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Combining stellar occultation observations probing Pluto's atmosphere from 1988 to 2013, and models of energy balance between Pluto's surface and atmosphere, we find the preferred models are consistent with Pluto retaining a collisional atmosphere throughout its 248-year orbit. The occultation results show an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a high thermal inertia and a permanent N-2 ice cap at Pluto's north rotational pole. (C) 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license.
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Evidence that Pluto’s atmosphere does not collapse from occultations
including the 2013 May 04 event
C.B. Olkin
a,
, L.A. Young
a
, D. Borncamp
a,1
, A. Pickles
b
, B. Sicardy
c
, M. Assafin
d
, F.B. Bianco
e
, M.W. Buie
a
,
A. Dias de Oliveira
c,j
, M. Gillon
f
, R.G. French
g
, A. Ramos Gomes Jr.
d
, E. Jehin
f
, N. Morales
h
, C. Opitom
f
,
J.L. Ortiz
h
, A. Maury
i
, M. Norbury
b
, F. Braga-Ribas
j
, R. Smith
k
, L.H. Wasserman
l
, E.F. Young
a
,
M. Zacharias
m
, N. Zacharias
m
a
Southwest Research Institute, Boulder 80503, USA
b
Las Cumbres Observatory Global Telescope Network, Goleta 93117, USA
c
Observatoire de Paris, Meudon, France
d
Universidade Federal do Rio de Janeiro, Observatorio do Valongo, Rio de Janeiro, Brazil
e
Center for Cosmology and Particle Physics, New York University, NY 10003, USA
f
Institut d’Astrophysique de I’Université de Liège, Liège, Belgium
g
Wellesley College, Wellesley, 02481, USA
h
Instituto de Astrofísica de Andalucía-CSIC, Granada, Spain
i
San Pedro de Atacama Celestial Explorations (S.P.A.C.E.), San Pedro de Atacama, Chile
j
Observatório Nacional/MCTI, Rio de Janeiro, Brazil
k
Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK
l
Lowell Observatory, Flagstaff 86001, USA
m
United States Naval Observatory, Washington, DC 20392, USA
article info
Article history:
Received 31 August 2013
Revised 28 February 2014
Accepted 15 March 2014
Available online xxxx
Keywords:
Pluto, atmosphere
Pluto, surface
Atmospheres, evolution
Occultations
abstract
Combining stellar occultation observations probing Pluto’s atmosphere from 1988 to 2013, and models of
energy balance between Pluto’s surface and atmosphere, we find the preferred models are consistent
with Pluto retaining a collisional atmosphere throughout its 248-year orbit. The occultation results show
an increasing atmospheric pressure with time in the current epoch, a trend present only in models with a
high thermal inertia and a permanent N
2
ice cap at Pluto’s north rotational pole.
Ó 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/3.0/).
1. Introduction
Pluto has an eccentric orbit, e = 0.26, and high obliquity,
102–126° (Dobrovolskis and Harris, 1983), leading to complex
changes in surface insolation over a Pluto year, and, therefore, in
surface temperatures. When the first volatile ice species, CH
4
,
was discovered on Pluto’s surface, researchers quickly recognized
that these insolation and temperature variations would lead to
large annual pressure variations, due to the very sensitive depen-
dence of equilibrium vapor–pressure on the surface temperature.
Pluto receives nearly three times less sunlight at aphelion than
perihelion, prompting early modelers to predict that Pluto’s atmo-
sphere would expand and collapse over its orbit (Stern and Trafton,
1984). More sophisticated models were made in the 1990s
(Hansen and Paige, 1996), after the definitive detection of Pluto’s
atmosphere in 1988 (Millis et al., 1993) and the discovery of N
2
as the dominant volatile in the atmosphere and on the surface
(Owen et al., 1993). Similar models were run recently (Young,
2013), systematically exploring a range of parameter space. These
models predict changes on decadal timescales, dependent on the
thermal inertia of the substrate and the total N
2
inventory. Only
in a subset of the models did pressures increase by a factor of
two between 1988 and 2002/2006, consistent with observations
(Sicardy et al., 2003; Elliot et al., 2003; Young et al., 2008a). These
models include important physical processes including the global
http://dx.doi.org/10.1016/j.icarus.2014.03.026
0019-1035/Ó 2014 The Authors. Published by Elsevier Inc.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).
Corresponding author. Address: 1050 Walnut Street, Boulder, CO 80302, USA.
Fax: +1 303 546 9685.
E-mail address: colkin@boulder.swri.edu (C.B. Olkin).
1
Current address: Space Telescope Science Institute, Baltimore 21218, USA.
Icarus xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Icarus
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Please cite this article in press as: Olkin, C.B., et al. Evidence that Pluto’s atmosphere does not collapse from occultations including the 2013 May 04 event.
Icarus (2014), http://dx. doi.org/10.1016/j.icarus.2014.03.026
migration of N
2
through a seasonal cycle and the varying heat
sources which include insolation changes due to Pluto’s varying
heliocentric distance, the effect of time varying albedo patterns
on insolation, the obliquity of Pluto which changes the frost
pattern facing the Sun and finally the heat flow from or to the
substrate. These are described in more detail in Young (2013). Over
the course of a Pluto year, changes in global insolation drives the
migration of 1 m of frost, therefore, seasonal changes in frost distri-
bution are likely. Continuing observations of Pluto’s atmospheric
pressure on decadal timescales constrain thermal inertia, provid-
ing insight into deeper layers of the surface that are not visible
in imaging.
2. Observations
Stellar occultations, where a body such as Pluto passes between
an observer and a distant star, provide the most sensitive method
for measuring Pluto’s changing atmospheric pressure. Pluto was
predicted to occult a 14th magnitude (R filter) star on May 4, 2013
(Assafin et al., 2010). This was one of the most favorable Pluto occ-
ultations of 2013 because of the bright star, slow shadow velocity
(10.6 km/s at Cerro Tololo), and shadow path near large telescopes.
