identified by Dawn. For instance, spectral observations have revealed
evidence of a form of graphite called graphitized carbon on its sur-
face.
However, Ceres’ surface shows important differences from asteroids.
It is covered in hydrated materials, including phyllosilicates (clays) and
localised deposits of salts such as carbonates, which indicate the pres-
ence of significant amounts of water in its interior. Some of these clays
appear to be ammoniated – that is, they contain ammonia. This is a
significant finding, because ammonia, a volatile substance, is scarce in
the inner Solar System, but abundant further out, which has implications
when trying to determine the origin of Ceres (see below ‘What is the
Origin of Ceres?’).
Dark material is left behind on Ceres’ surface when ice at the surface
sublimates. Ceres’ surface is relatively warm. The maximum tempera-
ture with the Sun overhead may even reach about 235 K (–38 °C). As
surface water ice is unstable at such temperatures and distances of less
than 5 AU from the Sun, it sublimes upon direct exposure to solar radia-
tion. So, while there is abundant ice below the surface, any exposed
surface ice should have sublimed long ago.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/
One of Dawn’s objectives is to try to detect and map water on the
dwarf planet. Has any water or ice been found on the surface? Ice
may have been exposed relatively recently by impacts or even by geo-
logical processes such as cryovolcanism or slumping. However, as yet,
water has only been unambiguously identified at one surface site. This
is within the small crater Oxo - named after the Candomblé (Brazilian)
and Yoruba (African) god of agriculture – located almost 45 degrees
north.
Nevertheless, water from sublimed ice may have fallen back to the
cerean surface at higher latitudes, where it could be protected from
sunlight within deep craters in shadowed areas which don’t receive
much exposure to the Sun. ‘These “cold traps”’ says Marc Rayman
‘may harbour ice that has accumulated over thousands of years (or
even longer).’ The most recent analyses of data do indicate deposits
of bright material in 10 such craters on Ceres, and data from Dawn’s
infrared mapping spectrometer has confirmed the presence of ice in
one of these.
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