Thursday, 23 January 2014

Water found on dwarf planet Ceres

An artist's impression of water outgassing from two sources on the dwarf planet Ceres

Astronomers have discovered direct evidence of water on the dwarf planet Ceres in the form of vapor plumes erupting into space, possibly from volcano-like ice geysers on its surface. Using European Space Agency's Herschel Space Observatory, scientists detected water vapor escaping from two regions on Ceres, a dwarf planet that is also the largest asteroid in the solar system. The water is likely erupting from icy volcanoes or sublimation of ice into clouds of vapor. "This is the first clear-cut detection of water on Ceres and in the asteroid belt in general," said Michael Küppers of the European Space Agency, Villanueva de la Cañada, Spain, leader of the study.

The research has implications for how Ceres formed, and supports models that suggest the planets moved around a lot within the solar system during its formation, Küppers said. Ceres, a dwarf planet or giant asteroid, is the largest object in the asteroid belt, orbiting at 2.8 astronomical units (the distance from Earth to the sun). The "snowline" is thought to partition the solar system into dry objects inside the asteroid belt, and icy objects such as comets further out. But the finding of water on Ceres suggests more mixing has occurred. Scientists have suspected that there is a substantial amount of water on Ceres for about 30 years. A study found hints of water in the form of hydroxide, a product of water's dissociation, on Ceres in 1991, but the finding wasn't confirmed by later observations. Now, Küppers and his colleagues have confirmed the finding.

The researchers used the Herschel Space Observatory's spectrometer to look for signals of water. Clouds of water vapor around Ceres absorbed the heat that radiates from the dwarf planet, which Herschel's instrument detected. The team found that Ceres produces about 2×10^26 molecules, or 13 lbs. (6 kilograms), of water vapor per second from its surface. One possible source of the water is icy volcanism. "It is like volcanism in that hot material from the interior is 'spat out' to the surface," Küppers said, much like a geyser. But these icy volcanoes eject water vapor instead of molten rock, he said.

Another possibility is that ice near the surface of Ceres sublimes, or goes directly from a solid to a gas, dragging with it dust from the surface and exposing more ice. A similar process occurs on comets. "I personally consider cometary-style sublimation the most likely source, because I find it difficult to maintain the internal heat over the age of the solar system to maintain volcanoes," Küppers said, but he added that more studies were needed.

NASA's Dawn spacecraft, set to go into orbit around Ceres in early 2015, could answer some questions about the water on Ceres. Dawn recently visited the asteroid Vesta, a baked world whose surface is covered with volcanic eruptions. "One of the most puzzling questions about the origin and evolution of asteroids is why Vesta and Ceres are so different," astrophysicists Humberto Campins and Christine Comfort at the University of Central Florida in Orlando wrote in an article. Water vapor can transport a lot of heat, so when Ceres formed 4.6 billion years ago, sublimation of water ice might have dissipated much of its heat into space, Campins and Comfort wrote. "This would have stopped Ceres from ending up with an igneous surface like that of Vesta."

Detecting water on Ceres supports models of the solar system in which giant planets, such as Jupiter, migrated to their current positions, mixing material from the outer and inner regions of the solar system. This mixing could have moved Ceres and Vesta far from the sites where they formed. Ceres probably formed close to its current position, but accreted material from further out, Küppers said. The findings also suggest that asteroids may have delivered some of the water in Earth's oceans.

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