An unusual opportunity to refine the predicted path of the shadow
presented itself in March 2013 when Pluto passed within 0.5 arcsec
of the occulted star six weeks before the occultation. The Portable
High-Speed Occultation Telescope group (based at Southwest
Research Institute, Lowell Observatory and Wellesley College) coor-
dinated observations of the appulse from multiple sites including
the 0.9-m astrograph at Cerro Tololo Inter-American Observatory
(CTIO), the 1-m Liverpool Telescope on the Canary Islands, as well
as the Las Cumbres Observatory Global Telescope Network (LCOGT)
sites at McDonald Texas, CTIO Chile, SAAO South Africa, SSO Austra-
lia and Haleakala Hawaii. The appulse observations improved the
knowledge of the shadow path location such that the final predic-
tion was within 100 km of the reconstructed location.
Occultation observations were obtained from the three 1.0-m
LCOGT telescopes at Cerro Tololo (Brown et al., 2013). The three
telescopes have 1.0-m apertures and used identical instrumenta-
tion, an off-axis Finger Lakes Instrumentation MicroLine 4720
frame transfer CCD cameras, unfiltered. The cameras have a 2-s
readout time, and autonomous observations were scheduled with
different exposure times to provide adequate time resolution and
minimize data gaps in the ensemble observation. We measured
the combined flux from the merged image of Pluto, Charon and
occultation star as a function of time using aperture photometry,
and accounted for variable atmospheric transparency using
differential photometry with five field stars. The light curves were
normalized using post-occultation photometry of the field stars
relative to the occultation star.
Observations were also attempted from the Research and
Education Cooperative Occultation Network (RECON) from the
western United States. This was an excellent opportunity to test
the network and provided backup observing stations in case the
actual path was further north than predicted. Observations were
attempted at 14 sites and data were acquired at all sites, although
in the end, all RECON sites were outside of the shadow path.
3. Modeling
In order to interpret an occultation light curve we need to have
accurate knowledge of the precise location of the star relative to
Pluto. The geometric solution was obtained by a simultaneous fit
to 7 light curves from the following five sites: Cerro Burek Argen-
tina, LCOGT at Cerro Tololo Chile (3 light curves), Pico dos Dias
Brazil, La Silla Observatory Chile and San Pedro de Atacama Chile.
The observation at San Pedro de Atacama was made using Caisey
Harlingten’s 0.5-m Searchlight Observatory Network Telescope.
Details of the geometric solution will be given in a future paper.
These sites span 900 km across the shadow covering more than
35% of Pluto’s disk with chords both north and south of the center-
line. The reconstructed impact parameter for LCOGT at Cerro Tololo
(i.e., the closest distance of that site from the center of the occulta-
tion shadow) is 370 ± 5 km with a mid time of 08:23:21.60 ± 0.05 s
UT on May 4, 2013.
We fit the three LCOGT light curves simultaneously using a
standard Pluto atmospheric model (Elliot and Young, 1992) that
separates the atmosphere into two domains: a clear upper atmo-
sphere with at most a small thermal gradient, and a lower atmo-
sphere that potentially includes a haze layer. This model was
developed after the 1988 Pluto occultation, which showed a distinct
kink, or change in slope, in the light curve indicating a difference in
the atmosphere above and below about 1215 km from Pluto’s cen-
ter. The lower atmosphere can be described with either a haze layer,
or by a thermal gradient (Eshleman, 1989; Hubbard et al., 1990;
Stansberry et al., 1994) or a combination of the two to match the
low flux levels in the middle of the occultation light curves. We
focus on the derived upper atmosphere parameters in this paper,
but give the lower atmospheric parameters for completeness.
Fig. 1 shows the LCOGT light curves and the best fitting model with
a pressure of 2.7 ± 0.2 microbar and a temperature of 113 ± 2 K for
an isothermal atmosphere at 1275 km from Pluto’s center. The
lower atmosphere was fit with a haze onset radius of 1224 ± 2 km,
a haze extinction coefficient at onset of 3.2 ± 0.3 10
3
km
1
and a
haze scale height of 21 ± 5 km (see Elliot and Young, 1992, for
details). This atmospheric pressure extends the trend of increasing
surface pressure with temperature since 1988.
Previous work (Young, 2013) combined stellar occultation
observations from 1988 to 2010 and new volatile transport models
to show that Pluto’s seasonal variation can be fit by models that fall
into one of three classes: a class with high thermal inertia, which
results in a northern hemisphere that is never devoid of N
2
ice
(Permanent Northern Volatile, PNV, using the rotational north pole
convention where the north pole is currently sunlit), a class with
moderate thermal inertia and moderate N
2
inventory, resulting in
two periods of exchange of N
2
ice between the northern and south-
ern hemispheres that extend for decades after each equinox
(Exchange with Pressure Plateau, EPP), and a class with moderate
thermal inertia and smaller N
2
inventory, where the two periods
of exchange of N
2
ice last only a short time after each equinox
(Exchange with Early Collapse, EEC). These models do not include
longitudinal variation in frost distribution and the runs in Young
(2013) investigated only one value for the substrate albedo (0.2).
All of the low-albedo substrate models have a low-albedo terrain
at the south pole in 1989 during the mutual event season. How-
ever, the mutual event maps show a high albedo surface in the
south pole. We have expanded the parameter-space search to
include a high value for the substrate albedo (0.6) and find Perma-
nent Northern Volatile models with substrate albedo of 0.6 on the
south pole and volatiles with an assumed albedo of 0.4 on the
northern hemisphere. This pattern would appear to have a brighter
southern pole at the epoch of the mutual events (equinox in 1989).
