Search

The Aswan cliff on Comet 67P/Churyumov–Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of |$\sim 12\, \mathrm{m}$| was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta /Microwave Instrument for Rosetta Orbiter (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 |$3\, \mathrm{cm}$|⁠. The post-collapse data are consistent with (1) a dust/water–ice mass ratio of 0.9 ± 0.5 and a molar CO2 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 CO2 ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to |$20\pm 6\, \mathrm{cm}$|⁠ ; (4) a thermal inertia ranging 10–45 MKS; (5) a gas diffusivity that decreased from |$0.1$| to |$0.001\, \mathrm{m^2\, s^{-1}}$|⁠ ; and (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. [ABSTRACT FROM AUTHOR]

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 (<50 Myr) planetary systems, if large (10–100 km) planetesimals are present. [ABSTRACT FROM AUTHOR]

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 CO2:CO mixtures and during crystallization of amorphous H2O is calculated, as well as mobilization and internal relocation of CO2. If an amorphous H2O host existed, and was protected by a CO2:CO heat sink, only diameter |$D\lt 20\, \mathrm{km}$| (inner disc) and |$D\lt 64\, \mathrm{km}$| (outer disc) bodies could have been involved in a collisional cascade. If CO2 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 Myr at 15 au and <50–70 Myr at 30 au. Alternatively, if a 450 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. [ABSTRACT FROM AUTHOR]

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 disc–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 disc–average solution are interpreted as a proxy |$\mathcal {W}$| of the local single–scattering albedo. We find a wide range |$0.02 \lesssim \mathcal {W}\lesssim 0.09$| around an average of |$\mathcal {W}=0.055$|⁠. The observed albedo variegation is strongly correlated with nucleus morphology – smooth terrain is brighter, and consolidated terrain is darker, than average. Furthermore, we find that smooth terrain darken prior to morphological changes, and that stratigraphically low terrain (with respect to the centre of each nucleus lobe) is brighter than stratigraphically high terrain. We propose that the observed albedo variegation is due to differences in porosity and the coherent effect: compaction causes small brighter particles to act collectively as larger optically effective particles that are darker. Accordingly, we consider the dark consolidated terrain materials more compacted than smooth terrain materials, and darkening of the latter is due to subsidence. [ABSTRACT FROM AUTHOR]

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 μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically |$\lesssim 2\, \mathrm{cm}$|⁠. The average CO2 sublimation front depths near aphelion were |$\sim 3.8\, \mathrm{m}$| and |$\sim 1.9\, \mathrm{m}$| on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverization 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. [ABSTRACT FROM AUTHOR]

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○ 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. [ABSTRACT FROM AUTHOR]

The primordial disc of small icy planetesimals, once located at 15– |$30\, \mathrm{au}$| from the Sun, was disrupted by giant planet migration in the early Solar system. The primordial disc thereby became the source region of objects in the present-day Kuiper belt, scattered disc, and Oort Cloud. I present the thermophysical code 'Numerical Icy Minor Body evolUtion Simulator', or nimbus , and use it to study the thermophysical evolution of planetesimals in the primordial disc 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 disc, that return pre-processed to the vicinity of the Sun. I find that bodies in the midst of the primordial disc with diameters ranging 4– |$200\, \mathrm{km}$| lost all their CO ice on time-scales of order 0.1– |$10\, \mathrm{Myr}$| 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 disc bodies may have sustained significant crystallization, CO:CO2 segregation, and CO2 sublimation in the uppermost few tens of metres. I discuss how this may affect coma abundance ratios and distant activity in dynamically new comets. [ABSTRACT FROM AUTHOR]

The classical way to treat absorption of solar light in thermophysical modeling of cometary nuclei (and other ice-rich bodies such as jovian satellites) has been to assume complete opaqueness of the surface material. However, as shown by Davidsson and Skorov (2002, Icarus156, 223–248), substantial light penetration can occur in porous ice even if it is very dusty, implying that gradual absorption of energy in a surface layer should be accounted for.We present a thorough comparison between a surface energy absorption model and a layer energy absorption model, for various combinations of heliocentric distances, conductivities, opacities, pore sizes, and rotational periods relevant for cometary nuclei, by fully solving the coupled differential equations of heat transfer and gas diffusion. We find substantial differences between the models in terms of gas production rate, thermal lag angle, surface temperature, and the origin of coma molecules. For example, the surface energy absorption model overestimates the total gas production by a factor of 2–7, underestimates the lag angle by a factor of 2–3, and places the origin of coma molecules at the surface, instead of the near-surface interior. [Copyright &y& Elsevier]

A small but increasing volume of observations of cometary nuclei has accumulated during the past two decades. This development is accelerating with upcoming space missions such as Stardust, Contour, and Rosetta. In response to the growing need for a theoretical understanding of optical properties of cometary nuclei, we have calculated synthetic reflectance spectra in the wavelength region 0.2–2.0 μm, photometric colors in the Johnson–Kron–Cousins UBVRI system, and visual geometric albedos for a large number of porous ice-dust mixtures with differing composition, regolith grain sizes, and grain morphologies, such as core-mantle grains, dense clusters of such grains, and large irregular particles with internal scatterers. The calculations are based on Mie theory, the discrete dipole approximation, Hapke theory, and a numerical solution to the equation of radiative transfer in particulate media. In addition, wavelength-integrated directional-hemispherical albedos and flux attenuation profiles in the regolith as functions of depth have been calculated in order to improve the energy budget and treatment of energy boundary conditions in thermal models of cometary nuclei.Our results are compared with spectra and colors of observed cometary nuclei. Our main conclusions are that only regolith consisting of relatively large core-mantle grains, or clusters of smaller core-mantle grains, is capable of reproducing the red colors seen in comets; that ice-dust mixtures actually can be darker than ice-free regolith in certain circumstances; and that solar radiation sometimes penetrates to a depth that is comparable to the region in which diurnal temperature variations occur. [Copyright &y& Elsevier]

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s-1 in early June 2014 to 1.2 kg s-1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the "neck" region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K-1 m-2 s-0.5 ), consistent with a thermally insulating powdered surface, is inferred. [ABSTRACT FROM AUTHOR]