. (2017); [8] = Bockelée-Morvan et al. (2009); [9] = Crovisier et al. (1997); [10] = de Val-Borro et al. (2010); [11] = Drahus et al. (2012);
[12] = Crovisier et al. (2013); [13] = Lupu et al. (2007); [14] = Weaver et al. (1994); [15] = Opitom et al. (2017); [16] = Knight and Schleicher (2015); [17] = Paganini et al.
(2013); [18] = Biver et al. (2012)
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5Page 22 of 59 C. Snodgrass et al.
campaign of observations supporting the Rosetta mission (Snodgrass et al. 2017),
while for many species observations were only possible for very bright or very nearby
comets (e.g., Hale–Bopp or 103P/Hartley 2; the latter was also a popular target due to
the flyby of the NASA EPOXI mission close to perihelion—Meech et al. 2011).
5.1 UV/visible emission features
Photodissociation of water produces a lot of hydrogen in cometary comae, both directly
and via further dissociation of OH, and the Lyman-αtransition is one of the strongest
emission features. However, it can only be observed from space, and even in Earth-orbit
is difficult to detect in comets due to the overwhelming background from geocoro-
nal emission. For the exceptionally bright and nearby comet C/1996 B2 (Hyakutake)
Hubble Space Telescope (HST) spectroscopy detected the Ly-αemission, as the rel-
ative velocity (54 km s1) of the comet was sufficient to Doppler shift it away from
the background (Combi et al. 1998). Observations with small field-of-view (FOV),
like HST, are difficult to interpret directly because the innermost coma is optically
thick to Lyman-α(Combi et al. 1998), so most observations have been very wide field
like sounding rockets and the SOHO SWAN all-sky Lyman-αcamera (Bertaux et al.
1998;Mäkinen et al. 2001;Combi et al. 2011b). Because the H coma can cover 10
or more degrees of the sky, background stars can become a serious limitation, espe-
cially for fainter comets. The lowest water production rates determined from SOHO
SWAN measurements have been 1027 mols1, and these were only for comets
very close to SOHO, e.g., comet 103P (Combi et al. 2011a), which was at a heliocen-
tric distance of only slightly more than 1 au. In Table 4we give an example from a
Ly-αimager on another spacecraft, the 2FOV LAICA camera on the experimental
Japanese PROCYON micro-satellite, which was designed to test deep-space naviga-
tion of a 50 kg satellite. The LAICA camera was designed to study the geocorona,
but was also successfully employed to measure water production rates in 67P near
perihelion (Shinnaka et al. 2017). The small sizes of these telescopes (2.7 and 4.2 cm
diameter for SWAN and LAICA, respectively) implies that more sensitive instruments
could be built relatively easily, but background noise will remain a limiting factor for
Ly-αobservations.
There are also emission lines from atomic oxygen in the UV, at 1304 and 1356
Å, which we list in Table 4for completeness, but note that these are weak and have
not been used to derive production rates. Also only observable from space, they were
detected in C/1995 O1 (Hale–Bopp) from sounding rocket observations (McPhate
et al. 1999), and from close range in 67P by Rosetta/ALICE (Feldman et al. 2015),
but the excitation process (photons vs electrons), and therefore the production rate, is
highly model dependent (see Sect. 7). There is no realistic prospect of remote detection
of these lines in an MBC.
Emission bands of OH, the direct product of water photo-dissociation, can be
detected at optical wavelengths. The strongest group of OH lines visible in this wave-
length range is in the 3070–3105 Å region (Swings et al. 1941). This OH emission is
often used to estimate the water production rate of comets, via spectroscopy or pho-
tometry. Low-resolution spectroscopy gives total production rates (e.g., Cochran et al.
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The Main Belt Comets and ice in the Solar System Page 23 of 59 5
2012), while higher resolution with either large telescopes or from space can improve
signal-to-noise (S/N) for bright comets and reveal the structure within the band (e.g.,
Weaver et al. 2003;Jehin et al. 2006). Using narrow-band filters that isolate the region
around the OH emission can give production rates and can be used to image the 2D
structure of the gas coma, with OH production rates as low as 2.2×1027 mols1
detected for comet 88P/Howell at 2 au from the Sun using the 60 cm TRAPPIST
telescope (Opitom et al. 2017, in prep.), but the most sensitive production rates come
from photometry with a traditional photomultiplier (e.g., Schleicher and Bair 2011).
However, at these wavelengths atmospheric extinction is high and the efficiency of
most optical telescope detectors and optics is low, so OH emission is difficult to detect
for faint comets. One attempt at detecting OH emission from an MBC was made,
using the medium-resolution X-shooter spectrograph on the 8 m VLT, but only upper
limits were obtained, at 8 9×1025 mol s1,in2.5hofintegration(Snodgrass et al.
2017b). A search for OH emission from Ceres using VLT/UVES produced a similar
upper limit (Rousselot et al. 2011), which is below the production rate found by Her-
schel observations (Küppers et al. 2014), suggesting some variability in the outgassing
rate from Ceres.
Atomic oxygen emissions are detected in the optical range through three forbidden
oxygen lines at 5577.339 Å for the green line and 6300.304 and 6363.776 Å for
the red doublet. Those lines are produced by prompt emission following the photo-
dissociation of various parent molecules (H2O, CO2,CO,butalsoO
2) into a short-
lived excited oxygen atoms (Festou and Feldman 1981;Cessateur et al. 2016). The
combination of measurements of two forbidden lines O1D and O1S shows that is
possible to even estimate the CO2/H2O ratio (McKay et al. 2012;Decock et al. 2013;
Cessateur et al. 2016). The 6300.304 Å line, which is the brightest of the three forbidden
oxygen lines has been successfully used to derive water production rates (e.g., Spinrad
1982;McKay et al. 2012). Observation of the forbidden oxygen lines require high-
resolution spectroscopy and a sufficient Doppler shift of the comet emission lines to
be distinguished from telluric oxygen lines. Those observations are challenging and
cannot be done for all comets. However, they may be a very sensitive way to detect
water (or CO and CO2) on faint comets, as observations of the Rosetta target 67P
showed that those oxygen lines were among the first emissions to be detected from
Earth (Jehin 2015). Such observations have been attempted on the MBC 133P with the
UVES instrument at the VLT, but none of the forbidden oxygen lines were detected
(Bodewits, Private Communication, 2017). Observations of Ceres and Themis also
produced only upper limits of 4.6×1028 and 4.5×1027 mol s1, respectively (McKay
et al. 2017).
Finally, for the UV/visible range, there is the emission line of Hαat 6563 Å. Despite
its common use in astronomical observations, it has been detected in relatively few
comets, and the best example of which we are aware was for Hyakutake, a very
bright comet with Q=3×1029 mols1, in spectroscopy with the 2.7 m telescope
at McDonald observatory (Combi et al. 1999). Hαdoes not appear promising for
detection of outgassing from an MBC.
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5Page 24 of 59 C. Snodgrass et al.