We present these model runs only to demonstrate that the models
can produce solutions with a bright southern pole. Another consid-
eration is the source of the bright south pole at equinox. The south
pole could be bright due to CH
4
ice at the south pole and this would
not be reflected in the volatile-transport models because the mod-
els only consider the dominant volatile, N
2
.
With this most recent stellar occultation of May 4 2013, we are
able to distinguish between these three classes (Fig. 2) of seasonal
variation. The new data clearly preclude the EEC (Fig. 2C) and
EPP (Fig. 2B) classes. Only the PNV class is consistent with the
2 C.B. Olkin et al. / Icarus xxx (2014) xxx–xxx
Please cite this article in press as: Olkin, C.B., et al. Evidence that Pluto’s atmosphere does not collapse from occultations including the 2013 May 04 event.
Icarus (2014), http://dx. doi.org/10.1016/j.icarus.2014.03.026
observations that show an increasing surface pressure in the current
epoch. Both the EEC and EPP classes result in condensation of Pluto’s
atmosphere after solstice with surface pressures at the nanobar
level or less (Young, 2013). The PNV model has a high thermal iner-
tia, such that the atmosphere does not collapse over the course of a
Pluto year with typical minimum values for the surface pressure of
roughly 10 microbar. At this surface pressure the atmosphere is col-
lisional and present globally, and we conclude that Pluto’s atmo-
sphere does not collapse at any point during its 248-year orbit.
We consider that an atmosphere has not collapsed if it is global, col-
lisional, and opaque to UV radiation. An atmosphere that is global
and collisional can efficiently transport latent heat over its whole
surface. The cutoff for a global atmosphere is 0.06 microbars
(Spencer et al., 1997) or more than 2 orders of magnitude smaller
than the typical minimum pressure for PNV models.
4. Discussion
The PNV model runs that show an increasing atmospheric pres-
sure with time over the span of stellar occultation observations
(1988–2013) have substrate thermal inertias of 1000 or
3162 J m
2
s
1/2
K
1
(tiu). These values are much larger than the
thermal inertia measured from daily variation in temperature on
Pluto of 20–30 tiu for the non-N
2
ice regions (Lellouch et al.,
2011), or on other bodies such as Mimas, 16–66 tiu (Howett
et al., 2011). The range of thermal inertias derived for Pluto from
this work is comparable to that for pure, non-porous H
2
O ice at
30–40 K, 2300–3500 tiu (Spencer and Moore, 1992). This points
to a variation of thermal inertia with depth on Pluto. The variation
of temperature over a day probes depths of 1 m, while the sea-
sonal models depend on conditions near 100 m, indicating that
the thermal inertia is lower near the surface (1 m) than at depth
(100 m).
Evidence for large thermal inertias at the depths probed by
seasonal variation has also been seen on Triton. Models that best
explain the presence of a N
2
cap on the summer hemisphere of
Triton during the 1989 Voyager encounter have thermal inertias
greater than 1000 tiu (Spencer and Moore, 1992). Also large-thermal
inertia models for Triton (Spencer and Moore, 1992) are further
supported by the large increase in atmospheric pressure observed
Fig. 2. Comparison of pressures derived from occultation measurements to pressures from volatile transport models that are consistent with the 1988 and 2006 occultations,
and also roughly consistent with visible, infrared, and thermal measurements (the preferred runs of Young, 2013). Points indicate pressures in Pluto’s atmosphere at 1275 km
from Pluto’s center, derived from fits of occultation data using the model of Elliot and Young (1992), and the points are repeated in each panel. These include six previously
published measurements (Young, 2013) and the new measurement reported here. Lines correspond to modeled pressures for the Permanent Northern Volatiles (PNV) cases
(2A), the Exchange with Pressure Plateau (EPP) cases (2B), and the Exchange with Early Collapse (EEC) cases (2C). As the pressures throughout the Pluto year for EPP1 (Young,
2013) resemble the PNV cases (with a minimum pressure of 21 microbars for EPP1), and the pressures for PNV8 resemble the EPP cases, these two models are plotted with the
alternative category. The only class of model consistent with the increasing pressure from 1988 to 2013 is the Permanent Northern Volatile class. The vertical line in each
panel is the closest approach of the New Horizons spacecraft to Pluto in July 2015.
Fig. 1. The observed occultation light curves overlaid with the best fitting model. Time plotted is seconds after 2013 May 04 08:00:00 UTC. The line at normalized flux of 0.4
corresponds to 1275 km in Pluto’s atmosphere. The transition from the upper atmosphere to the lower atmosphere in the fitted model occurs at a flux level of 0.25 in these
data (see text for details). All three telescopes are 1.0-m telescopes located at the Cerro Tololo LCOGT node. The WGS 84 Coordinates of the three telescopes are (1) Dome A:
Latitude: 30°.167383S, Longitude: 70°.804789W, (2) Dome B: Latitude: 30°.167331S, Longitude: 70°.804661W and (3) Dome C: Latitude: 30°.167447S, Longitude:
70°.804681W. All telescopes are at an altitude of 2201 m. The Dome A telescope used a 2-s integration time; Dome B used a 3-s integration time and Dome C used a 5-s
integration time.
C.B. Olkin et al. / Icarus xxx (2014) xxx–xxx
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on Triton from 1989 to 1995 (Olkin et al., 1997; Elliot et al., 2000).
Pluto and Triton are similar in size, density and surface composition.
They may also be similar in their substrate thermal inertia
properties.
Pluto’s atmosphere is protected from collapse because of the
high thermal inertia of the substrate. The mechanism that prevents
the collapse is specific to Pluto, because it relies on Pluto’s high
obliquity and the coincidence of equinox with perihelion and aph-
elion. In the PNV model, volatiles are present on both the southern
and northern hemispheres of Pluto just past aphelion. Sunlight
absorbed in the southern hemisphere (the summer hemisphere)
from aphelion to perihelion powers an exchange of volatiles from
the southern hemisphere to the northern (winter) hemisphere.
