Ever since NASA’s Dawn spacecraft started snapping pictures of the dwarf planet Ceres in the Asteroid Belt in 2015, scientists have been perplexed by the space rock’s relatively smooth surface; the object should have way more impact craters than it does now. To get to the bottom of this mystery, scientists have used computer simulations to model how Ceres most likely evolved over time, showing just how many different types of craters the dwarf planet should have on its surface. Many of the craters predicted in the models have seemingly been erased, meaning that some kind of major geological activity has wiped them away.

Many of the craters predicted in the models have seemingly been erased

The results of the computer modeling, published today in the journal Nature Communications, confirm that Ceres is rather unique. Scientists expected the dwarf planet to be similar to the heavily cratered Vesta — a large asteroid in the Asteroid Belt that Dawn visited before putting itself in orbit around Ceres. "Even Vesta, only about half of Ceres' size, has two big basins at its south pole," said Dawn researcher David Williams, director of the Ronald Greeley Center for Planetary Studies at Arizona State University. During the 4.5 billion years that Ceres has been around, the dwarf planet should have been hit by a lot of rocks and they should have left a lot of marks, according to Williams.

And the models agree. According to the computer simulations, Ceres should have at least 40 craters that are larger than 62 miles wide, and somewhere between 10 and 15 craters that are larger than 250 miles across. But that’s not what the surface of Ceres shows. Pictures from Dawn indicate that the dwarf planet has only 16 craters that are bigger than 62 miles in diameter. And Ceres’ largest crater, Kerwan basin, only reaches around 177 miles across.

"It is as though Ceres cures its own large impact scars."

Scientists have a few ideas as to how these craters disappeared. Data from Dawn indicates that there are three large depressions on Ceres, measuring up to 500 miles wide, that are filled with smaller craters. The researchers think these depressions — also known as planitiae — may be huge impact basins that have been slowly erased and covered up by newer craters. "It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over," Dawn research Simone Marchi, a senior research scientist in SwRI's Space Science and Engineering Division, said in a statement.

It’s possible that salty ice underneath Ceres’ surface has weakened the crust, causing it to smooth out over time. Plus, there are signs of cryovolcanoes — volcanoes that spew molten ice instead of hot lava. These may help spread ice on to the surface of Ceres, slightly covering up older impact craters. Though scientists aren’t exactly sure the root cause of the craters’ disappearance, they may get answers soon enough. NASA recently extended the Dawn mission, so the spacecraft will remain in orbit around Ceres. Further analysis of Dawn’s data may hold the answers to how the dwarf planet smoothed out.