The largest and most mysterious resident of the debris belt between Mars and Jupiter is an icy world called Ceres, and it’s on the threshold of being explored up close for the first time by NASA’s Dawn mission, which is scheduled to enter Ceres’s orbit on March 6.
Discovered in 1801 by the Italian astronomer Giuseppe Piazzi, Ceres was initially thought to be a full-fledged planet. That began to change when one of Piazzi’s rivals, the astronomer William Herschel, noted that Ceres only appeared as a point of light in his telescope rather than a resolved disk, like the other known planets. To Herschel that meant Ceres was probably too small to be considered a planet, and he coined the term “asteroid” to describe its starlike appearance. Ceres received a minor upgrade to “dwarf planet” in 2006, part of the same process that demoted Pluto to the same status.
Whatever you call may call it, Ceres is one of the most geologically interesting and strange objects in the solar system. Its shape, size and composition—round, roughly the size of Texas and at least 20 percent water ice—place it at the poorly understood transition point between rocky worlds like Earth and icy worlds like Jupiter’s Europa, Saturn’s Enceladus, and other large moons of the outer solar system. Other than blurry Hubble Space Telescope images from 2004, its surface had scarcely been glimpsed until Dawn’s approach. As the spacecraft’s ion engines slowly push it toward Ceres, the dwarf planet’s details are now coming into focus, revealing tantalizing new details with practically every new image.
In the latest images, taken from 46,000 kilometers away and released on February 25, Dawn has sharpened its view of mysterious bright spots dotting Ceres’s crater-pocked surface, some of which were previously seen in the Hubble images. What used to appear as Ceres’s brightest blotch now appears to be two—a brighter, larger spot next to a smaller, dimmer one, both in the same crater. “Bright” is a relative term—all the bright spots are actually quite dark but still far brighter than the rest of Ceres, which is blacker than coal. No one knows what the bright spots are but guesses abound: Perhaps they are scars from recent impacts or minerals deposited by active geysers or water ice erupted by “cryovolcanoes”—or something even wilder. In 2014 the Herschel space telescope spied transient plumes of water vapor tentatively linked to the approximate locations of the white spots in Hubble images.
“Ceres’s bright spot can now be seen to have a companion of lesser brightness, but apparently in the same basin,” says Chris Russell, Dawn’s principal investigator, based at the University of California, Los Angeles. “This may be pointing to a volcanolike origin of the spots but we will have to wait for better resolution before we can make such geologic interpretations.”
“The images now are just at that intriguing resolution that lets you make stuff up,” says Mike Brown, the California Institute of Technology astronomer whose work helped motivate the reclassification of Pluto and Ceres as dwarf planets. The white spots would seem to be exposed ice, Brown says, but observations of Ceres with ground-based telescopes don’t show any evidence of ice at the bright spots’ locations.
But ice shouldn’t be stable at Ceres’s surface, says Andy Rivkin, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. “So its presence there would mean it’s only gotten there recently, either by having impacts expose it or…. Well, probably that’s the only way,” Rivkin says. “I suppose cryovolcanism could also bring it to the surface, but impacts would be the safer bet.”
All anyone really knows for sure right now is that the spots are getting brighter as Dawn’s view sharpens, says Mark Sykes, director of the Planetary Science Institute and a Dawn mission scientist. And the brighter they become “the more interesting they get,” because water ice is one of the brightest things researchers could possibly see on Ceres. “If we discover something like cryovolcanism on Ceres, that would be spectacular because it would be an indicator that there are subsurface reservoirs of water,” Sykes says. “This isn’t what you normally think of as an asteroid, a dead potato just being smacked around by its neighbors out in space. There is a lot happening on this object, and that could make Ceres very astrobiologically important.”
If Ceres proves to be a possible home for extraterrestrial life and is venting water into space, Sykes says, “this raises the possibility that we could send another spacecraft there in the near future to go down to one of these spots, scoop up soil, take a look and ask whether there are any dead bugs in there.” Such a mission would be relatively inexpensive, Sykes says, because Ceres is so close by and accessible. “It’s between Mars and Jupiter, it’s not a nasty radiation environment like Jupiter’s Europa, it’s not really far away like Saturn’s Enceladus and it’s got low gravity so it doesn’t require lots of energy to land on! It’s too early to say whether this stuff is there, but the prospect is very exciting.”
Dawn is still in the earliest phases of investigating Ceres, and the best images and scientific data are yet to come. It will eventually swing down to within about 400 kilometers of the surface to study its composition and to generate high-resolution maps. The spacecraft will spend at least the next 16 months studying Ceres, but because the dwarf planet could potentially be an abode for subsurface life Dawn will not be crashed into it at the end of its life, as is the usual procedure. Instead, it will be left in orbit, becoming a long-lived mechanical moonlet of Ceres once its mission ends.