Why are there so few large craters on dwarf planet Ceres?

BOULDER, Colo., July 26 (UPI) -- Life in the asteroid belt isn't conducive to a smooth complexion. Ceres' surface is proof of that. But researchers say the dwarf planet's face isn't nearly as chiseled as they expected.

New analysis reveals a conspicuous absence of large impact craters. Collision models predicted astronomers would find 10 to 15 craters wider than 250 miles and at least 40 craters wider than 62 miles.

Surveys conducted by the Dawn probe tell a different story. New data revealed zero craters with a diameter larger than 175 miles. Just 16 craters wider than 62 miles were identified.

Meanwhile, analysis of neighboring asteroids like Vesta have revealed massive craters, measuring as large as 300 miles wide.

It's unlikely Ceres has avoided large impacts during its 4.5 billion years in one of the most crowded parts of the solar system. Apparently, Ceres has found a way to erase the past.

"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," Simone Marchi, a scientist with the Southwest Research Institute, said in a news release.

Though the surveys haven't identified large craters on Ceres, astronomers have found more subtle depressions stretching 500 miles across. Scientists say these may be the signatures of ancient asteroid collisions.

"These depressions -- or planitiae -- may be 'relict' impact basins, left over from large collisions that took place early in Ceres' history," Marchi explained. "It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over."

Scientists -- who published their latest findings in the journal Nature Communications -- say more research is needed to determine the internal mechanism responsible.

It's possible Ceres' crust features a deep, ice-rich layer or low viscous material that allows the sharp edges of a crater rim to relax over time. It's also possible cryolava once flowed across Ceres' surface, filling in large craters.