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Cliff collapses on Comet 67P/Churyumov-Gerasimenko expose relatively pristine nucleus matter and offer rare opportunities to characterise ice-rich comet material. Here, Microwave Instrument for \emph{Rosetta} Orbiter (MIRO) observations of two collapsed or crumbling cliffs in the Imhotep and Hathor regions have been assembled. The empirical diurnal antenna temperature curves are analysed with thermophysical and radiative transfer models in order to place constraints on the physical properties and degrees of stratification in the near-surface material. The Imhotep site consists of an exposed dust/water-ice mixture with thermal inertia 100-$160\,\mathrm{J\,m^{-2}\,K^{-1}\,s^{-1/2}}$, having sublimating $\mathrm{CO_2}$ ice located $11\pm 4\,\mathrm{cm}$ below the surface. Its estimated age is consistent with an outburst observed on 2014 April 27-30. The Hathor site has a $0.8\pm 0.2\,\mathrm{cm}$ dust mantle, a thermal inertia of $40\pm 20\,\mathrm{J\,m^{-2}\,K^{-1}\,s^{-1/2}}$, no $\mathrm{CO_2}$ ice to within $1.0\,\mathrm{m}$ depth, and a mantle bulk density of $340\pm 80\,\mathrm{kg\,m^{-3}}$ that is higher than the theoretically expected $180\pm 10\,\mathrm{kg\,m^{-3}}$, suggesting that compression has taken place., Comment: 16 pages, 12 figures with 21 panels. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

The Aswan cliff on Comet 67P/Churyumov-Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of ~12 m was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/MIRO have been retrieved from 8 months prior to collapse, as well as from 5, 7, and 11 months post-collapse. The MIRO data are analysed with thermophysical and radiative transfer models. The pre-collapse observations are consistent with a 30 MKS thermal inertia dust mantle with a thickness of at least 3 cm. The post-collapse data are consistent with: 1) a dust/water-ice mass ratio of 0.9$\pm$0.5 and a molar $\mathrm{CO_2}$ abundance of ~30 per cent relative to water; 2) formation of a dust mantle after ~7 months, having a thickness of a few millimetres or a fraction thereof; 3) a $\mathrm{CO_2}$ ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to 20$\pm$6 cm; 4) a thermal inertia ranging 10-45 MKS; 5) a gas diffusivity that decreased from $0.1\,\mathrm{m^2\,s^{-1}}$ to $0.001\,\mathrm{m^2\,s^{-1}}$; 6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a first empirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating., Comment: 22 pages, 24 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

Planetesimals in the primordial disc may have experienced a collisional cascade. If so, the comet nuclei later placed in the Kuiper belt, scattered disc, and Oort Cloud would primarily be fragments and collisional rubble piles from that cascade. However, the heating associated with the collisions cannot have been strong enough to remove the hypervolatiles that are trapped within more durable ices, because comet nuclei are rich in hypervolatiles. This places constraints on the diameter of the largest bodies allowed to participate in collisional cascades, and limits the primordial disc lifetime or population size. In this paper, the thermophysical code NIMBUS is used to study the thermal evolution of planetesimals before, during, and after catastrophic collisions. The loss of CO during segregation of $\mathrm{CO_2:CO}$ mixtures and during crystallisation of amorphous $\mathrm{H_2O}$ is calculated, as well as mobilisation and internal relocation of $\mathrm{CO_2}$. If an amorphous $\mathrm{H_2O}$ host existed, and was protected by a $\mathrm{CO_2:CO}$ heat sink, only diameter $D<20\,\mathrm{km}$ (inner disc) and $D<64\,\mathrm{km}$ (outer disc) bodies could have been involved in a collisional cascade. If $\mathrm{CO_2}$ was the only CO host, the critical diameters drop to $D<20$-$32\mathrm{km}$. Avoiding disruption of larger bodies requires a primordial disc lifetime of $<9\,\mathrm{Myr}$ at $15\,\mathrm{au}$ and $<50$-$70\,\mathrm{Myr}$ at $30\,\mathrm{au}$. Alternatively, if a $450\,\mathrm{Myr}$ disc lifetime is required to associate the primordial disc disruption with the Late Heavy Bombardment, the disc population size must have been 6-60 times below current estimates., Comment: 20 pages, 11 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

