We know there’s ice inside Ceres, but it may be chillin’ on the surface, too. Scientists with NASA’s Dawn mission have identified permanently shadowed craters on the dwarf planet that have likely been cold enough to trap water ice for a billion years.
“The conditions on Ceres are right for accumulating deposits of water ice,” said Norbert Schorghofer, a Dawn guest investigator at the University of Hawaii. “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.”
Ceres’ polar regions remain in permanent shadow because its axis is only tilted 4°, much less that Earth’s 23.5°. In other words, Ceres rotates nearly straight up and down rather than tilted over on the side like Earth and several of the other planets. Likewise Mercury (2.1°) and the moon (1.5° to the plane of Earth’s orbit), both of which feature shadowed craters in their polar regions that contain ice. All parts of a well-tipped orbiting body including the insides of any polar craters get exposed to sunlight; polar craters on bodies that spin nearly straight up and down lie in perennial shadow and can provide a haven for ice.
Permanently shadowed regions don’t receive direct sunlight and are typically located on the crater floor or along a section of the crater wall facing toward the pole. While there’s indirect sunlight reflecting from crater rims, without direct sunlight, temperatures can remain well below zero. If the temperature stays below about –240° F (–151°C), the permanently shadowed area is a cold trap, a place for water ice to accumulate and remain stable.
Permanent Shadows on Ceres
In their study, Schorghofer and team studied Ceres’ northern hemisphere, which was better illuminated than the south. Images from Dawn’s cameras were combined to yield the dwarf planet’s shape, showing craters and their depths in three dimensions. Scientists fed the information into a sophisticated computer model 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.
Bingo! The researchers found dozens of sizable permanently shadowed regions across the northern hemisphere. The largest one is inside a 10-mile-wide (16 km) crater located less than 40 miles (65 km) from the north pole. The team expects the permanently shadowed regions on Ceres to be colder than those on Mercury or the moon because Ceres is considerably farther from the sun, and the shadowed parts of its craters receive little indirect radiation.
“On Ceres, these regions act as cold traps down to relatively low latitudes,” said Erwan Mazarico, a Dawn guest investigator. “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.”
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 (1,682 days). That’s enough to build up thin but detectable ice deposits over 100,000 years or so. It’s thought that Mercury and the moon received their water from comets and meteorites crashing down on the surface. Most of that H2O would have vaporized upon impact, but models show that small amount would have migrated to the poles. Ceres on the other hand already has ice — it’s estimated that 25% of its mass is made of water — so the dwarf planet itself may be the source of its cratery water reserves. You can read more about the new findings here.
Although Dawn completed its primary mission on June 30th, the probe will remain at Ceres through March or April 2017. The Dawn team had proposed an extended mission to asteroid 145 Adeona with a flyby date of May 2019, but a review panel decided instead to keep doing science at Ceres especially as the dwarf planet approaches perihelion, its closest point to the sun.
While the news undoubtedly disappointed the Dawn team, NASA approved plans for an extended mission to send the New Horizons spacecraft, which flew by Pluto last July, to the distant Kuiper Belt asteroid 2014 MU69 on Jan. 1, 2019.