Fig. 3 Spectrum of water emission features in the NIR–radio (1 µmto10cm)atR=100, generated for
an MBC with Q=1024 mol s1,r=3au,= 2 au, and an atmospheric temperature of 80 K. The beam
width is 1at all wavelengths
5.2 Infrared and sub-mm/radio emission features
Molecular species in the gas comae of comets, including water, produce a large number
of emission features in the infrared (Crovisier 1984;Mumma et al. 2003;Bockelée-
Morvan et al. 2004;Cernicharo and Crovisier 2005) and sub-millimetre (Biver et al.
2007;Bockelée-Morvan 2008), which are caused by the vibrational and rotational
modes. We show the NIR–radio spectrum of a model MBC atmosphere that comprised
water, generated using the Goddard Planetary Spectrum Generator3(Villanueva et al.
2015), in Fig. 3. Water itself is difficult to observe from Earth due to its presence in
the atmosphere, but at high resolution and with sufficient Doppler shift comet lines
can be separated from telluric ones. Sufficiently high spectral resolution also allows
the structure of vibrational bands to be resolved, or even isotopic measurements to
be made for bright comets (e.g., Paganini et al. 2017). The advantage of observing
in this region is that water is directly detected, rather than production rates being
derived via daughter species. For fainter comets, where high-resolution spectroscopy is
impossible, or where there is too low radial velocity with respect to Earth, observations
from space telescopes are necessary. Some of the most sensitive observations were
made with the Akari satellite, with detections of the 2.7 µmν3band of water at
Q=1.2×1027 mols1made in comet 22P/Kopff when it was at 2.4 au from the Sun
(Ootsubo et al. 2012). This band was also observed in bright comets in both imaging
and spectroscopy by the International Space Observatory (ISO) satellite (Colangeli
et al. 1999;Crovisier et al. 1997) and the Deep Impact spacecraft (Feaga et al. 2014).
At longer wavelengths, the ν2band at 6.3 µm was observed by ISO and Spitzer, again
for relatively bright comets with Q1028 1029 mol s1(Crovisier et al. 1997;
3https://ssed.gsfc.nasa.gov/psg/index.php
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The Main Belt Comets and ice in the Solar System Page 25 of 59 5
Bockelée-Morvan et al. 2009). None of these space telescopes are still operating at
these wavelengths, but the James Webb Space Telescope (JWST) will (see Sect. 8).
At sub-mm and radio wavelengths very high resolution spectroscopy is possible,
enabling detection of spectrally resolved individual rotational lines used to determine
the kinematics of gas flows and excitation conditions in the coma, although mostly
these observations are sensitive to larger molecules. With large dishes or arrays on
Earth a large number of molecules have been identified in comets (e.g., Biver et al.
2002,2014), especially hydrocarbons, with methanol and HCN being most regularly
observed, and abundances normally measured relative toHCN. Water is generally not
identified from the ground, although Drahus et al. (2012) were able to detect the 183
GHz band with the 30 m IRAM telescope during the close approach of comet 103P, at
Q=1.7×1025 mols1. The launch of sub-mm space telescopes, in particular Odin
and Herschel, allowed observations of several ortho- and para-water lines (e.g., at 557
and 1113 GHz; de Val-Borro et al. 2010,2014;Hartogh et al. 2010;Biver et al. 2012).
The 3.5 m mirror of Herschel gave it excellent sensitivity, and the high-resolution
Heterodyne Instrument for the Far Infrared (HIFI; de Graauw et al. 2010) instrument
meant that D/H could be measured for the first time in a bright enough JFC (103P;
Hartogh et al. 2011).
Direct detection of H2O emission from MBCs has been attempted using Herschel.
Based on the visibility and anticipated gas emission activity during the Herschel mis-
sion lifetime (2009–2013), two MBCs, 176P and P/2012 T1, were observed by HIFI
(de Val-Borro et al. 2012;O’Rourke et al. 2013). 176P passed its perihelion on 2011
June 30 and was observed by Herschel/HIFI on UT 2011 August 8.78 when it was at
a heliocentric distance of 2.58 au and a distance of 2.55 au from the spacecraft. At the
end of the mission, the newly discovered P/2012 T1 was observed as part of director’s
discretionary time on UT 2013 January 16.31, about 3 months after its perihelion pas-
sage, when the object was at a heliocentric distance of 2.50 au and a distance of 2.06
au from Herschel. The line emission of the fundamental 110–101 rotational transition
of ortho-H2O at 557GHz was searched for in both objects. There was no detection of
H2O line emission in either of the targets. However, sensitive 3-σupper limits were
inferred for the H2O production rate of <4×1025mol.s1and <7.6×1025 mol.s1
for 176P and P/2012 T1, respectively. While 176P was shown to be inactive during
its 2011 passage from ground-based observations, dust emission activity was clearly
observed in P/2012 T1 at the time of the Herschel observation suggesting that the gas
production was lower than the derived upper limit (O’Rourke et al. 2013).
Finally, at very long (radio) wavelengths, there is OH emission at 18 cm. There
are two lines (1.665 and 1.667 GHz) that are regularly observed in comets, especially
using the Nançay array (Crovisier et al. 2013). The (at the time) largest single dish
telescope in the world, the 300 m Arecibo radio telescope, has also targeted comets at
18 cm (Lovell et al. 2002), but even using these facilities only the brightest comets are
detectable, with the most sensitive detection again coming from the close approach of
103P to Earth in 2010. Detection of MBC-level outgassing at 18 cm does not appear
likely.
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5Page 26 of 59 C. Snodgrass et al.
5.3 Absorption features from surface ice or coma ice grains
It is also possible to detect water ice directly on surfaces of small bodies, or on grains
in the comae of comets, through absorption features in the reflected solar continuum.
Water ice may take many forms, depending on the temperature and pressure at the
time of formation (Petrenko and Whitworth 1999). If temperatures were low (T <
50K), water would have frozen out into its amorphous phase, but had they been higher
(120–180 K), water molecules would have been able to arrange themselves into a crys-
talline structure (Mukai 1986). Observationally, amorphous ice and crystalline ice are
recognisable through infrared spectroscopy. The position, shape, intensity and width
of absorption bands in an ice spectrum are indicators of the structure, temperature
and thermal history of the ice (Newman et al. 2008). For ground-based observations,
there are two key bands for recognising crystalline (as opposed to amorphous) water
ice. One is the 3.1 µm Fresnel reflection peak, which is caused by a stretching mode
of the water molecule, and another one is a temperature-sensitive absorption band at
1.65 µm that is only seen in crystalline ice (Grundy and Schmitt 1998). The 1.65 µm
feature is absent in most available NIR spectra of the observed comets, except for the
outbursting comet P/2010 H2 (Yang and Sarid 2010). On the other hand, several ther-
modynamical models have proposed that the heat, generated through crystallisation
of amorphous ice, is an important energy source thought to trigger distant cometary
activity even when comets are beyond the critical sublimation heliocentric distance
(Prialnik 1992;Prialnik et al. 2004). Given the current observational constraints on
water ice in comets, it remains uncertain whether the ice in comets is amorphous even
before their entry to the hot, inner region of the Solar System.
