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Overview/Goals
and Objectives
Dawn
Dictionary
Why Ceres and Vesta?
Technology
Instrumentation
Science Payload/Data Return
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Dawn focuses on two of the first bodies formed in the solar system, the surviving
protoplanets, Ceres and Vesta. Radioisotope chronology from the howardite, eucrite,
and diogenite (HED) meteorites believed to be from Vesta suggests it accreted
in only 5-15 million years.
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Eucrite meteorite
possibly from Vesta
Photo Credit: R. Kempton
(New England Meteoritical Services)
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Similar evidence indicates that Mars continued to accrete for close to 30 million
and Earth for 50 million years. The early cessation of accretion in the asteroid
belt was presumably due to the formation of Jupiter whose gravitational forcing
countered the accretionary process, and today is causing the disruption of the
bodies that did accrete. Although we do not have similar meteorite evidence directly
linked to Ceres, it too is expected to have formed in the first approximately
10 million years. In addition the asteroid belt may have been scoured by comets,
scattered by the formation of the remaining gas giants. Today only some of the
largest asteroids remain relatively undisrupted. The most massive of these are
Ceres and Vesta, two most complementary minor planets. The former has a very
primitive surface, water-bearing minerals, and possibly a very weak atmosphere
and frost. The latter is a dry, differentiated body whose surface has been resurfaced
by basaltic lava flows possibly possessing an early magma ocean like the Moon.
Most importantly Vesta has experienced significant excavating events, most notably
indicated by the huge crater near its southern pole. Cosmic ray exposure dating
of HEDs indicates that impacts have released meteoritic material at least five
times in the last 50 million years. Meteorites from these impacts have been used
to piece together a most probable scenario for Vesta's thermal evolution.
No meteorites have unmistakably come from Ceres. Possibly the excavating events
or dynamics that provided the HED meteorites did not occur at Ceres, but also
possibly, the reflectance spectrum of the surface of Ceres is not indicative
of its crustal rocks. Microwave studies suggest that Ceres is covered with
a dry clay, in contrast to Vesta's basaltic dust layer that reflects its crustal
composition. To determine if Ceres-derived meteorites are in our collections
and to understand the origin of Ceres, we must travel there and obtain spatially
resolved spectra inside fresh craters.
Meteorites provide an incomplete glimpse of their parent bodies. For Vesta
and Ceres we need to know their interior structure, their thermal history as
manifested in the geological and geophysical record, and the processes that
are acting on and affecting their surfaces. We need to determine the geologic
context for the HED meteorites from Vesta, and search for similar data for
Ceres. We are especially interested in contrasting dry, differentiated Vesta
with its wet counterpart, Ceres, just a little further from the Sun. It appears
that a rather short additional radial separation allowed Ceres to accrete wet
and stay cool while early heat sources in the accreting material melted Vesta.
Most importantly because they both lie near the ecliptic plane in near circular
orbits we can rendezvous with and study both quite cost effectively with a
single Discovery mission, one with ample reserves. |
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accountability report