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Large and deep depressions, also known as pits, are observed at the surface of all Jupiter Family Comets (JFCs) imaged by spacecraft missions. They offer the opportunity to glimpse into sub-surface characteristics of comet nuclei, and study the complex interplay between surface structures and cometary activity. This work investigates the evolution of pits at the surface of 81P/Wild 2, 9P/Tempel 1 and 103P/Hartley 2, in continuation of the work by Benseguane et al. (2022), on 67P/Churyumov-Gerasimenko. Pits are selected across the surface of each nucleus, and high-resolution shape models are used to compute the energy they receive. A thermal evolution model is applied to constrain how cometary activity sustained under current illumination conditions could modify them. Similarly to what was found for 67P, we show erosion resulting from water-driven activity is primarily controlled by seasonal patterns, unique to each comet as a consequence of their shape and rotational properties. However, progressive erosion sustained after multiple perihelion passages is not able to carve any of the observed pits. Instead, cometary activity tends to erase sharp morphological features: they become wider and shallower over time. Our results reinforce the evolutionary sequence evidenced from independent measurables to transform "young" cometary surfaces, with sharp surface topography prone to outbursts, into "old" cometary surfaces. Finally, we suggest that the mechanism at the origin of pits on JFCs should be able to carve these structures in a region of the solar system where water ice does not sublimate: the Centaur phase thus appears critical to understand JFCs surface properties.

One of the common approximations in long-term evolution studies of small bodies is the use of circular orbits averaging the actual eccentric ones, facilitating the coupling of processes with very different timescales, such as the orbital changes and the thermal processing. Here we test a number of averaging schemes for elliptic orbits in the context of the long-term evolution of comets, aiming to identify the one that best reproduces the elliptic orbits' heating patterns and the surface and subsurface temperature distributions. We use a simplified thermal evolution model applied on simulated comets both on elliptic and on their equivalent averaged circular orbits, in a range of orbital parameter space relevant to the inner solar system. We find that time averaging schemes are more adequate than spatial averaging ones. Circular orbits created by means of a time average of the equilibrium temperature approximate efficiently the subsurface temperature distributions of elliptic orbits in a large area of the orbital parameter space, rendering them a powerful tool for averaging elliptic orbits., Comment: 12 pages, 7 figures, accepted for publications on the AJ

It was recently proposed that there exists a "gateway" in the orbital parameter space through which Centaurs transition to Jupiter-family Comets (JFCs). Further studies have implied that the majority of objects that eventually evolve into JFCs should leave the Centaur population through this gateway. This may be naively interpreted as gateway Centaurs being pristine progenitors of JFCs. This is the point we want to address in this work. We show that the opposite is true: gateway Centaurs are, on average, more thermally processed than the rest of the population of Centaurs crossing Jupiter's orbit. Using a dynamically-validated JFC population, we find that only $\sim 20\%$ of Centaurs pass through the gateway prior to becoming JFCs, in accordance with previous studies. We show that more than half of JFC dynamical clones entering the gateway for the first time have already been JFCs -they simply avoided the gateway on their first pass into the inner solar system. By coupling a thermal evolution model to the orbital evolution of JFC dynamical clones, we find a higher than 50\% chance that the layer currently contributing to the observed activity of gateway objects has been physically and chemically altered, due to previously sustained thermal processing. We further illustrate this effect by examining dynamical clones that match the present-day orbits of 29P/Schwassmann-Wachmann 1, P/2019 LD2 (ATLAS), and P/2008 CL94 (Lemmon)., Comment: 16 pages, 13 Figures, Accepted for publication on the ApJ

The observation of pits at the surface of comets offers the opportunity to take a glimpse into the properties and the mechanisms that shape a nucleus through cometary activity. If the origin of these pits is still a matter of debate, multiple studies have recently suggested that known phase transitions alone could not have carved these morphological features on the surface of 67P/C-G. We want to understand how the progressive modification of 67P's surface due to cometary activity might have affected the characteristics of pits. In particular, we aim to understand whether signatures of the formation mechanism of these morphological features can still be identified. To quantify the amount of erosion sustained at the surface of 67P since it arrived on its currently observed orbit, we selected 380 facets of a medium-resolution shape model of the nucleus, sampling 30 pits across the surface. We computed the surface energy balance with a high temporal resolution, including shadowing and self-heating. We then applied a thermal evolution model to assess the amount of erosion sustained after ten orbital revolutions under current illumination conditions. We find that the maximum erosion sustained after ten orbital revolutions is on the order of 80 m, for facets located in the southern hemisphere. We thus confirm that progressive erosion cannot form pits and alcoves, as local erosion is much lower than their observed depth and diameter. We find that plateaus tend to erode more than bottoms, especially for the deepest depressions, and that some differential erosion can affect their morphology. As a general rule, our results suggest that sharp morphological features tend to be erased by progressive erosion. This study supports the assumption that deep circular pits, such as Seth1, are the least processed morphological features at the surface of 67P, or the best preserved since their formation.

