Bright Spots on Ceres: Mystery Solved?

Results from NASA's Dawn spacecraft hint that the stark brightenings in and near scores of craters on Ceres might result from salty brines seeping onto the surface.

Everyone loves a good mystery, planetary scientists included. For the past decade they've wondered about a solitary white spot seen on the surface of asteroid 1 Ceres. It first appeared in infrared views acquired from the ground with the Keck II telescope in 2002, then in Hubble images taken in 2005.

Aside from its brightness, researchers didn't have much to go on, so they put the puzzle aside knowing that NASA's Dawn spacecraft would eventually reveal the spot in glorious detail. Dawn arrived at Ceres last March, and that solitary spot became evident in its onboard camera months before that.

In time, as the spacecraft drew nearer, mission scientists realized that Ceres veritably bristled with more than 130 bright spots, most associated with impact craters. But none was as large, as bright, or as distinctive as the Big One, which turned out to lie inside a 90-km-wide crater now named Occator. Close-up views show the spot to be more of a splash, with its bright core corresponding to a pit about 10 km across. ("Bright" is relative: the big spot's core has a reflectivity of 25%, compared to less than 4% for the carbon-infused material covering the rest of Ceres.)

But what is that stuff? Speculation has run wild. Contests have sprung up. One pundit proposed eight possible causes, among them "aliens' solar concentrators."

Well, not likely. But a new analysis in December 10th's Nature certainly narrows the possibilities. A team led by Andreas Nathues (Max Planck Institute for Solar System Research) started with three possibilities: water ice, iron-depleted clay minerals, or salt deposits.

Water ice has had the inside track for some time. Ground-based spectroscopists had detected water-containing minerals more than a decade ago, and Ceres' relatively low density (2.1 g/cm³) and shape suggest that it contains plenty of water ice and that pretty much all of it must be on the outside. Back in July, Dawn principal investigator Chris Russell even divulged that the spacecraft had been noticing occasional hazes inside Occator dense enough to shroud its interior.

The Case for Salts

Nathues and his colleagues base their analysis on views from Dawn's Framing Camera, which has seven narrowband filters at visible and near-infrared wavelengths. That's a useful but not particularly diagnostic spectroscopic capability. In fact, the team notes their identification of specific compounds "must be considered tentative."

Still, a modest dip near 0.8 micron in the spectrum of the Occator spot's center isn't a good match for either water ice (which is spectrally bland at those wavelengths) or likely clay minerals. But it's a better fit to hexahydrite, a magnesium-sulfate salt that locks up six water molecules for each molecule of salt.

Apparently briny fluids are oozing from the interior onto the floor of Occator, whose estimated age is only 78 million years. There's no real gushing — Dawn hasn't detected any geyser-like plumes. Instead, the Sun-warmed brines give up their water through sublimation. The vapor gently lifts away from the surface, carrying with it fine dust particles. It's not much, perhaps a bucketful of water per second during daylight, but enough to form the hazes seen inside the crater. And the brine's salts are largely left behind as bright stains on the otherwise dark surface.

Of course, how Ceres manages to keep the salty fluids in liquid form below the surface — and yet not very far down — is perhaps an even bigger question. Does a briny ocean lie hidden beneath Ceres' dark exterior? If so, what keeps it from freezing, since there's nothing near Ceres to pump tidal energy into its interior.

More confident answers might be coming soon. First, Dawn is just days away from reaching its final and lowest mapping orbit, and from an altitude of 375 km (233 miles) it will circle Ceres every 5½ hours. So even more detailed images of the bright spot are coming soon.

Second, the compositional questions will get easier once Dawn's visible and infrared spectrometer (VIR) can probe the spots individually. VIR obtains spectra in 432 channels from 0.95 to 5.0 microns — the kind of spectral horsepower that should make disentangling the composition of the bright spots much easier.

In fact, in a companion Nature paper, Maria Cristina De Sanctis (INAF/IAPS, Rome) and others use Dawn's VIR spectra to reveal that clay minerals called ammoniated phyllosilicates are widespread across Ceres' surface — yet the instrument has yet to detect any exposed water ice.

Eventually we'll hear from Dawn's third instrument, a gamma-ray and neutron spectrometer called GRaND, which can detect buried water ice. But GRaND's field of view is very wide, and it needs to be close to Ceres to get meaningful results.

So the mysteries continue — does anyone still want to bet that the spots are aliens' solar concentrators?