NASA's Dawn spacecraft is about to make its second and final stop during its exploration of the asteroid belt and it is already returning some stunning images that are creating more questions than answers.
After leaving massive asteroid Vesta's orbit in 2012, Dawn has traveled through the asteroid belt that occupies the region between the orbits of Mars and Jupiter to rendezvous with 600 mile-wide dwarf planet Ceres - the first spacecraft ever to orbit two celestial bodies during its mission.
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The probe will be captured by Ceres' gravity on Friday, March 6, and in the run-up to this highly anticipated event, the probe has been sending back increasingly detailed observations of the solar system's innermost dwarf planet that are already puzzling planetary scientists. One puzzle focuses on bright patches on the Cererian landscape - one of which is a particularly bright spot, with a dimmer partner, inside an impact crater.
Speaking during Monday's press conference at NASA's Jet Propulsion Laboratory in Pasadena, Calif., Dawn deputy project scientist Carol Raymond outlined some possibilities that may explain these strange features. As previously reported, one mechanism that could be creating the bright patches is cryovolcanism, where sub-surface ice is forced to the surface. But as Dawn's imagery becomes sharper by the day, this mechanism is looking less likely.
"A cryovolcano will likely result in a constructional feature," said Raymond. "So we'd expect to see a mounded feature on the surface - some sort of deposit around a central vent or a crack. In the case of this crater, what we can say is that the brightest spot is not associated with a ‘positive relief feature' - i.e. a mound or peak ... so a cryovolcano is not at the top of the list for that feature."
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We'll have to wait until Dawn has completed its first science orbit (in late-April) so the probe's instrumentation can be better calibrated to understand just why these spots are so bright and, ultimately, understand their origin.
Although cryovolcanism may be looking less and less likely, the mystery of Ceres' water is one of most exciting unknowns on the minds of the Dawn team.
"One of the mysteries is that of liquid water," Dawn mission director and chief engineer Marc Rayman told Discovery News. "Are there sub-surface reservoirs of water - ponds or lakes or oceans? I think that's really exciting."
From observations and theoretical models, scientists have a pretty good idea that Ceres was a planet in the making in the early epochs of the solar system. It is composed of stratified material and from density models of the world, there are strong indications that there should be sub-surface reservoirs of water.
As Ceres isn't heated by the the tidal heating that Saturn's moon Enceladus or Jupiter's moon Europa experience and receives weak sunlight as its only heat source, it is most likely that if Ceres did have sub-surface liquid water reservoirs early in its history, they are likely now long frozen, unless interactions with minerals in the rocks produced salts to maintain a liquid state.
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The expectation that Dawn would see Ceres covered with ice was further bolstered last year when the ESA Herschel space telescope detected water vapor in the vicinity of Ceres. Although it wasn't a huge quantity, it did boost hopes that Ceres may be venting water vapor into space from its sub-surface reservoirs, in a similar (but more understated) manner to Enceladus' impressive south pole geysers. Another theory is that, by chance, Herschel may have spotted the after effects of a meteorite impact on Ceres that kicked up surface ice into space.
Before Dawn started its approach of Ceres, it seemed highly possible that Ceres was going to have more in common with Enceladus and Europa - two icy worlds with sub-surface oceans. As it turns out, as Ceres came into focus, its ancient cratered surface, as opposed to an ice-covered crust, came as a surprise.
"That was the biggest surprise I think for me and many of the team members - when we saw those craters we were like ‘Okay...'," Raymond told Discovery News. "The ice should be close to the surface, it flows. So something else is going on. There's lots of ideas as to how we can explain that but we're going to have to sharpen our pencils, do a lot of detailed models and we're going to need a little more insight from high-resolution data (from Dawn)."
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There is still a possibility that water vapor may be escaping from the dwarf planet, but Dawn's instrumentation isn't designed to specifically seek out venting regions. But through the use of its infrared spectrometer, Dawn may be able to detect the back-scattered light created by dust that is being blown into space by the venting water vapor. A "tenuous atmosphere about Ceres" may also be a possibility, according to Raymond.
"Since I have the meteorology background, I most want to know what's going on with the water vapor, maybe there's an atmospheric thing going on," Keri Bean, Dawn mission operations engineer, told Discovery News. "Dawn will give us an answer, one way or another. It may not be able to see it (the water vapor), but that will also be a clue. So it will be interesting to see what Dawn does."
Rather than answering any questions early in Dawn's Ceres encounter, it seems even more questions are popping up.
"The real excitement is, what does Ceres have to tell us? It's not a specific question; it's rather that this is a mysterious alien world that, for two centuries has just been this faint smudge of light," added Rayman. "Now we're finally getting this in-depth, richly detailed portrait. That's what I think is exciting.
"What questions is Ceres going to answer that we're not even smart enough to ask now?"
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Understanding how Ceres is storing its water, what mechanics are driving the possible water vapor (and a potential atmosphere) and, of course, whether ice is behind the mysterious bright spots, are just a few components of our desire to seek out whether Ceres is (or was) a place that life as we know it would consider to be habitable. Following the water in any solar system body ultimately has this aim - to search for niches where life may take hold beyond Earth.
"I really hope that we are going to be able to say something definitive about Ceres' habitability and the way we would get at that is by reading the record of the surface - it would provide us clues as to what was going on at this ancient water-rock interface," said Raymond. "It's the types of minerals and how they got to the surface and what kind of convection or mixing was going on within the interior."