You thought those bright spots on dwarf planet Ceres were cool? That was only the half of it. Ceres is also covered in dark spots, craters that -- because of their position -- never see the light of day. Now, astronomers have discovered that at least one and perhaps many of these shadowy regions are filled with water ice.
Ceres' bright spots have captivated imaginations for more than a year, ever since NASA's Dawn spacecraft began approaching the dwarf planet in the spring of 2015. Initially, scientists suspected the bright spots were made of ice, but the first good spectral data from Dawn revealed otherwise. The bright spots are actually giant piles of salt, dashing our hopes that Ceres might one day serve as an interplanetary gas station.
But now, evidence has emerged that water ice could be widespread on the surface of Ceres, after all. It's just been hiding from us, according to Norbert Schorghofer of the University of Hawaii, who presented his research on Ceres' "persistently shadowed regions" in a talk at the American Geophysical Union Conference this morning. The research, which was led by Thomas Platz of the Max Planck Institute, also appears today in Nature Astronomy.
"There are over 600 persistently shadowed regions on Ceres," Schorghofer said, noting that these shadowlands are mostly associated with craters near the dwarf planet's poles. "I call it Ceres' darkest secret."
Because persistently shadowed regions, by definition, get no sunlight, they're considered "cold traps," meaning they suck ice and water vapour in and keep it locked away forever. That is exactly what has happened in at least one dark crater near Ceres' north pole.
Map showing permanently shadowed regions around Ceres' north pole. Image: Platz et al. 2016
During its flyovers of the north pole, the Dawn's Framing Camera was able to capture enough scattered light from this crater -- unimaginatively called PSR2, though I prefer Narnia -- to "see" inside of it. What it saw was a giant sheet of water ice.
So far, PSR2 is the only dark spot which we've been able to peer into, but there could be others like it. "We assume other bright deposits in shadowed regions are water ice as well," Schorghofer said. Other planets like Mercury feature water ice in virtually every cold trap on their surface.
Where the ice on the surface of Ceres originated is still a matter of debate, although the dark spots may be connected to the bright spots. Scientists suspect that the salt heaps found in craters like Occator (shown in the video above) were excavated from a subsurface brine layer during ancient impacts. Initially, those impacts would have brought a mix of water and ice to the surface. It's possible that over time water escaped the bright spots, bounced around the planet, and eventually got sucked into Ceres' cold traps.
It's no surprise that Ceres' dazzling bright spots captured our attention during the Dawn mission. But never let anyone say they're the only cool thing about Ceres. After all, as soon as humans start travelling beyond Earth's orbit, water is going to become the most precious resource in the solar system.