Ice volcanoes have probably exploded for billions of years on Ceres

All the bodies of our solar system started to heat up, the energy being created by their gravitational collapse and their subsequent bombardment. The radioactivity then contributed to additional heating. For a planet like Earth, it allowed the inside to stay warm enough to support plate tectonics. The smaller bodies like Mars and Moon, however, have cooled down and become geologically silent. This set expectations for the dwarf planets, where it was thought that they were cold and dead.

Pluto, however, turned out to be anything but. It turns out that water and nitrogen ice need much less energy to participate in active geology, and that radioactive decay and sporadic collisions seem to be enough. to maintain it. Which brings us to Ceres, a dwarf planet that is the largest body of the asteroid belt. The Dawn spacecraft has identified an unusual peak called Ahuna Mons, some of which has suggested it was a cryovolcano, erupting from ice of slimy water. But why would Ceres have enough energy to support just one volcano?

A new document suggests no. Instead, there may be more than two dozen cryovolcanoes on the surface of Ceres. We simply did not spot them because the geology of the dwarf planet did not stop when the cryovolcano stopped bursting.

Ceres himself was a bit of a surprise. According to its density, it should be rich in water. And that, at typical body surface temperatures, water ice flows slowly, like in the glaciers of the Earth. Over time, this should result in a return to a more average form of all large areas (craters and peaks). Yet, Ceres is covered with great old features, suggesting that its icy crust is stiffer than we thought.

But a team of US-based researchers has acknowledged that cryovolcans, as their name suggests, produce materials rich in water and ice, which should be able to gradually return to flat terrain. So they started with a peak shaped like Ahuna Mons and modeled how it would warp over time. They then took a database of images taken by the Dawn spacecraft and scanned for structures similar to those produced by their model.

They found 32. Some of them did not have sufficient data to perform a complete reconstruction of their features, while others were partially wiped out by craters of impact. Even eliminating them, they still had a collection of 22 dome-like features that could be old cryovolcanoes. Their diameter ranged from 15 to 85 kilometers and extended up to 4.4 km above the surrounding terrain. Typical volumes were hundreds to thousands of cubic kilometers.

The collection included a report called Yamor Mons, which was most similar to that of Ahuna Mons. But the authors do not think it is a recently active cryovolcano. Instead, the poles are 55K colder than mid-latitudes – at this temperature, the water ice is much stiffer. So, it could be a much older volcano that does not just go down to the level of its environment.

Elsewhere, all the domes have sagged, so that they are much wider than they are tall. The degree to which they collapsed allows the authors to determine that they are probably hundreds of millions of years old, suggesting that cryovolcanism has been active on Ceres for a long time, perhaps since the beginning of its history . According to the expected number and time of survival, a new cryovolcane is formed about every 50 million years in the last billion years.

The orbit of Dawn's dwarf planet allowed the researchers to record subtle changes in gear speed, providing details on the interior of Ceres. The researchers therefore compared these data to the cryovolcano site and found that there was no relationship between the thickness of the Ceres crust and the presence of a volcano. Thus, they conclude that volcanism probably comes from the earth's crust, rather than being driven by deep warm materials within the planet, as is the case on Earth.

It's also very different in terms of volume. Even if you increase the rate of volcanism on Ceres to account for the difference in size with the Earth and other terrestrial planets, the rate of volcanic activity is at least 10 times lower on Ceres.

However, the new document provides strong evidence that Ceres is geologically active. Its activity will be different from that of Pluto, where the conditions are colder and the ice is not water, as well as Io, where the volcanism is driven by the gravitational stresses of the large neighboring bodies. Thus, the results suggest that there can be a wide variety of ways to drive volcanism.

Astronomy of nature, 2018. DOI: 10.1038 / s41550-018-0574-1 (About DOIs).