Most direct evidence of the presence of water-bearing minerals on asteroids comes
from infrared observations, particularly in the so-called 3 µm region, where the
hydroxyl fundamental absorption and the strong first overtone of water are both
present. Characteristics of absorption features in this region, such as wavelength posi-
tion of the band centres, the shape of the absorption bands and their relative intensities
are diagnostics of mineralogy as well as abundance. However, the 3 µmregionis
notoriously difficult for ground-based observations because of intense atmospheric
absorptions and faint solar radiation in this wavelength regime. Nevertheless, spectral
surveys of large asteroids in the 3 µm region have flowered, mostly using the SpeX
instrument at the IRTF telescope (Takir and Emery 2012;Rivkin et al. 2015).
Compared with the NIR observations, visible spectroscopy is relatively easy to
obtain using small-to-moderate-sized telescopes. Therefore, numerous efforts have
been made to search for diagnostic features in MB asteroids between 0.4 and 0.9 µm
(Vilas and Smith 1985;Luu and Jewitt 1990;Sawyer 1991;Xu 1994;Bus and Binzel
2002). A shallow but fairly broad (width 0.25 µm) absorption feature centred
near 0.7 µm is found to be very common among the low-albedo asteroids (Sawyer
1991). This feature is attributed to an Fe2+Fe3+charge transfer transition in
hydrated minerals (Vilas and Gaffey 1989). Consistently, a 0.7 µm feature is also seen
in the laboratory spectra of CM2 carbonaceous chondrite meteorites and terrestrial
phyllosilicates (a hydrated mineral), which is similar to the asteroidal feature both in
band centre and strength (Vilas and Gaffey 1989). Although King and Clark (1997)
warned that a feature near 0.7 µm has been seen in many different minerals, and so
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The Main Belt Comets and ice in the Solar System Page 27 of 59 5
this feature alone is not sufficient for identifying aqueous alteration, the majority of
minerals that show an absorption centred at 0.7 µm are iron- and OH-bearing silicates
(Rivkin et al. 2002).
Additionally, 13 C-, P-, and G-class asteroids were observed in the UV/blue spec-
tral regions and they exhibit an absorption feature near 0.43 µm(Vilas et al. 1993;
Cochran and Vilas 1997). The blue/UV drop-off observed in these low-albedo aster-
oids is thought to be attributed to a ferric spin-forbidden absorption in aqueously
altered iron-containing minerals (Burns 1981). Among low-albedo asteroids, the UV
drop-off is found to be correlated with the 3 µm absorption due to hydrated mineral
components (Feierberg et al. 1985). Similarly, the same correlation is observed in car-
bonaceous chondrites, which contain a significant fraction of hydrated silicates, such
as phyllosilicates. Therefore, it has been suggested that the (U-B) colour might be also
useful as an indicator of the presence hydrated materials (Gaffey and McCord 1978).
These UV/visible wavelength features have not yet been observed in MBCs.
5.4 In situ detection of water
The last set of techniques for detecting water are those that can be applied by visiting
the target with a spacecraft. Various cometary missions have done so, with the recent
Rosetta mission combining mass spectroscopy with remote sensing instruments that
were sensitive to water via the various emission and absorption features described in
previous sections. The mass spectrometer and pressure gauge instrument ROSINA on
Rosetta first detected water in August 2014, at a distance of 100 km from the comet
and 3.5 au from the Sun, at Q5×1025 mols1(Hansen et al. 2016), although
earlier detection was possible from the remote sensing instruments (see Sect. 7). Mass
spectroscopy can measure both precise relative abundancesof water and other volatiles,
and isotopic ratios (Altwegg et al. 2015).
Suitably equipped spacecraft can also search for evidence of buried ice, in addition
to identifying surface ice or outgassing water. Radar detection of subsurface water
relies on the marked difference in dielectric properties between liquid water and other
common geo-materials such as rocks and water ice. Whereas the relative dielectric
permittivity of ice is 3.1, and that of rocks ranges between 4 and 10, liquid water has
a permittivity of 80 (Ulaby et al. 1986, Appendix E). The marked dielectric contrast
between a layer of liquid water and the surrounding rocks results in a high radar
reflection coefficient, and thus in a strong radar echo. On Earth, this feature is routinely
used in the identification of subglacial lakes (see e.g., Palmer et al. 2013). While liquid
water is not expected to be present on an MBC, radar would be useful in identifying
rock and ice layers. The radar reflection coefficient of a surface is determined both
by dielectric properties and by geometry. Rough surfaces diffuse the impinging radar
pulse and weaken the backscatter towards the radar (Ogilvy 1991), thus making the
identification of water more and more ambiguous as the water/rock interface becomes
rougher or more curved.
The depth at which radar can detect echoes from a subsurface interface is affected
by the properties of the material between the surface and the interface. Electromagnetic
waves are absorbed in a dielectric medium, such as rocks and ices, and can be scat-
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5Page 28 of 59 C. Snodgrass et al.
tered by cracks, voids and other irregularities at scales comparable to the wavelength.
However, assuming the results of the CONSERT experiment on board Rosetta can be
used to predict the expected bulk dielectric properties of MBCs, then both dielectric
attenuation and volume scattering should be low (Kofman et al. 2015).
At this moment, there are no useful precedents for the detection of water within
an MBC. The CONSERT experiment is based on the transmission of a radar signal
through the cometary material, rather than on its reflection at a dielectric interface.
If used on an MBC, CONSERT would probably be unable to identify a ice layer,
although it would likely measure a significant signal attenuation. The measurements
at Phobos of the MARSIS experiment on board Mars Express (Picardi et al. 2005)are
of limited value in predicting the expected performance of radar in detecting water
within an MBC. MARSIS was originally designed solely for the observation of Mars,
and Phobos is smaller than the MARSIS footprint (Safaeinili et al. 2009).
A higher-frequency radar, such as SHARAD on board NASAs Mars Reconnais-
sance Orbiter (Seu et al. 2007), could be used to acquire full coverage of an MBC
and produce its three-dimensional radar tomography. This is a well-known technique
onEarth(seee.g.,Knaell and Cardillo 1995), but the irregular shape of an MBC will
require some developments. In spite of the difficulties in deconvolving of the effects
of geometric and dielectric interface properties on the echo, the detection of water
pockets could then be demonstrated based on their greater radar backscatter cross
section, compared to similar structures elsewhere in the subsurface, their relation to
the surrounding stratigraphy, and the general geologic context.
6 Expected activity levels for MBCs
When considering how best to detect evidence of sublimation from MBCs, it is impor-
tant to consider what levels of activity can be reasonably expected, as well as which
MBCs are most likely to exhibit detectable levels of activity.
