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Abstract: The petrologic diversity of meteorites demonstrates that planetesimals ranged from unmelted, variably metamorphosed aggregates to fully molten, differentiated bodies. However, partially differentiated bodies have not been unambiguously identified in the asteroid belt. New constraints on the density, composition, and morphology of 21 Lutetia from the Rosetta spacecraft indicate that the asteroid''s high bulk density exceeds that of most known chondritic meteorite groups, yet its surface properties resemble those of some carbonaceous and enstatite chondrite groups. This indicates that Lutetia likely experienced early compaction processes like metamorphic sintering. It may have also partially differentiated, forming a metallic core overlain by a primitive chondritic crust. [Copyright &y& Elsevier]

Abstract: Methane is key to sustaining Titan''s thick nitrogen atmosphere. However, methane is destroyed and converted to heavier hydrocarbons irreversibly on a relatively short timescale of approximately 10–100 million years. Without the warming provided by CH4-generated hydrocarbon hazes in the stratosphere and the pressure induced opacity in the infrared, particularly by CH4–N2 and H2–N2 collisions in the troposphere, the atmosphere could be gradually reduced to as low as tens of millibar pressure. An understanding of the source–sink cycle of methane is thus crucial to the evolutionary history of Titan and its atmosphere. In this paper we propose that a complex photochemical–meteorological–hydrogeochemical cycle of methane operates on Titan. We further suggest that although photochemistry leads to the loss of methane from the atmosphere, conversion to a global ocean of ethane is unlikely. The behavior of methane in the troposphere and the surface, as measured by the Cassini–Huygens gas chromatograph mass spectrometer, together with evidence of cryovolcanism reported by the Cassini visual and infrared mapping spectrometer, represents a “methalogical” cycle on Titan, somewhat akin to the hydrological cycle on Earth. In the absence of net loss to the interior, it would represent a closed cycle. However, a source is still needed to replenish the methane lost to photolysis. A hydrogeochemical source deep in the interior of Titan holds promise. It is well known that in serpentinization, hydration of ultramafic silicates in terrestrial oceans produces H2(aq), whose reaction with carbon grains or carbon dioxide in the crustal pores produces methane gas. Appropriate geological, thermal, and pressure conditions could have existed in and below Titan''s purported water-ammonia ocean for “low-temperature” serpentinization to occur in Titan''s accretionary heating phase. On the other hand, impacts could trigger the process at high temperatures. In either instance, storage of methane as a stable clathrate–hydrate in Titan''s interior for later release to the atmosphere is quite plausible. There is also some likelihood that the production of methane on Titan by serpentinization is a gradual and continuous on-going process. [Copyright &y& Elsevier]

With the surface properties and shapes of solar system small bodies (comets and asteroids) now being routinely revealed by spacecraft and Earth-based radar, understanding their interior structure represents the next frontier in our exploration of these worlds. Principal unknowns include the complex interactions between material strength and gravity in environments that are dominated by collisions and thermal processes. Our purpose for this review is to use our current knowledge of small body interiors as a foundation to define the science questions which motivate their continued study: In which bodies do “planetary” processes occur? Which bodies are “accretion survivors”, i.e., bodies whose current form and internal structure are not substantially altered from the time of formation? At what characteristic sizes are we most likely to find “rubble-piles”, i.e., substantially fractured (but not reorganized) interiors, and intact monolith-like bodies? From afar, precise determinations of newly discovered satellite orbits provide the best prospect for yielding masses from which densities may be inferred for a diverse range of near-Earth, main-belt, Trojan, and Kuiper belt objects. Through digital modeling of collision outcomes, bodies that are the most thoroughly fractured (and weak in the sense of having almost zero tensile strength) may be the strongest in the sense of being able to survive against disruptive collisions. Thoroughly fractured bodies may be found at almost any size, and because of their apparent resistance to disruptive collisions, may be the most commonly found interior state for small bodies in the solar system today. Advances in the precise tracking of spacecraft are giving promise to high-order measurements of the gravity fields determined by rendezvous missions. Solving these gravity fields for uniquely revealing internal structure requires active experiments, a major new direction for technological advancement in the coming decade. We note the motivation for understanding the interior properties of small bodies is both scientific and pragmatic, as such knowledge is also essential for considering impact mitigation. [Copyright &y& Elsevier]