Latent heat of sublimation cools the southern hemisphere and
warms the northern hemisphere, keeping the N
2
ice on both
hemispheres the same temperature. This exchange of volatiles con-
tinues until all the N
2
ice on the southern hemisphere sublimates
and is condensed onto Pluto’s northern hemisphere. Once this
occurs at approximately 1910 in Fig. 3b and 1890 in Fig. 3d, the
northern (winter at this time) hemisphere is no longer warmed
by latent heat, and begins to cool. However, the thermal inertia
of the substrate is high, so the surface temperature on the northern
Fig. 3. Results for two different PNV runs (PNV9 and PNV12 from Young (2013)). Both of these cases have a high thermal inertia (3162 tui), but one case has a large inventory
of N
2
(16 g cm
2
for PNV9) and the other has a small inventory of N
2
(2 g cm
2
for PNV12). For each run, the plot on the left shows Pluto over a season. The circles represent
Pluto at each of 12 equally spaced times in the orbit, indicated by date. The short vertical bar behind the circles represents the rotational axis, oriented so that the axis is
perpendicular to the Sun vector at the equinoxes, with the northern pole at the top (currently pointed sunward). Latitude bands are colored with their geometric albedos. The
red line indicates the globe at the time of the New Horizons encounter (July 2015). The plots on the right show surface pressure and temperature as a function of year. The
temperatures of the N
2
ice (solid line) and of a mid-southern latitude (60°, dashed line) are indicated. At any given time, all the N
2
ice on Pluto’s surface is at the same
temperature due to the transfer of energy from condensation and sublimation. Bare, N
2
-ice free regions can have temperatures higher than the ice temperature, as seen from
1910 to 2030 in panel (b). The surface pressure reaches a minimum of 10 microbar for each of these cases and this is typical for PNV models. Southern solstice, equinox at
perihelion, northern solstice, and equinox at aphelion are indicated for the current Pluto year. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
4 C.B. Olkin et al. / Icarus xxx (2014) xxx–xxx
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hemisphere does not cool quickly. The ice temperature drops by
only a few degrees K before the N
2
-covered areas at mid-northern
latitudes receive insolation again, in the decades before perihelion,
as shown in Fig. 3b from 1910 to 1970 or in Fig. 3d from 1890 to
1980.
Near the perihelion equinox, the southern hemisphere surface
is warm, 42 K, because the N
2
-free substrate was illuminated
for the preceding eight decades (1910–1990, in Fig. 3a and b,
1890–1990 in Fig. 3c and d). Approaching and after equinox (at
perihelion), the southern hemisphere receives less sunlight, and
radiatively cools slowly due to high thermal inertia. Once the sur-
face cools to the N
2
ice temperature (in approximately 2035–2050,
see Fig. 3), the N
2
gas in the atmosphere will condense onto the
southern hemisphere, and there begins a period of exchange
transferring N
2
from the summer (northern) hemisphere to the
winter (southern) hemisphere. However, this period of flow lasts
only until equinox at aphelion. The period of exchange is not long
enough to denude the northern hemisphere, thus as Pluto travels
from perihelion to aphelion, N
2
ice is always absorbing sunlight
on the northern hemisphere keeping the ice temperatures
relatively high throughout this phase and preventing collapse of
Pluto’s atmosphere.
5. Robustness of the results
Unfortunately, we cannot measure the atmospheric pressure at
the surface of Pluto from the ground so we need to use the pressure
at a higher altitude as a proxy for the surface pressure. We inves-
tigated the validity of this proxy measurement. We started with
synthetic occultation light curves derived from GCM models
(Zalucha and Michaels, 2013) at a variety of different methane
column abundances and surface pressures ranging from 8 to
24 mbars. We fit the synthetic light curves with the Elliot and
Young (1992) model to derive a pressure at 1275 km. We found
that the ratio of the pressure at 1275 km to the surface pressure
was a constant within the uncertainty of the model fit (0.01 mbar).
Because of this, we concentrate on those occultations for which the
pressures at 1275 km have been modeled by fitting Elliot and
Young (1992) models, which is a subset of the occultation results
presented in Young (2013).
We have also considered whether there are intermediate cases
where there is an increase in atmospheric pressure in the current
epoch (as the occultation data show) and then a collapse of the
atmosphere in later years. We have found no set of conditions that
is consistent with this. In order for the pressure increasing cur-
rently, one must have increasing insolation on the ices in the
northern hemisphere (current summer pole) while there is not
yet formation of a southern pole. If the gases could condense on
the southern pole currently, this becomes a sink and the atmo-
sphere would be decreasing in bulk pressure. One might ask if
there is a case where there is currently no condensation onto the
south pole but that it would begin in the next few decades and lead
to significant reduction in the bulk atmospheric pressure. For this
to happen the atmosphere would have to collapse approximately
before the year 2080 because that is when the southern pole starts
to be illuminated by the Sun given the obliquity of Pluto. At this
time, the south pole begins sublimating and supplying the atmo-
sphere. Such a case would require a very specific combination of
thermal inertia and N
2
inventory. In fact, we have not yet found
any such cases in our parameter-space searches.
Fig. 3 shows two different cases of Permanent Northern Volatile
models. A significant difference between the top panel (PNV9) and
the lower panel (PNV12) is the mass of N
2
available for the surface
and atmosphere. PNV9 has 16 g/cm
2
while PNV12 has only 2 g/
cm
2
. The effect of this difference is seen in the globes that indicate
the distribution of N
2
frost on the surface. There is obviously less
N
2
available in the second case and yet the pressures and temper-
atures have very similar variation over the Pluto year.
