We are all familiar with images of volcanoes on Earth spewing out red hot molten rock. But picture a volcano where, instead of lava, freezing material slowly pours out. Such an icy volcano – known as a cryovolcano – has been discovered on the dwarf planet Ceres by NASA’s Dawn spacecraft.
Ahuna Mons is an isolated mountain that stands four kilometres high. Volcanoes on Earth have many different formation mechanisms; one of the most common being a tectonic plate slipping under another. As the misfortunate plate melts, magma is forced upwards to form a volcano and this plate subduction is responsible for the volcanoes in the “Pacific Ring of Fire”.
Ceres is nothing like the Earth. It is a rocky and icy body only 945 kilometres in diameter – and it does not have plate tectonics. However, it is still possible to draw parallels between Ahuna Mons and certain volcanoes on Earth. “Ahuna Mons is a cryovolcanic construct similar in morphology (shape and texture) to what we call silicic domes on Earth: volcanoes built from very pasty, high viscosity [thickness] lavas, rich in silica,” explains David Williams of Arizona State University, co-author of the paper that appeared in the journal Science.
Partially molten material beneath the surface is more buoyant than the surrounding rocks, and therefore rose up through the surface and oozed out, forming the dome of the cryovolcano.
A volcano with a twist
Cryovolcanism was previously thought to only involve water and water ice. However, Ahuna Mons is actually a “salty mud” mix – and the only known volcano of its kind.
“The discovery at Ceres suggests that cryovolcanism can occur also with a different composition, involving salty water, carbonates, and clays,” said Ottaviano Ruesch of NASA’s Goddard Space Flight Center, and lead author of the study. “We do not observe ice on the cryovolcano suggesting that water is not a major component. So this tell us we have more diversity, more geologic and volcanic diversity within the Solar System as previously recognised.”
Ahuna Mons is also surprisingly young in geological terms. It most likely formed no more than a billion years ago, and possibly even within the last few hundred million years. For cryovolcanism – or indeed terrestrial volcanism – to occur, there must be an internal heat source. Planets and moons in the Solar System can retain heat from their formation billions of years ago, although they will gradually cool down. This is what happened on Mars.
While it boasts the largest volcano in the Solar System – Olympus Mons – this volcano is long extinct because Mars has cooled due its small size. The internal heat necessary for volcanism can also occur due to the gravitational pull of a nearby body creating tides. The moon Io is the most volcanic body in the Solar System due to its orbit around Jupiter.
But Ceres is a small body that should have cooled long ago, and due to its isolated position orbiting between Mars and Jupiter it doesn’t experience any tides. “Recent cryovolcanism on Ceres is telling us that the energy was not easily lost into space as previously thought,” Ruesch tells Skymania News. “Still, temperatures in the interior of Ceres are not warm, we are talking about temperatures around 230 degree Kelvin [-43 decrees Celsius]. Ahuna Mons demonstrates that such conditions are nevertheless enough to sustain some amount of melt, perhaps locally, in pockets in the subsurface.”
More than mountains
Cryovolcanism is also present elsewhere on Ceres, and it doesn’t always build mountains. There are craters that appear too shallow compared to what would be expected from a meteorite impact. Some craters also show cracks where cryomagma could have been pushed out to resurface the crater floor. The type of cryovolcanism at these craters is different from that which built Ahuna Mons.
“Ahuna Mons was built by extrusive flow of cryolavas apparently without any explosive eruptions,” said Williams. “In contrast, the bright spots in Occator crater appear to have involved explosive ‘venting’ of briny water rich materials, where the water vapour boiled away and left bright minerals like salts and carbonates behind.”
The Dawn spacecraft will continue to orbit Ceres for at least another year, no doubt bringing many more exciting discoveries.
Watch NASA’s video about the ice volcano discovery.