Haze on Ceres

Independent observations of Ceres by Dawn spacecraft and by HARPS spectrograph at La Silla, Chile, reveal the extremely variable behavior of the mysterious bright spots on the surface of the dwarf planet, which were discovered by the spacecraft in February 2015. Variability measured with HARPS and recent closer images reveal a mysterious haze in the Occator crater (92 kms), which disappears within a few hours.

It is assumed that something comes out from inside the planet where a huge amount of water is present. Water could possibly evaporate and fill the crater and then disappear under the effect of solar radiation. The team of the space mission showed great interest for the results of ground-based observations, and joint coordinated observations are being planned to continue research also after the space mission is completed.

Ceres is the largest object in the asteroid belt located between Mars and Jupiter and is also the dwarf planet closer to Earth. Unlike the other celestial bodies around it, it seems to have traces of activity on its surface. This is suggested by the beautiful high resolution images collected by Dawn spacecraft published in Nature last December, and presently by recent observation with ground based telescopes by a team of INAF astronomers including Paolo Molaro and Marco Fulle of OATs.

Dawn spacecraft entered into stable orbit around Ceres almost one year ago and since then it has observed its surface in detail at close distance. The first close images revealed a constellation of particularly bright spots, especially those located in the Occator crater which has a diameter of 92 kms, as can be seen in Ceres map in Figure 1.

Figure 1 - Map of Ceres


Searching for further clues about the nature of the spots, Molaro and co-authors measured the radial velocity distortions produced in the spectrum of the solar light reflected by Ceres by the bright spots which rotate like a headlight. Using HARPS spectrograph at ESO 3.6 telescope, the authors could reach the limit of about one meter per second necessary to reveal the extremely small distortions. The results of observations confirm the variations of the solar spectrum with a period slightly higher than nine hours, which corresponds to the rotation period of Ceres. However, observations also reveal marked and unexpected variations in radial velocity in different nights. Data analysis brings to conclude that the observed effect 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 spots which reflect the solar light very effectively. Since they evaporate very quickly, they lose their reflecting power and generate the observed variation in radial velocity. This effect changes every night and generates a non-reproducible trend.

In accordance with ground based spectroscopic data, Nathues and co-authors by analyzing data from Dawn spacecraft in an article published in Nature assumed that the appearance of the bright areas in Occator crater would be consistent with the presence of hydrated magnesium sulphates. On the floor of the crater there seems to be something that pours out from inside and probably sublimates, thus generating a periodical haze that appears and disappears within a few hours.

If this interpretation were confirmed, the nature of Ceres would be quite different from the nature of Vesta and the other asteroids in the main belt. Ceres is a celestial body which, despite being so isolated and unaffected by the direct action of other planets, could have an internal activity on its own.
We know that Ceres hides huge amounts of water inside it, but we do not know either if the material reaching the surface is water, or the nature of the energy source that allows its continuous discharge. In the next few months, the Dawn mission will probably shed light on these mysteries and on the nature of the reflecting material, while the technique to measure radial velocity will allow to monitor this cycle of activity also after the space mission will be completed.

The crater is lined with bright material which shows a diurnal cycle (Fig.2). At noon (a) it is filled by a diffused haze which disappears completely at twilight (b). At very close views (see (c) and (d)) it is evident that the haze remains inside the crater and does not expand towards the South-West area over the higher elevation of the crater. In an article published in Nature Nathues and co-authors assume that the aspect of the bright areas in Occator crater is consistent with the presence of hydrated magnesium sulphates. On the floor of the crater there seems to be something that pours out from inside and could possibly sublimate generating a periodical haze, i.e. a haze that appears and disappears within a few hours. The figure on the right side shows a diffused opacity which fills Occator at Ceeres’ dawn but disappears almost completely at twilight.

For further information, see "Daily variability of Ceres' Albedo detected by means of radial velocities changes of the sunlight reflected" of Paolo Molaro, Antonino Lanza, Lorenzo Monaco, Federico Tosi, Gaspare Lo Curto, Marco Fulle, Luca Pasquini in Monthly Notices of the Royal Astronomical Society Letters 2016 458 (1): L54-L58

 Contact: Paolo Molaro, This email address is being protected from spambots. You need JavaScript enabled to view it.

Figure 2 - top: images of Occator crater (diameter 92 kms). The surface of the crater is lined with bright material which shows a diurnal cycle: at noon (a) it is filled with diffused haze  which disappears completely at twilight (b). Credits: Nathues et al., Nature, 2015. Bottom: radial velocities versus the observer longitude. The longitude decreases with time (indicated in hours on the top axis) or the rotation phase. Black diamonds are the measurements taken in July, filled diamonds are for the beginning of the night and empty diamonds are for the end of the night. Triangles are the observations of August 26 and 27. The black continuous line shows the model based on the light curve. The shadowed area is when Occator is visible, which coincides with the maximum variations of the observed radial velocities and when moving towards the observer. Credits: Molaro et al., MNRAS 458 L54, 2016.