Guilbert-Lepoutre, Prialnik, Métayer (2020) In The Trans-Neptunian Solar System, Eds. Prialnik, Barucci, Young, 183-201

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Modeling the evolution and internal structure of large- and mid-size Trans-Neptunian objects (TNOs) is a challenging task, owing to the strong dependence on the thermophysical and structural parameters involved, which are poorly constrained. Thus the outcome of evolutionary tracks for a given object may range from fully pristine to completely differentiated and need to be further constrained by detailed observations of TNOs’ physical and chemical properties. The key question remains, whether knowing surface and bulk properties is sufficient for inferring internal properties and more ambitiously, the evolutionary course of a TNO from formation to present day. Recent models of Charon involve a complex evolution, including the formation of subsurface oceans and past cryovolcanism. This perspective should trigger the development of new comprehensive models, which —combined with new observations— should lead to significant progress in our understanding of how the whole population formed and evolved.

Guilbert-Lepoutre, Rosenberg, Prialnik, Besse (2016) MNRAS, 462, 146-155

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Guilbert-Lepoutre, Besse, Mousis, Ali-Dib, Hofner, Koschny, Hager (2015) SSRv, 197, 271

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Studying comets is believed to bring invaluable clues on the formation and evo- lution of our planetary system. In comparison to planets, they have undergone much less alteration, and should have therefore retained a relatively pristine record of the conditions prevailing during the early phases of the solar system. However, comets might not be entirely pristine. As of today, we have not been able to determine which of the observed physical, chemical and orbital characteristics of comets, after they have evolved for more than 4 Gyr in a time-varying radiative and collisional environment, will provide the best clues to their origin. Comet physical characteristics as inherited from their formation stage may be very diverse, both in terms of composition and internal structure. The subsequent evolution of comet nuclei involves some possible processing from radiogenic heating, space weathering and large- and small-scale collisions, which might have modified their primordial structures and compositions with various degrees. When comets enter the inner solar system and be- come active, they start to lose mass at a very high rate. The effects of activity on comet nuclei involve a layering of the composition, a substantial non-even erosion and modifica- tion of their size and shape, and may eventually result in the death of comets. In this review, we present the dominating processes that might affect comet physical and chemical prop- erties at different stages of their evolution. Although the evolutionary track may be specific to each comet, we can focus on long-lasting modifications which might be common to all nuclei after their formation stage, during their storage in reservoirs in the outer solar system, and once comets enter the inner solar system and become active objects.

Guilbert-Lepoutre, Schulz, Rozek, Lowry, Tozzi, Stuwe (2014) A&A, 567, 2

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Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion.

Guilbert-Lepoutre (2014) Icarus, 231, 232

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Jupiter Trojans appear to be a key population of small bodies to study and test the models of the Solar System formation and evolution. Because understanding the evolution of Trojans can bring strong and unique constraints on the origins of our planetary system, a significant observational effort has been undertaken to unveil their physical characteristics. The data gathered so far are consistent with Trojans having volatile-rich interiors (possibly water ice) and volatile-poor surfaces (fine grained silicates). Since water ice is not thermodynamically stable against sublimation at the surface of an object located at 5 AU, such layering seems consistent with past outgassing. In this work, we study the thermal history of Trojans after the formation of a dust mantle by possible past outgassing, so as to constrain the depth at which water ice could be stable. We find that it could have survived 100 m below the surface, even if Tro- jans orbited close to the Sun for 10,000 years, as suggested by the most recent dynamical models. Water ice should be found 10 m below the surface in most cases, and below 10 cm in the polar regions in some cases.

Guilbert-Lepoutre (2012) Astronomical Journal, 144, 97

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Centaurs are believed to be Kuiper Belt objects in transition between Jupiter and Neptune before possibly becoming Jupiter family comets. Some indirect observational evidence is consistent with the presence of amorphous water ice in Centaurs. Some of them also display a cometary activity, probably triggered by the crystallization of the amorphous water ice, as suggested by Jewitt and this work. Indeed, we investigate the survival of amorphous water ice against crystallization, using a fully three-dimensional thermal evolution model. Simulations are performed for varying heliocentric distances and obliquities. They suggest that crystallization can be triggered as far as 16 AU, though amorphous ice can survive beyond 10 AU. The phase transition is an efficient source of outgassing up to 10–12 AU, which is broadly consistent with the observations of the active Centaurs. The most extreme case is 167P/CINEOS, which barely crystallizes in our simulations. However, amorphous ice can be preserved inside Centaurs in many heliocentric distance–obliquity combinations, below a ∼5–10 m crystallized crust. We also find that outgassing due to crystallization cannot be sustained for a time longer than 10^4–10^5 years, leading to the hypothesis that active Centaurs might have recently suffered from orbital changes. This could be supported by both observations (although limited) and dynamical studies.

