The origin and composition of the dwarf planet Ceres, the most massive body in the asteroid belt, have long been the subject of debate. As wide as the state of Texas, the nearly spherical body has a density less than half that of Earth’s, suggesting either that Ceres could host large volumes of ice in its mantle or that it may be composed of very porous silicate minerals. The potential presence—or absence—of ice on Ceres, as well as other small planetary bodies, may also affect whether the remnants of asteroids and other cosmic impactors remain on the surface or bounce back off into space, according to a new study by Daly and Schultz.
Since previous estimates of how much impactor material remains on the surface have focused on rocky targets denser than Ceres, the researchers conducted a series of experiments at the NASA Ames Vertical Gun Range to examine the effects of high-speed collisions on more porous, Ceres-like targets. Using a 4-meter-long cannon that can launch projectiles at speeds of up to 22,000 kilometers per hour, the scientists simulated impacts of both metallic and stony meteorites into frozen surfaces whose properties represented the two possibilities for Ceres’s surface: ice rich or porous silicate.
The results show that regardless of the angle of impact, the type of impactor, or the composition of the surface, a large proportion of the projectiles remained in and near the impact craters, providing strong experimental evidence that Ceres’s surface is extensively contaminated with cosmic debris. Because the team observed that more of the projectile remained in and near the crater when it impacted the snowy targets, they concluded that an ice-rich surface will accumulate cosmic debris even more efficiently than one composed of porous silicates.
The findings also have implications for understanding the evolution of Ceres’s surface. If high-speed impactors excavate ice, it could rapidly sublimate, a process that would concentrate impactor material on the body’s surface, according to the researchers. If, however, there is minimal ice present, the team predicts the impact debris will be more evenly distributed throughout the depth of the unconsolidated surface layer. High-resolution images scheduled to be taken by the Dawn spacecraft after it drops into its final orbit around the dwarf planet later this year should provide the first visual evidence that will help to determine the extent to which Ceres’s surface is dominated by exotic or locally derived material. (Geophysical Research Letters, doi:10.1002/2015GL065601, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Objects that slam into Ceres remain on its surface, Eos, 97, doi:10.1029/2016EO046215. Published on 19 February 2016.