22 January 2014
Water vapor plumes observed erupting from dwarf planet
by Will Parker
The Herschel space observatory has used its far-infrared capabilities to capture a clear spectral signature of water vapor venting from the largest object in the asteroid belt, Ceres. Ceres is classified as a dwarf planet, a body bigger than an asteroid and smaller than a planet.
"This is the first time water vapor has been unequivocally detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere," said Michael Kuppers of the European Space Agency (ESA), lead author of a paper documenting the observations in the journal Nature.
The results come at the right time for NASA's Dawn mission, which is on its way to Ceres now after spending more than a year orbiting the large asteroid Vesta. Dawn is scheduled to arrive at Ceres in the spring of 2015, where it will take the closest look ever at its surface.
"We've got a spacecraft on the way to Ceres, so we don't have to wait long before getting more context on this intriguing result, right from the source itself," said Carol Raymond, the deputy principal investigator for Dawn at NASA. "Dawn will map the geology and chemistry of the surface in high-resolution, revealing the processes that drive the outgassing activity."
Ceres was considered the largest asteroid in our solar system, but in 2006, the International Astronomical Union reclassified Ceres as a dwarf planet because of its large size (around 600 miles in diameter). Scientists believe Ceres contains rock in its interior with a thick mantle of ice that, if melted, would amount to more fresh water than is present on Earth.
Until now, ice had been theorized to exist on Ceres but had not been detected. It took Herschel's far-infrared vision to see, finally, a clear spectral signature of the water vapor. But Herschel did not see water vapor every time it looked. While the telescope spied water vapor four different times, on one occasion there was no signature.
Scientists think that when Ceres swings through the part of its orbit that is closer to the Sun, a portion of its icy surface becomes warm enough to cause water vapor to escape in plumes at a rate of about 13 pounds per second. When Ceres is in the colder part of its orbit, no water escapes.
The strength of the signal also varied over hours, weeks and months, because of the water vapor plumes rotating in and out of Herschel's views as the object spun on its axis. This enabled the scientists to localize the source of water to two darker spots on the surface of Ceres. The dark spots might be more likely to outgas because dark material warms faster than light material.
The results are unexpected because comets, the icier cousins of asteroids, are known typically to sprout jets and plumes, while objects in the asteroid belt are not. "The lines are becoming more and more blurred between comets and asteroids," said NASA's Seungwon Lee. "We knew before about main belt asteroids that show comet-like activity, but this is the first detection of water vapor in an asteroid-like object."