Guilbert-Lepoutre & Jewitt (2011) Astrophysical Journal, 743, 31

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We use a fully three-dimensional thermal evolution model to examine the effects of a non-uniform surface albedo on the subsurface thermal structure of comets. Surface albedo markings cast “thermal shadows” with strong lateral thermal gradients. Corresponding compositional gradients can be strong, especially if the crystallization of amorphous water ice is triggered in the hottest regions. We show that the spatial extent of the structure depends mainly on the obliquity, thermal conductivity, and heliocentric distance. In some circumstances, subsurface structure caused by the thermal shadows of surface features can be maintained for more than 10 Myr, the median transport time from the Kuiper Belt to the inner solar system. Non-uniform compositional structure can be an evolutionary product and does not necessarily imply that comets consist of building blocks accumulated in different regions of the protoplanetary disk.

Guilbert-Lepoutre (2011) Astronomical Journal, 141, 103

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Centaur 10199 Chariklo appears to have a varying spectral behavior. While three different spectral studies detect the presence of water ice at the surface, two more recent studies do not detect any absorption bands. In this article, we consider the possibility that Chariklo undergoes cometary activity that could be responsible for the observed spectral variations. We simulate its thermal evolution, finding that crystalline water ice should be present in the object core, and amorphous water ice should be found at the surface. Upon entering the inner solar system, Chariklo might experience some cometary activity due to ice crystallization if the obliquity is high, due to the adjustment of the internal structure to a new thermal equilibrium. No other activity is expected from this source, unless an external source like an impact provides the heat needed. In the case of such an event, we find that dust emitted in a coma is unlikely to be responsible for the observed spectral variations. In contrast, water ice grains in the coma would reproduce this pattern, meaning that the water ice detected after Chariklo’s discovery was present in these grains and not on the object surface. Nonetheless, any activity would require an external additional heat source to be triggered, through an outburst, which might favor the spatial variations hypothesis.

Guilbert-Lepoutre, Lasue, Federico et al. (2011) A&A, 529, A71

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Thermal evolution models have been developed over the years to investigate the evolution of thermal properties based on the transfer of heat fluxes or transport of gas through a porous matrix, among others. Applications of such models to trans-Neptunian objects (TNOs) and Centaurs has shown that these bodies could be strongly differentiated from the point of view of chemistry (i.e. loss of most volatile ices), as well as from physics (e.g. melting of water ice), resulting in stratified internal structures with differentiated cores and potential pristine material close to the surface. In this context, some observational results, such as the detection of crystalline water ice or volatiles, remain puzzling. In this paper, we would like to present a new fully three-dimensional thermal evolution model. With this model, we aim to improve determination of the temperature distribution inside icy bodies such as TNOs by accounting for lateral heat fluxes, which have been proven to be important for accurate simulations. We also would like to be able to account for heterogeneous boundary conditions at the surface through various albedo properties, for example, that might induce different local temperature distributions. In a departure from published modeling approaches, the heat diffusion problem and its boundary conditions are represented in terms of real spherical harmonics, increasing the numerical efficiency by roughly an order of magnitude. We then compare this new model and another 3D model recently published to illustrate the advantages and limits of the new model. We try to put some constraints on the presence of crystalline water ice at the surface of TNOs. The results obtained with this new model are in excellent agreement with results obtained by different groups with various models. Small TNOs could remain primitive unless they are formed quickly (less than 2 Myr) or are debris from the disruption of larger bodies. We find that, for large objects with a thermal evolution dominated by the decay of long-lived isotopes (objects with a formation period greater than 2 to 3 Myr), the presence of crystalline water ice would require both a large radius (>300 km) and high density (>1500 kg m−3). In particular, objects with intermediate radii and densities would be an interesting transitory population deserving a detailed study of individual fates.

Guilbert, Barucci, Brunetto, et al. (2009) A&A, 501, 777

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An ESO Large Program was undertaken in October 2006 (P.I.: M. A. Barucci) to provide as complete observations as possible of about 40 Trans-Neptunian Objects and Centaurs, to investigate their surface properties. Hence, new visible and near- infrared observations of Centaur 10199 Chariklo (1997 CU26)) were performed. We investigate Chariklo’s surface composition. It has already been suspected of being inhomogeneous. We try to confirm this assumption by comparing our results with previously published works, and find an explanation related the observed variations. A spectral modeling is applied to the spectra, using different types of mixtures, to place constraints on the amount of water ice present in our new spectrum. Several spectra, obtained at different moments by different groups, are compared by studying the variations in the depth of absorption bands attributable to water ice. The irradiation doses received by Chariklo’s surface are also considered to interpret the observed variations. The presence of water ice is not confirmed by our featureless near-infrared spectra. The main component on the surface, identified by our spectral modeling, is amorphous carbon, which may have been produced by irradiation if Chariklo originated in the transneptunian region. The suspected surface heterogeneity is also confirmed. We show that the variations in Chariklo’s spectral behaviour could be explained by a variation in the number of craters across the surface. Comet-like activity is not detected in our data, though it cannot be excluded.

