NASA hopes Dawn mission can answer the big question: could life lurk in icy volcanoes on Ceres, the asteroid belt's biggest resident?
CERES is a dwarf planet with giant potential. As NASA's Dawn spacecraft gears up for the first in-depth look at this tiny world, speculation is rife. Could Ceres be an overgrown comet? Host an ocean made of mud? Or even possess icy volcanoes that make it an unexpected host for life in the asteroid belt?
"When we complete our observations, we will show that Ceres is every bit a planet as its terrestrial neighbours Mars, Earth, Venus and Mercury." That's what Christopher Russell, who leads the Dawn mission, told the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas, last week.
The first signs of excitement came earlier this month when Dawn spotted a mysterious bright spot just 1 pixel wide inside a crater as it pulled into orbit around the dwarf planet. There were suspicions that the spot could be a sign of water spewing into space, and now fresh views, presented for the first time at the LPSC, lend weight to the idea.
In Dawn's latest pictures, the bright spot is visible even from the side, meaning it probably protrudes above the crater. "What is amazing is you can see this feature while the rim is very likely in front of the line of sight," said Andreas Nathues, who is in charge of the mission's camera. "We believe this could be some kind of outgassing."
Remote observations using the Herschel space telescope show Ceres spitting water from somewhere on its surface, probably towards the equator. We think other icy bodies in the solar system, like Jupiter's moon Europa and Saturn's moon Enceladus, spew water in spectacular plumes from subsurface oceans (see diagram). If Ceres also has a buried sea, that could boost its chances of playing host to life – so astronomers are keen to track the plumes to their source.
Images taken from dusk until dawn on Ceres seem to indicate something more transient, since the spot brightens throughout the day and completely disappears at night. So the patch could be a pocket of ice on the surface that releases gas as it warms up in the sun, which is similar to how a comet behaves. However, Nathues said only higher resolution data will confirm its true nature. This won't come for a while, as Dawn is on the dark side of Ceres and won't emerge until mid-April.
But a model of Ceres presented at the LPSC added a complication, suggesting comet-like behaviour is only possible at the dwarf planet's poles, not the lower latitudes where the bright spot has been seen.
Timothy Titus of the US Geological Survey in Flagstaff, Arizona, presented a thermal model that considered where ice could remain stable on Ceres's surface over the lifetime of the solar system, rather than boiling away more quickly. If Ceres is behaving like a comet, it must have ice patches that can survive for a long time until the sun's heat reaches them as the dwarf planet moves into a warmer part of its orbit.
Titus found that long-lasting ice could only be possible in chilly latitudes above 40°. But the plumes spotted by Herschel seemed to come from nearer the equator. "The water ice is just not stable at the latitudes that the plumes are supposedly coming from," Titus says.
What about cryovolcanism? In this scenario, ice and water are ejected from the surface by processes similar to those that drive magma volcanoes on Earth. Ceres doesn't have enough muscle to drive these eruptions, according to a second model presented at the LPSC by David O'Brien of the Planetary Science Institute in Tucson, Arizona.
Let's suppose that Ceres has a subsurface ocean covered by an icy shell. As the bottom of the shell freezes, it expands, putting pressure on the ocean and the shell itself. To create a cryovolcano, says O'Brien, the water pressure has to build up enough to shoot up through the shell before the ice cracks and relieves the pressure.
Since we don't know exactly how deep the ice is on Ceres, O'Brien modelled a range of plausible depths. None recreated the conditions for spewing cryovolcanoes – the ice always cracked before enough pressure accumulated. In the best-case scenario, water reached about 90 per cent of the way to the surface.
Intriguingly, that might mean water could reach the surface from a deep crater, where there would be less ice to negotiate – perhaps even from a crater like the one where Dawn saw the bright spot. "Everybody wants to know what's going on there," says O'Brien. A cryovolcano could be producing enough of a plume to replenish the ice on the surface. So Ceres could be producing comet-like emissions, driven by a weak cryovolcano. "It's sort of a midpoint between comets and cryovolcanic icy worlds," says Titus.
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