Asteroid Vesta is the second largest asteroid in the Solar System’s asteroid belt, with a diameter of around 330 miles. (Ceres is the biggest.) It’s also the brightest asteroid up there, sometimes visible to the naked eye from Earth. Astronomers consider it a planetesimal because, like a mini-Earth, it has an iron core and rock in its crust and mantle.
The asteroid has long been an object of interest to astronomers. The first book published by Isaac Asimov was called Submerged off Vesta, and in 2011, NASA’s Dawn spacecraft visited him en route to Ceres.
Dawn found two massive impact craters on Vesta – Rheasilvia and Veneneia – evidence of collisions large enough to eject about one percent of Vesta into space. Indeed, about six percent of the meteorites we have found on Earth come from Vesta. Dawn also observed that there are two huge hollows roughly around Rheasilvia and Veneneia. It was speculated that they were somehow related to the two giant impacts.
A new study revisits this hypothesis and offers a new hypothesis on what exactly these mysterious hollows are.
Vesta’s topography, enhanced with color, of Dawn.Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI
If the troughs were produced by the impacts of Rheasilvia and Veneneia, then they must be roughly the same age as the craters. Counting craters is one way to determine age.
“Our work used crater counting methods to explore the relative age of basins and troughs,” says co-author Jupiter Cheng. Since a newly formed body is free from impact craters, its age can be estimated by counting the number of craters present. While this is obviously an imprecise way to determine the absolute age of an asteroid, it is useful in determining the relative age of specific features. If the items have a similar number of hits surrounded by them, they are probably about the same age.
“Our result,” says Cheng, “shows that the hollows and basins have a similar number of craters of different sizes. [sic], indicating that they share the same age. However, the uncertainties associated with counting craters allow hollows to form well after impacts. ”
This chronology corresponds to the explanation proposed by the researcher for the hollows.
Low gravity and troughs
Credit: University of Georgia / NASA / JPL
It has been speculated, says Cheng, that “the hollows are fault-bound valleys with a distinct escarpment on either side that together mark the descent (slip) of a boulder.
However, there is a problem with this theory. It is based on how rocks and debris behave under the force of gravity on Earth; Vesta’s gravitational pull is much less. Indeed, Dawn found Vesta’s gravity to correspond to an iron core with a diameter of 140 miles; that of Earth, by comparison, is about 2,165 miles in diameter.
Cheng notes that “rock can also crack and form such hollows, an origin that has not been considered before. Our calculations also show that Vesta’s gravity is not sufficient to induce surrounding stresses favorable to shallow sliding. Instead, physics shows that the rocks there are favored to crack. ”
Cheng sums up: “Taken as a whole, the overall project offers alternatives to the previously proposed trough origin and the geological history of Vesta, results that are also important for understanding similar landforms on other small planetary bodies. elsewhere in the solar system.
So while remaining consistent with the dominant theory that the impacts caused the troughs, the researchers suggest that they did not cause landslides on Vesta. The impacts cracked it.