between a slope and a flat terrain showing a
regular topography in OSIRIS images (31). The
morphology and illumination conditions at this
place are similar to those of many nearby areas
observed by VIRTIS-M.
The presence of CO2 ice at the surface of the
nucleus thus appears to be an ephemeral occurrence, which provides clues to the emplacement mechanism. After perihelion passage, the
activity of a cometary nucleus starts to decrease,
with water sublimation decreasing first. Nucleus
thermodynamical modeling (1) shows that a stratigraphy associated to the volatility of the major
gaseous species is produced in the outer layers of
67P/CG. However, the combination of spin axis
inclination and nucleus shape means that the
Anhur CO2 ice–rich area experiences a fast drop
in illumination, going into permanent shadow
quickly after equinox and, consequently, undergoing a rapid reduction in surface temperatures
in winter to less than 80 K, whereas the interior
remains warmer for a longer time because of the
low thermal inertia (8, 29). Sublimation of water
ice at depth is prevented, but sublimation of CO2
ice is not; CO2 can continue to flow from the interior to the surface, where it begins to freeze as a
result of the low surface temperatures. Moving
further toward the aphelion, the low surface temperatures preserve the CO2 ice on the surface,
which grows in >100-mm grains until, on the next
orbit, it is exposed again to sunlight and sublimates away. This inverse temperature profile of
cometary surfaces (warmer inside and cooler on
the surface) going into winter after perihelion (in
permanently shadowed regions) could potentially
freeze other volatiles that are sublimed from the
warmer interior as well. Based on the temperature of these surface areas, more volatiles species
such as CO and CH4 could also be frozen until the
next exposure to solar photons occurs. The same
phenomenon could also explain why no water ice
was seen at this site during the initial exposure to
the Sun, because the water ice would have been
frozen at lower depths than the CO2 ice.
The 67P/CG nucleus shows two different temporal activity cycles respectively caused by H2O
and CO2 ices in different regions: Whereas water
ice has diurnal variability, with a surface sublimation and condensation cycle occurring in the
most active areas (7), the surface condensation of
CO2 ice has a seasonal dependence. Similar processes are probably common among many Jupiter-family comets, which share with 67P/CG short
revolution periods and eccentric orbits (32).
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The authors thank the institutions and agencies that supported
this work: the Italian Space Agency (ASI), Centre National
d'Etudes Spatiales (CNES, France), DLR (Germany), and NASA
(USA). VIRTIS was built by a consortium from Italy, France, and
Germany, under the scientific responsibility of INAF-IAPS, Rome,
Italy, which also led the scientific operations. The VIRTIS
instrument development for ESA has been funded and managed
by ASI, with contributions from Observatoire de Meudon
(financed by CNES) and DLR. The VIRTIS instrument industrial
prime contractor was formerly Officine Galileo and is now
Leonardo SpA in Campi Bisenzio, Florence, Italy. The authors
thank the Rosetta Liaison Scientists, the Rosetta Science
Ground Segment, and the Rosetta Mission Operations Centre for
their support in planning the VIRTIS observations. T.M.
acknowledges additional funding from NASA JPL, W.-H.I. from
the National Science Council of Taiwan (grant 102-2112-M-008-
013-MY3) and Science and Technology Development Fund of
Macao Special Administrative Region (grant 017/2014/A1),
and L.M. from Deutsche Forschungsgemeinschaft (grant
MO 3007/1-1). The VIRTIS calibrated data are available through
ESA's Planetary Science Archive ( www.cosmos.esa.int/web/
psa/rosetta). This research has made use of NASA's
Astrophysics Data System.
Materials and Methods
Figs. S1 to S8
8 June 2016; accepted 28 October 2016
Published online 17 November 2016
Rosetta’s comet 67P/Churyumov-
Gerasimenko sheds its dusty mantle
to reveal its icy nature
S. Fornasier,1 S. Mottola,2 H. U. Keller,2,3 M. A. Barucci,1 B. Davidsson,4 C. Feller,1
J. D. P. Deshapriya,1 H. Sierks,5 C. Barbieri,6 P. L. Lamy,7 R. Rodrigo,8,9 D. Koschny,10
H. Rickman,11,12 M. A’Hearn,13 J. Agarwal,5 J.-L. Bertaux,14 I. Bertini,6 S. Besse,15
G. Cremonese,16 V. Da Deppo,17 S. Debei,18 M. De Cecco,19 J. Deller,5 M. R. El-Maarry,20
M. Fulle,21 O. Groussin,22 P. J. Gutierrez,8 C. Güttler,5 M. Hofmann,5 S. F. Hviid,2
W.-H. Ip,23,24 L. Jorda,22 J. Knollenberg,2 G. Kovacs,5,25 R. Kramm,5 E. Kührt,2
M. Küppers,15 M. L. Lara,8 M. Lazzarin,6 J. J. Lopez Moreno,8 F. Marzari,6
M. Massironi,26,27 G. Naletto,28,27,17 N. Oklay,5 M. Pajola,29,27 A. Pommerol,20 F. Preusker,2
F. Scholten,2 X. Shi,5 N. Thomas,20 I. Toth,30 C. Tubiana,5 J.-B. Vincent5
The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large
heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal
evolution of the colors of the 67P nucleus, finding changes driven by sublimation and
recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as
increasing activity removed the surface dust, implying that water ice is widespread
underneath the surface. We identified large ( 1500 square meters) ice-rich patches appearing
and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus.
Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid
variations in color are seen on extended areas close to the terminator. These cyclic processes
are widespread and lead to continuously, slightly varying surface properties.
All cometary nuclei observed to date have appeared to be dark, with only a limited amount of water ice detected in small patches (1, 2), although water is the dominant vola- tile observed in their coma. The Rosetta spacecraft has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, providing the opportunity to continuously inves- tigate its nucleus. The comet has a distinct bilobate shape and a complex morphology (3–5), with a
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