�鶹��ý

Hiding 800-kilometre scars isn’t as tricky as you might think – if you’re a dwarf planet, all you need is a mud facial scrub.

The absence of such large impact craters on the dwarf planet Ceres, which is located in the asteroid belt between Mars and Jupiter, has puzzled planetary scientists since NASA’s Dawn probe arrived there in March 2015. The latest mapping images from Dawn, still in orbit around the dwarf, may just provide the answer.

Objects in the asteroid belt are the fragmented leftovers of clashing planetesimals, the building blocks of planets. Studying their impact history can therefore yield important clues to the formation and evolution of our solar system.

Ceres is by far the largest object found in the asteroid belt with a diameter of 939 km, and has witnessed most of the evolution of our solar system, so should have a rich history of collisions. Models developed by and his colleagues at the Southwest Research Institute in Colorado indicate that Ceres should have around 10 to 15 impact craters at least 400 km wide alongside its many smaller craters, but early pictures from Dawn did not show any craters larger than 280 km wide.

Now Marchi’s team have used the latest data from Dawn in an attempt to unravel the mystery. The team combined multiple hi-res images to create a three-dimensional model of Ceres’s outer layer, allowing them to focus in on the nuances of the surface. They discovered that many of Ceres’s larger craters have been obliterated beyond recognition.

“Our detailed simulations were telling us that these craters should be there when they simply weren’t, which was very puzzling,” says Marchi. “Using the topography models from Dawn we confirmed that there were no defined large craters. What we can see, however, is large-scale depressions – some almost 800 km across and 4 to 5 km deep – which may be the remains of impact craters that have been wiped off of the surface over billions of years.”

Marchi and his team suggest that Ceres’s unusual internal composition and evolution is to blame for the missing craters. It’s thought that beneath its surface lies a slushy cocktail of low viscosity substances such as ice and clay that form a kind of mud, which permits the outer layer to shift and relax, potentially smoothing out any large craters.

The results confirm Ceres is a long-term resident of the asteroid belt, says at Imperial College London.

“Some had theorised that Ceres actually slotted into the asteroid belt late from the outer solar system, and hence had not sustained the impacts necessary for large craters to form” he says. “This new research discredits that idea, so it’s an important step towards bettering our understanding of this unusual dwarf planet.”

Journal reference: Nature Communications, DOI: