A partially differentiated interior for (1) Ceres deduced from its gravity field and shape.

Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated. A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation together with either precession rate or a validated assumption of hydrostatic equilibrium. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected. Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies. Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient. To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460-2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites), which yields an outer-shell thickness of 70-190 kilometres. The density of this outer shell is 1,680-1,950 kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts. Although the gravity and shape data confirm that the interior of Ceres evolved thermally, its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000 kilometres across) ice-rich rocky bodies.

Science potential from a Europa lander.

The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

Eruptions arising from tidally controlled periodic openings of rifts on Enceladus.

In 2005, plumes were detected near the south polar region of Enceladus, a small icy satellite of Saturn. Observations of the south pole revealed large rifts in the crust, informally called 'tiger stripes', which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions. Models of the ultimate interior source for the eruptions are under consideration. Other models of an expanding plume require eruptions from discrete sources, as well as less voluminous eruptions from a more extended source, to match the observations. No physical mechanism that matches the observations has been identified to control these eruptions. Here we report a mechanism in which temporal variations in tidal stress open and close the tiger-stripe rifts, governing the timing of eruptions. During each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions. In a complementary process, periodic shear stress along the rifts also generates heat along their lengths, which has the capacity to enhance eruptions. Plume activity is expected to vary periodically, affecting the injection of material into Saturn's E ring and its formation, evolution and structure. Moreover, the stresses controlling eruptions imply that Enceladus' icy shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick.

The state and future of Mars polar science and exploration.

As the planet's principal cold traps, the martian polar regions have accumulated extensive mantles of ice and dust that cover individual areas of approximately 10(6) km2 and total as much as 3-4 km thick. From the scarcity of superposed craters on their surface, these layered deposits are thought to be comparatively young--preserving a record of the seasonal and climatic cycling of atmospheric CO2, H2O, and dust over the past approximately 10(5)-10(8) years. For this reason, the martian polar deposits may serve as a Rosetta Stone for understanding the geologic and climatic history of the planet--documenting variations in insolation (due to quasiperiodic oscillations in the planet's obliquity and orbital elements), volatile mass balance, atmospheric composition, dust storm activity, volcanic eruptions, large impacts, catastrophic floods, solar luminosity, supernovae, and perhaps even a record of microbial life. Beyond their scientific value, the polar regions may soon prove important for another reason--providing a valuable and accessible reservoir of water to support the long-term human exploration of Mars. In this paper we assess the current state of Mars polar research, identify the key questions that motivate the exploration of the polar regions, discuss the extent to which current missions will address these questions, and speculate about what additional capabilities and investigations may be required to address the issues that remain outstanding.