All of the bodies in our Solar System started out hot, with energy built up by their gravitational collapse and subsequent bombardment. Radioactivity then contributed further heating. For a planet like Earth, that has kept the interior hot enough to sustain plate tectonics. Smaller bodies like Mars and the Moon, however, have cooled and gone geologically silent. That set the expectations for the dwarf planets, which were thought to be cold and dead.

Pluto, however, turned out to be anything but. It turns out that water and nitrogen ices need far less energy input to participate in active geology, and radioactive decay and sporadic collisions seem to be enough to sustain it. Which brings us to Ceres, a dwarf planet that is the largest body in the asteroid belt. The Dawn spacecraft identified an unusual peak called Ahuna Mons that some have suggested is a cryovolcano, erupting viscous water ice. But why would Ceres only have enough energy to support a single volcano?

A new paper suggests it doesn't. Instead, there may be more than two dozen cryovolcanoes on Ceres' surface. We just haven't spotted them because geology on the dwarf planet didn't stop when the cryovolcanoes stopped erupting.

Ceres itself was a bit of a surprise. Based on its density, it is expected to be water-rich. And, at the temperatures typical of the body's surface, water ice slowly flows, much as it does in glaciers on Earth. Over time, this should cause any large surface features (like craters and peaks) to slump back to a more average shape. Yet Ceres is covered in large, old features, suggesting that its icy crust is more rigid than we thought.

But a team of US-based researchers recognized that any cryovolcanoes would, as their name implies, be erupting water-ice-rich materials, which should be able to gradually flow back into shapes that look like level terrain. So they started with a peak shaped like Ahuna Mons and modeled how it would deform over time. They then took a database of images taken by the Dawn spacecraft and scanned the dwarf planet for structures similar to those their model produced.

The result of their model was that they found 32 possible cryovolcano structures. A few of the images provided insufficient data for a full reconstruction of their features, and others have been partly wiped out by impact craters. Even eliminating those, however, left them with a collection of 22 dome-like features that could be former cryovolcanoes. These ranged in diameter from 15 to 85 kilometers and extended up to 4.4km above the surrounding terrain. Typical volumes were hundreds to thousands of cubic kilometers.

The collection included a feature called Yamor Mons, which was the most peak-like of them, similar in proportions to Ahuna Mons. But the authors don't think it's a recently active cryovolcano. Instead, the poles are 55K colder than the mid-latitudes—at that temperature, water ice is much more rigid. So, this could be a much older volcano that simply isn't slumping back down to the level of its surroundings.

Elsewhere, all the domes had slumped down so that they are much broader than they are tall. The degree to which they've slumped allowed the authors to determine that they are likely to be hundreds of millions of years old, suggesting that the cryovolcanism has been active on Ceres for a long time, possibly since early in its history. Based on the count and their expected survival time, it looks like a new cryovolcano has formed about every 50 million years for the last billion years.

Dawn's orbit of the dwarf planet allowed researchers to register subtle changes in the craft's speed that provided details about Ceres' interior. So the researchers compared this data to the site of cryovolcanoes and found that there was no relationship between the thickness of Ceres' crust and the presence of a volcano. Thus, they conclude that the volcanism probably originates in the crust rather than being driven by hot material deep within the planet, as it is in Earth.

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

Still, the new paper provides strong evidence that Ceres is geologically active. Its activity is going to be different from Pluto, where conditions are colder and the ices aren't water, as well as Io, where volcanism is driven by the gravitational stresses from nearby large bodies. Thus, the results suggest there may be a large variety of ways to drive volcanism.

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