Ceres mysterious bright spots = icy volcanoes? Probably not, but the alternatives are just as intriguing. In a press conference Tuesday with scientists behind the Dawn mission, deputy principal investigator Carol Raymond explained that the brightest spot isn’t associated with a mound or peak as it would be if it were produced by oozing or eruption of ice in a cryovolcano.
Instead, they’re more likely ice or salts exposed on Ceres’ dark surface through impact.
“The brightest may contain ice or salt,” said Raymond, adding that in 2014 the European Space Agency’s Herschel Space Telescope detected water vapor from two sectors on Ceres, one of them where the crater ringing the bright spots is located. Ice is a good fit based on how highly reflective the spots are, but if the original ice vaporized, it could have left bright salts behind that would also do the trick.
“This mystery will be resolved, but it’s really got us on the edge of our seats,” said Raymond. We’ll have to sit tight for a little while before we know more. Dawn will be captured by Ceres’ gravity on Friday (March 6). As it spirals in closer to the dwarf planet we’ll see only its shadowed side for the time being. Not until April will we get see the sunlit face along with clearer, closer views of those spots.
Ice is strongly suspected because Ceres is thought to harbor a thick layer of ice beneath its crust. We know this because it’s density is so much less than typical rocky asteroids and moons. Compared to the asteroid Vesta, Dawn’s first study target, Ceres is only 61% as dense, leading scientists to estimate that a quarter of its mass is in the form of water.
Based on Ceres’ size and location relatively near the Sun and the heat given off by the decay of the short-lived radioactive elements it possessed in its youth, the dwarf planet likely harbored a sub-surface ocean capped by an icy crust. That ocean would since have cooled down in the intervening 4 billion years into ice. This would explain why some of Ceres’ craters look relatively smooth with low rims. Ice allows for a certain amount of “relaxation” in crater shapes. Rugged floors smooth out and rims elevations decline.
Ceres’ current ice layer most likely contains salts that dissolved in the water at the time. Similar to what a salt lake leaves behind when it dries up, vaporizing ice can leave a crust or deposits of bright salt crystals.
Raymond also mentioned the possibility of dust levitation by gases emanating from ice vaporizing just under the surface. Might we someday see small, bright “clouds” caught in the last rays of sunlight at Ceres’ day-night terminator?
Dawn project manager Robert Mase explained that Dawn’s first “science” orbit will happen in April. Throughout the year, mission control will be gradually lowering the spacecraft’s orbit all the way down to 235 miles (378 km) – nearly identical to that of the International Space Station. The main science mission will continue through June 2016.
“It’s clear that discoveries lie ahead as Ceres is revealed in stunning detail just like Vesta,” said Raymond.