When Nasa's Dawn spacecraft arrived to orbit the dwarf planet Ceres in March last year, mission scientists expected to find a heavily cratered body resembling the protoplanet Vesta, Dawn's previous port of call.
Instead, as the spacecraft drew near, a somewhat different picture began to emerge. Ceres is almost entirely devoid of all impact basins.
In particular, the absence of craters larger than about 175 miles (280km) on the surface of dwarf planet – located in the asteroid belt between Mars and Jupiter – are notable. The dwarf planet is only pockmarked with smaller impact craters less than 100 miles (161km) wide.
Computer models, designed to plot collisions during the evolution of planets, predict that between 10 and 15 craters larger than 250 miles (400km) should have formed on Ceres over the past 4.55 billion years.
Similarly, comparison with the asteroid Vesta suggests at least six to seven craters larger than 250 miles should be present today. This has baffled astronomers for years and their conspicuous absence was unexplained – until now.
Simone Marchi and colleagues from Southwest Research Institute and Arizona State University used data from the Nasa Dawn mission on Ceres to study global crater distribution. They ran collisional simulations of the evolution of Ceres, assuming it held its current position in the main belt over the past 4.55 billion years.
Their models predict the presence of craters over 62 miles to 93 miles (100km to 150km) in size, but the authors found these are mostly absent, and they believe a significant number of large craters have been "obliterated beyond recognition over geological timescales."
This likely occurred as a result of Ceres' peculiar composition and, how it evolved internally. In particular, the researchers say that cryovolcanoes may have been responsible for smoothing the crater rims. Unlike standard volcanoes, cryovolcanoes – also known as ice volcanoes – typically erupt water, ammonia or methane, instead of molten rock.
This is referred to as cryomagma or ice-volcanic melt and the presence of liquid water on the surface, as well as the evolution of such cryovolcanoes, could have eroded and smoothed the rocks in and around the craters. In addition, rims could also be obscured by the high density of smaller craters.
The authors also present evidence for the presence of one — possibly two — depressions 500 miles (800km) in diameter that may be relict impact basins from large collisions that took place early in Ceres' history.
"Regardless of the specific mechanisms for crater rim removal, our result requires that large crater obliteration was active well after the late heavy bombardment," said the researchers. "This conclusion reveals that Ceres' cratering record is inextricably linked to its peculiar composition and internal evolution."
The findings are published in the journal Nature Communications.