The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA's Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates ("pebbles"), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies.

All atmosphere-less planetary bodies are covered with a dust layer, the so-called regolith, which determines the optical, mechanical and thermal properties of their surface. These properties depend on the regolith material, the size distribution of the particles it consists of, and the porosity to which these particles are packed. We performed experiments in parabolic flights to determine the gravity dependency of the packing density of regolith for solid-particle sizes of 60 µm and 1 mm as well as for 100-250 µm-sized agglomerates of 1.5 µm-sized solid grains. We utilized g-levels between 0.7 m s −2 and 18 m s −2 and completed our measurements with experiments under normal gravity conditions. Based on previous experimental and theoretical literature and supported by our new experiments, we developed an analytical model to calculate the regolith stratification of celestial rocky and icy bodies and estimated the mechanical yields of the regolith under the weight of an astronaut and a spacecraft resting on these objects.

We present laboratory experiments of a vertically vibrated granular medium consisting of 1-mm-diameter glass beads with embedded 8-mm-diameter intruder glass beads. The experiments were performed in the laboratory as well as in a parabolic flight under reduced-gravity conditions (on Martian and Lunar gravity levels). We measured the mean rise velocity of the large glass beads and present its dependence on the fill height of the sample containers, the excitation acceleration, and the ambient gravity level. We find that the rise velocity scales in the same manner for all three gravity regimes and roughly linearly with gravity.

Context. Radiative heat transfer occurs in a porous medium, such as regolith on planetary bodies. Radiation enhances the efficiency of heat transport through the subsurface, effecting a strong temperature dependence of thermal conductivity. However, this effect has been omitted in many studies of comet 67P/Churyumov-Gerasimenko (67P). Aims. We concisely review the method for characterizing radiative heat transfer and present a generic treatment in thermal modeling. In particular, we study the impact of radiative heat transfer on 67P subject to both diurnal and seasonal variations of insolation. Methods. We adapted a numerical model based on the Crank-Nicolson scheme to estimate the subsurface temperatures and water production rate of 67P, where conductivity may vary with depth. Results. Radiative heat transfer is efficient during the day near the surface but it dicreases at night, which means that more energy is deposited underneath the diurnal thermal skin. The effect increases with pore size and accordingly, with the size of the constituent aggregates of the nucleus. It also intensifies with decreasing heliocentric distance. Close to perihelion, within 2 au, for example, radiation may raise the temperature by more than 20 K at a depth of 5 cm, compared with a purely conductive nucleus. If the nucleus is desiccated and composed of centimeter-sized aggregates, the subsurface at 0.5 m may be warmed to above 180 K. Conclusions. Radiative heat transfer is not negligible if the nucleus of 67P consists of aggregates that measure millimeters or larger. To distinguish its role and ascertain the pore size of the subsurface, measurements of temperatures from a depth of ∼1 cm down to several decimeters are most diagnostic. The water production rate of the nucleus, on the other hand, does not provide a useful constraint.

Aims. Photophoretic motion of dust agglomerates can play a role for the re-distribution and mixing of material in protoplanetary disks. The dust agglomerates can consist of various materials and may possess a variety of morphologies and sizes. This experimental study intends to investigate the influence of different dust materials and dust aggregate sizes on the photophoretic motion. Methods. Dust agglomerates were subjected to different light intensities and their respective photophoretic motion was observed under microgravity conditions and in a rarefied gas. Results. The measured velocities for dust aggregates are on average proportional to the size of the dust aggregate, vary largely with material, and for a given material the velocity distribution for a single dust aggregate size is very broad and can be described by a Gaussian with a width comparable to its mean velocity. Remarkably, a fraction of a few 10 percent of all particles investigated exhibit a motion in the opposite direction. The mean photophoretic velocity of dust aggregates can be explained by the model of Beresnev et al. (1993, Phys. Fluids, 5, 2043) with a surprisingly high value for the ratio of heat conductivity to the asymetry factor of λ/J 1 0.1 W/m/K. Earlier work on photophoretic particle transport in protoplanetary disks assumed values of λ/J 1 0.001 W/m/K so that the real transport efficiency should me much lower and the corresponding timescale much longer.