That strange haze on Ceres
Ceres, originally called Cerere Ferdinandea, is the largest asteroid in the main belt of the solar system and the first to be discovered. For half a century Ceres has been considered the eighth planet until, in 2006, it was reclassified as a dwarf planet, the only one among asteroids.
In September 2007, NASA launched the Dawn spacecraft, which entered into stable orbit around Ceres on 6 March 2015 to observe the planet surface, initially from an altitude of 5,900 km, which was then decreased to 1,300 km and finally brought to 700 km for additional five months.
The unprecedented images of Ceres surface collected by the Dawn spacecraft show the features of the dwarf planet surface: mountains, craters and fractures and reveal a constellation of particularly bright spots especially those located in the Occator crater. Now that the probe is at the right distance, researchers hope to clarify the nature of the bright spots that stud the planet surface.
The high-resolution images of Ceres look different from those of the other celestial bodies surrounding it, and show traces of activity on its surface. Now, a new study with ground-based telescopes conducted by a team of INAF astronomers strengthens the hypotheses provided by the Dawn spacecraft images: the dwarf planet seems to have an internal activity on its own which becomes evident when the light intensity of the mysterious bright spots varies.
The article published in Nature with the picture of Ceres sent by the spacecraft assumes that the way the areas inside the Occator crater look is determined by the presence of hydrated magnesium sulphates. On the floor of the crater there seems to be something that comes out from the inside and sublimates,thus generating a periodical haze that appears and disappears within a few hours.
«When the Dawn spacecraft revealed the presence of mysterious bright spots on the surface of Ceres, I immediately thought about the presence of effects that could be possibly measures from the Earth» said Paolo Molaro, INAF researcher at the Astronomic Observatory in Trieste and first author of the study, published in the online journal Monthly Notices of the Royal Astronomical Society Letters. «Ceres reflects the solar electromagnetic radiation and the bright spots, by rotating like a headlight, produce distortions at radial velocity in the spectrum of the solar light reflected. Calculations showed that these distortions were very small, about a metre per second, but still possible to measure with high-precision instruments such as the ESO 3.6 m HARPS telescope in La Silla, Chile or the twin instrument HARPS-N operated at the National Galileo Telescope facility in La Palma, Canary Islands. So, with a programme expressly approved by the ESO Director (Director Discretionary Time), we have observed Ceres with HARPS for little more than two nights on 31 July and 26 and 27 August 2015. Based on the experience made over the last few years in observing similar objects, we expected Ceres to show variations in the radial velocity of the solar spectrum reflected with a variability equal to its rotation period that is little more than nine hours».
«However, the results were surprising» added Antonino Lanza, from INAF Astronomic Observatory in Catania and co-author of the study. «We observed the variations in the radial velocity of the solar spectrum reflected, but with marked variations one night from another. After discarding a number of hypotheses, we concluded that, not knowing the results obtained by Dawn yet, the effect observed could be due to the presence of volatile substances which evaporate due to solar radiation. When the rotation of Ceres takes them on the side illuminated by the Sun, they sublimate and form some spots that reflect the solar light in a very effective way. Since they evaporate very quickly, they lose their reflecting power and generate the variation observed at radial velocity. This effect changes every night and generates a non-reproducible trend».
If this interpretation was confirmed, Ceres nature would be very different from the nature of the other asteroids in the main belt, and we could also confirm the assumption that, despite being so isolated and far from the direct action of other planets, it could have an internal activity on its own.
We know that Ceres hides huge amounts of water inside it, but we do not know if the material reaching the surface is water or not, nor the energy source that allows its continuous discharge. The Dawn mission will probably shed light on these mysteries and on the nature of the reflecting material in the next few months, while the technique to measure radial velocity will allow to monitor this cycle of activity also after the space mission is completed.