Guilbert, Alvarez-Candal, Merlin, et al. (2009) Icarus 201, 272

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We present in this work the observations performed with SINFONI in the framework of a new ESO-Large Program (2006–2008) on Trans-Neptunian Objects (TNOs) and Centaurs. We obtained 21 near-infrared (1.49 to 2.4 microns) spectra of high quality, including 4 spectra of objects never observed before. We search for the presence of features due to ices, particularly water ice. Eris is the only object showing deep methane ice absorption bands. The spectra of 4 objects are featureless, and 6 others show clearly the presence of water ice. For 7 objects, the detections are more ambiguous, but absorption bands could be embedded in the noise. The 3 remaining spectra are too noisy to draw any reliable conclusion. The possible amount of water ice on each object’s surface has been computed. The analysis shows that some objects present strong compositional heterogeneities over the surface (e.g. Chariklo), while some others are completely homogeneous (e.g. Quaoar).

The ‘Oumuamua ISSI Team (2019) Nature Astronomy, 3, 594-602

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Métayer, Guilbert-Lepoutre, Ferruit et al. (2019) Frontiers in Astronomy and Space Sciences, 6, 8

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Cabral, Lagarde, Reylé, Guilbert-Lepoutre, Robin (2019) Astronomy & Astrophysics, 622, 49

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Cabral, Guilbert-Lepoutre, Fraser, Marsset, Volk et al. (2019) Astronomy & Astrophysics, 621, 102

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Snodgrass, Agarwal, Combi, Fitzsimmons, Guilbert-Lepoutre et al. (2017) Astronomy & Astrophysical Reviews, 25, 5

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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.

Rousselot, Korsun, Kulyk, Guilbert-Lepoutre, Petit (2016) MNRAS, 462, 432

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Centaur 174P/Echeclus, initially designated as (60558) 2000 EC₉₈, presented three outbursts. A first and main one detected in 2005 December, another smaller one detected in 2011 May and a last one at the end of 2016 August. The first outburst was the largest one ever detected for a Centaur, of the order of 30 times that seen in other similar bodies. Because of the special interest of this target, and its brightness, we now have a large set of observational data were obtained before, during and after the two first outbursts. We present here new observational data obtained after the main outburst or coming from archives and an analysis of them. The main results of our study are (i) an absence of light curve in our 2013 data (while it was ˜0.24 mag in the R-band in 2002-2003) and (ii) a satisfactory fit of the main outburst with two short events and a longer one (three sources of dust). Both results suggest a high obliquity of the rotation axis. We also discuss the origin of these outbursts and conclude that they are probably related to internal inhomogeneities of the nucleus.

Barucci, the OSIRIS and VIRTIS teams et al. (2016) A&A, 595, 102

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Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus. The aim of this work is to search for the presence of H2O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H2O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination conditions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotometrically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes. Out of the 13 selected bright spots, eight of them present positive H2 O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H2O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H2O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.

Opitom, Guilbert-Lepoutre, Jehin, Manfroid, Hutsemékers et al. (2016) A&A, 589, 8

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We present a unique data set of more than one year’s worth of regular observations of comet C/2013 A1(Siding Spring) taken with TRAPPIST, along with low-resolution spectra obtained with the ESO/VLT FORS 2 instrument. The comet made a close approach to Mars on October 19, 2014, and was then observed by many space-borne and ground-based telescopes. We followed the evolution of the OH, NH, CN, C3 , and C2 production rates as well as the A f ρ parameter, a proxy for the dust production. We detected an outburst two weeks after perihelion, with gas and dust production rates increased by a factor of five within a few days. By modelling the shape of the CN and C2 radial profiles, we determined that the outburst happened on November 10 around 15:30 UT (±5 h) and measured a gas expansion velocity of 1.1 ± 0.2 km s−1. We used a thermal evolution model to reproduce the activity pattern and outburst. Our results are consistent with the progressive formation of a dust mantle explaining the shallow dependence of gas production rates, which may be partially blown off during the outburst. We studied the evolution of gas composition, using various ratios such as CN/OH, C2/OH, or C3/OH, which showed little or no variation with heliocentric distance, including at the time of the outburst. This indicates a relative level of homogeneity of the nucleus composition.

Vernazza, Beck, Lamy, Guilbert-Lepoutre (2016) RoAJ, 26, 35

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In this paper, we present a sample of high-priority science objectives for future interplanetary missions towards asteroids that were submitted to ESA as a white paper in 2013.

Mousis, Guilbert-Lepoutre, Brugger, Jorda, Kargel et al. (2015) Astrophysical Journal Letters, 81

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We investigate the thermal evolution of comet 67P/Churyumov-Gerasimenko’s subsurface in the Seth_01 region, where active pits have been observed by the ESA/Rosetta mission. Our simulations show that clathrate destabilization and amorphous ice crystallization can occur at depths corresponding to those of the observed pits in a timescale shorter than 67P/Churyumov-Gerasimenko’s lifetime in the comet’s activity zone in the inner solar system. Sublimation of crystalline ice down to such depths is possible only in the absence of a dust mantle, which requires the presence of dust grains in the matrix small enough to be dragged out by gas from the pores. Our results are consistent with both pits formation via sinkholes or subsequent to outbursts, the dominant process depending on the status of the subsurface porosity. A sealed dust mantle would favor episodic and disruptive outgassing as a result of increasing gas pressure in the pores, while high porosity should allow the formation of large voids in the subsurface due to the continuous escape of volatiles. We finally conclude that the subsurface of 67P/Churyumov-Gerasimenko is not uniform at a spatial scale of ˜100-200 m.