6. Conclusions
The PNV model is testable in multiple ways. In 2015, The New
Horizons spacecraft will fly past Pluto providing the first close-up
investigation of Pluto and its moons (Stern et al., 2008; Young
et al., 2008b). The infrared spectrometer on New Horizons will
map the composition across Pluto’s surface. New Horizons will
observe all of Pluto’s terrain that is illuminated by the Sun and will
attempt an observation of Pluto’s winter pole using reflected
Charon-light. We will be able to compare the N
2
ice distribution
predicted by the Permanent Northern Volatile model with the
observed ice distribution determined by New Horizons including
perhaps even the southern pole of Pluto to determine if frost is
present on the currently winter pole. The REX instrument on
New Horizons will provide thermal measurements to compare
with the surface temperatures predicted by the PNV models. From
the UV solar and stellar occultations of Pluto, the New Horizons
mission will determine the composition of Pluto’s atmosphere as
well as the thermal structure in the thermosphere. From the Radio
Science experiment, the pressure and temperature profiles in Plu-
to’s lower atmosphere will be determined. All of these data provide
a test of the PNV model.
In addition to this close-up comprehensive investigation of
Pluto by the New Horizons spacecraft, the model results can be
tested by regular stellar occultation observations from Earth. The
current epoch is a time of significant change on Pluto. Most of
the PNV models show a maximum surface pressure between
2020 and 2040. Regular observations over this time period will
constrain the properties of Pluto’s substrate and the evolution of
its atmosphere.
Acknowledgments
This work was supported in part by NASA Planetary Astronomy
Grant NNX12AG25G.
The Liverpool Telescope is operated on the island of La Palma by
Liverpool John Moores University in the Spanish Observatorio del
Roque de los Muchachos of the Instituto de Astrofisica de Canarias
with financial support from the UK Science and Technology Facili-
ties Council.
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Icarus (2014), http://dx. doi.org/10.1016/j.icarus.2014.03.026
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Icarus (2014), http://dx. doi.org/10.1016/j.icarus.2014.03.026
... Global volatile transport models of Pluto have been used to explain how changes in insolation over the course of Pluto's orbit affect the surface and subsurface temperatures (which plays a key role in the Pluto environment), resulting in latitudinal variations of distribution of volatile ices (Young, 1993;Hansen and Paige, 1996;Spencer et al., 1997;Young, 2012, Young, 2013Hansen et al., 2015;Toigo et al., 2015;Olkin et al., 2015;Bertrand and Forget, 2016;Bertrand et al., 2018). ...
... These results suggest that the southern hemisphere of Pluto is not entirely covered by N 2 -rich ice, otherwise the peak surface pressure would have occurred much earlier than 2015 (a similar result is found in Young, 2013;Olkin et al., 2015). At most, a thin mid-latitude band of N 2 -rich ice (similar to that observed in the northern hemisphere) could be present in the southern hemisphere in 2015. ...
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Pluto's surface is covered in numerous CH 4 ice deposits, that vary in texture and brightness, as revealed by the New Horizons spacecraft as it flew by Pluto in July 2015. These observations suggest that CH 4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N 2 and CH 4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N 2 cycles. Results show that bright CH 4 deposits can create cold traps for N 2 ice outside Sputnik Planitia, leading to a strong coupling between the N 2 and CH 4 cycles. Depending on the assumed albedo for CH 4 ice, the model predicts CH 4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH 4 -rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH 4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH 4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N 2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH 4 and N 2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH 4 to absorb most of the incoming Lyman-α flux.
... This was initially modeled for Triton and Pluto (see reviews by Spencer et al. 1997 andYelle et al. 1995). Since those reviews, trends of increasing atmospheric pressure for both Triton and Pluto were observed using the technique of stellar occultation, with an increase by factors of two and three respectively (Elliot et al. 1998;Elliot et al. 2000;Elliot et al. 2003a;Olkin et al. 1997Olkin et al. , 2015; Meza et al 2019; see section 6). The new time-base of atmospheric observations and the New Horizons flyby of Pluto inspired new models of seasonal variation (e.g., Young 2012Young , 2013Young , 2017Hansen et al. 2015;Olkin et al. 2015), including general circulation models (e.g., Forget et al. 2017) and evolution of atmospheres on the timescale of millions of years (e.g., Bertrand et al. 2016Bertrand et al. , 2018. ...
... Since those reviews, trends of increasing atmospheric pressure for both Triton and Pluto were observed using the technique of stellar occultation, with an increase by factors of two and three respectively (Elliot et al. 1998;Elliot et al. 2000;Elliot et al. 2003a;Olkin et al. 1997Olkin et al. , 2015; Meza et al 2019; see section 6). The new time-base of atmospheric observations and the New Horizons flyby of Pluto inspired new models of seasonal variation (e.g., Young 2012Young , 2013Young , 2017Hansen et al. 2015;Olkin et al. 2015), including general circulation models (e.g., Forget et al. 2017) and evolution of atmospheres on the timescale of millions of years (e.g., Bertrand et al. 2016Bertrand et al. , 2018. ...
Preprint
At 30-50 K, the temperatures typical for surfaces in the Kuiper Belt (e.g. Stern & Trafton 2008), only seven species have sublimation pressures higher than 1 nbar (Fray & Schmitt 2009): Ne, N$_2$, CO, Ar, O$_2$, CH$_4$, and Kr. Of these, N$_2$, CO, and CH$_4$ have been detected or inferred on the surfaces of Trans-Neptunian Objects (TNOs). The presence of tenuous atmospheres above these volatile ices depends on the sublimation pressures, which are very sensitive to the composition, temperatures, and mixing states of the volatile ices. Therefore, the retention of volatiles on a TNO is related to its formation environment and thermal history. The surface volatiles may be transported via seasonally varying atmospheres and their condensation might be responsible for the high surface albedos of some of these bodies. The most sensitive searches for tenuous atmospheres are made by the method of stellar occultation, which have been vital for the study of the atmospheres of Triton and Pluto, and has to-date placed upper limits on the atmospheres of 11 other bodies. The recent release of the Gaia astrometric catalog has led to a "golden age" in the ability to predict TNO occultations in order to increase the observational data base.
