When the NASA Dawn mission witnessed strange bright spots on Ceres from close range for the first time, planetologists were confused. What material can produce such a bright feature on a usually gray surface? Now, a year after the probe arrived in orbit around the dwarf planet, scientists are approaching the answer.
However, it was not Dawn that found the last clue as to what these bright spots might be; it was a powerful observatory on Earth, which noticed very slight changes when the surface of Ceres was gently heated by the Sun.
The bright spots in question are mainly grouped inside a large crater called the Occator. Now that Dawn is in the lowest orbit around the small world, located in the main Belt of asteroids (between the orbits of Mars and Jupiter), sweeping over the crater landscape at an altitude of only 240 miles, he captured an amazingly detailed picture of the crater bottom. The bright feature is actually a cluster of bright spots with a diffuse, almost powdery material surrounding the bright spots. The leading theory, so far, has been that it is ice material, such as water ice, but it can also be mineral deposits. Now, using the HARPS spectrograph attached to the ESO 3, 6-meter telescope at La Silla, Chile, it seems the ice theory has just become much stronger.
"As soon as Dawn's spacecraft showed mysterious bright spots on the surface of Ceres, I immediately thought of possible measurable effects from the surface of the Earth," said Paolo Molaro, an INAF astronomical observatory in Trieste and lead author of a study published in the Monthly Notices magazine of the Royal Astronomical of society. "As the Ceres rotate, the spots approach the Earth and then recede again, which affects the spectrum of reflected sunlight coming to Earth."
When Ceres makes a revolution every 9 hours, HARPS is so sensitive that it can detect a very small Doppler shift in the frequency of the spectrum when moving bright spots away from and toward the Earth, but during observations over 2 nights in July and August 2015, changes were found unrelated to the rotation of ceres.
“The result was a surprise,” said co-author Antonino Lanza, also a staff member of the astrophysical observatory INAF-Catania. "We found the expected changes in the spectrum from the rotation of Ceres, but with significant other changes from night to night." And it turns out that these changes correspond to some volatile substance (ice) that is exposed to sunlight and throws vapors into space, causing an increase in reflectivity. It seems that when the Occator is experiencing solar heating, plumes are produced, and then they evaporate, creating a complex spectroscopic signal that develops in this hemisphere during the daytime. This conclusion seems to be consistent with earlier observations made by Zorya, who showed a mysterious haze over Occator.
We know that Ceres is rich in water, but whether these bright spots are explained by water ice erupting to the surface have yet to be proved, although the evidence seems to be becoming more convincing as we study the hidden dwarf planet of the Solar System.
