Observations made using the HARPS spectrograph at the European Southern Observatory’s (ESO) La Silla Observatory in Chile 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 Earth, 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.
Ceres is the largest body in the asteroid belt between Mars and Jupiter and the only such object classed as a dwarf planet. NASA’s Dawn spacecraft has been in orbit around Ceres for more than a year and has mapped its surface in great detail. One of the biggest surprises has been the discovery of bright spots, which reflect far more light than their much darker surroundings. The most prominent of these spots lies inside the crater Occator and suggests that Ceres may be a much more active world than most of its asteroid neighbors.
New and precise observations using the HARPS spectrograph at the ESO 3.6-meter telescope at La Silla have now not only detected the motion of the 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.
The lead author of the new study, Paolo Molaro, from the INAF–Trieste Astronomical Observatory, said: "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 Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth.”
Ceres spins every nine hours, and calculations showed that the velocities of the spots towards and away from the Earth due to this rotation would be small — about 12 miles (20 kilometers) 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," said Antonino Lanza at the INAF–Catania Astrophysical Observatory. "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 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 behavior 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.