Evidence for cometary activity beyond Jupiter and Saturn's orbits -- such as that observed for Centaurs and long period comets -- suggests that the thermal processing of comet nuclei starts long before they enter the inner Solar System, where they are typically observed and monitored. Such observations raise questions as to the depth of unprocessed material, and whether the activity of JFCs can be representative of any primitive material. Here we model the coupled thermal and dynamical evolution of Jupiter Family Comets (JFCs), from the moment they leave their outer Solar System reservoirs until their ejection into interstellar space. We apply a thermal evolution model to a sample of simulated JFCs obtained from dynamical simulations (arXiv:1706.07447) that successfully reproduce the orbital distribution of observed JFCs. We show that due to the stochastic nature of comet trajectories toward the inner solar system, all simulated JFCs undergo multiple heating episodes resulting in significant modifications of their initial volatile contents. A statistical analysis constrains the extent of such processing. We suggest that primordial condensed hypervolatile ices should be entirely lost from the layers that contribute to cometary activity observed today. Our results demonstrate that understanding the orbital (and thus, heating) history of JFCs is essential when putting observations in a broader context., Comment: 30 pages, 10 figures, to be published in ApJ

Gas has been detected in many exoplanetary systems ($>$10 Myr), thought to be released in the destruction of volatile-rich planetesimals orbiting in exo-Kuiper belts. In this letter, we aim to explore whether gas is also expected in the Kuiper belt (KB) in our Solar System. To quantify the gas release in our Solar System, we use models for gas release that have been applied to extrasolar planetary systems, as well as a physical model that accounts for gas released due to the progressive internal warming of large planetesimals. We find that only bodies larger than about 4 km can still contain CO ice after 4.6 Gyr of evolution. This finding may provide a clue as to why Jupiter-family comets, thought to originate in the Kuiper belt, are deficient in CO compared to Oort-clouds comets. We predict that gas is still produced in the KB right now at a rate of $2 \times 10^{-8}$ M$_\oplus$/Myr for CO and orders of magnitude more when the Sun was younger. Once released, the gas is quickly pushed out by the Solar wind. Therefore, we predict a gas wind in our Solar System starting at the KB location and extending far beyond with regards to the heliosphere with a current total CO mass of $\sim 2 \times 10^{-12}$ M$_\oplus$. We also predict the existence of a slightly more massive atomic gas wind made of carbon and oxygen (neutral and ionized) with a mass of $\sim 10^{-11}$ M$_\oplus$. We predict that gas is currently present in our Solar System beyond the Kuiper belt and that although it cannot be detected with current instrumentation, it could be observed in the future with an in situ mission using an instrument similar to Alice on New Horizons with larger detectors. Our model of gas release due to slow heating may also work for exoplanetary systems and provide the first real physical mechanism for the gas observations., Comment: accepted for publication as a Letter to the editor in A&A; abstract shortened; 15 pages, 5 figures, 4 tables

We report the clear detection of C$_2$ and of abundant NH$_2$ in the first prominently active interstellar comet, 2I/Borisov. We observed 2I on three nights in November 2019 at optical wavelengths 4800--9300 \AA with the Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectrograph on the ESO/Very Large Telescope. These data, together with observations close in time from both 0.6-m TRAPPIST telescopes, provide constraints on the production rates of species of gas in 2I's coma. From the MUSE detection on all epochs of several bands of the optical emission of the C$_2$ Swan system, a rich emission spectrum of NH$_2$ with many highly visible bands, and the red (1-0) bandhead of CN, together with violet CN detections by TRAPPIST, we infer production rates of $Q$(C$_2$) = $1.1\times10^{24}$ mol s$^{-1}$, $Q$(NH$_2$) = $4.8\times10^{24}$ mol s$^{-1}$ and $Q$(CN) = $(1.8\pm0.2)\times 10^{24}$ mol s$^{-1}$. In late November at 2.03~au, 2I had a production ratio of C$_2$/CN$=0.61$, only barely carbon-chain depleted, in contrast to earlier reports measured further from the Sun of strong carbon-chain depletion. Thus, 2I has shown evolution in its C$_2$ production rate: a parent molecule reservoir has started sublimating. At $Q$(NH$_2$)/$Q$(CN) = 2.7, this second interstellar object is enriched in NH$_2$, relative to the known Solar System sample., Comment: 5 figures. Submitted to AAS Journals

