Ceres – a 950 km wide dwarf planet in the main asteroid belt between Mars and Jupiter. As Dawn approached in early 2015 it detected strange bright spots on the surface in a 90 km wide crater later named the Occator Crater. They were so bright they dominated the pixel they resided in on the camera. Now, many months later, NASA’s Dawn team has still been unable to place their origin, though they are finding similar, but smaller, bright patches and streaks all over the surface. Initially, these were predicted to be the result of highly reflective ice, supported by a haze (possibly sublimated water) visible over the bright spots. The best guess at this point is a highly reflective salt, though a mechanism for how and why this salt is exposed at the surface is yet to be confirmed. Possible explanations include surface impacts removing surface material covering the salt (possible for the Occator Crater, but unlikely for the smaller streaks), or some internal mechanism for the salt rising to the surface, which would suggest a geologically active body. Such small bodies have historically been thought to be long since inactive, but Pluto is only 236 km wider and showed remarkable features that are almost certainly due to recent geological activity.
The surface of Ceres appears to be covered with a hydrated rock alteration product, and may have some areas covered with frost. Thermal models also indicate that Ceres is an icy object subject in the past to hydrothermal activity and differentiation which would give it layers similar to Earth’s inner/outer core, mantle and crust, and even suggest the possibility of a liquid subsurface.
Dawn has generated a topographic map of Ceres’ surface in extraordinary detail. Carol Raymond, deputy mission chief from NASA’s Jet Propulsion Laboratory has noted that the shape of the craters on Ceres are irregular, resembling those on Saturn’s moon Rhea more than those on Vesta, the second largest body in the main belt. A mineral composition map reveals streaks across the surface around the Occator Crater that researchers believe may be relevant.
Another great unsolved mystery is the origin of the feature known as ‘The Lonely Mountain’, a 6 km high feature with an approximately pyramidal shape. There is no evidence of volcanic activity or plate tectonics on Ceres that might have thrust up such a feature.
I noticed that the mountain is somewhat reminiscent of the centre of some craters on the Moon, for example the crater Alphonsus, pictured below. The centre feature of Alphonsus is also pyramid shaped, but unfortunately the similarities seem to end there. It only rises 1.5 km from the Lunar surface and is surrounded by ejecta material from the impact which created the crater. As seen in topographic images, The Lonely Mountain does not appear to sit within any consistent surface depression the might suggest an impact. It’s possible that subsequent impacts and events have erased evidence of a crater, but if so they have carefully avoided the mountain in the centre. The small crater adjacent to The Lonely Mountain is of approximately the same size and appears to be too close for coincidence. I’m not saying the material from the crater was directly translated to the mountain by some kind of impact, but there may be a relationship.
Dawn will cease operations in mid-late 2016 and remain in orbit as a permanent satellite of Ceres. Let’s hope we will collect enough data from Ceres’ surface to solve these questions and more before then.