Data from a NASA spacecraft suggest that Ceres—a 950-kilometer-wide dwarf planet estimated to contain about one-third of all the mass in our solar system’s asteroid belt—occasionally sports a cloud of ice and dust within one of its largest craters. Although another study released today hints that Ceres has little if any water on its exterior, together the findings suggest that the small world may harbor significant amounts of water ice just below its surface.
One of Ceres’s biggest surprises—first spotted by NASA’s Dawn spacecraft as it approached the dwarf planet in February—is its unusually shiny spots. Some of them are several kilometers across, and the largest are located within large craters. Most of the cerean surface is as dark as asphalt, reflecting only about 9% of the sunlight that falls on it. But new data show that one 10-kilometer-wide bright spot inside the Occator crater reflects about 25%, says Andreas Nathues, a planetary scientist at the Max Planck Institute for Solar System Research in Göttingen, Germany.
Early reports suggested that Occator’s bright spot could be emitting water vapor into space—vapor that could loft dust above the surface and also condense into bright particles of ice.
The team’s new data, reported online today in Nature, back up this notion. At surface temperatures ranging between –93°C and –33°C in the daytime, about one-third of any water molecules sublimating into vapor wouldn’t have the speed needed to escape the dwarf planet’s gravity, the team found. So, the water molecules would likely concentrate inside the crater bowl during the daytime and condense as frost at night.
The sunlight-driven sublimation of water ice inside Occator “makes the most sense to explain what might be happening,” says Andrew Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who was not involved in the study.
Indeed, Nathues and his colleagues argue that Ceres could be experiencing something similar to the jetting that occurs on the sunlit surfaces of comets. (Although several icy moons of Jupiter and Saturn spew plumes of ice particles into space, Ceres isn’t flexed and heated by gravitational interactions with other celestial bodies, and so the energy driving its activity is suspected to come from another source.)
Water ice is only one possibility for the material in the bright spots, Nathues says. Iron-poor clays and certain mineral salts are other candidates, but none of those materials would likely sublime into space when lit by the sun. The apparent clincher: the European Space Agency’s Herschel Space Observatory has spotted diffuse plumes of water vapor emanating from two regions on Ceres—and one of those regions includes Occator, Nathues says.
The presence of large amounts of water would help explain why Ceres is relatively light. Because the dwarf planet has an overall density of about 2.2 grams per cubic centimeter, previous studies have suggested that Ceres is about 25% water or other light materials.
Nevertheless, another study, also reported online today in Nature, challenges the idea that there’s water on Ceres—at least on its surface. That analysis uses spectral data that Dawn gathered in a broad view of the dwarf planet, says Maria Cristina De Sanctis, a planetary scientist at the Institute for Space Astrophysics and Planetology in Rome, who led the effort. Overall, the surface of Ceres seems to be made up of silicate minerals whose molecules have bonded with large amounts of ammonia, her team reports. The discovery of the ammonia, a typical cometary material that is only stable in the outer solar system, suggests that Ceres may have formed much farther out and moved into the asteroid belt later.
“But these results don’t contradict each other,” De Sanctis notes. That’s because her team’s data came from the planet as a whole and were gathered at a distance, not from close orbit. So, she explains, any signs of water emanating from regions such as Occator would likely be overwhelmed by the spectra from the rest of the surface materials—similar to the way the light from a candle would be difficult to discern if it were next to a mighty spotlight.
Nathues agrees that the two teams’ results don’t necessarily disagree. Because Occator’s rim and crater walls are sharp and well-defined, the crater appears relatively fresh, he notes. It’s likely that the impact that excavated Occator also exposed deep layers of water ice—ice that’s now spewing into space at low rates, but quickly enough to form the haze that his team observes. Based on the number of smaller craters inside Occator, he and his team estimate that the crater is about 78 million years old.