HOW
TIMES FLY, it does not seem all that long ago since the NASA spacecraft DAWN
arrived at Ceres, yet the first anniversary of its arrived passed on 6 March
2016. Since then DAWN has settled into its science orbit 233 miles above the
asteroid and has send back some amazing images.
At
760 miles in size Ceres is the largest member of the minor planets that orbit
around the Sun midway between the planets Mars and Jupiter.
It
turns out that Ceres far more dynamically active than the asteroid
Vesta, which DAWN first visited in November 2011.
Astronomers
from Italy, Germany and Chile, made a series of observations of Ceres in July
2015, in which they measured the radial velocity of some of the surface
features. The observations confirmed that plumes volatile gases were being
pumped into space from the location the main white spot in the crater named Occator.
The Team of astronomer led by Paolo
Molaro, at the INAF–Trieste Astronomical Observatory, and Antonino Lanza, at
the INAF–Catania Astrophysical Observatory, have been using the European Southern
Observatories 3.6-metre telescope at La Silla observatory situated in the
southern part of the Atacama desert of Chile.
Attached to the ESO 3.6-metre telescope
is the HARPS spectrograph that is normally used in the search for extra solar
planets to detect minute wobbles in a stars’ motion. HARPS picks up small
changes in the star’s radial velocity along the line of sight.
Observations made using the HARPS
spectrograph have revealed unexpected changes in the bright spots on the dwarf
planet Ceres. Although Ceres appears as little more than a point of light from
the Earth, very careful study of its light shows not only the changes expected
as Ceres rotates, but also that the spots brighten during the day and also show
other variations. These observations suggest that the material of the spots is
volatile and evaporates in the warm glow of sunlight.
The recent Dawn observations suggest that the bright spots could provide some atmosphere in this particular region of Ceres confirming previous water vapour detection. It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk.
That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast of some kind of plume or other feature.
The lead author of the new study,
Paolo Molaro, at the INAF–Trieste Astronomical Observatory, takes up the story:
"As soon as the Dawn spacecraft revealed the mysterious bright spots on
the surface of Ceres, I immediately thought of the possible measurable effects
from Earth. As Ceres rotates the spots approach the Earth and then recedes
again, this affects the spectrum of the reflected sunlight arriving at Earth.”
The volatiles are being released every time that the white spots enter direct sunlight during its 9 hour rotation period.
The recent Dawn observations suggest that the bright spots could provide some atmosphere in this particular region of Ceres confirming previous water vapour detection. It has been noted that the spots appear bright at dawn on Ceres while they seem to fade by dusk.
That could mean that sunlight plays an important role, for instance by heating up ice just beneath the surface and causing it to blast of some kind of plume or other feature.
Ceres spins every nine hours and
calculations showed that the effects due to the motion of the spots towards and
away from the Earth caused by this rotation would be very small, of order 20
kilometres per hour. But this motion is big enough to be measurable via the
Doppler Effect with high-precision instruments such as HARPS.
The team observed Ceres with HARPS
for a little over two nights in July and August 2015. "The result was a
surprise," adds Antonino Lanza, at the INAF–Catania Astrophysical
Observatory and co-author of the study. "We did find the expected changes
to the spectrum from the rotation of Ceres, but with considerable other
variations from night to night.”
The team concluded that the observed
changes could be due to the presence of volatile substances that evaporate
under the action of solar radiation. When the spots inside the Occator crater
are on the side illuminated by the Sun they form plumes that reflect sunlight
very effectively. These plumes then evaporate quickly, lose reflectivity and
produce the observed changes. This effect, however, changes from night to
night, giving rise to additional random patterns, on both short and longer
timescales.
If this interpretation is confirmed Ceres
would seem to be very different from Vesta and the other main belt asteroids.
Despite being relatively isolated, it seems to be internally active. Ceres is
known to be rich in water, but it is unclear whether this is related to the
bright spots. The energy source that drives this continual leakage of material
from the surface is also unknown.
Dawn is continuing to study Ceres
and the behaviour of its mysterious spots. Observations from the ground with
HARPS and other facilities will be able to continue even after the end of the
space mission.
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