Prior to our visit to the dwarf planet Ceres, we knew that it had a density about twice that of water. Considering it likely has a rocky core, that means much of its outer crust is likely to be water. Since water is pretty malleable at the temperatures we'd expect to prevail on Ceres, there were some expectations that any features on the planet had long since flattened out as the ice they were made of flowed downwards as it does in glaciers.
This turned out to be anything but the case, as Ceres is covered in craters, ridges, and the remains of what appear to be ice volcanoes. All of which suggests that the crust is far more rigid than we thought and has a composition that's more complex than simply water with some dust mixed in.
Now, a researcher named Pasquale Tricarico has analyzed the distribution of material on Ceres and found that it is consistent with the densest material having caused the relocation of the planet's equator. He suggests that a series of ridges that ring the dwarf planet represent the former equator, which would mean that Ceres tipped over on its side at some point early in its history.
The simplest evidence that there are potential oddities in Ceres' crust comes from a simple mapping of its density. Ceres' average density is about two grams per cubic centimeter, but the crust can be below 1.7 g/cm>3 and range up to above 2.3 g/cm>3. So there are a lot of spatial differences in the distribution of material in the crust. Mapping that out on the surface of Ceres shows that the most dense regions—and therefore the ones likely to be the most massive—are generally located along the equator. This includes the likely cryovolcano Ahuna Mons.
This makes sense from what we know of a body's rotation, which would naturally align so the heaviest material is the farthest out from the axis of rotation. Typically, this would happen naturally during planet formation. But if planets are geologically active, then the area that's favored to be at the equator can end up forming later in its history. In that case, it's possible for the massive regions to exert a force that wrenches the whole planet around, putting itself at the equator and creating a new axis of rotation. This causes the poles to shift, a phenomenon called "true polar wander."
Tricarico provides two pieces of evidence that this is what happened on Ceres. The first relates to the fact that, as the planet tips to create a new equator, the process creates stresses in the crust. Tricarico calculated areas where the crust should have been compressed and others where it should have stretched. He then found some previously identified features on Ceres that seem to be consistent with the sort of faulting these forces should create.
But perhaps more significantly, Tricarico found what might be the former equator. A series of elevated features that extend around the planet, forming what would have been an equatorial ridge, formed as Ceres' rotation influenced the distribution of materials during its formation. Given the shape of the ridge, Tricarico was able to precisely place the former pole of the dwarf planet. The ridge is significantly inclined from the present equator, at 36° out of a possible 90°. He suggests the ridge formed due to compressional forces pushing material toward the equator.
Looking at the features near where the "paleo-pole" would have been, there are a couple of features that may have represented temporary poles on an erratic path to the present one, each at a somewhat lower elevation. This suggests the process may have been discontiguous, and possibly the product of a process that changed the distribution of Ceres' mass gradually.
Tricarico favors the idea of an ice-based equivalent of what we'd call a mantle plume if it were on Earth: a large body of warm, semi-molten material that forces its way to the surface from deep in the planet's interior. On Ceres, this could explain both the excess mass at the equator and the presence of the Ahuna Mons cryovolcano there. But features like Ahuna Mons aren't expected to last very long on Ceres, as their ice will gradually flow down to the level of the terrain around them, and Tricarico thinks the true polar wander happened early in Ceres' history. So the identity of what caused Ceres to be wrenched over is still a matter of speculation.
Nature Geoscience, 2017. DOI: 10.1038/s41561-018-0232-3 (About DOIs).
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Ars Technica
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