Far from inert rock, hidden hydrogen points to Ceres being icy and active
When I first told my kids about the asteroid Ceres, their first concerns were entirely about its size. The fact that it’s the largest object in the asteroid belt, constituting one third of all the mass found out there, just made them worry. How does this thing compare to the asteroid that wiped out the dinosaurs? was their primary thought, which sort of makes sense since hitting Earth is one of the most exciting things asteroids seem to do. To get away from that mental roadblock, it seems that it might be easier to forget that Ceres is an asteroid and think of it as its other designation— a dwarf planet.
Frigid but flowing
Data from the Dawn spacecraft has actually been making Ceres sound more and more like common conceptions of planets, and less like an inert hunk of rock and metal we associate with asteroids. By measuring reflected gamma-rays and neutrons, scientists have been able to calculate how much hydrogen, most likely stored as ice, is hidden beneath the outer later of rock and dust. There’s a lot there, apparently, buried only three feet below the surface at the equator, and under a thin veneer of dust near the north pole.
All this water isn’t just frozen in place either, as it seems to be tied to a fair amount of geological activity on Ceres. Heat in the dwarf planet’s core seems to have helped the water churn and separate from other materials, forming strata of ice below the surface a bit like Earth’s layers of rock and magma. This ‘ice magma’ concept even includes at least one cryovolcano, called Ahuna Mons. At around half the height of Mount Everest, this cone doesn’t isn’t exploding with plumes of icy slush as much as leaking a briny slush. Various salts mixed with the water help keep things flowing, as the temperatures on the surface of Ceres are usually around -171° Fahrenheit.
Interestingly, my kids’ weren’t completely off the mark to wonder about asteroid impacts, only they were worried about the wrong planet. One of the tasks on the Dawn spacecraft’s extended mission is to photograph and map new impact craters on Ceres itself. Whether you think of it as a tiny planet or huge asteroid, it’s large enough to be hit and damaged by other objects, some of which make craters deep enough that they’re never fully exposed to sunlight, which may play a role in persevering some ice and salt.
Source: Solar System’s biggest asteroid is an ancient ocean world by Alexandra Witze, Nature News