the same massive feature will slightly retard its progress, slowing it down
just a little.’
By the beginning of September 2016 Dawn had completed more
than eight months of ‘virtually flawless activities at this altitude, over
1,100 orbital revolutions, returning far, far more data than ever antici-
pated.’ For instance, of the more than 51,000 photos of Ceres taken
since December 2014, over 37,000 of those have been taken in this low-
est orbit at very high resolution, as little as 35m per pixel, or some 850
times better than that of the Hubble Space Telescope’s view of Ceres
from the vicinity of the Earth.
GLOBAL MAP: This map of Ceres shows its topography ranging from 7.5km (4.7 miles) low in indigo to 7.5km (4.7 miles) high in white. The names of features
have been approved by the International Astronomical Union. Following the theme of the asteroid’s original naming after the Roman goddess of agriculture,
names of agricultural deities have been given to craters on Ceres, and agricultural festivals for everything else. This map was constructed from analysing images
from Dawn’s framing camera taken from varying sun and viewing angles, combined with an image mosaic of Ceres and projected as a simple cylindrical projec-
tion. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Little world, big surprises!
Dawn’s instruments have confirmed Ceres’ unique status in the main
asteroid belt. While its surface is heavily marked by craters of all types
and ages, as well as mountains, fractures and faults, like other larger
members of this region of the Solar System, the varied crater distribu-
tion across its surface ‘indicates crustal heterogeneities and a complex
geologic evolution’ (H. Hiesinger et al, Science 353, aaf4759, 2016). The
probe has revealed not only a world with a rich and varied history, but
also one which shows evidence of recent and even continuing geo-
logical activity.
Globally, with a bulk density of only 2.161 g/cm3 , Ceres is composed
of a mixture of rock and ice, as compared to Vesta, with its silicate
mantle and metallic core and a higher mean density of 3.456 g/cm3
(for comparison, the Earth, with its massive iron-nickel core has a den-
sity of 5.51 g/cm3 ). Ceres appears to be differentiated into a rocky
(silicate) core, overlain by a 100km thick icy mantle, with a thin, dusty
outer crust.
Prior to the Dawn mission, it was believed that Ceres had a deep
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