Scientists have found a bit of a mystery at the dwarf planet Ceres. Yes, there are those intriguing bright spots inside numerous craters, the mystery that has mostly been solved, as being made of bright salts, likely leftover from a briny solution of sodium carbonate and ammonium chloride. But a new puzzle involves the craters themselves. In the rough and tumble environment of the asteroid belt, ancient Ceres was certainly pummeled by numerous large asteroids during its 4.5 billion-year lifetime. But yet, there are just a few large craters on Ceres.
How could that be?
“It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over,” said Dr. Simone Marchi, a senior research scientist at the Southwest Research Institute.
Ceres has lots of little craters, but the Dawn spacecraft, orbiting Ceres since early 2015, has found only 16 craters larger than 100 km, and none larger than 280 km (175 miles) across. Scientists who model asteroid collisions in our Solar System predicted Ceres should have amassed up to 10 to 15 craters larger than 400 kilometers (250 miles) wide, and at least 40 craters larger than 100 km (62 miles) wide. By comparison, Dawn’s other target of study, the smaller asteroid Vesta, has several large craters, including one 500 kilometers (300 miles) in diameter, covering almost the entire south pole region.
While they aren’t visible now, the scientists say there are clues that large impact basins may be hidden beneath Ceres’ surface.
“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, likely the result of Ceres’ peculiar composition and internal evolution,” Marchi said.
There are hints of about three shallow depressions around 800 km (500 miles) wide, and Marchi said they could be what are called or planitiae, or ancient impact basins, left over from large collisions that took place early in Ceres’ history.
There are a few reasons why the big craters have been erased, and the scientists now have to figure out which reason or combination of reasons best explains their findings. One reason could be because large amounts of water or ice in Ceres’ interior, which has long been suspected. Because ice is less dense than rock, the topography could “relax” over time — just like what happens if you push on your skin, then take the pressure off, and it relaxes back to its original shape. The scientists said that over geological timescales of several million years the water or ice would slowly flow and the craters would smooth out.
Additionally, recent analysis of the center of Ceres’ Occator Crater — where the largest bright areas are located — suggests that the salts found there could be remnants of a frozen ocean under the surface, and that liquid water could have been present in Ceres’ interior.
Another reason could be hydrothermal activity, such as geysers or cryovolcanoes, which could have flowed across the surface, possibly burying pre-existing large craters. Smaller impacts would have then created new craters on the resurfaced area.
And then, all the smaller, later impacts could have erased the bigger older impact basins. But if that were the case, the older basins would seemingly be more visible than they are now.
“Regardless of the specific mechanism(s) for crater removal, our result requires that large crater obliteration was active well after the late heavy bombardment era, or about 4 billion year ago. This conclusion reveals that Ceres’ cratering record is inextricably linked to its peculiar composition and internal evolution,” Marchi said.
And finding out more about Ceres’ interior is one of the more intriguing aspects of Dawn’s continued mission there.
Marchi is lead author of the paper, “The Missing Large Impact Craters on Ceres,” published in the July 26, 2016, issue of Nature Communications.
Sources: SwRI, JPL
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