... Over Pluto's 248 yr orbit, changes in solar insolation due to eccentricity and obliquity govern the surface pressure (currently 11 μbar) of its primarily N 2 atmosphere, which is in vapor pressure equilibrium with the surface ice (e.g., Meza et al. 2019). Upon arrival in 2058, we expect Pluto's surface atmospheric pressure to be lower than it was in 2015, but still global in extent, and likely higher than at the time of discovery of the atmosphere in 1988, based on postencounter models and ongoing ground-based stellar occultations (Olkin et al. 2014;Meza et al. 2019). Thus, studying Pluto's atmosphere, in particular by performing the first direct determination of its composition through mass spectrometry, remains a key objective. ...
... N 2 atmosphere, which is in vapor pressure equilibrium with the surface ice (e.g.,Meza et al., 2019). Upon arrival in 2058, we expect Pluto's surface atmospheric pressure to be lower than it was in 2015, but still global in extent, and likely higher than at the time of discovery of the atmosphere in 1988, based on post-encounter models and ongoing ground-based stellar occultations(Olkin et al., 2014;Meza et al., 2019). Thus, studying ...
Preprint
Full-text available
Persephone is a NASA concept mission study that addresses key questions raised by New Horizons' encounters with Kuiper Belt objects (KBOs), with arguably the most important being "Does Pluto have a subsurface ocean?". More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 years long, with launch in 2031 on a Space Launch System (SLS) Block 2 rocket with a Centaur kick stage, followed by a 27.6 year cruise powered by existing radioisotope electric propulsion (REP) and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50- to 100-km-class KBO encounter before starting a 3.1 Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8-year extended mission, which would enable the exploration of another KBO in the 100- to 150-km-size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager; Low-Light Camera; Ultra-Violet Spectrometer; Near-Infrared (IR) Spectrometer; Thermal IR Camera; Radio Frequency Spectrometer; Mass Spectrometer; Altimeter; Sounding Radar; Magnetometer; and Plasma Spectrometer. The nominal cost of this mission is $3.0B, making it a large strategic science mission.
... Mars is not the only planetary body to experience partial atmospheric collapse (Soto et al., 2015). Pluto (Bertrand et al., , 2019Hansen and Paige, 1996;Olkin et al., 2015) and Titan (Lorenz et al., 1997) likewise have strong seasonal atmospheric cycles (lasting hundreds of Earth years) and orbital variations that could cause similar ice layering as is found on Mars, and atmospheric collapse has been proposed for tidally locked planets around TRAPPIST-1 (Turbet et al., 2018). Earth, with anthropomorphic influences, abundant biology, and liquid phases, does not provide a good analog for such layered ice deposits or climate models. ...
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Extensive evidence of landform-scale martian geomorphic changes has been acquired in the last decade, and the number and range of examples of surface activity have increased as more high-resolution imagery has been acquired. Within the present-day Mars climate, wind and frost/ice are the dominant drivers, resulting in large avalanches of material down icy, rocky, or sandy slopes; sediment transport leading to many scales of aeolian bedforms and erosion; pits of various forms and patterned ground; and substrate material carved out from under subliming ice slabs. Due to the ability to collect correlated observations of surface activity and new landforms with relevant environmental conditions with spacecraft on or around Mars, studies of martian geomorphologic activity are uniquely positioned to directly test surface-atmosphere interaction and landform formation/evolution models outside of Earth. In this paper, we outline currently observed and interpreted surface activity occurring within the modern Mars environment, and tie this activity to wind, seasonal surface CO2 frost/ice, sublimation of subsurface water ice, and/or gravity drivers. Open questions regarding these processes are outlined, and then measurements needed for answering these questions are identified. In the final sections, we discuss how many of these martian processes and landforms may provide useful analogs for conditions and processes active on other planetary surfaces, with an emphasis on those that stretch the bounds of terrestrial-based models or that lack terrestrial analogs. In these ways, modern Mars presents a natural and powerful comparative planetology base case for studies of Solar System surface processes, beyond or instead of Earth.
... Results from the mission identified the hydrocarbons acetylene, ethylene, ethane, and methylacetylene (C 3 H 4 ) in the atmosphere, and as many as 20 separate haze layers extending beyond 200 km (Stern et al. 2015;Gladstone et al. 2016;Cheng et al. 2017;Steffl et al. 2020). Moreover, the atmosphere was far more complex in structure than that inferred from ground-based occultations (Figure 1; Elliot et al. 2007;Young et al. 2008;Young 2013;Olkin et al. 2015). Observations from the Atacama Large Millimeter/submillimeter Array (ALMA) identified CO and HCN in the atmosphere; the quantification of CO at a level of 515±40 has important implications for the abundance of oxygen available for the formation of aerosols (Lellouch et al. 2017). ...
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We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution = 2 s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing atmospheric model of Pluto. We find the lowest pressure value at a reference radius of r = 1215 km among those reported after 2012. These reports indicate a possible rapid (approximately 21 −5⁺⁴ % of the previous value) pressure drop between 2016, which is the latest reported estimate, and 2019. However, this drop is detected at a 2.4 σ level only and still requires confirmation from future observations. If real, this trend is opposite from the monotonic increase of Pluto’s atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing N 2 condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.