Holm, Oze, Mousis, Waite, Guilbert-Lepoutre (2015) Astrobiology, 15, 587

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Serpentinization involves the hydrolysis and transformation of primary ferromagnesian minerals such as olivine ((Mg,Fe)2SiO4) and pyroxenes ((Mg,Fe)SiO3) to produce H2-rich fluids and a variety of secondary minerals over a wide range of environmental conditions. The continual and elevated production of H2 is capable of reducing carbon, thus initiating an inorganic pathway to produce organic compounds. The production of H2 and H2-dependent CH4 in serpentinization systems has received significant interdisciplinary interest, especially with regard to the abiotic synthesis of organic compounds and the origins and maintenance of life in Earth’s lithosphere and elsewhere in the Universe. Here, serpentinization with an emphasis on the formation of H2 and CH4 are reviewed within the context of the mineralogy, temperature/pressure, and fluid/gas chemistry present in planetary environments. Whether deep in Earth’s interior or in Kuiper Belt Objects in space, serpentinization is a feasible process to invoke as a means of producing astrobiologically indispensable H2 capable of reducing carbon to organic compounds.

Ellinger, Pauzat, Mousis, Guilbert-Lepoutre, Leblanc et al. (2015) Astrophysical Journal, 801, 30

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Observations of comet C/1995O1 Hale-Bopp during the spring of 1997 led to the discovery of a neutral sodium tail whose origin is still not clearly understood. Here, we propose an interpretation for the origin of this sodium tail, which is based upon chemical grounds. Starting from Na+ trapped chemically during the condensation of refractory material in the protosolar nebula to its incorporation in the building blocks of comets and its transfer from refractory to volatile phases in the nucleus due to aqueous alteration, we follow the chemical path of sodium until its transformation into a neutral atom when released from the sublimating cometary ice. We propose that two Na reservoirs should coexist in a comet: one coming from the refractory dust, the other one from the icy matrix. Their relative importance would depend on the extent of the zone where liquid water formed within the nucleus and the time during which water remained liquid, thus favoring the Na+ exchange between rocks and ice. These two key parameters would in turn strongly depend on the thermal history of the comet (amounts of radiogenic nuclides, orbital history, etc.). If our model is correct, the detection of Na originating from water ice would be a testimonial of the past aqueous alteration of the comet or its parent body.

Mousis, Guilbert-Lepoutre, Lunine et al. (2012) Astrophysical Journal, 757, 146

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We propose a scenario that explains the apparent nitrogen deficiency in comets in a way that is consistent with the fact that the surfaces of Pluto and Triton are dominated by nitrogen-rich ice. We use a statistical thermodynamic model to investigate the composition of the successive multiple guest clathrates that may have formed during the cooling of the primordial nebula from the most abundant volatiles present in the gas phase. These clathrates agglomerated with the other ices (pure condensates or stoichiometric hydrates) and formed the building blocks of comets. We report that molecular nitrogen is a poor clathrate former, when we consider a plausible gas-phase composition of the primordial nebula. This implies that its trapping into cometesimals requires a low disk temperature (∼20 K) in order to allow the formation of its pure condensate. We find that it is possible to explain the lack of molecular nitrogen in comets as a consequence of their postformation internal heating engendered by the decay of short-lived radiogenic nuclides. This scenario is found to be consistent with the presence of nitrogen-rich ice covers on Pluto and Triton. Our model predicts that comets should present xenon-to-water and krypton-to-water ratios close to solar xenon-to-oxygen and krypton-to-oxygen ratios, respectively. In contrast, the argon-to-water ratio is predicted to be depleted by a factor of ∼300 in comets compared to solar argon-to-oxygen, as a consequence of poor trapping efficiency and radiogenic heating.

Peixinho, Delsanti, Guilbert-Lepoutre, et al. (2012) A&A, 546, 86

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Ever since the very first photometric studies of Centaurs and Kuiper belt objects (KBOs) their visible color distribution has been controversial. This controversy has triggered to a prolific debate on the origin of the surface colors of these distant icy objects of the solar system. Two scenarios have been proposed to interpret and explain the large variability of colors, hence surface composition. Are the colors mainly primordial and directly related to the formation region, or are they the result of surface evolution processes? To date, no mechanism has been found that successfully explains why Centaurs, which are escapees from the Kuiper belt, exhibit two distinct color groups, whereas KBOs do not. We readdress this issue using a carefully compiled set of B − R colors and HR(α) magnitudes (as proxy for size) for 253 objects, including data for 10 new small objects. We find that the bimodal color distribution of Centaurs is a size-related phenomenon, common to both Centaurs and small KBOs, i.e. independent of dynamical classification. Furthermore, we find that large KBOs also have a bimodal distribution of surface colors, albeit distinct from the small objects and strongly dependent on the “Haumea collisional family” objects. When plotted in B − R, HR (α) space, the colors of Centaurs and KBOs display a peculiar N shape.

