Cryovolcanism on Dwarf Planet Ceres

Among the most striking features on the surface of Ceres are the bright spots in the center of Occator crater which stood out already as NASA's space probe Dawn approached the dwarf planet. Scientists under the leadership of the Max Planck Institute for Solar System Research (MPS) have now for the first time determined the age of this bright material, which consists mainly of deposits of special mineral salts.

With about four million years only, these deposits are about 30 million years younger than the crater itself. This, as well as the distribution and nature of the bright material within the crater, suggests that Occator crater has been the scene of eruptive outbursts of subsurface brine over a long period and until almost recently. Ceres is thus the body closest to the Sun that shows cryovolcanic activity.

For nearly two years, the NASA's space probe Dawn has been accompanying dwarf planet Ceres, which orbits the Sun within the asteroid belt between Mars and Jupiter. During the first part of the mission, the probe advanced to lower and lower orbits until between December 2015 and September 2016 only approximately 375 kilometers separated it from the surface.

During this so-called Low Altitude Mapping Orbit the Dawn Framing Cameras produced highly resolved images of Ceres' surface displaying a resolution of 35 meters per pixel. The Dawn Framing Cameras, Dawn's scientific imaging system, were developed and built and are operated under the leadership of the MPS.

MPS researchers have now thoroughly investigated the complex geological structures that are shown in the detailed images of Occator crater. These structures include fractures, avalanches, and younger, smaller craters. "In these data, the origin and evolution of the crater as it presents itself today can be read more clearly than ever before", says Andreas Nathues, Framing Camera Lead Investigator.

Additional indications were provided by measurements of the infrared spectrometer VIR onboard Dawn.

Occator crater located in the northern hemisphere of Ceres measures 92 kilometers in diameter. In its center a pit with a diameter of about 11 kilometers can be found. On some parts of its edges, jagged mountains and steep slopes rise up to 750 meters high. Within the pit a bright dome formed. It has a diameter of 3 kilometers, is 400 meters high and displays prominent fractures.

"This dome contains the brightest material on Ceres," says MPS scientist Thomas Platz. Researchers call the bright material in the central pit Cerealia Facula. VIR data show that it is rich in certain salts, so-called carbonates. Since later impacts in this area did not expose any other material from the depth, this dome possibly consists entirely of bright material. The scattered bright spots (Vinalia Faculae) located further outside in the crater are paler, form a thinner layer and - as VIR and camera data show - turn out to be a mixture of carbonates and dark surrounding material.

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