The mystery of the missing craters on Ceres

Via theguardian.com by Ian Sample

When NASA's Dawn spacecraft arrived at Ceres last year the images it beamed back were puzzling. The nearest dwarf planet to Earth was missing the massive craters that astronomers thought would heavily scar the surface.

As the Dawn probe swung around the body, the largest in the asteroid belt, its cameras recorded pictures of pockmarked terrain. But even though small craters dotted the Cerean surface, none were larger than the 175-mile-wide dent that is the Kerwan impact crater.

This left scientists scratching their heads. Models of collisions in the main asteroid belt between Mars and Jupiter held that Ceres should have accumulated 10 to 15 craters more than 250 miles across in its 4.5 billion year history. Instead, it had none. And rather than hosting at least 40 craters more than 60 miles wide, Dawn had spotted only sixteen.

To investigate, Simone Marchi at Southwest Research Institute in Colorado ran a series of computer simulations of the impacts Ceres might endure over its lifetime. They showed that the dwarf planet was missing craters all the way down to pits 100 miles wide. Only the smaller craters were found in the expected numbers.


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

From other data collected by Dawn, the scientists reasoned that large craters on the 620-mile-wide Ceres had been “obliterated beyond recognition” over geological time scales. “For the most part, they are totally obscured,” said Marchi, whose report appears in Nature Communications.

The disappearance of large impact craters has “huge implications”, Marchi adds. The report suggests that the icy interior of Ceres is mobile to some extent, allowing the ground to refresh itself over long periods of times. Another possibility is that Ceres was geologically active in the past, with “cryovolcanoes” blasting out water and ice that gradually erased large craters from the surface.

Dawn’s images of Vesta, another giant asteroid, revealed immense impact craters, including one that covered almost an entire side of the 300-mile-wide body. On closer inspection of the Ceres images, Marchi and his team believe they may have found the remains of huge craters, one being the Vendimia Planitia depression that reaches 500 miles across the surface.

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