Meteorites and their components have anomalous oxygen isotopic compositions characterized by large variations in 18O/16O and 17O/16O ratios. On the basis of recent observations of star-forming regions and models of accreting protoplanetary disks, we suggest that these variations may originate in a parent molecular cloud by ultraviolet photodissociation processes. Materials with anomalous isotopic compositions were then transported into the solar nebula by icy dust grains during the collapse of the cloud. The icy dust grains drifted toward the Sun in the disk, and their subsequent evaporation resulted in the 17O- and 18O-enrichment of the inner disk gas.

Oxygen isotopic composition of our solar system is believed to have resulted from mixing of two isotopically distinct nebular reservoirs, 16 O-rich and 17,18 O-rich relative to Earth. The nature and composition of the 17,18 O-rich reservoir are poorly constrained. We report an in situ discovery of a chemically and isotopically unique material distributed ubiquitously in fine-grained matrix of a primitive carbonaceous chondrite Acfer 094. This material formed by oxidation of Fe,Ni-metal and sulfides by water either in the solar nebula or on a planetesimal. Oxygen isotopic composition of this material indicates that the water was highly enriched in 17 O and 18 O (δ 17,18 O SMOW = +180‰ per mil), providing the first evidence for an extremely 17,18 O-rich reservoir in the early solar system.

[1] We developed a new numerical model which can simulate the thermal history and metal-silicate separation of a growing Mars. In this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating, crater excavation, and isostatic rebound. A metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature. Sinking of the metal blobs is traced assuming Stokes' velocity. Their coalescence during sinking is treated by a Monte Carlo approach. A series of simulations is carried out assuming that Mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process. Our numerical results show that (1) no global magma ocean is formed during accretion, (2) metal-silicate separation takes place without global scale melting, and (3) instead of a metallic core, a metal-rich layer is formed at the late stage of accretion.

Many icy solar system bodies possess subsurface oceans. At Pluto, Sputnik Planitia's location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in ice shell thickness, Pluto's ice shell needs to be cold. Achieving such an interior structure is problematic. Here we show that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from complete freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane either contained in precursor bodies and/or produced by cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would likely not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer capping a subsurface ocean may be an important generic mechanism maintaining long-lived subsurface oceans in relatively large but minimally-heated icy satellites and Kuiper Belt Objects.Liquid water oceans are thought to exist inside icy satellites of gas giants such as Europa and Enceladus and the icy dwarf planet Pluto 1 . Understanding the survival of subsurface oceans is of fundamental importance not only to planetary science but also to astrobiology. One indication of a subsurface ocean on Pluto is Sputnik Planitia, a ~1000-km-wide basin. It is a topographical low and is located near the equator, indicating that it is a positive gravity anomaly. To make this basin a positive gravity anomaly, a subsurface ocean beneath a locally thinned ice shell (by ~90 km) is inferred 2 .

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