DURHAM, N.H. -- A University of New Hampshire
space scientist examining the atmosphere of the
Moon says new findings might prove useful in future
colonization of the lunar surface.
Antoinette Galvin, UNH research associate professor in the Institute for the Study of Earth, Oceans and Space, is co-author of a research paper detailing new measurements of small amounts of oxygen, silicon and aluminum in the moon's atmosphere. The findings are based on observations by the Suprathermal Ion Composition Spectrometer (STICS) aboard the WIND spacecraft and appear in the American Geophysical Union's publication "Geophysical Research Letters."
Galvin, who recently joined the UNH faculty from the University of Maryland, is the lead co-investigator of the STICS sensor.
She explains that the moon does not have an atmosphere like Earth. "If you look in almost any textbook listing planetary atmospheres, the moon will be listed as having 'none' or 'essentially none,'" she points out.
The low lunar gravity (one-sixth that of Earth) means that the moon has a smaller "escape velocity" -- 1.5 miles per second compared to the Earth's 7 miles per second -- "so almost any atmosphere the moon gets, it immediately drifts off or escapes into space. That means the atmosphere must be continuously replenished."
The lunar atmosphere is also only 1/100,000,000,000,000 that of Earth, and in fact is often called the exosphere. For planets, the exosphere is the tenuous part of the atmosphere beyond the ionosphere that blends into space, says Galvin. "The Earth's exosphere starts at 480 kilometers up. For the moon, you have the surface of the moon and -- bang! -- the exosphere right next to it."
The lunar atmosphere was first found by the Apollo 17 crew, which detected helium and argon. Ground-based observations later found sodium and potassium, but the question remained: since these elements made up less than 10 percent of the moon's atmospheric density, what other elements were there?
In answer to that question, STICS detected what are called "lunar pick-up ions." Galvin explains that when atoms are struck by ultraviolet radiation from the Sun, they absorb the energy and kick out an electron, becoming ions. "Ions are subject to the solar wind electric and magnetic fields -- we call it being 'picked up' by the solar wind," says Galvin. The pick-up ions can receive quite a boost in speed this way and accelerate up to twice the 400-kilometer-per-second solar wind speed.
It was through its detection of pick-up ions that STICS identified the oxygen, silicon and aluminum.
"But where does this atmosphere come from?" asks Galvin. "The composition gives us a clue."
The helium probably comes from the solar wind hitting the moon, she explains. "In this way, some of the solar wind becomes part of the lunar atmosphere. But the other elements observed, both on the ground and by STICS, are either from objects impacting the moon and vaporizing, or from the moon itself." Some may be due to out-gassing from the lunar interior. As for the aluminum and silicon seen by STICS, the origin is most like sputtering: the solar wind hits the lunar surface, and knocks out atoms from that surface.
The STICS paper does not speculate as to the original source of the lunar oxygen. It may come from the disassociated molecules that contain oxygen, such as water, or like the aluminum and silicon may come from sputtering off the surface. Galvin points out that NASA's Lunar Prospector recently confirmed the presence of ice on the moon, and some theorize that the ice seen at the lunar poles may be accumulated from this thin atmosphere.
Scientists now believe that water molecules -- whether out-gassed or left from comet impacts -- migrate to colder regions, where they condense as ice back into the ground. They suggest this is the most likely way that ice may have collected in the polar shadows.
"So if the moon was really airless, there would be no collected H2O as ice," says Galvin. "This ice may be used by future colonists."
She adds, "The surface itself may be a colony's source for metals, such as the aluminum seen by STICS, so these aspects also could be important for colonization of the moon."
Lunar exploration also can help us learn more about other heavenly bodies, says Galvin.
"The moon is not the only 'airless' body that in fact has a thin atmosphere. Some planets, asteroids and other moons have similar atmospheres. Any flyby or orbit of these objects by other spacecraft should look for pick-up ions. Then one will get an idea of the origin of the atmosphere and even some information on the composition of the surface."
By Carmelle Druchniak
UNH News Bureau
Toni Galvin can be contacted at 603-862-3511 or at 603-431-0492.