A decades-long search by astronomers for water on an asteroid has unexpectedly turned up frozen water on a 120-mile-wide rock located well beyond Mars.
While scientists generally agree that asteroids and comets probably brought some water to Earth, as well as the carbon-based materials thought to be necessary for the origin of life on our planet, finding them has proven difficult. When these materials and water arrived on the warm Earth, four billion years ago, the conditions were suitable for the formation of living organisms.
"Finding widespread water ice on an asteroid so close to the sun was a completely surprising result. We expect ice to evaporate quickly into space from the surfaces of asteroids," said Noemi Pinilla-Alonso, a research scientist at NASA's Ames Research Center, Moffett Field, Calif. Pinilla-Alonso is the co-author of a research paper titled, "Water ice and organics on the surface of the asteroid 24 Themis," published this week in the British-American scientific journal Nature.
On a clear night in January 2008, Pinilla-Alonso and her colleagues were working at NASA's Infrared Telescope Facility (IRTF) at the observatory on Mauna Kea, Hawaii. Although, the Earth's atmosphere usually blocks much of the infrared light they record with the large telescope, on this night, the signals came through with little interference. The scientists were looking for evidence of minerals on an asteroid that might have been altered by contact with liquid water in the distant past. The asteroid, called 24 Themis, was a high priority target because it is the largest member of a group of smaller asteroids that sometimes behave like comets, which are known to contain ice.
For the first time, the infrared measurements taken at the IRTF showed the characteristics of frozen water on Themis.
"To analyze our measurements made with the telescope, we calculated a synthetic spectrum using water ice and other materials we thought could be present on the asteroid," said Pinilla-Alonso. "Through a process of elimination and repeated calculations, we confirmed the presence not only of frozen water, but of complex organic materials as well."
Although, the frozen water on asteroid 24 Themis should have disappeared into space more than a billion years ago, it is still seen uniformly covering the surface. "This discovery tells us that water vapor is slowly leaking out of the inside of the asteroid at the present time and freezing on the surface in a paper-thin layer that we can measure with our telescope," observed Pinilla-Alonso. “We also see the signs of the organic materials that are necessary for the formation of life on Earth."
Other scientists have been scouring the solar system for other worlds where ice and organic materials occur, including the moons of Saturn and Jupiter. Dale Cruikshank, a research scientist at NASA's Ames experienced in these studies, and a colleague of Pinilla-Alonso, noted that "Dr. Pinilla-Alonso is one of a small band of planetary scientists pushing the limits of our knowledge of the tiny rocks and icy lumps in the solar system. These small objects are of growing importance in our understanding of the current state of all the planets, including our own planet Earth.”
Ruth Dasso Marlaire
Ames Research Center, Moffett Field, Calif.