Exploring the craters at the moon's north and south poles may be even more challenging than previously thought for future astronauts. New NASA calculations now show that solar wind streaming over the rough lunar surface may electrically charge polar craters on the moon.
The moon's polar craters are of particular interest to researchers because resources, including water ice, exist at these lunar structures. The moon's orientation to the sun keeps the bottoms of polar craters in permanent shadow, allowing temperatures there to plunge below minus 400 degrees Fahrenheit (minus 240 degrees Celsius), cold enough to store volatile material like water for billions of years.
"However, our research suggests that, in addition to the wicked cold, explorers and robots at the bottoms of polar lunar craters may have to contend with a complex electrical environment as well, which can affect surface chemistry, static discharge, and dust cling," said William Farrell of NASA's Goddard Space Flight Center in Greenbelt, Md., the lead author of the study.
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These new observations contribute to our evolving understanding of the moon, said Gregory Schmidt, deputy director of the NASA Lunar Science Institute at NASA's Ames Research Center in Moffett Field, Calif.
"This important work by Dr. Farrell and his team is further evidence that our view on the moon has changed dramatically in recent years," Schmidt said. "It has a dynamic and fascinating environment that we are only beginning to understand."
Solar wind hits the moon
Solar wind is a thin stream of electrified components of atoms – negatively charged electrons and positively charged ions – that constantly blows from the surface of the sun into space. Since the moon is only slightly tilted compared to the sun, the solar wind flows almost horizontally over the lunar surface at the poles, and along the region of the moon called the terminator, where day transitions to night.
The researchers discovered that in some ways, solar wind behaves like wind on Earth – flowing into deep polar valleys and crater floors. But, unlike wind on Earth, the dual electron-ion composition of the solar wind may create an unusual electric charge on the side of the mountain or crater wall; that is, on the inside of the rim directly below the solar wind flow.
As solar wind flows into craters, it can erode the surface, which affects recently discovered water molecules. Static discharge could disturb sensitive equipment, while the sticky and extremely abrasive lunar dust could wear out spacesuits and may even be hazardous if tracked inside spacecraft and inhaled by astronauts over long periods of time.
Since electrons are over 1,000 times lighter than ions, these lighter electrons in the solar wind rush into a lunar crater ahead of the heavier ions, creating a negatively-charged region within the crater.
The ions eventually fill the crater, but at consistently lower concentrations than that of the electrons. This imbalance in the crater makes the inside walls and floor acquire a negative electric charge. The researchers calculated that the electron/ion separation effect is most extreme on a crater's leeward edge – along the inside crater wall and at the crater floor nearest the solar wind flow.
Along this inner edge, the heavier ions have the greatest difficulty reaching the surface, unable to make the sharp turns over the mountain tops the way the lighter electrons can.
"The electrons build up an electron cloud on this leeward edge of the crater wall and floor, which can create an unusually large negative charge of a few hundred volts relative to the dense solar wind flowing over the top," Farrell said.
Still, the negative charge along this leeward edge won't build up indefinitely. Eventually, the attraction between the negatively-charged region and positive ions in the solar wind will cause some other unusual electric current to flow.
Electrified moon dust
The research team believes that a possible source for this current could be negatively-charged dust that is repelled by the negatively-charged surface that then gets levitated and flows away from the electrified region.
"The Apollo astronauts in the orbiting Command Module saw faint rays on the lunar horizon during sunrise that might have been scattered light from electrically lofted dust," Farrell said. "Additionally, the Apollo 17 mission landed at a site similar to a crater environment – the Taurus-Littrow valley. The Lunar Ejecta and Meteorite Experiment left by the Apollo 17 astronauts detected impacts from dust at terminator crossings where the solar wind is nearly-horizontal flowing, similar to the situation over polar craters."
To further this research, scientists want to create more complex computer models.
"We want to develop a fully three-dimensional model to examine the effects of solar wind expansion around the edges of a mountain," Farrell said. "We now examine the vertical expansion, but we want to also know what happens horizontally."
NASA is planning to launch the Lunar Atmosphere and Dust Environment Explorer (LADEE) as early as 2012, in a mission that will orbit the moon and could look for the dust flows predicted by the team's research
The details of the study were published March 24 in the Journal of Geophysical Research. The research is part of the Lunar Science Institute's Dynamic Response of the Environment at the moon (DREAM) project.
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