6.1 Predictions from energy balance
As described in Hsieh et al. (2015a) and elsewhere, the equilibrium temperature and
unit-area water sublimation rate for a sublimating grey-body at a given heliocentric dis-
tance can be computed iteratively from the energy balance equation for a sublimating
grey-body (neglecting heat conduction),
F
r2(1A)=χεσ T4
eq +Lf
D˙mw(T),(2)
the sublimation rate of ice into a vacuum,
˙mw=Pv(T)μ
2πkT ,(3)
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The Main Belt Comets and ice in the Solar System Page 29 of 59 5
and the Clausius–Clapeyron relation,
Pv(T)=611 ×exp Hsubl
Rg1
273.16 1
T.(4)
In the energy balance equation, Teq is the equilibrium surface temperature, F=
1360 W m2is the solar constant, ris the heliocentric distance of the object in au,
A=0.05 is the assumed Bond albedo of the body, χdescribes the distribution of
solar heating over an object’s surface (χ=1 for a flat slab facing the Sun where this
so-called subsolar approximation produces the maximum attainable temperature for
an object, χ=πfor the equator of a rapidly rotating body with zero axis tilt, and
χ=4 for an isothermal sphere, as in the limiting approximation of an extremely fast
rotator and strong meridional heat flux), ε=0.9 is the assumed effective infrared
emissivity, and σis the Stefan–Boltzmann constant. In the sublimation rate equation,
L=2.83 MJ kg1is the latent heat of sublimation of water ice, which is nearly inde-
pendent of temperature, fDdescribes the reduction in sublimation efficiency caused
by the diffusion barrier presented by a rubble mantle, where fD=1 in the absence
of a mantle, ˙mw(T)is the water mass loss rate due to sublimation of surface ice,
μ=2.991×1026 kg is the mass of one water molecule, kis the Boltzmann constant.
The equivalent ice recession rate, ˙
i, corresponding to ˙mwis given by ˙
imw,
where ρis the bulk density of the object. Lastly, in the Clausius–Clapeyron relation,
Pv(T)is the vapour pressure of water in Pa, Hsubl =51.06 MJ kmol1is the
heat of sublimation for ice to vapour, and Rg=8314 J kmol1K1is the ideal gas
constant.
We use these equations to compute the peak expected sublimation rate using the
isothermal approximation for the known MBCs, and also convert unit-area sublimation
rates from the above equations to a total surface-wide sublimation rate for each object
assuming spherical bodies and active areas of 100% (Table 5). We can see from these
results that there is a range of expected maximum water sublimation rates expected
from various MBCs, and two important factors influencing these maximum rates are
perihelion distance and nucleus size. Hsieh et al. (2015a) noted that, on average, the
currently known MBCs have higher eccentricities (and therefore smaller perihelion
distances) than the overall outer main-belt asteroid population, suggesting that the
activity of those particular objects could be at least partly due to the fact that they
reach higher peak temperatures than other asteroids with similar semimajor axes. The
significance of having larger-than-average eccentricities and therefore smaller perihe-
lion distances is further illustrated by Fig. 4, which illustrates water ice sublimation
rates as a function of heliocentric distance, computed using Eqs. 2,3, and 4as well as
the positions of the perihelion, aphelion, and semimajor axes of seven of the known
MBCs. It is interesting to note that over the ranges of heliocentric distances traversed
by these MBCs over the course of their orbits, the theoretical water ice sublimation rate
varies by as much as four orders of magnitude in the isothermal case. This suggests
that the fact that all are seen to be active near perihelion is not just a coincidence or
observational bias.
Meanwhile, we see that larger maximum total water sublimation rates are expected
for larger objects than smaller objects due to the simple fact that larger objects have
123
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5Page 30 of 59 C. Snodgrass et al.
Tab le 5 MBC activity properties
Object qaRNbvesc cQ(H2O)qddM/dteActive rangefReferencesg
133P 2.664 1.9 2.2 3.5×1016 1.6×1024 1.4 350–109[1-4]
238P 2.366 0.4 0.5 1.1×1017 2.2×1023 0.2 306–123[5,6]
176P 2.580 2.0 2.4 4.9×1016 2.5×1024 0.1 1–19[1,7]
259P 1.794 0.3 0.4 5.0×1017 5.7×1023 – 315–49[8-10]
324P 2.620 0.6 0.7 4.2×1016 1.9×1023 4.0 300–96[11-13]
288P 2.436 1.3 1.5 8.4×1016 1.8×1024 0.5 338–47[14-16]
P/2012 T1 2.411 9.3×1016 17
–48[17-18]
P/2013 R3 2.204 1.8×1017 ––14
–43[19]
313P 2.391 0.5 1.2 1.0×1017 1.3×1024 0.4 354–53[20–23]
P/2015 X6 2.287 1.4×1017 1.6 329–344[24]
P/2016 J1-A 2.448 <0.9 <1.1 8.1×1016 0.7 346–12[25,26]
P/2016 J1-B 2.448 <0.4 <0.5 8.1×1016 0.5 346–12[25,26]
aPerihelion distance, in au
bEffective nucleus radius, in km
cTheoretical escape velocity, m s1, in assuming spherical, non-rotating nuclei with bulk densities of ρ2500 kg m3
dUnit-area and total (assuming 100% active area) theoretical water sublimation rates, in mol m2s1and mol s1, at perihelion, assuming a spherical sublimating isothermal
grey-body.
ePeak reported observed dust mass production rate in kg s1
fTrue anomaly range over which activity has been reported. Listed ranges are incomplete, sometimes reflecting limitations in observational coverage rather than the confirmed
absence of activity over orbit arcs not included in the listed true anomaly ranges
gReferences: [1] Hsieh et al. (2009a); [2] Jewitt et al. (2014b); [3] Hsieh et al. (2010); [4] Kaluna and Meech (2011); [5] Hsieh et al. (2009b); [6] Hsieh et al. (2011b); [7]
Hsieh et al. (2011a); [8] Jewitt et al. (2009); [9] MacLennan and Hsieh (2012); [10] Hsieh and Chavez (2017); [11] Moreno et al. (2011a); [12] Hsieh et al. (2012c); [13]
Hsieh (2014b); [14] Hsieh et al. (2012b); [15] Licandro et al. (2013); [16] Agarwal et al. (2016); [17] Hsieh et al. (2013); [18] Moreno et al. (2013); [19] Jewitt et al. (2014a);
[20] Hsieh et al. (2015b); [21] Jewitt et al. (2015a); [22] Jewitt et al. (2015c); [23] Pozuelos et al. (2015); [24] Moreno et al. (2016); [25] Hui et al. (2017); [26] Moreno et al.
(2017)
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The Main Belt Comets and ice in the Solar System Page 31 of 59 5
Fig. 4 Mass loss rate due to water ice sublimation from a sublimating grey body as a function of heliocentric
distance over the range of the main asteroid belt, where the semimajor axis ranges of the inner, middle,
and outer MB are labelled IMB, MMB, and OMB, respectively, and mass loss rates calculated using the
isothermal approximation (χ=4) and subsolar approximation (χ=1) are marked with solid and dashed
green lines, respectively. The positions of the major mean-motion resonances with Jupiter (4:1, 3:1, 5:2, and
2:1) that delineate the various regions of the main asteroid belt are shown with vertical dashed black lines.
