Prior to NASA’s Dawn mission,
the dwarf planet Ceres was widely believed to contain a substantial
ice-rich layer below its rocky surface.
The existence of such a layer has significant implications for Ceres’
formation, evolution, and astrobiological potential. Ceres’ surface
temperature virtually ensures that large impact craters should be erased
by viscous flow (a process called "viscous relaxation")
on short geologic timescales if Ceres is, in fact, ice-rich.
Measurements using digital terrain models derived from Dawn framing
camera images, show that most of Ceres’ largest craters are several
kilometers deep, and are therefore inconsistent with the existence
of an ice-rich subsurface. Preventing viscous relaxation requires that
Ceres’ subsurface has a viscosity at least one thousand times greater
than that of pure water ice. Ceres’ shallow subsurface is therefore no
more than 30%-40% ice by volume, with a mixture
of rock, salts, and/or clathrates accounting for the other 60%-70%.
However, several anomalously shallow craters suggest limited viscous
relaxation has occurred, possibly indicating spatial variations in the
ice content of the subsurface.