According to recent data collected by NASA’s Dawn spacecraft, that has been orbiting Ceres since March of 2015, the dwarf planet should have at least 10 to 15 large impact craters larger than 250 miles (400 kilometers) in diameter across its surface, and around 40 craters larger than 60 miles (100 kilometers) wide. The strange thing about Ceres, however, is that it doesn’t. What Dawn actually found was that Ceres only had 16 craters larger than 60 miles, and none larger than 175 (280 kilometers) in diameter.

So, what happened to the craters on Ceres? Well, therein lies the mystery.

Prior to flying into orbit around Ceres last year, Dawn spent 14 months orbiting Vesta, a heavily cratered protoplanet near Ceres, roughly half the size of the dwarf planet. Using Vesta as a model, Simone Marchi, a scientist at the Southwest Research Institute’s Space Science and Engineering Division in Colorado, created computer simulations of asteroid collisions Ceres should have endured over its 4.5 billion year lifespan, reports the Guardian.

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

The computer simulations showed that Ceres should have had some incredibly large impact craters pockmarking its surface, what scientists found, however, was a nearly smooth surface, dotted only by several small craters. Not at all what one would expect for a celestial body that has spent billions of years in an asteroid belt.

The mystery of the missing craters on Ceres has baffled scientists since Dawn initially dropped into orbit last year, though several hypotheses about the mystery abound. Originally, scientists hypothesized that Ceres could have formed farther out in the solar system, outside of the asteroid belt, near Neptune, but migrated to its current location. This theory was later disregarded however, as scientists determined that even if Ceres had formed elsewhere before migrating to its present location later, it still should have had a vast amount of large impact craters.

One theory about the missing crater mystery is that in its past, Ceres was incredibly geologically active with cryovolcanoes — also known as ice volcanoes, that erupt molten ice, rather than rock. According to NASA, These cryovolcanoes could have erupted extensive amounts of molten ice across the surface of Ceres, essentially re-burying pre-existing large impact craters, and resurfacing the dwarf planet. Smaller asteroid impacts would then create new craters on top of the resurfaced larger impact sites.

Another theory has to do with the “peculiar composition” of the dwarf planet Ceres, says Marchi.

“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres’ peculiar composition and internal evolution.”

Evidence sent back from Dawn shows that upper layers of Ceres contain ice and salt just beneath the planet’s surface. Over time, the upward flow of frozen brine could have initiated a process known as viscous relaxation — when an area dominated by ice and a lower density material, such as salt, causes the topography to “relax” or smooth out — could have flattened out the cratered surface of Ceres. “It is as though Ceres cures its own large impact scars and regenerates new surfaces, over and over,” Marchi says.

Though at the moment the case of the disappearing craters on Ceres remains a mystery, researches will have plenty of time to solve it, as NASA has announced that it will extend Dawn’s mission on Ceres through 2019.

Source: Inquisitor