July 27, 2016 – A team of scientists led by Simone Marchi of the Southwest Research Institute (SwRI) in Boulder, Colorado, made a puzzling observation while studying the size and distribution of craters on the dwarf planet Ceres. Using computer simulations based on data from NASA’s Dawn mission, the study suggest that Ceres has experienced significant geological evolution, possibly erasing the large basins.
Ceres is the largest object in the tumultuous Main Asteroid Belt between Mars and Jupiter. Collision models predicted Ceres should have accumulated up to 10 to 15 craters larger than 400 kilometers (250 miles) wide, and at least 40 craters larger than 100 km (62 miles) wide. Instead, NASA’s Dawn spacecraft found only 16 craters larger than 100 km, and none larger than the 280 km (175 miles) across.
Crater size and distribution offer planetary scientists important clues to the age, makeup, and geologic history of planets and asteroids. Ceres is believed to have originated about 4.5 billion years ago at the dawn of our solar system. It grew in size through a history of accretionary collisions of smaller bodies. Some of its largest siblings were subsequently incorporated into larger objects, such as planets. Today, Ceres and Main Belt asteroids remain as the leftovers of the planet-building process.
Although Ceres endured the most violent phase of the solar system’s collision-prone past, images of its surface taken by the Dawn spacecraft showed plenty of small impact craters, but the largest well-defined crater is only about 280-km in diameter.
This defied most models of crater size and distribution and is at odds with what is known from previously imaged asteroids. For example, Dawn visited Vesta for 14 months from 2011 to 2012 and the images revealed huge craters. Although Vesta is only half the size of Ceres, it has a well-preserved 300-mile- (500-kilometer) -wide crater called Rheasilvia, where an impacting asteroid knocked out a huge chunk of the body. This and other large craters suggest that Vesta has not had processes at work to smooth its surface, perhaps because it is thought to have much less ice.
“The ability to compare these two very different worlds in the asteroid belt — Vesta and Ceres — is one of the great strengths of the Dawn mission,” Marchi said.
A closer look at Ceres’ topography revealed subtle clues to a possible solution. Up to three roughly circular, shallow basins as much as 800 km (500 miles) wide may lie hidden beneath a surface subsequently marked with small craters. One of them, called Vendimia Planitia, is a sprawling area just north of Kerwan crater, Ceres’ largest well-defined impact basin. Vendimia Planitia must have formed much earlier than Kerwan.
“These depressions — or planitiae — may be ‘relict’ impact basins, left over from large collisions that took place early in Ceres’ history,” Marchi said.
“We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, likely the result of Ceres’ peculiar composition and internal evolution,” said lead investigator Dr. Simone Marchi, a senior research scientist in SwRI’s Space Science and Engineering Division.
The scientists think Ceres’ missing large craters may have been erased over time as a deep subsurface ice-rich layer or low viscous material caused the crater rims and bowls to relax, or that cryolava may have flowed across the surface. Past hydrothermal activity, which may have influenced the salts rising to the surface at Occator, could also have something to do with the erasure of craters. If Ceres had widespread cryovolcanic activity in the past — the eruption of volatiles such as water — these cryogenic materials also could have flowed across the surface, possibly burying pre-existing large craters. Smaller impacts would have then created new craters on the resurfaced area.
“Regardless of the specific mechanism(s) for crater removal, our result requires that large crater obliteration was active well after the late heavy bombardment era, or about 4 billion year ago. This conclusion reveals that Ceres’ cratering record is inextricably linked to its peculiar composition and internal evolution,” Marchi said.
Marchi is lead author of a paper, “The Missing Large Impact Craters on Ceres,” published in the July 26, 2016, issue of Nature Communications.
Dawn’s mission is managed by JPL for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.