A bright spot drifting in the dark Asteroid Belt between Mars and Jupiter, the dwarf planet Ceres becomes more intriguing the more astronomers learn about it. Recently scientists announced that NASA’s Dawn spacecraft in orbit around Ceres had made the first clear detection of water ice there, bolstering the possibility that a liquid ocean lies hidden underneath the hard crust of the dwarf planet’s surface.
Two years ago Dawn spotted hints that water exists at least briefly on the surface of the dwarf planet when the spacecraft detected plumes of water vapor rising above the ground as it approached Ceres. Small patches of liquid water should not survive long on the dwarf planet’s crust, where temperatures cause ice to quickly jump from solid to gas, so scientists concluded that the spray either came from shallow ice just below the surface or had welled up from the deeper mantle layer, which was thought to be made of frozen water ice, in a process known as cryovolcanism. But if the plumes came from beneath, they could indicate that the mantle is liquid rather than ice. "If we have water in liquid form, could we have life?" says planetary scientist Timothy Titus of the U.S. Geological Survey. "It's one of the more exciting things with these plumes, that they could be a potential exobiology target."
As Dawn made its way nearer to the dwarf planet, it spotted bright spots of material on the surface, raising hopes that ice had been found. Further observations revealed the features as salts likely left behind when ice containing them made the leap to gas at some point in the past. Like the plumes, that salt-laced icy material could have originated in a liquid ocean, or formed from ice sublimating after an impact scuffed the surface.
But when the spacecraft finally reached Ceres last spring, it found a far less active place than many hoped to observe. Dawn has not spotted other plumes since its arrival, and the bright spots appear clustered in a single area. Two weeks ago, however, Dawn team member Jean-Philippe Combe of the Bear Fight Institute announced at the Lunar and Planetary Science Conference, that Dawn had seen definitive evidence of water ice on Ceres. Using Dawn's Visible and Infrared Mapping Spectrometer (VIR), Combe and his colleagues probed the top few millionths of a meter of Ceres’s surface by bouncing infrared signals from the spacecraft to the surface and back. By studying how the infrared light was absorbed by the top layers, the scientists determined that a patch of water ice lay on the slope of a crater known as Oxo.
"The spectral features observed at Oxo are suggestive of ice," says Paul Hayne. Although he was not part of Combe's team, Hayne, a planetary geoscientist at the NASA Jet Propulsion Laboratory, is familiar with icy worlds and has modeled how long ice features could last on Ceres. If ices only survive for minutes or even years after being exposed, then either Dawn's detection would have been extremely lucky or ice should be continuously leaking up to the surface, although no other samples have been spotted after nearly a year.
Hayne found that the life span of ice on Ceres depends not only on how far it lies from the cold poles but also on how thick and bright the material is. Contrary to previous expectations, ice on Ceres’s surface could last for thousands of years—brief in geologic and astronomical time but long enough that Dawn would not have had to pass by at the exact moment the ice was exposed. "If it's more like a block of ice or some icy bedrock that's been exposed, then [the timescale] would be more like a thousand years," Hayne says.
So where did the ice come from? Combe reported that Oxo's ice lays on the edge of the crater, on a downhill slope pointed toward Ceres’s northern pole. The crater has existed for about eight million years, far longer than ices could last elsewhere on the surface, so the ice could not have been carved out when an object struck Ceres, creating the crater. Pointing to landslidelike features inside Oxo, Combe suggested that either material on the slope became loose on its own or a smaller impact triggered motion, exposing the ice beneath. Because Oxo is small and there are no signs of cryovolcanism, Combe deemed it unlikely that the water came from a liquid ocean mantle. Marc Neveu of Arizona State University, who studies cryovolcanism on icy worlds, agrees that a landslide is the likely explanation for Oxo's ice.
A buried ocean
If the ice did not well up from the shallow layers beneath the surface, it most likely came from the massive ice sheet predicted to lie only a few meters under the dust and rock covering Ceres. Understanding how widespread this sheet is, if it exists, will help scientists better model how the dwarf planet formed, which could then help them calculate if an ocean beneath could be liquid. Dawn's Gamma Ray and Neutron Detector (GRaND) instrument has just begun to probe one meter into Ceres’s surface layer to determine if such a sheet exists, and how extensive it might be.
The question remains, however—if Dawn can find ice in Oxo, why has it not seen ice elsewhere on Ceres? Is there something special about Oxo crater? According to Hayne, other shallow patches of ice could hide under the top layer of the surface, revealed by landslides much like in Oxo. As the topmost layer becomes exposed, that water will leap from ice to gas, leaving behind the dust and salts mixed in. As more layers make the leap, material builds up until it insulates the ice, stopping the process and leaving ice behind. But the debris, known as sublimation lag, is dark and would be deeper than Dawn’s infrared mapping instrument could penetrate. Surface ice could have produced the plumes Dawn observed before the debris halted the process. If these hidden patches exist, however, GRaND should be able to detect them while probing deeper for the larger ice sheet. Ultimately, the newly observed sample and other similar patches could help determine if a liquid ocean where life could evolve and thrive lies beneath the bright shell of Ceres or if it is frozen through.