Deriving properties of cometary nuclei from coma data is of significant importance for our understanding of cometary activity and has implications beyond. Ground-based data represent the bulk of measurements available for comets. Yet, to date these observations only access a comet's gas and dust coma at rather large distances from the surface and do not directly observe its surface. In contrast, spacecraft missions are one of the only tools that gain direct access to surface measurements. However, such missions are limited to roughly one per decade. We can overcome these challenges by recognising that the coma contains information about the nucleus's properties. In particular, the near-surface gas environment is most representative of the nucleus. It can inform us about the composition, regionality of activity, and sources of coma features and how they link to the topography, morphology, or other surface properties. The inner coma data is a particularly good proxy because it has not yet, or only marginally, been contaminated by coma chemistry or secondary gas sources. Additionally, the simultaneous observation of the innermost coma with the surface provides the potential to make a direct link between coma measurements and the nucleus. If we hope to link outer coma measurements obtained by Earth-based telescopes to the surface, we must first understand how the inner coma measurements are linked to the surface. Numerical models that describe the flow from the surface into the immediate surroundings are needed to make this connection. This chapter focuses on the advances made to understand the flow of the neutral gas coma from the surface to distances up to a few tens of nuclei radii. We describe both simple/heuristic models and state-of-the-art physically consistent models. Model limitations and what they each are best suited for are discussed., Comment: 29 pages, 9 figures, Chapter in press for the book Comets III, edited by K. Meech and M. Combi, University of Arizona Press

Kuiper-like belts of planetesimals orbiting stars other than the Sun are most commonly detected from the thermal emission of small dust produced in collisions. Emission from gas, most notably CO, highlights the cometary nature of these planetesimals. Here we present models for the release of gas from comet-like bodies in these belts, both due to their thermophysical evolution, most notably the decay of long-lived radioactive nuclides and collisional evolution, including catastrophic and gentler resurfacing collisions. We show that the rate of gas release is not proportional to the rate of dust release, if non-catastrophic collisions or thermal evolution dominate the release of CO gas. In this case, care must be taken when inferring the composition of comets. Non-catastrophic collisions dominate the gas production at earlier times than catastrophic collisions, depending on the properties of the planetesimal belt. We highlight the importance of the thermal evolution of comets, including crucially the decay of long-lived radioactive nuclides, as a source of CO gas around young (<50Myr) planetary systems, if large (10-100s kms) planetesimals are present., Comment: Accepted by MNRAS, 16 pages. Updated following review comments

Between 2014 December 31 and 2015 March 17, the OSIRIS cameras on Rosetta documented the growth of a 140m wide and 0.5m deep depression in the Hapi region on Comet 67P/Churyumov-Gerasimenko. This shallow pit is one of several that later formed elsewhere on the comet, all in smooth terrain that primarily is the result of airfall of coma particles. We have compiled observations of this region in Hapi by the microwave instrument MIRO on Rosetta, acquired during October and November 2014. We use thermophysical and radiative transfer models in order to reproduce the MIRO observations. This allows us to place constraints on the thermal inertia, diffusivity, chemical composition, stratification, extinction coefficients, and scattering properties of the surface material, and how they evolved during the months prior to pit formation. The results are placed in context through long-term comet nucleus evolution modelling. We propose that: 1) MIRO observes signatures that are consistent with a solid-state greenhouse effect in airfall material; 2) CO2 ice is sufficiently close to the surface to have a measurable effect on MIRO antenna temperatures, and likely is responsible for the pit formation in Hapi observed by OSIRIS; 3) the pressure at the CO2 sublimation front is sufficiently strong to expel dust and water ice outwards, and to compress comet material inwards, thereby causing the near-surface compaction observed by CONSERT, SESAME, and groundbased radar, manifested as the "consolidated terrain" texture observed by OSIRIS., Comment: 32 pages, 41 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

We here study the level of albedo variegation on the nucleus of Comet 67P/Churyumov-Gerasimenko. This is done by fitting the parameters of a standard photometric phase function model to disk-average radiance factor data in images acquired by the Rosetta/OSIRIS Narrow Angle Camera in the orange filter. Local discrepancies between the observed radiance factor and the disk-average solution are interpreted as a proxy W of the local single-scattering albedo. We find a wide range 0.02

The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov-Gerasimenko in 2014-2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well-understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code 'Numerical Icy Minor Body evolUtion Simulator', or NIMBUS, to search for conditions under which the observed H2O and CO2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water-ice mass ratio of relatively pristine nucleus material is mu~1, that airfall material has mu~2, and that the molar abundance of CO2 relative H2O near 30 per cent. The dust mantle thickness is typically < 2 cm. The average CO2 sublimation front depths near aphelion were ~3.8 m and ~1.9 m on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverisation due to thermal fatigue during the aphelion passage. Sub-surface compaction of material due to CO2 activity near perihelion seems to have reduced the diffusivity in a measurable way., Comment: 22 pages, 19 figures. This is a pre--copyedited, author--produced PDF of an article accepted for publication in MNRAS following peer review