In 1977, while Apple II and Atari computers were being sold, a tiny dot was observed in an inconvenient orbit. The minor body 1977 UB, to be named (2060) Chiron, with an orbit between Saturn and Uranus, became the first Centaur, a new class of minor bodies orbiting roughly between Jupiter and Neptune. The observed overabundance of short-period comets lead to the downfall of the Oort Cloud as exclusive source of comets and to the rise of the need for a Trans-Neptunian comet belt. Centaurs were rapidly seen as the transition phase between Kuiper Belt Objects (KBOs), also known as Trans-Neptunian Objects (TNOs) and the Jupiter-Family Comets (JFCs). Since then, a lot more has been discovered about Centaurs: they can have cometary activity and outbursts, satellites, and even rings. Over the past four decades since the discovery of the first Centaur, rotation periods, surface colors, reflectivity spectra and albedos have been measured and analyzed. However, despite such a large number of studies and complementary techniques, the Centaur population remains a mystery as they are in so many ways different from the TNOs and even more so from the JFCs., Comment: Review chapter to be published in the book "The Transneptunian Solar System", Editors: Dina Prialnik, Maria Antonietta Barucci, and Leslie Young, Publisher: Elsevier (20 pages, 2 figures, 1 long table)

Connecting star and planet properties in a single model is not straightforward. Stellar population synthesis models are key to explore combined statistical constraints from stars and planets observations. The Besan\c{c}on stellar population synthesis model (Robin et al. 2003, Lagarde et al. 2017) includes now the stellar evolutionary tracks computed with the stellar evolution code STAREVOL (Lagarde et al. 2012, Amard et al. 2016). It provides the global (M, R, Teff, etc) and chemical properties of stars for 54 chemical species. It enables to study the different galactic populations of the Milky Way (the halo, the bulge, the thin and thick disc) and a specific observational survey. Here, we couple the Besan\c{c}on model with a simple stoichiometric model (Santos et al. 2017) in order to determine the expected composition of the planet building blocks (PBB). We investigate the trends and correlations of the expected chemical abundances of PBB in the different stellar populations of the Milky Way (Cabral et al. 2018)., Comment: 4 pages, 3 figures, conference

The Outer Solar System Origins Survey (OSSOS), a wide-field imaging program in 2013-2017 with the Canada-France-Hawaii Telescope, surveyed 155 deg$^{2}$ of sky to depths of $m_r = 24.1$-25.2. We present 838 outer Solar System discoveries that are entirely free of ephemeris bias. This increases the inventory of trans-Neptunian objects (TNOs) with accurately known orbits by nearly 50%. Each minor planet has 20-60 Gaia/Pan-STARRS-calibrated astrometric measurements made over 2-5 oppositions, which allows accurate classification of their orbits within the trans-Neptunian dynamical populations. The populations orbiting in mean-motion resonance with Neptune are key to understanding Neptune's early migration. Our 313 resonant TNOs, including 132 plutinos, triple the available characterized sample and include new occupancy of distant resonances out to semi-major axis $a \sim 130$ au. OSSOS doubles the known population of the non-resonant Kuiper belt, providing 436 TNOs in this region, all with exceptionally high-quality orbits of $a$ uncertainty $\sigma_{a} \leq 0.1\%$; they show the belt exists from $a \gtrsim 37$ au, with a lower perihelion bound of $35$ au. We confirm the presence of a concentrated low-inclination $a\simeq 44$ au "kernel" population and a dynamically cold population extending beyond the 2:1 resonance. We finely quantify the survey's observational biases. Our survey simulator provides a straightforward way to impose these biases on models of the trans-Neptunian orbit distributions, allowing statistical comparison to the discoveries. The OSSOS TNOs, unprecedented in their orbital precision for the size of the sample, are ideal for testing concepts of the history of giant planet migration in the Solar System., Comment: Invited paper, special issue Data: Insights and Challenges in a Time of Abundance. Data tables and example survey simulator are in the supplementary materials (see arXiv source under Downloads > Other formats)

