Think of an asteroid, and a dry, rocky body may spring to mind. But Ceres, king of the asteroid belt, is helping to throw cold water on that stereotype. Whether frozen in the permanent shadows of its polar craters or mixed into its icy surface layers, water appears to abound on the dwarf planet.
Two different findings based on data from NASA's Dawn spacecraft, presented at last week's American Geophysical Union meeting in San Francisco, may fill out a portrait of the frigid dwarf planet and could help shed light on its internal evolution.
As the largest member of the main belt, the rocky ring of debris that lies between the orbits of Mars and Jupiter, Ceres can open a special window onto the solar system's evolution. Asteroids are thought to be the leftovers from the planets' formation more than 4 billion years ago. Largely free of the chemical and mineral alterations that came with being incorporated into a planet, they represent "fossils" that could reveal pristine details about the solar system's early days.
But Ceres is different from its smaller brethren: It's a dwarf planet that never quite made it to the big leagues — though it probably underwent some of the processes that the full planets did. Forever frozen in planetary adolescence, this little world could provide a snapshot of our fully formed planets' early growing pains.
Ceres has proved to be very unlike Vesta, the other giant asteroid visited by the Dawn spacecraft. Ceres may have formed farther out than Vesta, far enough from the sun that water could remain solid on the surface without completely escaping into space over time. The differences between the two could sketch out two different evolutionary paths in early planet formation.
"Both Vesta and Ceres are what you could call protoplanets — so they started off just like any other planet, but at some [point] in time they didn't grow any larger, and we want to understand why," said Thomas Platz, a geologist at the Max Planck Institute for Solar System Research in Germany and leader of a study in the journal Nature Astronomy on polar ice deposits hidden in some of Ceres' shadowy craters.
Since Dawn arrived at Ceres in March 2015, scientists have seen many hints that the dwarf planet could be rich in water ice: "Bright spots" in the Occator crater near the equator appear to be salt deposits probably left after the water holding them escaped, and the 2.5-mile-tall Ahuna Mons is thought to be an "ice volcano" that spewed salty water and mud from the interior to the surface.
Finding direct evidence of water, however, has proved to be a challenge. But now, a pair of studies have found strong signs of its presence, both in specific polar reservoirs and distributed across the dwarf planet's surface.
Dawn's low-altitude orbit, which began in December 2015, provided a new opportunity for researchers to dive into the water question. The spacecraft's Gamma Ray and Neutron Detector, or GRaND, examined the composition of materials in the upper meter of the surface, finding an abundance of hydrogen. (Hydrogen can serve as a telltale sign of the presence of water molecules, which are made of two hydrogen atoms and one oxygen atom.)
The survey revealed that a significant amount of water ice lies within the dwarf planet, particularly at the middle to high latitudes. The data, published in the journal Science, also showed that the concentrations were more abundant at higher latitudes, ultimately making up a whopping 30% of the material at the poles.
"I was delighted — you have the first elemental measurements using nuclear spectroscopy of the water-rich asteroid, which happens to be a dwarf planet," said study leader Thomas Prettyman, leader of the GRaND instrument based at the Planetary Science Institute in Tucson. "What amazes me at this moment was just how uniform the surface is, compared to Vesta. If you look at the distribution of hydrogen and you just sort of walk around the equator, there's not a lot of variability in the hydrogen content. It's only when you look north-south that you see a high degree of variability."
When rocky planets gain mass and take shape, their interiors (heated by the decay of radioactive isotopes) begin to melt and their materials separate and differentiate. Asteroids are mostly too small to have experienced this, but Prettyman and his colleagues found that Ceres does seem to have had its ice and rock separate over time. This could help explain why Ceres' surface composition doesn't seem to match its bulk composition — in other words, why its surface might not match its insides, as is typically the case with full-fledged planets.
In the other paper, published in Nature Astronomy, researchers found water deposits in the permanently shadowed regions of the northern hemisphere. Because Ceres has only the slightest axis tilt (about 4 degrees, compared with Earth's 23.5 degrees), there are areas in its polar craters that remain in permanent shadow. In these spots that never see direct sunlight, it can get very, very cold, study leader Platz said — about 60 Kelvin, or roughly minus 351 degrees Fahrenheit. If any spot on Ceres had any surface water ice, it would be in these regions. (Water ice has been found in similarly shadowed spots on the moon and even on Mercury, the sun-scorched innermost planet.)
Using images from Dawn's Framing Camera instrument, the scientists carefully tracked these permanently shadowed regions in 634 craters. It's a deliberate, careful process, Platz said.
"You need to take many images of the same locations, ideally over the entire length of one day," he said.
Ten of those craters have bright features that appear to be water ice — and in one partially sunlit spot, thanks to the spacecraft's infrared mapping spectrometer, the scientists picked up the clear presence of ice.
The finding serves as "another confirmation that there's ice on Ceres," Platz said.
The scientists were surprised, however, by the fact that they found fewer than a dozen bright spots in the hundreds of permanently shadowed craters they examined.
"Usually you would expect to be a wider distribution of those spots, and that's a bit puzzling," he said. "One reason could be that Ceres has an axis-tilt variation."
Ceres is almost straight now, but if it has tilted more dramatically in the past, many of those shadowed spots might no longer exist, which would mean fewer surface havens for ice.
That's one of many possible explanations, however. For now, water appears to be plentiful on the frigid planet — though whether it all remains locked in ice, or whether some of it flows in a briny ocean deep beneath the surface (as it probably does beneath moons Enceladus and Europa and perhaps even Pluto) remains to be seen.
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