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.
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.
Nathues and his team interpret the central pit with its rocky, jagged ridge as a remnant of a former central mountain. It formed as a result of the impact that created Occator Crater some 34 million years ago and collapsed later. The dome of bright material is much younger: only approximately four million years. The key to determining these ages was the accurate counting and measuring of smaller craters torn by later impacts. This method's basic assumption is that surfaces showing many craters are older than those that are less strongly "perforated". Since even very small craters are visible in highly resolved images, the new study contains the most accurate dating so far.
"The age and appearance of the material surrounding the bright dome indicate that Cerealia Facula was formed by a recurring, eruptive process, which also hurled material into more outward regions of the central pit", says Nathues. "A single eruptive event is rather unlikely," he adds. A look into the Jupiter system supports this theory. The moons Callisto and Ganymede show similar domes. Researchers interpret them as volcanic deposits and thus as signs of cryovolcanism.
|This 3d-anaglyph for the first time shows a part of Occator crater in a combination of anaglyphe and false-color image |
The last of these eruptions must have created the present surface of the dome four million years ago. Whether the cryovolcanic activity has ceased completely or is ongoing on a lower level, is still unclear. Pictures of the crater showing haze when imaged at certain angles seem to speak for the latter. At the end of 2015 already, MPS researchers explained this phenomenon with the sublimation of water.
Recent investigations support this interpretation. The MPS researchers evaluated numerous images of Occator crater from an early phase of the mission taken from a distance of 14,000 kilometers and from low angles. They clearly show variations in brightness following a diurnal rhythm. "The nature of the light scattering at the bottom of Occator differs fundamentally from that at other parts of the Ceres surface," MPS researcher Guneshwar Singh Thangjam describes the result of his analysis. "The most likely explanation is that near the crater floor an optically thin, semi-transparent haze is formed," he adds. The researchers believe that the haze is possibly formed by sublimating water emerging from fractures in the floor of the crater when exposed to sunlight.
The findings are published in The Astronomical Journal.
Source: Max Planck Institute for Solar System Research [March 06, 2017]