Infrared readings from a space telescope confirm that water vapor is rising from the surface of Ceres, and that discovery is likely to heat up interest in a strange world that's the biggest asteroid as well as the smallest known dwarf planet.
The find comes just in time. Next year, NASA's Dawn spacecraft is due to go into orbit around Ceres and is likely to address some of the questions raised in Thursday's issue of the journal Nature: Where is the water vapor coming from? How is it getting into space? And what are the implications for Ceres' place in the solar system?
"This is what you might call the 'smoking gun,'" Mark Sykes, CEO and director of the Arizona-based Planetary Science Institute, told NBC News. "The implications could be huge for the future of astrobiology and planetary exploration."
Sykes wasn't involved in the Nature study, which is based on data from the European Space Agency's Herschel Space Observatory. Nevertheless, Ceres is close to his heart: He's a co-investigator on the $466 million Dawn mission and has long suggested that the dwarf planet might possess subsurface liquid water — and perhaps even traces of life.
Reservoirs of waterThe findings from Herschel don't go nearly that far, but they do provide the strongest evidence yet that Ceres contains reservoirs of water ice. Past missions turned up indirect evidence for water, but nothing conclusive. It took Herschel's sensitive HIFI infrared detector to pick up the spectral signature of water vapor, apparently emanating from two dark spots on Ceres' surface.
"Possibly, the dark regions are warmer than the average surface, resulting in efficient sublimation of small water-ice reservoirs," the researchers, led by ESA's Michael Küppers, wrote in the Nature paper. (Ice can consist of different ingredients on different worlds: Mars, for example, has water ice as well as carbon dioxide ice.)
Küppers and his colleagues estimated that Ceres was giving off about 13.2 pounds (6 kilograms) of water vapor per second. If that much vapor were condensed into liquid, it would amount to a little more than a gallon and a half (6 liters) of water.
The researchers speculate that the water vapor could come from a cometlike layer of surface ice, or from volcanoes that eject ice instead of lava.
"The cryovolcanism hypothesis requires a warm interior, and it is possible there there is a layer of water (subsurface ocean) somewhere," Küppers told NBC News in an email. "In the cometary sublimation scenario, there is 'just' an ice layer that is locally close to the surface and heated by the sun. In this case, there may be conditions for liquid water somewhere in the interior as well, if pressure and temperature happen to be right."
Implications of an oceanThe researchers saw more evidence for the comet hypothesis, based on the fact that the signature of water vapor became stronger when Ceres came closer to the sun in its orbit. However, there's not yet enough data to settle the matter. Dawn's observations should provide the required data.
Ceres is the biggest object in the main asteroid belt that lies between Mars and Jupiter, orbiting at a distance of about 257 million miles (414 million kilometers) from the sun. Like Pluto, Ceres is considered a dwarf planet because it's massive enough to take on a round shape.
Küppers said his group's findings lend weight to the idea that impacting asteroids and comets contributed water to the early Earth's oceans. And if Ceres really does harbor a subsurface ocean — something that the Jovian moon Europa and the Saturnian moon Enceladus are already thought to possess — that could strengthen the argument for life on Ceres.
"This raises the possibility that Ceres could replace Europa as the prime target for planetary investigation," Sykes said. "It's going to upend the cart a bit, but that's science."