The bright white spot on Ceres that has puzzled astronomers for a decade is not one but two spots, the Dawn spacecraft has revealed - but astronomers remain bemused as to what the spots actually are.
In 2003 and 2004, the Hubble Space Telescope took the highest resolution images of Ceres up to that point and revealed a spot on its surface that was distinctly lighter than anything else. However, nothing besides its location could be established, including how large the spot actually is. As Dawn approached, it confirmed the spot's existence, but threw no figurative light on the topic.
Now, however, as Dawn readies to go into orbit around Ceres, photos of unprecedented clarity have split the spot into two marks of unequal size. The image also reveals that the spots are smaller and much brighter than might have been thought from the pale pixels previously observed.
"Ceres' bright spot can now be seen to have a companion of lesser brightness, but apparently in the same basin," UCLA's Professor Chris Russell announced. "This may be pointing to a volcano-like origin of the spots, but we will have to wait for better resolution before we can make such geologic interpretations." When the image was taken on February 19, Dawn was 46,000 kilometers, (29,000 miles) from the dwarf planet.
"The brightest spot continues to be too small to resolve with our camera, but despite its size it is brighter than anything else on Ceres. This is truly unexpected and still a mystery to us," says Dr. Andreas Nathues of the Max Planck Institute for Solar System Research. While a few light patches that may include smaller white spots can be seen elsewhere, it is not yet clear if these spots are unique or the largest of a widespread phenomenon.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Dawn's view of both sides of Ceres.
Although the ice remains the most likely constituent of the dots, no one knows why it should be concentrated at these two nearby locations. However, the fact that these spots, along with their suspected counterparts, are in a basin that may be a crater is probably significant.
"During the course of the mission, the craters will enable us to take an indirect look into Ceres' interior,” Nathues says. His colleague Michael Schäfer explains: "The way the impactors deform the dwarf planet's subsurface and the way the subsurface reacts to this on long time scales allows conclusions about the material hidden beneath the topmost layer.”