The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures. The second most massive asteroid--4 Vesta--has differentiated to a crust, mantle and core. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N 2 ) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH 4 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and ethane (C 2 H 6 ) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state-over seasonal or geologic time scales.

No abstract

No abstract

Pluto's first known moon, Charon, was discovered in 1978 1 and has a diameter about half that of Pluto 2-4 , which makes it larger relative to its primary than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful 5-7 , but they were not sensitive to objects ∼ <150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites 6 . Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), which makes Pluto the first Kuiper belt object (KBO) known to have multiple satellites. These new satellites are much smaller than Charon (diameter ∼1200 km), with P1 ranging in diameter from 60-165 km depending on the surface reflectivity, and P2 about 20% smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ∼38 days (P1) and ∼25 days (P2). The implications of the discovery of P1 and P2 for the origin and evolution of the Pluto system, and for the satellite formation process in the Kuiper belt, are discussed in a companion paper 8 . We observed Pluto with the Hubble Space Telescope (HST) using the Wide-Field Channel (WFC) mode of the Advanced Camera for Surveys (ACS) on UT 2005 May 15 and May 18 (Fig. 1). The ACS/WFC consists of two 4096 × 2048 pixel CCDs (WFC1 and WFC2) butted together, effectively forming a single 4096 × 4096 pixel camera with a gap of ∼50 pixels between the two CCDs. The F606W ("Broad V") filter, which has a center wavelength of 591.8 nm and a width of 67.2 nm, was used for all images. At the time of the observations, Pluto was 31.0 astronomical units (AU) from the sun, 30.1 AU from the Earth, and had a solar phase angle of 0.96 deg on May 15 and 0.88 deg on May 18. Identical strategies were employed on each observing date. First, a single short exposure (0.5 s) was taken to enable accurate positions of Pluto and Charon to be measured on unsaturated images. Then, two identical, long exposures (475 s) were taken at the same pointing to provide high sensitivity to faint objects. Finally, the telescope was moved by ∼5 pixels in one dimension and ∼60 pixels in the other dimension, and two identical, long exposures (475 s) were taken to provide data in the region of the sky falling in the inter-chip gap during the first two long exposures. The telescope was programmed to track the apparent motion of Pluto (∼3 arcsec hr −1 ) for all exposures. The two new satellites are detected with high signal-to-noise ratio (S/N ≥ 35) and have a spatial morphology consistent with the ACS point spread function (PSF; this is the spatial brightness