New observations using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph – a high-precision instrument on the 3.6-m telescope at ESO’s La Silla Observatory in Chile — have revealed unexpected daily changes in the mysterious bright spots on Ceres, suggesting that the spots change under the influence of sunlight as the dwarf planet rotates.
This artist’s impression shows the very bright patches of material in the crater Occator and elsewhere. Image credit: ESO / L.Calçada / NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Steve Albers / N. Risinger, skysurvey.org.
Ceres’ diameter of 590 miles (950 km) makes it the largest body in the main asteroid belt as well as the smallest dwarf planet in the Solar System. At least one-quarter of its mass is composed of water, a proportion greater than in other asteroids and even on our planet.
NASA’s Dawn robotic spacecraft reached its final destination to Ceres in 2015 and the first images of the dwarf planet revealed the unexpected presence of mysterious bright spots.
Precise observations using HARPS have now not only detected the motion of these spots due to the rotation of Ceres about its axis, but also found unexpected additional variations suggesting that the material of the spots is volatile and evaporates in sunlight.
“As soon as Dawn revealed the bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth,” said Dr. Paolo Molaro from the INAF-Trieste Astronomical Observatory.
“As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth.”
“Ceres spins every 9 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 12.4 mph (20 km per hour).
“But this motion is big enough to be measurable via the Doppler effect with high-precision instruments such as HARPS.”
Dr. Molaro and his colleagues observed Ceres with HARPS for a little over several nights in July and August 2015.
“We observed Ceres on 30 July 2015 taking a sequence of 40 HARPS exposures of 780 sec each,” they said. “On the 26th of August we took a second sequence of 29 exposures of 900 sec each. Five exposures were also taken at the end of the night on 27th August when we could shortly open the dome during a pause of bad weather.”
“The result was a surprise. We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night,” said Dr. Antonino Lanza from the INAF-Catania Astrophysical Observatory.
The astronomers 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 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,” the scientists explained.
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.
The findings were published online February 7, 2016, in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
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P. Molaro et al. Daily variability of Ceres’ Albedo detected by means of radial velocities changes of the reflected sunlight. MNRAS, published online February 7, 2016; doi: 10.1093/mnrasl/slw017
