Ceres, the largest object in the asteroid belt between Mars and Jupiter, should be riddled with craters.

But when Nasa's Dawn spacecraft started orbiting the dwarf planet last year, scientists were surprised to find it was relatively smooth.

In a study published yesterday in Nature Communications, researchers conclude that something mysterious must have erased the marks.

Given the pockmarked appearance of most asteroid belt objects - including Vesta, the asteroid that Dawn visited before arriving at Ceres - scientists expected many large craters to pepper their planetary target.


"Ceres is thought to have formed at the dawn of the solar system, some one to ten million years or so after the onset of formation," lead researcher Simone Marchi at Southwest Research Institute told the Guardian.

"Thus, Ceres is a witness to the tumultuous early days where collisions were much more frequent and violent than today."

Instead, while many small craters mark up the surface, the largest impact crater they found was just 280km across. In contrast, Vesta - half the size of Ceres - has a whopper of an impact crater some 800km wide.

When they crunched the numbers, Marchi and the rest of the study team found that there was a less than 2 per cent chance of Ceres getting through 4 billion odd years with the assortment of craters it seems to sport.

"Thus, it points to something special about Ceres, something that we could not have guessed," Marchi told Space.com.

With the help of computer models and data from Dawn, Marchi and his colleagues conclude that the "peculiar composition and internal evolution" of Ceres must have erased larger impact basins somehow.

The researchers found signs of multiple large, circular craters hiding just below a surface smoothed and marked up with smaller impacts.

"It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over," Marchi said in a statement.


The researchers can't be sure yet exactly how Ceres managed to turn back the clock and achieve this youthful look, but they're floating two ideas: Briny, subsurface liquids and ice volcanoes. It could be that this icy, salty layer just below the surface is viscous and flowing.

This substance might allow the craters to smooth out over time as the ground settled, or it could even have flowed over the crust and formed an entirely new surface. Alternatively, ice volcanoes - which may have been more active when the planet was younger and warmer - could have spewed out roiling water that resurfaced the globe.