Might the dwarf planet Ceres have permanent ice deposits? Using NASA’s Dawn mission, a team led by Norbert Schorghofer, an astronomer at the University of Hawaiʻi at Mānoa Institute for Astronomy, has identified permanently shadowed regions on the dwarf planet Ceres. Most of these areas likely have been cold enough to trap water ice for a billion years, suggesting that ice deposits could exist there now.
“The conditions on Ceres are right for accumulating deposits of water ice,” said Schorghofer. “Ceres has just enough mass to hold on to water molecules, and the permanently shadowed regions we identified are extremely cold—colder than most that exist on the moon or Mercury.”
Permanently shadowed regions do not receive direct sunlight but get indirect radiation. These regions are typically located on a crater floor or along a section of the crater wall facing the pole. If the temperature stays below minus 243 degrees Fahrenheit (minus 153 degrees Celsius), the area is a cold trap—a good place for water ice to accumulate and remain stable. Cold traps were predicted for Ceres, but this is the first time they have been identified.
Investigating Ceres’ northern hemisphere
In this study, Schorghofer and colleagues studied Ceres’ northern hemisphere. Images from Dawn’s cameras were combined to map the dwarf planet’s shape, showing craters, plains and other features in three dimensions. Utilizing this input, a sophisticated computer model developed at NASA’s Goddard Space Flight Center, Greenbelt, Maryland, was used to determine which areas receive direct sunlight, how much solar radiation reaches the surface, and how the conditions change over the course of a year on Ceres.
The researchers found dozens of sizable, permanently shadowed regions across the northern hemisphere. The largest one is inside a 10-mile-wide (16-kilometer) crater located less than 40 miles (65 kilometers) from Ceres’ north pole. Taken together, all of these regions on Ceres cover about 695 square miles (1,800 square kilometers)—much less than 1 percent of the surface area of the northern hemisphere.
The team expects the permanently shadowed regions on Ceres to be colder than those on Mercury or the moon. That’s because Ceres is quite far from the sun, and the shadowed parts of its craters receive little indirect radiation.
Cold traps and stable ice
“On Ceres, these regions act as cold traps down to relatively low latitudes,” said Erwan Mazarico, a Dawn guest investigator at Goddard. “On the moon and Mercury, only the permanently shadowed regions very close to the poles get cold enough for ice to be stable on the surface.”
The situation on Ceres is quite similar to that on Mercury, where permanently shadowed regions account for roughly the same fraction of the northern hemisphere. The trapping efficiency—the ability to accumulate water ice—is also comparable.
By the team’s calculations, about 1 out of every 1,000 water molecules generated on the surface of Ceres will end up in a cold trap during a year on Ceres (1,680 days). That’s enough to build up thin but detectable ice deposits over 100,000 years or so.
“While cold traps may provide surface deposits of water ice as have been seen at the moon and Mercury, Ceres may have been formed with a relatively greater reservoir of water,” said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles. “Some observations indicate Ceres may be a volatile-rich world that is not dependent on current-day external sources.”
The findings are available online in Geophysical Research Letters.