Prior to our visit to the dwarf planet Ceres, we knew it had a density about twice that of water. Since it likely has a rocky core, that means much of the outer crust is likely made up of water. Because water is quite malleable at the temperatures we would expect on Ceres, there were some expectations that all the features on the planet had long since flattened out by the time the ice from which they were made poured down, like in glaciers.
This turned out to be everything but the case, as Ceres is covered in craters, ridges, and the remains of what appear to be ice volcanoes. All this suggests that the crust is much stiffer than we thought and has a composition more complex than just water with some dust in it.
Now a researcher named Pasquale Tricarico has analyzed the distribution of material on Ceres and determined that it corresponds to the densest material that caused the displacement of the planet’s equator. He suggests that a series of ridges surrounding the dwarf planet represent the former equator, which would mean Ceres fell on its side sometime early in its history.
The simplest evidence that there are potential quirks in Ceres’ crust comes from a simple map of its density. The average density of Ceres is about two grams per cubic centimeter, but the crust can be less than 1.7 g/cm>3 and reach above 2.3 g/cm>3. Thus, there are many spatial differences in the distribution of material in the crust. Mapping that on Ceres’s surface reveals that the most dense regions — and thus the most massive — are generally along the equator. This includes the likely cryovolcano Ahuna Mons.
This makes sense based on what we know about the rotation of a body, which would naturally align so that the heaviest material is furthest from the axis of rotation. Normally this would happen naturally during the formation of the planet. But if planets are geologically active, the area preferred to be on the equator may form later in its history. In that case, it is possible that the massive regions exert a force that spins the entire planet, placing itself on the equator and creating a new axis of rotation. This causes the poles to shift, a phenomenon called “true polar error.”
Tricarico provides two proofs that this is what happened on Ceres. The first relates to the fact that, as the planet tilts 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 been stretched. He then found some previously identified features on Ceres that seem to be consistent with the kind of flaws these forces are supposed to create.
But perhaps more importantly, Tricarico found what could be the former equator. A series of elevated elements extending across the planet, forming what would have been an equatorial ridge, formed when Ceres’ rotation affected the distribution of materials during its formation. Given the shape of the ridge, Tricarico was able to precisely locate the former pole of the dwarf planet. The rim slopes significantly from the current equator, at 36° from a possible 90°. He suggests the ridge formed by compressive forces pushing material toward the equator.
Looking at the features near where the “paleopole” would have been, there are a few features that may have suggested temporary poles on a jagged path to the current one, each at a slightly lower elevation. This suggests that the process may have been non-contiguous and may have been the product of a process that gradually changed the distribution of Ceres’ mass.
Tricarico favors the idea of an ice-based equivalent of what we’d call a mantle plume if it were on Earth: a large amount of warm, semi-molten material that makes its way to the surface from deep within 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 long on Ceres, as their ice will gradually flow to the level of the terrain around them, and Tricarico thinks the real polar wandering happened early in Ceres’s history. So the identity of what caused Ceres to be torn apart is still a matter of speculation.
Natural Geosciences2017. DOI: 10.1038/s41561-018-0232-3 (About DOIs).