Ice volcano spotted on Ceres, the asteroid belt’s dwarf planet

Behold: Ahuna Mons. It’s the best example in the solar system of a cryovolcano, a mountain made of ice that spews water instead of magma. Although cryovolcanoes probably exist on Pluto, and there are hints as well on Titan, this peculiar, 4-kilometer-tall mountain on Ceres, the largest object in the asteroid belt, is the real thing, say researchers with NASA’s Dawn spacecraft in one of six papers published today in Science. A lack of craters on nearby surfaces point to an eruption in the recent geological past—within about 200 million years. The researchers suggest that salts help lower the melting point for ice, deep underground where it is warmer, allowing brines to rise up as a cryomagma. They also say that an impact basin on the other side of the dwarf planet, a 280-kilometer-wide area called Kerwan, may have sent shockwaves through Ceres and triggered the eruption. Other papers in the package also touch on the presence of water ice on Ceres, which had already been reported by the Dawn team and by astronomers observing the dwarf planet from afar. Dawn began orbiting Ceres in 2015, following its exploration of Vesta, the asteroid belt’s second largest body. A second paper finds widespread evidence for clay minerals, which require water. Another paper finds that, though Ceres is scarred by many small craters, it tends to lack big impact basins. The researchers say this indicates an upper crust made of a mixture of rock and ice—in which small craters could be carved—supported by a more viscous mantle of ice lower down, which would allow the biggest impact basins to relax away with time, like silly putty returning to its original shape. The science coming from Ceres will be Dawn’s last hurrah. Mission leaders had wanted the spacecraft to use its ion thrusters to visit yet another asteroid, but were denied in July following a review of ongoing planetary missions.