Rethinking Ceres’s Physical Properties

Dawn’s images show that Ceres is a slightly ellipsoidal body measuring 965 by 891 km (600 by 554 miles). That’s smaller than we’d assumed, but not by much — so its overall density must be 2.16 g/cm3, about 4% higher than previous estimates. This increases the assumed percentage of water in the dwarf planet. “Ceres is a prototype wet protoplanet, intact from the earliest days of the solar system,” Russell concludes.

This artist concept shows NASA’s Dawn spacecraft above dwarf planet Ceres, as seen in images from the mission. Source: Sky & Telescope

Second, its axial tilt (or obliquity) is about 3° with respect to its orbital plane. That’s the right amount, reversed in direction, from what astronomers had thought. In other words, we had its seasons swapped. Compared to Earth’s 23.4° tilt, an astronaut on Ceres might not even notice the days change length throughout its 4.6-year-long orbit around the Sun. “The obliquity being small it doesn’t make for a big difference in the lighting on the planet,” says Dawn principal investigator Christopher Russell.

The surface of Ceres is peppered with impact craters, though few are very large (Yalode, 271 km across, tops the list). Most fall into three types: simple, central-peak, and central-pit craters. Many show evidence of landslides and flow features, which again provide tantalizing hints of past geologic activity. This theory is further bolstered by observations from ESA’s Herschel observatory, which in 2011–13 found solid evidence for water vapor hovering over specific regions of the big rock.

There’s even a lonely mountain, which has been dubbed “The Pyramid” for its strange, steep-sloped geometry. It measures 30 km across its base and 5 km high. Some sides are stained white. “We don’t understand it,” admits Russell. The Dawn team is still waiting on spectra of the surface to analyze the quasi-faceted peak.

In January 2016, Dawn will be lowered into its closest (and final) vantage point over Ceres, a Low Altitude Mapping Orbit (LAMO) that’s 375 km (235 miles) above the surface. This last level will give scientists their best spectral maps, gravitational measurements, and gamma-ray detections, as well as clearer snapshots of what’s been seen so far.

And some of the features already seen have gotten permanent names. Since Ceres is a god of agriculture in Roman mythology, the naming theme adopted by the International Astronomical Union involves harvest deities. More specifically, craters will be named for gods and goddesses of agriculture and vegetation from world mythology, while other features will be named for agricultural festivals.