16 Mar 2016

Ground based observations confirm Ceres is still 'an active world'

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 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.  
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.”

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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