Ceres presented mysteries before the NASA Dawn spacecraft even reached it. Long range images of Ceres taken by the probe revealed unexpected points of light on the surface. There was speculation this could be ice, but now the consensus is leaning toward salt. That doesn’t mean Ceres is lacking in water ice. Two new studies of the Dawn data have shown strong evidence for significant ice deposits on Ceres. This can tell us much about the early solar system.
Ceres is the second (and final) object in the asteroid belt studied by Dawn. On its way to Ceres, the ion engine-powered craft swing by the asteroid Vesta. Ceres is the largest object in the asteroid belt at nearly 600 miles in diameter, but Vesta is no lightweight at 326 miles across. There was initially hope that the bright spots in craters on Ceres’ surface were ice, but the current thinking is that impacts in the past released salty water (brine) that was trapped under the surface. The water sublimated into space (turned to vapor), leaving the salt behind crystallized on the surface.
A new analysis of gamma ray data collected by the GRaND instrument on Dawn shows that there’s probably a lot of ice on Ceres anyway — it may be just about everywhere, in fact. The dwarf planet appears to have very high levels of hydrogen in the mid to high latitudes, and that points to the presence of water ice. Importantly, the amount of hydrogen detected increases the higher you get. Scientists estimate as much as 30% of the material at Ceres’ poles is ice. This ice isn’t technically on the surface, but it’s mixed in with the regolith. That keeps it from being lost to space, and could mean Ceres is around 10% ice by weight. See above for a comparison with Vesta (ice is blue).
What about ice that is on the surface and easily accessible to our instruments? Researchers from the Max Planck Institute focused on the shadowy rim of craters on Ceres where sunlight never reaches (see below). We know similar formations (sometimes called “cold traps”) on the moon have ice, so the hope was something similar was going on in the Ceres craters. The frigid temperatures in these areas should be low enough to keep the ice from sublimating.
The team found deposits of bright reflective material in 10 cold traps using Dawn’s infrared mapping spectrometer, indicating the presence of ice. It is believed that Ceres has an extremely thin atmosphere that includes water molecules. As they hop around the surface, there’s a chance they will be lost to space, but a few end up in a cold trap where they accumulate as ice.
Celestial bodies like Ceres can provide data about the early solar system because they haven’t undergone the sort of geological alternation common with full-fledged planets. Finding water ice on it is a good sign for the existence of life (past or present) elsewhere in the solar system.