Drahus, Jewitt, Guilbert-Lepoutre, et al. (2012) Astrophysical Journal, 756, 80

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One of the least understood properties of comets is the compositional structure of their nuclei, which can either be homogeneous or heterogeneous. The nucleus structure can be conveniently studied at millimeter wavelengths, using velocity-resolved spectral time series of the emission lines, obtained simultaneously for multiple molecules as the body rotates. Using this technique, we investigated the sources of CH3OH and HCN in comet 103P/Hartley 2, the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. Our monitoring with the IRAM 30 m telescope shows short-term variability of the spectral lines caused by nucleus rotation. The varying production rates generate changes in brightness by a factor of four for HCN and by a factor of two for CH3OH, and they are remarkably well correlated in time. With the addition of the velocity information from the line profiles, we identify the main sources of outgassing: two jets, oppositely directed in a radial sense, and icy grains, injected into the coma primarily through one of the jets. The mixing ratio of CH3OH and HCN is dramatically different in the two jets, which evidently shows large-scale chemical heterogeneity of the nucleus. We propose a network of identities linking the two jets with morphological features reported elsewhere and postulate that the chemical heterogeneity may result from thermal evolution. The model-dependent average production rates are 3.5 × 10^26 molecules s−1 for CH3OH and 1.25 × 10^25 molecules s−1 for HCN, and their ratio of 28 is rather high but not abnormal. The rotational temperature from CH3OH varied strongly, presumably due to nucleus rotation, with the average value being 47 K.

Jewitt & Guilbert-Lepoutre (2012) Astronomical Journal, 143, 21

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We present optical spectra of (24) Themis and (65) Cybele, two large main-belt asteroids on which exposed water ice has recently been reported. No emission lines, expected from resonance fluorescence in gas sublimated from the ice, were detected. Derived limits to the production rates of water are 400 kg s−1 (5σ ) for each object, assuming a cometary H2O/CN ratio. We rule out models in which a large fraction of the surface is occupied by high-albedo (“fresh”) water ice because the measured albedos of Themis and Cybele are low (∼0.05–0.07). We also rule out models in which a large fraction of the surface is occupied by low-albedo (“dirty”) water ice because dirty ice would be warm and would sublimate strongly enough for gaseous products to have been detected. If ice exists on these bodies it must be relatively clean (albedo 0.3) and confined to a fraction of the Earth-facing surface 10%. By analogy with impacted asteroid (596) Scheila, we propose an impact excavation scenario, in which 10 m scale projectiles have exposed buried ice. If the ice is even more reflective (albedo 0.6), then the timescale for sublimation of an optically thick layer can rival the ∼103 yr interval between impacts with bodies this size. In this sense, exposure by impact may be a quasi steady-state feature of ice-containing asteroids at 3 AU.

Drahus, Jewitt, Guilbert-Lepoutre et al. (2011) Astrophysical Journal Letters 734, L4

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The nuclei of active comets emit molecules anisotropically from discrete vents. As the nucleus rotates, we expect to observe periodic variability in the molecular emission line profiles, which can be studied through millimeter/ submillimeter spectroscopy. Using this technique we investigated the HCN atmosphere of comet 103P/Hartley 2, the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. We detected short-term evolution of the spectral line profile, which was stimulated by the nucleus rotation, and which provides evidence for rapid deceleration and excitation of the rotation state. The measured rate of change in the rotation period is +1.00 ± 0.15 minutes day−1 and the period itself is 18.32 ± 0.03 hr, both applicable at the epoch of the EPOXI encounter. Surprisingly, the spin-down efficiency is lower by two orders of magnitude than the measurement in comet 9P/Tempel 1 and the best theoretical prediction. This secures rotational stability of the comet’s nucleus during the next few returns, although we anticipate a catastrophic disruption from spin-up as its ultimate fate.

DeMeo, Barucci, Merlin, Guilbert-Lepoutre et al. (2010) A&A, 521, A35

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We present new visible and near-infrared photometric measurements and near-infrared spectroscopic measurements for three outer solar system small bodies, the Jupiter-coupled object (52872) Okyrhoe and the TNOs (90482) Orcus and (73480) 2002 PN34. We analyzed their surface compositions by modeling their spectra in the visible and near-infrared wavelength ranges. We then compared this new data with previous measurements of Okyrhoe and Orcus to search for heterogeneity on their surfaces. All observations were performed at the European Southern Observatory 8 m Very Large Telescope, UT1 and UT4 at the Paranal Observatory in Chile. We find varying amounts of H2O ice among these bodies, Okyrhoe shows no trace of it in our spectrum, 73480 has small amounts, and Orcus has large quantities. While we do clearly see for Orcus that a significant fraction of the H2O ice is in crystalline form from the 1.65-μm feature, we cannot detect the 2.21-μm feature supposedly due to ammonia hydrate, because of the low signal- to-noise of the data. We also do not see any indication of ices more volatile than H2O, such as CH4 or CO2, in the spectrum, so we limit their presence to no more than about 5% based on the data presented here and on high-quality data from Barucci et al. (2008, A&A, 479, L13).

