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[Occator crater on Ceres, sporting its bright spots likely due to briny water seeping up from below the surface. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA]
How many objects in the solar system are geologically active?
It’s a good question. Earth is; we have erupting volcanoes and sliding continents. Jupiter’s moon Io has active volcanoes, and Saturn’s moon Enceladus has water geysers (Neptune’s moon Triton is likely active, as well). Venus also shows evidence of recent volcanic rumblings.
Moons and planets are big, and while it’s amazing that they are active like Earth is, in hindsight, we should’ve expected it.
But what about asteroids? They’re small(ish), and you wouldn’t think they could have any sort of geologic activity on (or in) them. But then, you’d need to consider Ceres.
Two major pieces of evidence of recent geologic activity on Ceres are evident in this one image, recently released by the folks working on the Dawn spacecraft. It shows Ceres half-lit, the Sun far off to the right, the surface moody and forbidding. But what catches the eye is the crater with the bright spots in it … and, if you look carefully, a huge mesa poking up from the limb of the world to the right at about the 3:30 position.
New research from two different teams has now shown that both these features are very likely due to geologically recent activity. And by “recent” I mean not only in the geological sense (like in the past few million years), but maybe even today. Yes, like right now, maybe even as you read this.
First, a little background: In between the orbits of Mars and Jupiter lies the asteroid belt. Most of the objects there are small; there may be billions smaller than 100 meters across, with bigger ones increasingly rare.
The biggest object there is Ceres, which properly isn’t really an “asteroid” at all. That term denotes the rubble left over from the formation of the solar system, countless internecine impacts since then having hewn them into irregular shapes.
Ceres, though, grew rapidly in those early days, and was well on its way to becoming an actual planet before available material ran out. Because of this, planetary scientists tend to refer to it as a protoplanet.
It’s mostly rock, but we’ve known for a while there’s substantial water ice there., too. This became even more clear when the Dawn spacecraft arrived there in 2015. The surface was saturated with impact craters, as expected, but many of them had bright features in them. At first, this was puzzling, but now it’s thought these are briny patches, left over from salty water bubbling up from the protoplanet’s interior. When the water ice sublimates away (turning directly into a gas as the Sun warms it), it leaves behind the salt crust.
The most famous of these craters is Occator, a 92-kilometer-wide crater with numerous bright spots in it. The floor of the crater is also littered with cracks, probably due to internal pressure of ice under the surface. You can actually see spidery cracks emanating from a mound in the floor of the crater in hi-res images, too.
[Quick aside: Although the spots look very shiny, Ceres is actually very dark. It only reflects about 9% of the sunlight hitting it, making it roughly as dark as asphalt. The images from Dawn are exposed and processed to make the contrast better. The big image above actually gives you an idea of just how dark it really is.]
New research just published shows that the crater itself is about 34 million years old —very young, compared to the 4.5 billion year age of Ceres! — and the white mound in the middle is probably only about four million years old, far younger than the crater. The scientists determined this by counting craters; old regions will have suffered more impacts than younger ones. The central mound has very few craters in it compared to the crater floor, and by looking at the population of asteroids the impact cratering rate can be estimated, which then yields the absolute ages of the terrain.
Not only that, but images from Dawn and observations from both space- and ground-based observatories have given tantalizing hints that there is actual haze in the bottom of the crater that is diurnal; that is, comes and goes with the day/night cycle. That would be very strong evidence, indeed, that water ice is still seeping up from inside Ceres and sublimating.
Amazing. But there’s more!
The mound poking up out of the right side of the Ceres image is actually a huge mountain called Ahuna Mons, another weird and mysterious feature of the alien world. It’s big - 4-5 km high and 18 long. The most likely explanation of it is that it’s a cryovolcano: A volcano not of lava, like on Earth, but of water ice! As muddy ice pushed up from beneath it piled up, probably too rapidly for the ice to sublimate away. There’s no other feature quite like it on the protoplanet, so either it’s unique and likely old, or it’s young and any older ones have eroded away over billions of years. Given that there are few craters on Ahuna Mons, it’s probably young — a few hundred million years old or so.
This, again, is bolstered by new research. If Ahuna Mons is composed of more than 40% water ice (which seems likely), then it will subside over time, slowly settling due to viscous flow (also called relaxation, a term I like). It should settle at a rate of 10 – 50 meters per million years, meaning it can’t be more than a couple of hundred million years old at most. That also explains why it’s alone: All the other cryovolcanoes on Ceres from billions of years ago have already subsided completely. The researchers are currently examining images of Ceres to look for these flattened ancient ex-cryovolcanoes.
Obviously, Ceres is a pretty interesting place, unique in many ways from anything else we’ve seen. It makes me wonder what else we’ll find in the asteroid belt as we send more probes there to explore. A lot of asteroids are metal, or a mix of rock and metal. Will we find more ice? Perhaps so, locked inside some of them. As humans start to explore the solar system we’ll need ice for water and air, and metals for structures. The asteroid belt is a fantastic place to find these precious resources. A lot of science fiction supposes we’ll have a civilization out there someday. I can’t disagree…and what a view they’ll have!