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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
journal homepage: www.elsevier.com/locate/icarus
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
3
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
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
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
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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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
