Ceres is the largest object orbiting in the asteroid belt nestled between Mars and Jupiter. So when NASA’s Dawn spacecraft reached the dwarf planet in March 2015, the science team expected to find an object with extensive cratering. Large and small. They found plenty of craters, but large impact basins were conspicuously absent.

Computer simulations predicted Ceres should have at least 10 craters larger than 250 miles in diameter. And another 40 at least 60 miles wide. But that’s not what Dawn found. The spacecraft spotted just 16 craters larger than 60 miles and none larger than 177 miles wide.

The biggest defined impact basin is dubbed the Kerwan Basin and is just 177 miles across. The lack of large craters on Ceres doesn’t make sense. Arizona State University’s David Williams explains why.

“Even Vesta, only about half of Ceres’ size, has two big basins at its south pole, But at Ceres, all we saw was the Kerwan Basin, just 177 miles in diameter,” says Williams. “That was a big red flag that something had happened to Ceres.”

Ceres was around in the early days of the solar system when objects were routinely slamming into one another. The pieces larger than Ceres ultimately formed the planets. Ceres and the rest of the asteroid belt are the left-overs.

The 177-mile Kerwan Basin is much smaller than a 300-mile crater on Vesta. And Vesta is just half its size.

Ceres largest crater basin

Large depressions and Ceres’ makeup

The mystery of Ceres’ missing large craters may lie in three planitae, large depressions that measure up to 500 miles across. Younger, smaller craters are within each one suggesting the depressions could be what’s left from much larger impacts.

Jupiter’s Great Hot Spot

How do we go from clear impact craters to large depressions? Williams says it’s all about Ceres’ composition.

“If Ceres were highly rocky, we’d expect impact craters of all sizes to be preserved,” Williams explains. “Remote sensing from Earth, however, told us even before Dawn arrived that the crust of Ceres holds a significant fraction of ice in some form.”

Combine ice and the salt Dawn discovered and the crust could weaken and change the dwarf planet’s topography over time.

“Plus we do see evidence of cryovolcanism — icy volcanism — in the bright spots found scattered over Ceres, especially in Occator Crater.”

Another recent study pointed to Ceres having interior hydrothermal activity.

Dr. Simone Marchi, a senior research scientist in SwRI’s Space Science and Engineering Division, says Ceres’ largest craters were “obliterated beyond recognition over geological time scales, likely the result of Ceres’ peculiar composition and internal evolution.”

Dawn’s data is revealing the mysteries of Ceres slowly, but surely. And its study of the dwarf planet will continue. Earlier this month, NASA announced Dawn would remain in orbit around Ceres instead of heading to the asteroid Adeona. That means Dawn will be perfectly positioned to study Ceres as it gets closer to perihelion (the closest point to the sun).

Today, Dawn circles Ceres every 5.4 hours at an altitude of just 240 miles above the surface.

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