Ceres – unusual landslides and unstable crater walls
The lower the Dawn spacecraft flies over the dwarf planet Ceres with its on-board camera, the more puzzling – and exciting – the celestial body appears. “Some of the things we are seeing have never been seen anywhere else in the Solar System,” says Ralf Jaumann from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). “Except for on Earth.” Dawn is now looking down onto the surface of Ceres from an altitude of just 1470 kilometres. The first images acquired from its High Altitude Mapping Orbit (HAMO) show a ‘pyramid’ with unusual landslides, unstable crater walls and chains of mountains. “We can only speculate about these things at the moment.” Where the bright stripes along the pyramid-shaped mountain come from and whether the surface of the dwarf planet is comprised of different materials are questions that the planetary researchers are still trying to answer.
Pyramid with bright stripes
Being three times closer to Ceres than in its previous orbit and with a resolution of 140 metres per pixel, Dawn’s Framing Camera is already capturing interesting details. "We are looking down on things such as a pyramid-shaped, six-kilometre high mountain that has bright stripes down one side." The ‘pyramid’, which is in the southern hemisphere between the craters Kirnis, Rongo and Yalode, has a diameter of between 10 and 12 kilometres: “The mountain must have immensely steep flanks in view of its considerable height." Yet there are almost no boulders at the foot of the mountain. Directly adjacent to it is an impact crater that stretches up to the slopes of the mountain. "Presumably, the mountain is younger than the crater, but to determine this precisely we need to wait for the images from the next orbit and data from the spectrometer, which will hopefully determine what material is on the surface."
Unstable crater rims and flat plains
Images of the Gaue crater, named after a Germanic goddess, show that it is partially situated on top of a smaller, older crater. “Gaue crater exhibits material that has fallen into the crater interior on one of its sides – meaning that the walls are rather unstable,” says Jaumann, interpreting the first images acquired at 1470 kilometres above the surface of the dwarf planet. “Presumably there have also been changes in the centre, as it appears to be very flat.” The craters on the dwarf planet are comparable with craters on a rocky body such as the Moon, indicating that the crust of Ceres may not be very stable. One possible explanation for the flat crater interior is that molten material may once have filled the crater. “In any case, something must have happened there after the crater was formed.”
Detailed images of the interior of another crater, named Urvara, also show structures that prompt further questions. Next to a mountain chain, fine fissures can be seen, as well as more evidence of loose material on the crater rim. "The material here seems to have broken up into large blocks and fallen towards the interior of the crater. The unusually smooth plain is most probably formed by the deposition of fine material that was presumably once molten. These are, of course, just initial suppositions that we are discussing in the mission team," adds Jaumann
Endless journey around the dwarf planet
It takes the Dawn spacecraft 11 days to cover the entire surface of the dwarf planet and send the images back to Earth. This is set to happen six times before it leaves this orbit in two months. From late October to the end of January 2016, Dawn will be flying in its last and lowest orbit, where it will be circling around the celestial body at an altitude of 375 kilometres. The spacecraft will be using its ion propulsion system as it descends – this means the camera will not be operational for two months. After the mission ends, Dawn will continue to orbit Ceres in this stable orbit – from a safe enough distance so that the dwarf planet is not contaminated with terrestrial microbes.
Ceres in 3D
Researchers at the DLR Institute of Planetary Research are currently working on a three-dimensional terrain model of Ceres, which was generated using image data acquired during the previous orbit at a distance of 4400 kilometres. "We can then use the images from the current orbit to refine the 3D model, meaning that we can measure things such as the elevation and size of the various structures on the dwarf planet Ceres." Then, it will be a matter of solving a number of riddles. In the words of DLR planetary researcher Ralf Jaumann: "We would like to find out things such as why the plains are so flat and how the ‘pyramid’ was formed."
The Dawn mission to Vesta and Ceres is managed by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, which is a division of the California Institute of Technology, for NASA's Science Mission Directorate in Washington DC. The University of California, Los Angeles, is responsible for overall Dawn mission science. The camera system on the spacecraft was developed and built under the leadership of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, with significant contributions from the German Aerospace Center (DLR) Institute of Planetary Research in Berlin and the Institute of Computer and Communication Network Engineering in Braunschweig. The Framing Camera project is funded by the Max Planck Society, DLR, and NASA/JPL.