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., Comment: Invited review for Astron. Astrophys. Rev. 43 pages + references. Product of ISSI team http://www.issibern.ch/teams/mainbeltcomets/

We report the discovery and orbit of a new dwarf planet candidate, 2015 RR$_{245}$, by the Outer Solar System Origins Survey (OSSOS). 2015 RR$_{245}$'s orbit is eccentric ($e=0.586$), with a semi-major axis near 82 au, yielding a perihelion distance of 34 au. 2015 RR$_{245}$ has $g-r = 0.59 \pm 0.11$ and absolute magnitude $H_{r} = 3.6 \pm 0.1$; for an assumed albedo of $p_V = 12$% the object has a diameter of $\sim670$ km. Based on astrometric measurements from OSSOS and Pan-STARRS1, we find that 2015 RR$_{245}$ is securely trapped on ten-Myr timescales in the 9:2 mean-motion resonance with Neptune. It is the first TNO identified in this resonance. On hundred-Myr timescales, particles in 2015 RR$_{245}$-like orbits depart and sometimes return to the resonance, indicating that 2015 RR$_{245}$ likely forms part of the long-lived metastable population of distant TNOs that drift between resonance sticking and actively scattering via gravitational encounters with Neptune. The discovery of a 9:2 TNO stresses the role of resonances in the long-term evolution of objects in the scattering disk, and reinforces the view that distant resonances are heavily populated in the current Solar System. This object further motivates detailed modelling of the transient sticking population., Comment: Accepted to AJ

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 8m 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 spectroscopy 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., Comment: 12 pages, accepted in A&A

We report the discovery, tracking and detection circumstances for 85 trans-Neptunian objects (TNOs) from the first 42 deg$^{2}$ of the Outer Solar System Origins Survey (OSSOS). This ongoing $r$-band Solar System survey uses the 0.9 deg$^{2}$ 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 semi-major axis uncertainty $<0.1\%$. We achieve this precision in just two oppositions, as compared to the normal 3-5 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. (2011) 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., Comment: Accepted to AJ, 27 April 2016. 59 pp

We analyze albedo data obtained using the Herschel Space Observatory that reveal the existence of two distinct types of surface among midsized transneptunian 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., Comment: 16 pages, 4 figures, 1 table, published in ApJL (12 August 2014), The Astrophysical Journal (2014), vol. 793, L2

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 Trojans 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.

We propose a scenario that explains the apparent nitrogen deficiency in comets in a way 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 ($\sim$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 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 $\sim$300 in comets compared to solar argon-to-oxygen, as a consequence of poor trapping efficiency and radiogenic heating., Comment: Accepted for publication in The Astrophysical Journal

Ever since the very first photometric studies of Centaurs and Kuiper Belt Objects (KBOs) their visible color distribution has been controversial. That controversy gave rise to a prolific debate on the origin of the surface colors of these distant icy objects of the Solar System. Two different views attempt 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. In this letter, we readdress this issue using a carefully compiled set of B-R colors and H({\alpha}) magnitudes (as proxy for size) for 253 objects, including data for 10 new small objects. We find that the bimodal behavior seen among Centaurs is a size related phenomenon, common to both Centaurs and small KBOs, i.e. independent of dynamical classification. Further, we find that large KBOs also exhibit a bimodal behavior of surface colors, albeit distinct from the small objects and strongly dependent on the `Haumea collisional family' objects. When plotted in B-R, H({\alpha}) space, the colors of Centaurs and KBOs display a peculiar N shape., Comment: To appear in Astronomy & Astrophysics. 12 pages (including 6 pages of appendix), 2 figures, 3 tables

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 4 for HCN and by a factor of 2 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 3.5x10**26 molec/s for CH3OH and 1.25x10**25 molec/s 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., Comment: Published in ApJ 756, 80 (2012). Supplementary materials available at http://www.its.caltech.edu/~mdrahus/103p_paperII.html

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 mm/submm 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 \pm 0.15 min per day and the period itself is 18.32 \pm 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., Comment: Accepted for publication in ApJ Letters