Delsanti, Merlin, Guilbert-Lepoutre et al. (2010) A&A, 520, A40

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Orcus is an intermediate-size 1000 km-scale Kuiper belt object (KBO) in 3:2 mean-motion resonance with Neptune, in an orbit very similar to that of Pluto. It has a water-ice dominated surface with solar-like visible colors. We present visible and near-infrared photometry and spectroscopy obtained with the Keck 10 m-telescope (optical) and the Gemini 8 m-telescope (near-infrared). We confirm the unambiguous detection of crystalline water ice as well as absorption in the 2.2 μm region. These spectral properties are close to those observed for Pluto’s larger satellite Charon, and for Plutino (208996) 2003 AZ84. Both in the visible and near-infrared Orcus’ spectral properties appear to be homogeneous over time (and probably rotation) at the resolution available. From Hapke radiative transfer models involving intimate mixtures of various ices we find for the first time that ammonium (NH+4 ) and traces of ethane (C2H6), which are most probably solar irradiation products of ammonia and methane, and a mixture of methane and ammonia (diluted or not) are the best candidates to improve the description of the data with respect to a simple water ice mixture (Haumea type surface). The possible more subtle structure of the 2.2 μm band(s) should be investigated thoroughly in the future for Orcus and other intermediate size Plutinos to better understand the methane and ammonia chemistry at work, if any. We investigated the thermal history of Orcus with a new 3D thermal evolution model. Simulations over 4.5×109 yr with an input 10% porosity, bulk composition of 23% amorphous water ice and 77% dust (mass fraction), and cold accretion show that even with the action of long-lived radiogenic elements only, Orcus should have a melted core and most probably suffered a cryovolcanic event in its history which brought large amounts of crystalline ice to the surface. The presence of ammonia in the interior would strengthen the melting process. A surface layer of a few hundred meters to a few tens of kilometers of amorphous water ice survives, while most of the remaining volume underneath contains crystalline ice. The crystalline water ice possibly brought to the surface by a past cryovolcanic event should still be detectable after several billion years despite the irradiation effects, as demonstrated by recent laboratory experiments.

Barucci, Merlin, Guilbert et al. (2008) A&A, 479, 13

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The aim of this paper is to investigate the surface composition of the Transneptunian Object (TNO) Orcus. High quality observations have been carried out with the new instrument SINFONI at the Very Large Telescope (VLT) of ESO. Crystalline water ice, and possibly ammonia ice, have been found from spectroscopic observations of the TNO Orcus between 1.4 and 2.4 μm. The existence of such ices on the surface of Orcus may indicate a renewal mechanism on the surface and geological activity. The presence of ammonia on the surface of Orcus, if confirmed, could have important implications for the composition of the primitive solar nebula and the formation of the TNO population.

Alvarez-Candal, Barucci, Merlin, Guilbert, de Bergh (2007) A&A 475, 369

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Our aim is to investigate the surface composition of TNOs. In particular we would like to confirm diverse spectral absorption features and to search for rotational inhomogeneities on one scattered disk object (26375) 1999 DE9, two plutinos (38628) Huya and (47932) 2000 GN171, and one centaur (83982) Crantor. We observed the targets with a new instrument available at the VLT, the near-infrared integral field spectrograph SINFONI. We obtained complete near-infrared spectra between 1.4 and 2.4 μm and compared them with data recorded previously by various groups. Two objects, (38628) Huya and (83982) Crantor, have evidence of an absorption feature in their spectra at 2.0 μm, probably associated with water ice. (83982) Crantor shows a feature at 2.3 μm which could be associated with methanol. On the other hand, no features were found for (26375) 1999 DE9 and (47932) 2000 GN171 above the signal-to-noise ratio. Possible rotational heterogeneity can be seen on two objects: (38628) Huya as the 2.0 μm band is present on some spectra and absent on others, while (47932) 2000 GN171 shows a flat reflectance in contrast to an absorption in the H region previously seen.

Merlin, Guilbert, Dumas, et al. (2007) A&A, 466, 1185

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Spectroscopic observations of numerous trans-Neptunian objects (TNOs), considered to be among the most pristine objects of the solar system, have revealed the presence of several kinds of surface ices. The high-sensitivity spectra that can be measured for the brightest objects also provide constraints on the physical properties of the surface (e.g. ice phase, temperature). We observed one of the largest and brightest TNOs, 136108 (2003 EL61), to determine its surface composition properties and to constrain its surface properties. We obtained new visible spectra with EMMI on the ESO-NTT and near-infrared spectra with the new 3D spectrograph SINFONI at the ESO-VLT. Our analysis consists of radiative transfer modelling to constrain composition and surface properties and to identify the precise minimum of the 1.65 micron band to constrain the surface temperature. The observations reveal a surface essentially composed of water ice. An absorption feature at 1.65 micron clearly indicates the presence of ice in the crystalline phase. Spectral modelling suggests that a small fraction of the surface ice is in the amorphous state. We also derive the temperature of the crystalline ice at the surface.

