Ceres, the largest body in the asteroid belt, may once have had an ocean beneath its icy crust. NASA’s Dawn spacecraft has been orbiting Ceres since March 2015 and has been measuring the strength of the gravity across the dwarf planet.
But new research has revealed something strange about the gravity measurements around a large mountain and several large craters.
“Simply speaking, a crater is a hole in the ground, so since some mass is missing due to that hole, gravity should be less strong there,” explained Anton Ermakov of JPL, lead author of one of the papers discussing the new results.
“With a mountain, we have the opposite situation. Since we know the shape of the craters and mountains, we know exactly how gravity should be weaker in a crater and how much stronger it should be on top of a mountain.”
However, the results showed several “anomalies” – differences between the measurements and the models – which indicate that the change in gravity is due to the mass being unexpectedly different in the crust.
These gravity anomalies were used to measure the density of the crust, which turned out to be extremely low. The crustal density is consistent with that of ice, however ice alone would not be strong enough to be the main component of the crust.
Another oddity about Ceres can be revealed through studying its topography and it appears that the mountains on Ceres have been flattened over time. As a rule of thumb, the height of a mountain is related to the width of the base of the mountain, so that on Earth a mountain with a 1 kilometre base would be typically be expected to have a height roughly around 500 metres, rather than soar to a height such as 10 kilometres.
“What we observe on Ceres is that wider base mountains do not quite have as much height as we would expect,” said Ermakov. “So, these large mountains have been preferentially flattened by some process.”
“Combined with the observed lack of large impact basins on Ceres (even though they should be there), I think there’s a strong case for decay of mountain heights,” added Roger Fu of Harvard University, lead author of the other paper.
By putting the density and topography information into a computer model, the scientists discovered that the unusual strength of the crust can be explained if it contained a molecular structure known as “clathrate hydrate”. This is a cage-like structure made of water molecules which encases a gas molecule, and it is between 100 and 1000 times stronger than ordinary water ice even though it has almost the same density.
They were also able to model how the crust “flows” gradually over time. The tough layer of ice and clathrate hydrates sits on top of more easily pliable layer, which must have contained liquid. This easily deformable layer explains how the mountains were able to flatten, something that wouldn’t happen if the crust was comprised of rigid rock.
Heat from radioactive decay of elements would have been sufficient to melt some of the ice in order to allow liquid water to exist below the crust. Most of this ancient ocean is now likely to be frozen, but it is still possible that there is still some liquid water left over.
The unusual gravity and topography measurements from the Dawn spacecraft were pieces of a puzzle that finally fell into place, showing that an ancient subsurface ocean created a deformable layer which led to the demise of the mountains on Ceres.
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