After NASA's Dawn mission revealed the first close-up images of the strange bright spots on the surface of Ceres last year, the mystery of Ceres' bright spots now continues with new information gathered from an earth-based, high-precision astronomical observatory in Chile.
At 950 kilometers (950 miles) in diameter, Ceres is the largest celestial body orbiting the asteroid belt between Mars and Jupiter, which has gained it the classification of being a dwarf planet.
NASA's Dawn probe has been following Ceres for more than a year, creating a highly detailed portrait of its surfaces. One of Dawn's main discoveries has involved the little planet's very bright spots, which appear to reflect much more light than their darker surroundings — especially within the Occator crater. These spots provide evidence that Ceres might in fact be a much more active world than the majority of its neighboring asteroids.
The mystery of the spots appeared to have been solved when, in December of last year, researchers working with Dawn's data reported evidence that they appear to be made of hydrated magnesium sulphate. Additionally, haze that is observable inside the Occator crater was thought to be a mix of dust, ice particles and water vapor caused by solar radiation reaching Ceres' surface.
The new study, led by Paolo Molaro of the INAF-Trieste Astronomical Observatory, adds further depth to these previous observations. Molaro's research team, based in La Silla, Chile, used the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph's ESO 3.6-meter telescope to make new observations about the motion of the bright patches. They also found surprising information that the matter comprising the spots is volatile and can evaporate upon contact with sunlight.
"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," Molaro explained. "As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth."
Ceres' revolution pattern occurs every nine hours, and the team's calculations indicated that the velocities of the spots toward and away from the Earth due to this rotation would only be around 20 kilometers (12.4 miles) per hour. This motion can be captured through measurements made via the Doppler effect, using meticulous devices like HARPS.
"The result was a surprise," added Antonino Lanza, one of the study's co-authors. "We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night."
Ceres has been known for a while to be water-rich, but it still remains unclear whether this is directly connected to the bright spots — as well as what the main energy source behind their variation is. Molaro noted that further research is necessary to confirm the new findings: "These are all speculations for the moment."
The study was made publicly available on March 16 via the Monthly Notices of the Royal Astronomical Society.