Also plotted are the perihelion distances (red, right-facing triangles), semimajor axis distances (orange
circles), and aphelion distances (green, left-facing triangles) of the known outer main-belt MBCs, as well
as the range of heliocentric distances over which they have been observed to exhibit activity (thick black
horizontal lines) Image reproduced with permission from Hsieh et al. (2015a), copyright by Elsevier
greater surface areas over which sublimation can occur. This presents an interesting
dilemma since larger (and therefore more massive) objects also have larger escape
velocities (cf. Table 5) that ejected dust grains must reach in order to overcome the
gravity of the active body and escape into space where they can be observed as comet-
like dust emission. Ejection velocities for dust emission from many MBCs tend to
be relatively low compared to ejection velocities for classical comets, yet they are
often comparable to the escape velocities of the corresponding nuclei (cf. Hsieh et al.
2004). This competition between size preferences may be a highly consequential effect,
meaning that there may be an ‘ideal’ size or size range for detectable MBCs where
they are large enough to produce observable amounts of gas or dust emission but small
enough that ejected dust can actually escape into space to become observable in the
first place. Hsieh (2009) also noted that if MBC ice must be preserved in subsurface
layers in order to survive to the present day and only excavated recently (cf. Hsieh
et al. 2004), currently active MBCs must also tend to somewhat larger sizes so that
they have the surface area to experience impacts capable of excavating subsurface ice
on timescales consistent with their discovery rates in the asteroid belt, but still must
be small enough for ejected dust to reach the escape velocity of the emitting body.
Interestingly, we see that measures of activity strength such as the peak inferred dust
production rate or total activity duration listed in Table 5are not strongly correlated to
either the theoretical unit-area or total expected water sublimation rates at perihelion.
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5Page 32 of 59 C. Snodgrass et al.
This discrepancy could be due to a number of factors, including differences in initial
ice content or distribution, devolatilisation progression of active sites, overall fractions
of active surface area on each body, or thermal properties of each object. In particular,
rotation rates are not known for most MBC nuclei, yet may be significant factors
in whether it is most appropriate to model their thermal behaviour using either the
subsolar or isothermal approximations.
Notably, the total maximum predicted water sublimation rates listed in Table 5are
smaller than most of the detection limits for various sublimation detection techniques
discussed elsewhere in this paper, and also assume 100% active areas, which are almost
certainly not the case for most or all MBCs (for comets active areas of a few percent
or less are typical; A’Hearn et al. 1995). We note, however, that these values represent
calculations based on the isothermal (or ‘fast-rotator’) approximation, which produces
the lowest temperatures and lowest production rates. At the other extreme, the sub-
solar approximation gives production rates as much as two orders of magnitude larger,
suggesting that we may be more likely to detect gas emission from slower-rotating
MBC nuclei or those with higher thermal inertias. Additionally, the total maximum
predicted water sublimation rates listed here are computed assuming the surface area
of spherical nuclei, whose effective radii have been estimated from their scattering
cross-sections. If MBC nuclei are irregularly shaped, like 67P, or perhaps even binary,
like 288P (Agarwal et al. 2016), larger usable surface areas and therefore higher gas
production rates could be possible.
6.2 Predictions from observed dust activity
Although no direct detection of gas in an MBC has been made to date, we can use
the observed dust comae as indicators of the mass-loss rates from these bodies. One
method is to model the observed dust distribution to find the probable grain size
distribution, the dust mass ejected over time, and then use nominal gas to dust mass
loss ratios to constrain the sublimation rate. Moreno et al. (2011a) derived a maximum
sustained dust mass loss rate of 34kgs
1at 324P. In similar studies, Moreno
et al. (2013) found that P/2012 T1 emitted 6 25 ×106kg of dust over 4–6 months,
implying an average mass loss rate of 2kgs
1, while Pozuelos et al. (2015) found
dust mass loss rates of 0.20.8kgs
1at 313P. Assuming a dust/gas mass ratio
of 1:1, this would imply maximum water sublimation rates of Q(H2O)1026
mols1.
Another method is by direct comparison with observations of normal active comets.
The standard method of estimating the dust content of the coma is through measure-
ment of the Afρparameter (units of cm) as defined by A’Hearn et al. (1984). Although
transformation to a dust mass-loss rate requires further assumptions or modelling, Afρ
has the advantage that it is calculated solely on the basis of the observed photometric
brightness. At the same time, the production rate Q(gas)can be accurately constrained
through narrow-band photometry or spectrophotometry. A’Hearn et al. (1995) reported
photometric observations and analysis of tens of comets, finding that for a typical
comet log[Afρ/Q(OH)]=−25.8±0.4. The most active MBCs discovered so far
appear to have Afρ10 20 cm (P/2012 T1, Hsieh et al. 2013; 313P, Hui and Jewitt
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The Main Belt Comets and ice in the Solar System Page 33 of 59 5
2015). Using the above relationship would imply Q(H2O)(7+10
4)×1025 mols1
assuming Q(OH)0.9Q(H2O).
Comparing these estimates of Q(H2O)with the upper limits in Table 2implies
that current observational efforts may be close to directly detecting gas in MBCs.
But there are two important factors to take into account. First, many upper limits are
based on the non-detection of the bright CN emission band and assuming normal
CN/OH cometary ratios. As discussed in Sect. 4.2, modelling indicates that more
volatile species in subsurface ices may be depleted due to thermal processing. If HCN
is the photodissociation parent molecule of CN, then the CN/OH ratio in MBC comae
may be lower than in a normal comet. This would imply that Q(H2O)is higher than
expected from the measured upper limit to Q(CN). Second, mantling of the source
region could occur to due fallback of slowly moving dust grains back onto the surface
of the MBC. This ‘airfall’ was readily apparent in images of 67P from the Rosetta
spacecraft (Thomas et al. 2015). This could plausibly increase the dust/gas ratio over
time as the airfall layer on the sublimation site increases and the thermally active
ice surface diminishes. This would mean that Q(H2O)is less than anticipated from
measurements of the dust coma.
6.3 Relative strength of water signatures
We now consider which of the various water detection methods listed in Sect. 5is
most promising to detect MBC-level outgassing, which, following the discussion in
the previous subsections, we take as Q(H2O) = 1024 mol s1. Comparing the rel-
ative effectiveness of the listed techniques is not straightforward, due to the great
differences in observation type, geometry, and activity level of the comets used as
examples. Even observations of the same comet—C/2009 P1 (Garradd)—with differ-
ent telescopes/techniques gave different production rates, which was attributed to a
halo of icy grains and different FOVs (Combi et al. 2013). Nevertheless, we attempt to
draw some approximate conclusions by scaling these observations with a number of
simplifying assumptions. We discard the possibility of in situ investigation for now;
although it would certainly be effective, it is a very different prospect to astronomical
observation in terms of cost. We also leave detection of absorption features out of this
comparison, as this appears to be only possible for larger bodies: even for much more
active comets, surface ice features are not detected remotely, and even in situ explo-
ration shows only relatively small and variable ice patches on surfaces (De Sanctis
et al. 2015).
