August 16, 2012
LRO Spectrometer Finds Traces Of Helium Around The Moon
April Flowers for redOrbit.com - Your Universe Online
Using the Lyman Alpha Mapping Project (LAMP) spectrometer aboard NASA's Lunar Reconnaissance Orbiter (LRO), scientists have made the first spectroscopic observations of the noble gas helium in the atmosphere surrounding the Moon. These remote-sensing observations complement the situ measurements taken in 1972 by the Lunar Atmosphere Composition Experiment (LACE) deployed by Apollo 17.
LAMP was originally designed to map the lunar surface, but the team expanded its science investigation to examine the far ultraviolet emissions visible in the tenuous atmosphere of the Moon, detecting helium over a campaign spanning more than 50 orbits. Several techniques were applied to remove signal contributions from the background helium and determine the amount of helium native to the Moon. This was necessary because helium also resides in the interplanetary background.
The team's research findings, "Lunar Atmospheric Helium Detections by the LAMP UV Spectrograph on the Lunar Reconnaissance Orbiter," are published in Geophysical Research Letters.
"The question now becomes, does the helium originate from inside the Moon, for example, due to radioactive decay in rocks, or from an exterior source, such as the solar wind?" says Dr. Alan Stern, LAMP principal investigator and associate vice president of the Space Science and Engineering Division at Southwest Research Institute, Boulder, Colo.
Dr. Paul Feldman of Johns Hopkins University led a related study whose observations showed day-to-day variations in helium abundances, possibly varying with the solar wind, and also decreasing significantly when the Moon passed Earth out of sight from the solar wind. Feldman's study was published in Icarus.
"If we find the solar wind is responsible, that will teach us a lot about how the same process works in other airless bodies," says Stern.
If spacecraft observations show no such correlation, radioactive decay or other internal lunar processes could be producing helium that diffuses from the interior or that is released during lunar quakes.
"With LAMP's global views as it moves across the Moon in future observations, we'll be in a great position to better determine the dominant source of the helium," says Stern.
Helium abundances provide a further point for future research. In the 1970's, LACE measurements showed an increase in helium abundances as the night progressed. This could be explained by atmospheric cooling, which concentrates atoms at lower altitudes. LAMP will build on those early measurements by investigating how the abundances vary with latitude.
LACE also detected the noble gas argon on the lunar surface. Although argon is significantly fainter on the spectrograph, LAMP will also seek argon and other gases during future observations.
"These ground-breaking measurements were enabled by our flexible operations of LRO as a Science Mission, so that we can now understand the Moon in ways that were not expected when LRO was launched in 2009," said Richard Vondrak, LRO Project Scientist at NASA's Goddard Space Flight Center, Greenbelt, Md.
NASA Goddard developed and manages the LRO mission. LRO's current Science Mission is implemented for NASA's Science Mission Directorate. NASA's Exploration Systems Mission Directorate sponsored LRO's initial one-year Exploration Mission that concluded in September 2010.
Image 2 (below): The Lyman Alpha Mapping Project (LAMP) aboard LRO (shown here in a pre-flight photo) uses a novel method to peer into the perpetual darkness of the moon's so-called permanently shadowed regions. LAMP "sees" the lunar surface using the ultraviolet light from nearby space and stars, which bathes all bodies in space in a soft glow of ultraviolet light. Credit: NASA Goddard/Debbie McCallum