Water spewing from icy volcanoes or ice patches may adorn our solar system's largest asteroid and smallest dwarf planet, known as Ceres, an international astronomy team says, instantly raising questions about the possibility of life there.
Other objects in that elite club include icy worlds like Jupiter's moon Europa and Saturn's moon Enceladus, where signs of water plumes hint at water under an icy surface where alien life might survive. (See: "Could Some Alien Worlds Be More Habitable Than Earth?")
Past observations attempting to confirm indirect hints of water on Ceres had been thwarted by the asteroid's faintness when viewed from Earth, and the detection was "a real surprise, at least to me," Küppers says by email.
Alien Life Potential?
Astronomers have been staring at Ceres since 1801, curious about the tiny world, only 590 miles (950 kilometers) wide, which reigns as the largest object in the solar system's asteroid belt.
Perched between Mars and Jupiter, Ceres resides some 260.4 million miles (419 million kilometers) from the sun, about 2.8 times farther away from our star than Earth is. The new report suggests that at least two locations on its surface spew water vapor into its thin atmosphere.
The team reports that observations made from 2011 to 2013 at the ESA's Herschel Space Observatory picked up signs that Ceres was releasing about 13.2 pounds (6 kilograms) of water per second from its surface into space, originating from two sites near its equator.
"Very exciting news," says Carol Raymond of NASA's Jet Propulsion Laboratory in Pasadena, California.
Raymond is the deputy principal investigator on NASA's Dawn spacecraft, which is headed for an orbital rendezvous with Ceres in 2015. "Ceres now looks like one of the good places in the solar system with astrobiological [alien life] potential," she says.
Comet or Geyser?
The water vapor observations came during a time when Ceres was at its closest point to the sun, an occurrence known as the perihelion, on its 4.6-year-long orbit of the sun, Küppers says.
That timing suggests that the water vapor spewed from ice patches on Ceres's surface, sublimating into space like a comet shedding a tail as it warms on a plunge past the sun.
"It's not an atmosphere like Earth's in that most of the water escapes into space due to Ceres's comparably low gravity. In that respect it is more similar to a comet," Küppers says."I do not expect a stable atmosphere."
Instead of cometlike ice patches, Raymond hopes to see water-spewing geysers (a phenomenon known as "cryo-volcanism") spouting from the asteroid when Dawn arrives at Ceres. That's because Dawn could fly through a geyser plume and analyze its contents.
The spacecraft can't fly through an ice patch.
Dawn is expected to start orbiting Ceres when it will be near its farthest point from the sun on its orbit, the so-called aphelion. If the asteroid is only spewing water when it is closest to the sun, then it may disappoint Dawn's scientists' hopes for a cryo-volcano show.
If there are geysers, the study indicates they aren't strong ones. "The plume would be more like a puff," Raymond says, for Dawn to sail through and collect data on the interior of the asteroid.
Küppers thinks the cometlike ice patch explanation is more likely, explaining past negative results of water vapor observations (a partial observation was made in 1991) made close to aphelion on Ceres.
The asteroid likely lacks an internal dynamo to warm geysers, as well as the tidal stresses that heat the moons, Europa and Enceladus. And Küppers regards the notion of an internal ocean on Ceres as speculation at this point.
"I really expect Ceres is going to be exciting, whatever we find," says Raymond.
In particular, Dawn should closely map the origin sites for the water vapor emissions in the study. While missions for future landers have been proposed for Europa and Enceladus, Raymond also notes that Ceres is much closer to Earth than those moons are.
A recent space travel study by Purdue University engineers, for example, suggested that astronaut travel to Ceres, a two-year trip that would include a 110-day stay on the asteroid, would not pose significantly more challenges than proposed trips to Mars.
And there would be plenty of water to drink when they arrived, apparently.
The heavyweight of the asteroid belt, Ceres is also the smallest dwarf planet in the solar system, according to International Astronomical Union specifications. The other IAU-recognized dwarf planets are Pluto, Eris, Haumea, and Makemake in the inner comet belt beyond Neptune.
Dwarf planets are worlds massive enough for their gravity to pull them into a round shape, like Ceres, but they aren't so big that they have cleared out their orbital region of space of similarly sized competitors.
The discovery of water vapor on Ceres may point to another connection between comets and dwarf planets, suggest astronomers Humberto Campins
and Christine Comfort of the University of Central Florida in Orlando, in a commentary accompanying the study.
Ceres may simply represent a very large comet nudged closer to the sun by the migration of Jupiter's orbit in the solar system some four billion years ago, they suggest. That would make it a remnant of an era when Earth and the other planets of the solar system suffered fierce bombardments by comets. (See also: "Asteroids and Comets.")
"The 'Ceres is a comet' suggestion fits nicely with the recently discovered active asteroids or main-belt comets," Küppers says. "Those are much smaller objects [ones about a kilometer wide] in the asteroid main belt that show cometlike dust comae or tails."
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