To compare the strength of emission features across a range of wavelengths, we
scale the observations listed in Table 4to give the exposure time required to achieve
an S/N of 5 detection of Q=1024 mols1at r= 3 au and = 2 au. We ignore
velocity effects, both heliocentric (i.e., Swings effect) and geocentric (i.e., assuming
that lines are detectable without telluric interference). We make the assumption that
the aperture is of fixed angular size and that the coma distribution scales as ρ1, i.e.,
that the signal scales as 1, which is not necessarily the case. A better approximation
could be made with scaled Haser models (Haser 1957) for each observation, taking
into account the different slit or measurement aperture areas, but we judged this to be
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5Page 34 of 59 C. Snodgrass et al.
Fig. 5 Exposure time required to get 5σdetection of Q=1024 mol s1outgassing from an MBC, based
on scaling the comet detections at various wavelengths in Table 4
Fig. 6 Same as Fig. 5, but scaled to a telescope diameter of 30 cm in space
of secondary importance given the other uncertainties. We plot the scaled exposure
times needed to make 5σdetections as a function of wavelength in Fig. 5.Itisclear
that the exposure times required with current technology are infeasible, and that there
is a very large range (from 1 year to a Hubble time!). There are minima at Ly-α,
the 2.66 µm water band, and at 557 GHz, indicating that these are the most promising
places to try for future detections, although none of the space telescopes used to make
the sensitive detections at these wavelengths (PROCYON/LAICA, Akari, Herschel)
remain operational today. Of the ground-based observations, the most promising is
the detection of the 3080 Å OH band with the photometer at the Lowell 1.1 m.
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The Main Belt Comets and ice in the Solar System Page 35 of 59 5
To enable a slightly fairer comparison between the different telescopes, we further
scaled the exposure times to the same diameter of telescope. This also involves the
significant assumption that telescope sensitivity can simply be scaled by collecting
area, and obviously ignores the fact that a 10 m radio dish is a much cheaper instrument
than a 10 m optical telescope. For comparison, we chose to scale the observations to
a 30 cm diameter telescope (Fig. 6), as could be imagined in a relatively cheap space
telescope in Earth orbit (e.g., the ESA S-class CHEOPS). Here the strong Lyαbands are
clear winners, which is not surprising given the small size of current telescopes at this
wavelength, but this ignores the significant background issues in these observations—
S/N will not scale directly with telescope collecting area in this case. The NIR region
continues to be promising, and a small-to-mid-sized space telescope covering 2.66 µm
could be of use for a broad survey of weak outgassing. In the immediate future, the
JWST will cover this range, but will not be used for broad surveys (see Sect. 8).
7 Rosetta
The recent ESA mission Rosetta has been transformative in cometary science, which
includes implications for MBCs, at least by providing a very detailed source of infor-
mation on a JFC for comparison. In this section, we briefly review some of the more
relevant findings, in particular considering the early phase of the mission, when Rosetta
first encountered comet 67P as its activity was only just starting. We are still in the
early days of analysis of Rosetta data, but a review of some of the key results to date
is presented by Taylor et al. (2017).
Rosetta entered orbit around 67P in August 2014, when the comet was at 3.6 au
from the Sun, but was already able to study the beginning of activity while approaching
using its remote sensing instruments, with the first detections of the comet and its dust
in images taken as early as March 2014 (Tubiana et al. 2015). The first detection
of the gas coma was through sub-mm observations of the 557 GHz water band by
the MIRO instrument, in June 2014 at 3.9 au from the Sun, when the spacecraft was
around half a million km from the comet (Gulkis et al. 2015). At this time the water
production rate was slightly below the strongest limits on MBCs to date, at 1 ×1025
mols1, and outgassing was not detectable from Earth (via sensitive searches for CN
with large telescopes—Snodgrass et al. 2016;Opitom et al. 2017). This is the lowest
activity comet environment ever visited by a spacecraft, with all previous mission
performing fast flybys at 1 au, and there were some surprising results: the Rosetta
Plasma Consortium instruments discovered oscillations in the magnetic field at around
40 mHz attributed to interactions with cometary ions and the solar wind (Richter et al.
2015), the so-called ‘singing comet’ based on the public release of an audio version of
this interaction. Feldman et al. (2015) show that emission lines from atomic hydrogen
and oxygen, observed by the Alice instrument in the UV, could only be explained by
electron impact dissociation rather than the more typical photodissociation seen in
cometary comae. This effect was also necessary to explain the morphology of the gas
in the inner coma, as imaged by the OSIRIS cameras (Bodewits et al. 2016). Both
of these effects were no longer detectable closer to perihelion, when the comet had a
more typical activity.
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5Page 36 of 59 C. Snodgrass et al.
Fig. 7 a Calculated specific intensities of the OH(0–0) band at 3080 Å due to both fluorescence and
prompt emission following electron impact dissociation of water, as a function of radial distance from the
nucleus. The underlying H2O and OH densities were calculated assuming a Haser model with Q(H2O)=
1024mol.s1.bIntegrated surface brightness profiles observed at Earth derived from this model, assuming
the MBC is at r=3au,=2au
An MBC with activity levels similar to, or even lower than, 67P at 3.5 au, could be
expected to show similar physics. While testing its interaction with the solar wind will
require in situ probing with a spacecraft equipped with a magnetometer, it is probable
that electron impacts would affect the observed emission in gas lines. To estimate the
magnitude of this effect, we assume that electron impact dissociation of H2O into the
A2Σ+state of OH takes place, followed by prompt emission to the ground state X2Π.
The increase in extra emission is extremely sensitive to the production rate of water,
as the photoionisation of H2O is expected to be the primary source of electrons in the
inner coma. Scaling the conditions at 67P to an MBC at 3.0 au with a production
rate of Q(H2O)=1024 mols1, the specific luminosity of the OH coma in ergs cm2
s1isshowninFig.7. We model the density distribution using a simple Haser model,
and scale the Rosetta measured electron density distribution to estimate the electron
impact-induced emission using the technique described in Bodewits et al. (2016).
A nearby spacecraft should be sensitive to this emission. However, integrating this
luminosity distribution into potential surface brightnesses as seen from Earth shows
that while a significant brightness enhancement exists, it is located within 1 arcsec of
the MBC at low apparent flux levels.
Aside from exploring a very low activity comet environment, Rosetta is also impor-
tant in the context of MBCs as it reveals a larger picture of comet formation. The full
implications are likely to be debated for some time as further analysis of the Rosetta
data is performed, but first attempts to model comet formation based on Rosetta results
(e.g., Davidsson et al. 2016) point to the low density/high porosity of the comet (Sierks
et al. 2015) and the presence of hyper-volatile species such as O2and N2(Bieler et al.
2015;Rubin et al. 2015) to show that it must have formed far from the Sun and
avoided significant heating during its formation and subsequent evolution. This must
be significantly different to the MBC case discussed in Sect. 4, and further points to
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The Main Belt Comets and ice in the Solar System Page 37 of 59 5
an expected difference in the observed composition of outgassing species between
MBCs and JFCs.
8 Future prospects
In this section, we consider what options we will have for searching for water in
MBCs in the coming years, both from ground- and space-based observatories, and
from proposed dedicated space missions.
