Dry Ice Found on Uranian Moon
11 Oct 2002
(Source: American Astronomical Society - Division for Planetary Sciences)
DPS Press Release
Embargoed until 9:30 AM, Fri., Oct. 11, 2002 Planetary scientists have discovered carbon dioxide ice on the surface of Ariel, one of the moons of the planet Uranus. The results were presented by Will Grundy of Lowell Observatory (Flagstaff, AZ) and Leslie Young and Eliot Young of Southwest Research Institute (Boulder, CO) at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Birmingham, Alabama.
The researchers observed Ariel at NASA's Infrared Telescope Facility (IRTF), perched atop Hawaii's 14,000-foot Mauna Kea. They detected the carbon dioxide ice (also known as dry ice) by its characteristic pattern of near-infrared absorption bands.
"Carbon dioxide ice is not very colorful to human eyes," Dr. Grundy noted. "It looks whitish, just like ordinary water ice. But as with water ice and lots of other kinds of ice, dry ice gets much more colorful at wavelengths slightly longer than our eyes can see, making it relatively easy to distinguish using an infrared camera."
The discovery marks the first detection of dry ice on a Uranian moon, the surfaces of which are mostly composed of water ice. Carbon dioxide ice has recently turned up in other places, including Neptune's largest moon (Triton), the Martian polar caps, and two of Jupiter's moons. It is also found in comets and in some interstellar dust particles.
The distribution of Ariel's dry ice revealed an additional curiosity. Just as Earth's Moon always shows the same face to the Earth, Ariel always shows the same face to Uranus. Each of these moons has a leading side and a trailing side as they orbit their respective planet. The leading side received more bombardment by meteors, just as a car's front windshield is struck by more insects than the rear window. Grundy and his collegues observed both leading and trailing sides of Ariel, but dry ice only appeared on the trailing side.
A possible explanation for the dry ice being on the trailing hemisphere only is that the dry ice was originally distributed uniformly over the surface, but over time was buried or destroyed by the more intense bombardment of meteors on the leading hemisphere.
Another process could be involved as well. A plasma of protons and electrons surrounds Uranus, circulating at the same rate as the planet spins, with a period of 17 hours. Ariel's orbit around Uranus takes a much longer 60 hours, so the faster-moving plasma overtakes the moon from behind. The dry ice on Ariel's trailing side could be related to the greater plasma irradiation on the trailing side.
"We're still working to figure out what combination of processes controls the distribution of the dry ice," Grundy said, "but it is exciting to have directly detected carbon on Ariel's surface, in addition to the oxygen and hydrogen of its water ice. The four main elements of life are carbon, oxygen, hydrogen, and nitrogen. Being based on at least three of these four key building blocks, Ariel's surface chemistry could be remarkably complex."
The research was supported by grants from the National Science Foundation's Life in Extreme Environments program and by NASA's Planetary Geology and Geophysics program.