How did Ceres heal old impact craters?

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Ceres’ largest impact crater, Kerwan near the edge of a large depression that could be the remains of an older impact. The colour-coding indicates elevation (blue = low; red = high).
NASA/Southwest Research Institute/Simone Marchi

NASA’s scientists have been mystified by the images of dwarf planet Ceres, sent back by the Dawn spacecraft. While they show many craters, they are all small and tiny. There are none of the big old craters they expected from a body that has been hurtling through space for 4.5 billion years.

The Kerwan crater is the largest at 280 kilometres wide, but only another 15 are wider than 100 kilometres.

Modelling based on asteroid collisions with other bodies suggests that Ceres should have up to 15 craters wider than 400 kilometres and another 40 larger than 100 kilometres.

A new study using Dawn data attempts to get to grips with the problem. 

“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres’ peculiar composition and internal evolution,” lead investigator Simone Marchi, from the Southwest Research Institute in Colorado, says of the research that was published in the journal Nature Communications.

“Whatever the process or processes were, this obliteration of large craters must have occurred over several hundred millions of years.”

While it has few old craters, Ceres does have three large depressions called “planitiae”. They are covered with craters caused by relatively recent impacts, but the planitiae themselves could be the remains of bigger, older impacts.

One, Vendimia Planitia, lies just north of Kerwan and clearly formed much earlier than that crater.

Scientists believe the most likely explanation for the lack of impact marks is due to Ceres’ internal structure that is thought to contain layers of ice and salts near the surface. Being less dense than rock, ice or salts could “relax” after any impact and smooth out the surface.

Another theory is that cryogenic materials from cryovolcanic activity in the past could have buried the older, large craters. 

Either way, “somehow Ceres has healed its largest impact scars and renewed old, cratered surfaces”, Marchi said. 

Dawn has been orbiting Ceres since March 2015.

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