8.1 Ground-based telescopes
It is worth considering if a traditional photoelectric photometer mounted on a large
telescope might allow successful detection of gas from an MBC. A photometer per-
forms like a single large pixel, allowing it to achieve a particular signal-to-noise more
efficiently than a CCD, where the noise associated with bias and readout becomes
significant due to the large number of pixels in an aperture. Assuming a system could
be scaled up directly, a photometer on a 10 m class telescope would provide a factor
of 100 improvement over existing facilities, e.g., the 1.1 m telescope at Lowell
Observatory used by D. Schleicher. Knight and Schleicher (2015) detected CN in
30 min on comet C/2012 S1 ISON at r=4.55 au with a production rate of Q(CN)
1024 mols1, near the limit for this instrument. This implies a hypothetical detection
capability of 1021 1022 mols1when accounting for the larger mirror, longer inte-
grations, and an MBC at smaller rand . Such production rates are near or somewhat
below the most restrictive upper limits placed on MBC activity to date (as discussed
in Sect. 5). Although the capabilities are promising, the lack of spatial context needed
to definitively detect a very faint coma near a relatively bright nucleus likely makes a
photometer a sub-optimal choice.
We briefly consider narrowband imaging with a CCD, which would have advan-
tages over a photometer in providing spatial context, having generally higher quantum
efficiency, and offering the possibility of selecting arbitrarily sized apertures. How-
ever, despite the very low read noise of modern CCDs, the small number of photons
spread across a very large number of pixels makes the detection of faint gas emission
via imaging relatively inefficient: with a blue optimised system and judicious binning,
a CCD on a 10 m may be able to go 1–2 orders of magnitude fainter than a photometer
ona1mwiththesameexposure times. However, this still likely underperforms the
hypothetical photometer on a 10 m by about an order of magnitude.
With the start of scientific operations of the Atacama Large Millimetre/submillimetre
Array (ALMA) interferometer in the second half of 2011, several observations of mod-
erately bright Oort Cloud comets have been carried out with high sensitivity and high
spatial resolution (e.g., observations of HNC, HCN, H2CO and CH3OH; Cordiner
et al. 2014,2017). As of 2017, ALMA consists of 43 12 m antennas and has unique
capabilities to probe the physical and chemical structure of the innermost regions of
the coma with great accuracy, providing new impetus to theoretical investigations of
the coma. Spatially resolved observations allow for study of asymmetric outgassing,
acceleration and cooling of the coma gases, and variations in gas kinetic temperature
123
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5Page 38 of 59 C. Snodgrass et al.
resulting in greatly improved accuracy of the derived molecular abundances. In addi-
tion, the release site of a given cometary species, whether the nucleus or an extended
source in the coma, can be constrained by measuring the spatial distribution in the
inner coma on scales of the order of 1000 km from the nucleus for a compact array
configuration.
Beginning in the second half of ALMA observing cycle 5 in early 2018, the newly
installed Band 5 dual polarisations receivers, covering the frequency range 157–212
GHz, will become available creating exciting new observational possibilities. By pro-
viding a sensitive access to rotational transitions from H2O and its isotopologues,
ALMA has the capability to measure isotopic ratios in H2O to test for the thermal and
radiative processing history. These observations will contribute to our understanding
of where and how cometary materials originated, as well as offering insight into the
physical and chemical conditions of the solar nebula. Based on the upper limits on the
H2Oof<4×1025mol.s1and <7.6×1025 mol.s1for MBCs 176P and P/2012
T1 obtained by Herschel (de Val-Borro et al. 2012;O’Rourke et al. 2013), the total
estimated time for a <4×1025mol.s1detection of the H2O3
13–220 emission line at
183.310 GHz with ALMA is 80 h including calibration overheads. These observations
require excellent weather conditions with precipitable water vapour in the atmosphere
<0.4mm. Therefore, ALMA does not offer a promising way to detect water in MBCs
for sensitivity reasons, unless a new MBC is discovered that is 10 times brighter than
the ones observed by Herschel.
The next generation of extremely large telescopes (ELTs) are primarily designed to
work at red optical and NIR wavelengths. For the 39 m ESO ELT, currently scheduled
to be commissioned in 2024, the low- and intermediate-dispersion HARMONI spec-
trograph will have a wavelength coverage of 0.47 2.45 µm, missing the primary
fluorescence bands of OH(0–0) and CN(0–0). The 30 m Giant Magellan Telescope,
scheduled for commissioning in 2022, will have the low/intermediate resolution spec-
trograph GMACS and the high-resolution spectrograph G-CLEF, both of which will
operate at 0.35 0.95 µm and will be sensitive to CN(0–0). The proposed thirty
meter telescope would have the wide field optical spectrometer (WFOS) as a first light
instrument, operating at 0.31 1.0µm. Hence it would miss the primary OH(0–0)
band, although it would allow detection of the OH(1-0) band which has a relative
brightness of 5–10% depending on the distance and heliocentric velocity of the MBC.
8.2 Space telescopes
With sensitivity in the NIR region, the JWST, a 6.5 m telescope orbiting the Earth–Sun
L2 point, may provide our most immediate opportunity to directly detect water in an
MBC coma (Kelley et al. 2016). The two brightest fluorescence bands of water are
the ν3band at 2.7 µm and the ν2band at 6.3 µm, with fluorescence band g-factors
near 3.2×104and 2.4×104s1, respectively, when opacity effects are neglected
(Crovisier and Encrenaz 1983;Bockelée-Morvan et al. 2009;Debout et al. 2016).
JWST has spectral sensitivity at both bands: the NIRSpec instrument covers 2.7 µm
at spectral resolving powers, R=λ/λ,of100, 1000, or 2700; and MIRI
covers 6.3 µm with R70 and 3500 (Kendrew et al. 2015;Wells et al. 2015).
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The Main Belt Comets and ice in the Solar System Page 39 of 59 5
In general for comets and dark asteroids, the 2–3 µm region has a lower continuum
flux than the 6–7 µm region, due to the low albedos of dust and surfaces. In addition,
the sky background is stronger at the longer wavelengths due to zodiacal dust. Thus,
given the similarities in band g-factors, the ν3band should be easier to observe in a
MBC coma. We ran a JWST exposure time simulation with a NIRSpec fixed-slit (0.4
arcsec wide) and a V=19 mag MBC at r=2.7 au, producing water with a rate of 1025
mols1. The continuum would be detected with an SNR of 45 in 1 h. Assuming the
brightest water lines are about one-tenth the total band flux (1×1017 erg s1cm2
in a 0.2 arcsec radius aperture), they may be detected with a peak signal-to-noise ratio
of about 3 above the continuum in 104s of integration time. This is a challenging
observation, but with the right target and production rate, a direct water detection
should be possible.