Peixinho et al. (2020) In The Trans-Neptunian Solar System, Eds. Prialnik, Barucci, Young, 307-329

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Bowles et al. (2018) Advances in Space Research, 62, 1998-2025

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Fabian et al. (2018) ACS Earth Space Chem. 2, 521-531

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Bannister et al. (2018) ApJS. 236, 18

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Snodgrass et al. (2017) Phil. Trans. R. Soc. 375, 20160249

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Bannister et al. (2016) AJ 152, 212

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Bannister et al. (2016) Astronomical Journal, 152, 70

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We report the discovery, tracking, and detection circumstances for 85 trans-Neptunian objects (TNOs) from the first 42 deg2 of the Outer Solar System Origins Survey. This ongoing r-band solar system survey uses the 0.9 deg2 field of view MegaPrime camera on the 3.6 m Canada–France–Hawaii Telescope. Our orbital elements for these TNOs are precise to a fractional semimajor axis uncertainty <0.1%. We achieve this precision in just two oppositions, as compared to the normal three to five oppositions, via a dense observing cadence and innovative astrometric technique. These discoveries are free of ephemeris bias, a first for large trans-Neptunian surveys. We also provide the necessary information to enable models of TNO orbital distributions to be tested against our TNO sample. We confirm the existence of a cold “kernel” of objects within the main cold classical Kuiper Belt and infer the existence of an extension of the “stirred” cold classical Kuiper Belt to at least several au beyond the 2:1 mean motion resonance with Neptune. We find that the population model of Petit et al. remains a plausible representation of the Kuiper Belt. The full survey, to be completed in 2017, will provide an exquisitely characterized sample of important resonant TNO populations, ideal for testing models of giant planet migration during the early history of the solar system.

Brugger et al. (2016) Astrophysical Journal, 822, 98

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On 2014 November 12, the ESA/Rosetta descent module Philae landed on the Abydos site of comet 67P/ Churyumov–Gerasimenko. Aboard this module, the Ptolemy mass spectrometer measured a CO/CO2 ratio of 0.07 ± 0.04, which differs substantially from the value obtained in the coma by the Rosetta/ROSINA instrument, suggesting a heterogeneity in the comet nucleus. To understand this difference, we investigated the physicochemical properties of the Abydos subsurface, leading to CO/CO2 ratios close to that observed by Ptolemy at the surface of this region. We used a comet nucleus model that takes into account different water ice phase changes (amorphous ice, crystalline ice, and clathrates) as well as diffusion of molecules throughout the pores of the matrix. The input parameters of the model were optimized for the Abydos site, and the ROSINA CO/ CO2 measured ratio is assumed to correspond to the bulk value in the nucleus. We find that all considered structures of water ice are able to reproduce the Ptolemy observation with a time difference not exceeding ∼50 days, i.e., lower than ∼2% on 67P/Churyumov–Gerasimenko’s orbital period. The suspected heterogeneity of 67P/Churyumov–Gerasimenko’s nucleus is also found possible only if it is constituted of crystalline ices. If the icy phase is made of amorphous ice or clathrates, the difference between Ptolemy and ROSINA’s measurements would rather originate from the spatial variations in illumination on the nucleus surface. An eventual new measurement of the CO/CO2 ratio at Abydos by Ptolemy could be decisive to distinguish between the three water ice structures.

Parker et al. (2016) PASP, 128, 8010

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Studies of the physical properties of trans-Neptunian objects (TNOs) are a powerful probe into the processes of planetesimal formation and solar system evolution. James Webb Space Telescope (JWST) will provide unique new capabilities for such studies. Here, we outline where the capabilities of JWST open new avenues of investigation, potentially valuable observations and surveys, and conclude with a discussion of community actions that may serve to enhance the eventual science return of JWSTʼs TNO observations.

Snodgrass et al. (2016) A&A, 588, 80

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As the ESA Rosetta mission approached, orbited, and sent a lander to comet 67P/Churyumov-Gerasimenko in 2014, a large campaign of ground-based observations also followed the comet. We constrain the total activity level of the comet by photometry and spectroscopy to place Rosetta results in context and to understand the large-scale structure of the comet’s coma pre-perihelion. We performed observations using a number of telescopes, but concentrate on results from the 8 m VLT and Gemini South telescopes in Chile. We use R-band imaging to measure the dust coma contribution to the comet’s brightness and UV-visible spec- troscopy to search for gas emissions, primarily using VLT/FORS. In addition we imaged the comet in near-infrared wavelengths (JHK) in late 2014 with Gemini-S/Flamingos-2. We find that the comet was already active in early 2014 at heliocentric distances beyond 4 au. The evolution of the total activity (measured by dust) followed previous predictions. No gas emissions were detected despite sensitive searches. The comet maintains a similar level of activity from orbit to orbit, and is in that sense predictable, meaning that Rosetta results correspond to typical behaviour for this comet. The gas production (for CN at least) is highly asymmetric with respect to perihelion, as our upper limits are below the measured production rates for similar distances post-perihelion in previous orbits.