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On UT 29 June 2015, the occultation by Pluto of a bright star (r′ = 11.9) was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and several ground-based stations in New Zealand and Australia. Pre-event astrometry allowed for an in-flight update to the SOFIA team with the result that SOFIA was deep within the central flash zone (~22 km from center). Analysis of the combined data leads to the result that Pluto's middle atmosphere is essentially unchanged from 2011 and 2013 (Person et al. 2013; Bosh et al. 2015); there has been no significant expansion or contraction of the atmosphere. Additionally, our multi-wavelength observations allow us to conclude that a haze component in the atmosphere is required to reproduce the light curves obtained. This haze scenario has implications for understanding the photochemistry of Pluto's atmosphere.
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Context . From 1988 to 2016, several stellar occultations have been observed to characterise Pluto’s atmosphere and its evolution. From each stellar occultation, an accurate astrometric position of Pluto at the observation epoch is derived. These positions mainly depend on the position of the occulted star and the precision of the timing. Aims . We present 19 Pluto’s astrometric positions derived from occultations from 1988 to 2016. Using Gaia DR2 for the positions of the occulted stars, the accuracy of these positions is estimated at 2−10 mas, depending on the observation circumstances. From these astrometric positions, we derive an updated ephemeris of Pluto’s system barycentre using the NIMA code. Methods . The astrometric positions were derived by fitting the light curves of the occultation by a model of Pluto’s atmosphere. The fits provide the observed position of the centre for a reference star position. In most cases other publications provided the circumstances of the occultation such as the coordinates of the stations, timing, and impact parameter, i.e. the closest distance between the station and centre of the shadow. From these parameters, we used a procedure based on the Bessel method to derive an astrometric position. Results . We derive accurate Pluto’s astrometric positions from 1988 to 2016. These positions are used to refine the orbit of Pluto’system barycentre providing an ephemeris, accurate to the milliarcsecond level, over the period 2000−2020, allowing for better predictions for future stellar occultations.
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Las Cumbres Observatory Global Telescope (LCOGT) is a young organization dedicated to time-domain observations at optical and (potentially) near-IR wavelengths. To this end, LCOGT is constructing a world-wide network of telescopes, including the two 2m Faulkes telescopes, as many as 17 x 1m telescopes, and as many as 23 x 40cm telescopes. These telescopes initially will be outfitted for imaging and (excepting the 40cm telescopes) spectroscopy at wavelengths between the atmospheric UV cutoff and the roughly 1-micron limit of silicon detectors. Since the first of LCOGT's 1m telescopes are now being deployed, we lay out here LCOGT's scientific goals and the requirements that these goals place on network architecture and performance, we summarize the network's present and projected level of development, and we describe our expected schedule for completing it. In the bulk of the paper, we describe in detail the technical approaches that we have adopted to attain the desired performance. In particular, we discuss our choices for the number and location of network sites, for the number and sizes of telescopes, for the specifications of the first generation of instruments, for the software that will schedule and control the network's telescopes and reduce and archive its data, and for the structure of the scientific and educational programs for which the network will provide observations.
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Context. We investigate transneptunian objects, including Pluto and its satellites, by stellar occultations. Aims: Our aim is to derive precise, astrometric predictions for stellar occultations by Pluto and its satellites Charon, Hydra and Nix for 2008-2015. We construct an astrometric star catalog in the UCAC2 system covering Plutoarcmins sky path. Methods: We carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We made the astrometry of 110 GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50 mas precision. Results: Positions were obtained in the UCAC2 frame with errors of 50 mas for stars up to magnitude R = 19, and 25 mas up to R = 17. New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. We generated 2252 predictions of stellar occultations by Pluto, Charon, Hydra and Nix for 2008-2015. An astrometric catalog with proper motions was produced, containing 2.24 million stars covering Plutoarcmins sky path with 30arcmin width. Its magnitude completeness is about R = 18-19 with a limit about R = 21. Based on the past 2005-2008 occultations successfully predicted, recorded and fitted, a linear drift with time in declination with regard to DE418/plu017 ephemerides was determined for Pluto and used in the current predictions. For offset (mas) = A * (t (yr) - 2005.0) + B, we find A = +30.5 ± 4.3 mas yr-1 and B = -31.5 ± 11.3 mas, with standard deviation of 14.4 mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6 m and 1.6 m telescopes at the Laboratório Nacional de Astrofísica/Brazil. Conclusions: Recurrent issues in stellar occultation predictions were addressed and properly overcome: body ephemeris offsets, catalog zero-point position errors and field-of-view size, long-term predictions and stellar proper motions, faint-visual versus bright-infrared stars and star/body astrometric follow-up. In particular, we highlight the usefulness of the obtained astrometric catalog as a reference frame for star/body astrometric follow-up before and after future events involving the Pluto system. Besides, it also furnishes useful photometric information for field stars in the flux calibration of observed light curves. Updates on the ephemeris offsets and candidate star positions (geometric conditions of predictions and finding charts) are made available by the group at http://www.lesia.obspm.fr/perso/bruno-sicardy Tables of predictions for stellar occultations by Pluto, Charon, Nix and Hydra for 2008-2015 and Catalog of star positions for 2008-2015 sky path of Pluto are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A32Observations made through the ESO run 079.A-9202(A), 075.C-0154, 077.C-0283 and 079.C-0345.Also based on observations made at the Laboratório Nacional de Astrofísica (LNA), Itajubá-MG, Brazil.
