The entrance of NASA’s Dawn mission at the enormous asteroid
“1 Ceres” in early 2015 has turned out to have been well worth to come for.
This dwarf planet is the biggest body in the asteroid belt between Mars and
Jupiter and was the first to be found. But, until recently, we have only had evidence
from ground and space centered telescopes, which have given us tempting sights
of a dark, maybe water-rich object.
Now the NASA’s Dawn space probe has sent back a mammoth
harvest of answers, shortened in six new exploration papers printed in a
special issue of the journal Science. We now have a map of Ceres that discloses
uncommon minerals, a surface sprinkled with craters, and water in the form of
ice and perhaps an external atmosphere of vapour. There is also sufficient
uncertainty in the answers to sow the seeds for yet to come researches.
The data delivers a global geographical map of the asteroid displaying
that its whole surface seems to be covered in phyllosilicates, a significant set
of clay minerals. Two particular clays are discovered: one that is totally
magnesium-rich, the second an ammonium-rich types. There appears to be little
or no pattern to the scattering of the two minerals, they are both nearly
everywhere.
This ubiquity is what is essential. The minerals could not
have been produced in an ordinary incident, such as an impact into an
ice-filled crater. They must have been formed by planet-wide adaptation, probably
denoting there must have been volumes of water. It is clear that huge amounts
of liquid water are not present on Ceres currently. But the signal of water-ice
has been spotted in at least one crater.
The temperature of Ceres is reasonably warm (between -93℃
to -33℃), water-ice visible at the surface would quickly
convert into a gas in such a low pressure surroundings. So the discovered
traces of water-ice propose some underground ice was just out and that there
must be some mechanism to describe how the surface was bothered in this way.
Some scientists think that the solution is cry volcanism, where subsurface coatings
of mixed ice and minerals percolate gradually to the surface through fractures
and cracks, or more quickly following an effect. If the minerals are chlorides,
then low temperature brine can keep the
subsurface coating move-able.
As well as a geographical map of Ceres, we also have an image
of Ceres’overall geomorphology (its surface types). This demonstrates that the
surface of Ceres is interspersed with impact craters, though the craters are not scattered equally over the surface. Much more fascinating are the three different
types of mineral drift across the landscape, formed by the movement of ice-rich
material, mudslides or layers of ejected particles following impact into the
ice-rich material. The scattering of the flow types differs with latitude, and
the scientists consider this means different surface coatings of the asteroid
contain different quantities of ice.
One of the most extraordinary results is the discovery of an
unexpected burst of extremely energetic electrons over a period of nearly a
week in June 2015, corresponding with a solar proton storm. The scientists consider
the protons fired out by the sun intermingled with particles in Ceres' feeble
atmosphere, generating a shock wave that speeded the electrons. Based on explanations by the Hubble Space Telescope, Ceres is assumed to have a feeble exo-sphere (external
atmosphere) of water vapour. The results from Dawn propose that this may actually
be the case.
Together, this new set of evidences shows that Ceres is a
world that has been formed by a series of happenings, with a tough crust of
magnesium- and ammonium-bearing phyllosilicates covering an interior of salty
ice and hydrated minerals. What other unknown secrets will be revealed as study
remains on the trove of data from Ceres? Questions still remain about the range
of mineral deposits, the depth of the subsurface ice-rock layer, and, of
course, the latent for organic material on the small planet. The harvest from
Ceres so far has been promising and promises to keep us busy in future for years.
Monica Grady, Professor of Planetary and Space Sciences, The Open University.
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