The Primordial Disk of small icy planetesimals, once located at 15-30 AU from the Sun, was disrupted by giant planet migration in the early Solar System. The Primordial Disk thereby became the source region of objects in the current-day Kuiper Belt, Scattered Disk, and Oort Cloud. I present the thermophysics code "Numerical Icy Minor Body evolUtion Simulator", or NIMBUS, and use it to study the thermophysical evolution of planetesimals in the Primordial Disk prior to its disruption. Such modelling is mandatory in order to understand the behaviour of dynamically new comets from the Oort Cloud, as well as the activity of Centaurs and short-period comets from the Scattered Disk, that return pre-processed to the vicinity of the Sun. I find that bodies in the midst of the Primordial Disk with diameters ranging 4-200 km lost all their CO ice on time-scales of order 0.1-10 Myr depending on size, through a combination of protosolar and long-lived radionuclide heating. CO and other hypervolatiles therefore require a less volatile host for their storage. I consider two possible hosts: amorphous water ice and CO2 ice. Because of the high luminosity of the protosun, some Primordial Disk bodies may have sustained significant crystallisation, CO:CO2 segregation, and CO2 sublimation in the uppermost few tens of meters. I discuss how this may affect coma abundance ratios and distant activity in dynamically new comets., Comment: 33 pages, 13 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

The observed presence of water molecules in the dayside lunar regolith was an unexpected discovery and remains poorly understood. Standard thermophysical models predict temperatures that are too high for adsorbed water to be stable. We propose that this problem can be caused by the assumption of locally flat surfaces that is common in such models. Here we apply a model that explicitly considers surface roughness, and accounts for solar illumination, shadows cast by topography, self-heating, thermal reradiation, and heat conduction. We couple the thermophysical model to a model of first-order desorption of lunar surface water and demonstrate that surface roughness substantially increases the capability of the Moon to retain water on its sunlit hemisphere at any latitude, and within $45^{\circ}$ of the poles, at any time of the lunar day. Hence, we show that lunar surface roughness has a strong influence on lunar water adsorption and desorption. Therefore, it is of critical importance to take account of surface roughness to get an accurate picture of the amount of water on the Moon's surface and in its exosphere., Comment: 9 pages, 10 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

Solar system bodies with surface and sub-surface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. This is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. However, some bodies (notably the asteroid (3200) Phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. We investigate whether the sodium present in the mineral matrix could act as the volatile element responsible for this activity. We conduct theoretical modeling which indicates that sodium has the potential to sublimate in the conditions that Phaethon experiences, depending on the mineral phase it is held in. To test this, we then exposed samples of the carbonaceous chondrite Allende to varying heating events similar to what would be experienced by low perihelion asteroids. We measured the change in sodium present in each sample, and find that the highest temperature samples show a significant loss of sodium from specific mineral phases over a single heating event, comparable to a day on the surface of Phaethon. Under specific thermal histories possible for Phaethon, this outgassing could be sufficient to explain this object's observed activity. This effect would also be expected to be observed for other low-perihelia asteroids as well, and may act as a critical step in the process of disrupting small low-albedo asteroids., Comment: Published in PSJ with interactive figures

We have carried out a continuous multi-band photometric monitoring of the nuclear activity of comet 29P/Schwassmann-Wachmann 1 from 2008 to 2010. Our main aim has been to study the outburst mechanism on the basis of a follow-up of the photometric variations associated with the release of dust. We used a standardized method to obtain the 10 arc-sec nucleus photometry in the V, R, and I filters of the Johnson-Kron-Cousins system, being accurately calibrated with standard Landolt stars. Production of dust in the R and I bands during the 2010 Feb. 3 outburst has been also computed. We conclude that the massive ejection of large (optically-thin) particles from the surface at the time of the outburst is the triggering mechanism to produce the outburst. Ulterior sublimation of these ice-rich dust particles during the following days induces fragmentation, generating micrometer-sized grains that increase the dust spatial density to produce the outburst in the optical range due to scattering of sun light. The material leaving the nucleus adopts a fan-like dust feature, formed by micrometer-sized particles that are decaying in brightness as it evolved outwards. By analyzing the photometric signal measured in a standardized 10-arcsec aperture using the Phase Dispersion Minimization technique we have found a clear periodicity of 50 days. Remarkably, this value is also consistent with an outburst frequency of 7.4 outbursts/year deduced from the number of outbursts noticed during the effective observing time., Comment: 19 pages, 3 Tables, and 6 figures