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Pluto occultations are historically rare events, having been observed in 1988, 2002, 2006, and, as Pluto moves into the crowded Galactic plane, on several occasions in 2007. Here we present six results from our observations of the 2006 June 12 event from several sites in Australia and New Zealand. First, we show that Pluto's 2006 bulk atmospheric column abundance, as in 2002, is over twice the value measured in 1988, implying that nitrogen frost on Pluto's surface is 1.2-1.7 K warmer in 2006 than 1988 despite a 9% drop in incident solar flux. We measure a half-light shadow radius of 1216 ± 8.6 km in 2006, nominally larger than published values of 1213 ± 16 km measured in 2002. Given the current error bars, this latest half-light radius cannot discriminate between continued atmospheric growth or shrinkage, but it rules out several of the volatile transport scenarios modeled by Hansen & Paige. Second, we resolve spikes in the occultation light curve that are similar to those seen in 2002 and model the vertical temperature fluctuations that cause them. Third, we show that Pluto's upper atmosphere appears to hold a steady temperature of ~100 K, as predicted from the methane thermostat model, even at latitudes where the methane thermostat is inoperative. This implies that energy transport rates are faster than radiational cooling rates. Fourth, this occultation has provided the first significant detection of a non-isothermal temperature gradient in Pluto's upper atmosphere also reported by Elliot et al., possibly the result of CO gas in Pluto's upper atmosphere. Fifth, we show that a haze-only explanation for Pluto's light curve is extremely unlikely; a thermal inversion is necessary to explain the observed light curve. And sixth, we derive an upper limit for the haze optical depth of 0.0023 in the zenith direction at average CCD wavelengths.
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Spectral maps of Mimas’ daytime thermal emission show a previously unobserved thermal anomaly on Mimas’ surface. A sharp V-shaped boundary, centered at 0°N and 180°W, separates relatively warm daytime temperatures from a cooler anomalous region occupying low- to mid-latitudes on the leading hemisphere. Subsequent observations show the anomalous region is also warmer than its surroundings at night, indicating high thermal inertia. Thermal inertia in the anomalous region is 66±23Jm-2K-1s12, compared to
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We report on thermal observations of the Pluto–Charon system acquired by the Spitzer observatory in August–September 2004. The observations, which consist of (i) photometric measurements (8 visits) with the Multiband Imaging Photometer (MIPS) at 24, 70 and 160μm and (ii) low-resolution spectra (8 visits) over 20–37μm with the Infrared Spectrometer (IRS), clearly exhibit the thermal lightcurve of Pluto/Charon at a variety of wavelengths. They further indicate a steady decrease of the system brightness temperature with increasing wavelength. Observations are analyzed by means of a thermophysical model, including the effects of thermal conduction and surface roughness, and using a multi-terrain description of Pluto and Charon surfaces in accordance with visible imaging and lightcurves, and visible and near-infrared spectroscopy. Three units are considered for Pluto, respectively covered by N2 ice, CH4 ice, and a tholin/H2O mix. Essential model parameters are the thermal inertia of Pluto and Charon surfaces and the spectral and bolometric emissivity of the various units. A new and improved value of Pluto’s surface thermal inertia, referring to the CH4 and tholin/H2O areas, is determined to be ΓPl=20–30Jm−2s−1/2K−1 (MKS). The high-quality 24-μm lightcurve permits a precise assessment of Charon’s thermal emission, indicating a mean surface temperature of 55.4±2.6K. Although Charon is on average warmer than Pluto, it is also not in instantaneous equilibrium with solar radiation. Charon’s surface thermal inertia is in the range ΓCh=10–150 MKS, though most model solutions point to ΓCh=10–20 MKS. Pluto and Charon thermal inertias appear much lower than values expected for compact ices, probably resulting from high surface porosity and poor surface consolidation. Comparison between Charon’s thermal inertia and even lower values estimated for two other H2O-covered Kuiper-Belt objects suggests that a vertical gradient of conductivity exists in the upper surface of these bodies. Finally, the observations indicate that the spectral emissivity of methane ice is close to unity at 24μm and decreases with increasing wavelength to ∼0.6 at 100μm. Future observations of thermal lightcurves over 70–500μm by Herschel should be very valuable to further constrain the emissivity behavior of the Pluto terrains.
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We present a 3D general circulation model of Pluto and Triton's atmospheres, which uses radiative-conductive-convective forcing. In both the Pluto and Triton models, an easterly (prograde) jet is present at the equator with a maximum magnitude of 10-12 m/s and 4 m/s, respectively. Neither atmosphere shows any significant overturning circulation in the meridional and vertical directions. Rather, it is horizontal motions (mean circulation and transient waves) that transport heat meridionally at a magnitude of 1 and 3 x 10^7 W at Pluto's autumn equinox and winter solstice, respectively (seasons referenced to the Northern Hemisphere). The meridional and dayside-nightside temperature contrast is small (<5 K). We find that the lack of vertical motion can be explained on Pluto by the strong temperature inversion in the lower atmosphere. The height of the Voyager 2 plumes on Triton can be explained by the dynamical properties of the lower atmosphere alone (i.e., strong wind shear) and does not require a thermally defined troposphere (i.e., temperature decreasing with height at the surface underlying a region of temperature increasing with height). The model results are compared with Pluto stellar occultation light curve data from 1988, 2002, 2006, and 2007 and Triton light curve data from 1997.
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Since the last Pluto volatile transport models were published (Hansen and Paige 1996), we have (i) new stellar occultation data from 2002 and 2006-2012 that have roughly twice the pressure as the discovery occultation of 1988, (ii) new information about the surface properties of Pluto, (iii) a spacecraft due to arrive at Pluto in 2015, and (iv) a new volatile transport model that is rapid enough to allow a large parameter-space search. Such a parameter-space search coarsely constrained by occultation results reveals three broad solutions: a high-thermal inertia, large volatile inventory solution with permanent northern volatiles (PNV); a lower thermal-inertia, smaller volatile inventory solution with exchanges between hemispheres, and a pressure plateau beyond 2015 (exchange with pressure plateau, EPP); and solutions with still smaller volatile inventories, with an early collapse of the atmosphere prior to 2015 (exchange with early collapse, EEC). PNV is favored by stellar occultation data, but EEC cannot yet be definitively ruled out without more atmospheric modeling or additional occultation observations and analysis.