Spacewatch: Dawn’s arrival at Ceres

Nasa’s Dawn spacecraft is closing in on Ceres and speculation is mounting about what it will find after it enters orbit around the dwarf planet on 6 March. Since Dawn orbited the asteroid Vesta during 2011-2012, it will become the first spacecraft to orbit two bodies beyond the Earth.

Mainly spherical and with an average diameter of 950km, Ceres is the largest body in the asteroid belt between Mars and Jupiter and revolves on its axis every nine hours. As it loomed larger in images from Dawn, so attention became focused on a striking white spot that had been first sighted by the Hubble telescope more than a decade ago. Further white spots are now also in view, but what could they be?

The answer may lie in the nature of Ceres as a rock-ice world, with a rocky core overlain by a mantle of ices, carbonates and clay. It contrasts with the very different world, Vesta, which Dawn revealed to be a largely rocky body with a metallic core.

While a quarter of the mass of Ceres may be water, its outermost shell is more likely a solid rock-ice mixture. That layer looks to be heavily cratered in the latest images, but suggestions that the mantle contains an ocean, possibly a muddy one, were strengthened in 2013 when infrared observations by the now-decommissioned Herschel space observatory showed plumes of water vapour above Ceres.

These plumes may have erupted from icy volcanoes, or they may result from the sublimation of ice exposed at the surface, possibly by upwelling or recent impacts.