A Far-Infrared (Far-IR) Surveyor mission concept for NASA’s 2020 Astrophysics
Decadal Survey has been proposed recently (Meixner et al. 2016). Originally envisaged
in NASAs 2013 Roadmap, this mission intends to build upon previous successful space
and airborne infrared astronomy observatories such as ISO, Spitzer, Herschel, SOFIA
and JWST.The mission will have a single large aperture telescope or an interferometer
with a large gain in sensitivity of about 103–104over the Herschel Space Observatory,
and better angular resolution. The Far-IR Surveyor will be able to detect both water
vapour and water ice, using the 43 µm water–ice band and a large number of water
bands, as well as other volatile molecules. In small bodies in the Solar System, the Far-
IR Surveyor can be used to detect gas sublimation and accurately measure the water
production rates. Direct detection of water in MBCs will be possible with sensitivity
to production rates of 1022mols1at a heliocentric distance of 2.5 au. Alternatively,
preliminary work continues for a future large UV/optical space telescope, with a 10–15
m diameter. Variously known as HDST or LUVIOR, details are not yet well known,
but this will be very sensitive to many gas emission features, and will certainly be
useful in studying weakly active comets, but is many years from becoming reality
(Dalcanton et al. 2015).
An alternative approach to building increasingly large space telescopes is to take a
small space telescope closer to the MBCs. CASTAway was recently proposed (as an
ESA medium class mission) to launch a 50 cm diameter telescope, equipped with a
low-resolution spectrograph covering 0.6–5 µm and a CCD camera, into an orbit that
loops through the asteroid belt (Bowles et al. 2017). This mission is primarily designed
to map the diversity of bodies in the asteroid belt, by performing a spectroscopic survey
of more than 10 000 asteroids of all sizes and close flybys of 10–20 diverse objects.
Even if an MBC cannot be one of the flyby targets (the relatively small size of the
population makes it unlikely that a suitable multi-asteroid tour can include one), the
telescope is designed to search for ice or outgassing, in the NIR through spectroscopy
or via OH emission in the UV through narrowband imaging. The advantage of a
dedicated survey mission, compared with JWST or other major facilities, is that many
more potentially ice bodies can be targeted. Placing a telescope in the asteroid belt
has the advantage that all sizes of asteroid can be targeted, and also presents unique
viewing geometries. For MBCs, such view points can be useful to characterise dust
tails, as was demonstrated by imaging P/2010 A2 with the OSIRIS cameras on Rosetta
123
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5Page 40 of 59 C. Snodgrass et al.
(Snodgrass et al. 2010b)—any camera on any mission passing through the asteroid belt
can potentially contribute to MBC dust trail characterisation as a target of opportunity.
8.3 Missions to MBCs
These various planned and proposed telescope facilities show that, while challenging,
the detection of water outgassing from MBCs may be achieved in the near future.
However, further detailed exploration of the water content of these bodies, and espe-
cially more challenging observations such as measurement of isotopic ratios (e.g., D/H
in water), will be beyond even the next generation of telescope facilities. Investigating
D/H, and the associated constraints on where in the Solar System planetary disc the
ice condensed, will require in situ measurement. Missions to visit MBCs have been
proposed for this purpose, to both ESA and NASA.
The European proposal Castalia would follow on from ESAs success with Rosetta,
and take copies of some of the same key instruments, including the sensitive ROSINA
mass spectrometer, to 133P. The mission is described in detail by Snodgrass et al.
(2017a), but in summary this would be a rendezvous mission launched in the late 2020s,
to arrive before 133P’s 2035 perihelion. Its instrument complement comprises visible,
NIR, and thermal IR cameras; mass spectrometers sensitive to neutral and ionised gas;
a dust instrument that combines the strengths of the Rosetta GIADA and COSIMA
instruments to study flux and composition of grains; a sensitive magnetometer and
plasma package; and two ground-penetrating radars. Castalia would perform a detailed
characterisation of the nucleus, including quantifying the amount and depth of buried
ice via the first sub-surface radar measurements at a small body, and directly ‘sniff’
the escaping gas, measuring its composition at an isotopic level. Combining results
from the similar Castalia and Rosetta instruments would allow very direct comparison
between an MBC and JFC. As a proposal for the M-class of ESA missions Castalia is
necessarily a simpler spacecraft than Rosetta (it does not carry a lander, for example),
but the low activity and more circular orbit of MBCs helps make the mission ‘easier’.
The proposal is under consideration in September 2017, with the shortlist for phase A
study for the ESA M5 call expected to be announced in December 2017.
A similar, but even more bare-essentials approach is taken by the proposed NASA
Discovery mission Proteus (Meech and Castillo-Rogez 2015). This mission concen-
trates on its ability to measure composition and isotopy at very high precision in the
low activity MBC environment, with the only other instrument (apart from the sensi-
tive mass spectrometer) being a camera for context and surface characterisation. The
core scientific motivation is testing whether or not the water in MBCs has an isotopic
match to Earth’s oceans. It was proposed to visit 238P in the last NASA Discovery
round, and although not selected on that occasion is expected to be reproposed in a
future round.
Finally, a proposed mission to an earlier round of ESA M-class missions aimed to
not only visit an MBC, but to return a sample of dust from it, capturing this in aerogel
during a flyby in the same way the NASA Stardust mission did at comet 81P/Wild 2.
Although this mission (Caroline—Jones et al. 2017) would not have been sensitive
to the volatile component of the MBC, the opportunity to apply sensitive laboratory
123
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The Main Belt Comets and ice in the Solar System Page 41 of 59 5
techniques to grains would certainly be revealing about the origins of the parent body.
Currently the in situ investigation of volatiles is seen as the priority, with many of
the Caroline team working on the Castalia proposal instead, but this concept remains
interesting to investigate further in the future.
9 Conclusions
The repeated activity of MBCs is strong evidence that there is buried ice in numerous
small asteroids in the MB, despite surface temperatures being too high for it to be stable.
Thermal models suggest that this ice is buried up to 30 m deep, but it is thought
that impacts could excavate enough of the insulating layer to allow periods of activity.
Outgassing from this activity will produce characteristic spectral features across a wide
range of wavelengths, from the UV to radio, but attempts to detect these in MBCs have
not yet succeeded. Given the low activity levels expected from MBCs, it appears that
this detection is beyond the capabilities of current telescope technology, but may be
achievable in the coming years with new facilities, in particular the JWST. The 2.7 µm
and 557 GHz water bands, and possibly UV observations of hydrogen (Ly-α) and OH,
appear to be the most promising regions of the spectrum to attempt detections. To get
more detailed information, such as the isotopic ratios that can answer questions on the
relevance of MBCs as a source of Earth’s water, or their original formation location
in the protoplanetary disc, will require in situ investigation via a spacecraft visit.
Acknowledgements This work is a direct result of support by the International Space Science Institute,
Bern, Switzerland, through the hosting and provision of financial support for an international team to discuss
the science of MBCs. CS is supported by the UK STFC as a Rutherford fellow. AF was supported by UK
STFC grant ST/L000709/1. HHH and MMK were supported by NASA Solar System Observations grant
NNX16AD68G. MTH is financially supported by David Jewitt. MdVB was supported by NASA’s Planetary
Astronomy Program. MC is supported by NASA Solar System ObservationsGrant NNX15AJ81G. We thank
Dave Schleicher for useful discussions. We made use of the Planetary Spectrum Generator developed by
Geronimo Villanueva at NASA’s Goddard Space Flight Center, and thank Geronimo for making this useful
service available to the community.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 Interna-
tional License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution,
and reproduction in any medium, provided you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons license, and indicate if changes were made.
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