Vincent, Bodewits, Besse and the OSIRIS Team (2015) Nature, 523, 63

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Pits have been observed on many cometary nuclei mapped by spacecraft. It has been argued that cometary pits are a signature of endogenic activity, rather than impact craters such as those on planetary and asteroid surfaces. Impact experiments and models cannot reproduce the shapes of most of the observed cometary pits, and the predicted collision rates imply that few of the pits are related to impacts8,9. Alternative mechanisms like explosive activity have been suggested, but the driving process remains unknown. Here we report that pits on comet 67P/Churyumov–Gerasimenko are active, and probably created by a sinkhole process, possibly accompanied by outbursts. We argue that after formation, pits expand slowly in diameter, owing to sublimation-driven retreat of the walls. Therefore, pits characterize how eroded the surface is: a fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother. The size and spatial distribution of pits imply that large heterogeneities exist in the physical, structural or compositional properties of the first few hundred metres below the current nucleus surface.

Lacerda et al. (2014) Astrophysical Journal, 793, 2

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We analyze albedo data obtained using the Herschel Space Observatory that reveal the existence of two distinct types of surface among midsized trans-Neptunian objects. A color–albedo diagram shows two large clusters of objects, one redder and higher albedo and another darker and more neutrally colored. Crucially, all objects in our sample located in dynamically stable orbits within the classical Kuiper Belt region and beyond are confined to the bright red group, implying a compositional link. Those objects are believed to have formed further from the Sun than the dark neutral bodies. This color–albedo separation is evidence for a compositional discontinuity in the young solar system.

Alvarez-Candal et al. (2010) A&A, 511, 35

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The scattered disk object (42355) Typhon shows interesting features in visible and near-infrared spectra, in particular, the visible spectrum shows evidence of aqueously altered materials.This article presents a possible origin for absorption features on the surface of (42355) Typhon based on an episode of aqueous alteration, and it seeks to understand this event in the context of its dynamical evolution. We observed (42355) Typhon at the ESO/Very Large Telescope using FORS2 and ISAAC on telescope unit 1 and SINFONI on telescope unit 4. We compared these data with those previously published, in order to confirm features found in the visible and near infrared spectra and to study possible surface heterogeneities. We interpreted the surface composition using the Hapke radiative transfer model on the whole available spectral range ∼0.5−2.4 μm. To complete the portrait of (42355) Typhon, we followed its dynamical evolution using the code EVORB v.13 for 20 Myr. We confirm detection of a subtle absorption feature in the visible at ∼0.6 μm, which we interpret as caused by water-altered silicates. In the near infrared, we confirm the presence of water ice by the 2.0 μm absorption feature. The best-fit models to our data point the presence of water ice. (42355) Typhon is too small to have suffered water alteration, but this event could happen in a larger parent body from which (42355) Typhon is a remnant after a catastrophic disruption.

Fornasier et al. (2009) A&A, 508, 457

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A second large programme (LP) for the physical studies of TNOs and Centaurs, started at ESO Cerro Paranal on October 2006 to obtain high-quality data, has recently been concluded. In this paper we present the spectra of these pristine bodies obtained in the visible range during the last two semesters (November 2007−November 2008) of the LP. We investigate the spectral behaviour of the TNOs and Centaurs observed, and we analyse the spectral slopes distribution of the full data set coming from this LP and from the literature. Spectroscopic observations in the visible range were carried out at the UT1 (Antu) telescope using the instrument FORS2. We computed the spectral slope for each observed object, and searched for possible weak absorption features. A statistical analysis was performed on a total sample of 73 TNOs and Centaurs to look for possible correlations between dynamical classes, orbital parameters, and spectral gradient. We obtained new spectra for 28 bodies (10 Centaurs, 6 classical, 5 resonant, 5 scattered disk, and 2 detached objects), 15 of which were observed for the first time. All the new presented spectra are featureless, including 2003 AZ84, for which a faint and broad absorption band possibly attributed to hydrated silicates on its surface has been reported. The data confirm a wide variety of spectral behaviours, with neutral-grey to very red gradients. An analysis of the spectral slopes available from this LP and in the literature for a total sample of 73 Centaurs and TNOs shows that there is a lack of very red objects in the classical population. We present the results of the statistical analysis of the spectral slope distribution versus orbital parameters. In particular, we confirm a strong anticorrelation between spectral slope and orbital inclination for the classical population. Nevertheless, we do not observe a change in the slope distribution at i ∼ 5◦, the boundary between the dynamically hot and cold populations, but we find that objects with i < 12◦ show no correlation between spectral slope and inclination, as has already been noticed on the colour-inclination relation for classical TNOs. A strong correlation is also found between the spectral slope and orbital eccentricity for resonant TNOs, with objects having higher spectral slope values with increasing eccentricity.

Dumas et al. (2007) A&A, 471, 331

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The surface composition of the largest TNO, the dwarf planet 136199 Eris, is studied and compared to Pluto’s. High signal-to-noise visible and near-infrared reflectance spectra were obtained at the TNG and ESO-VLT observatories. The nature and properties of the compounds present on the surface of Eris are investigated by applying Hapke and Shkuratov radiative transfer models to our spectra. The surface of Eris can be modeled using two areas of distinct composition: about 50% appears to be covered with pure methane ice, while the rest of its surface would be made of an intimate mixture of methane, nitrogen and water ices, and ice tholin. The use of nitrogen in our model is shown to improve significantly the data fit, in particular for high surface albedo values. The icy grains are found to be large, from sub-mm to a few tens of mm in size.