Image courtesy SDO/NASA
Published December 9, 2011
The sun is constantly emitting charged particles, or ions, in all directions in a stream called the solar wind. Scientists previously knew that solar ions can collide with and eject material on the moon's surface in a process dubbed sputtering.
But a new computer simulation finds that this sandblasting effect kicks into high gear during intense bursts of solar plasma—charged gas—known as coronal mass ejections (CMEs).
A strong CME can hurl about a billion tons of solar particles at up to a million miles (1.6 million kilometers) an hour in a cloud that is many times the size of Earth.
Normal solar wind is made up mostly of lightweight protons—hydrogen atoms that have been stripped of their electrons. But CMEs contain a much higher percentage of heavier ions such as helium, oxygen, and even iron.
These heavier atoms slam into the moon with greater force than protons, so they can dislodge a larger number of atoms from the surface.
"We found that when this massive cloud of plasma strikes the moon, it acts like a sandblaster and easily removes volatile material from the surface," study co-author William Farrell, leader of the Dynamic Response of the Environment At the Moon, or DREAM, team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.
"The model predicts 100 to 200 tons of lunar material—the equivalent of ten dump truck loads—could be stripped off the lunar surface during the typical two-day passage of a large CME."
Once ejected, about 90 percent of sputtered moon particles escape into space, where they become ionized and are drawn into the solar wind, said study co-author Rosemary Killen, also of NASA Goddard.
"The material is in atomic form," Killen added in an email to National Geographic News. "It is not meteoric and does not produce meteor showers" on Earth.
Sputtering Can Help Probe Moon's Chemistry?
CMEs are most likely to happen during solar maximum, a period of high magnetic activity on the sun that occurs about once every 11 years.
"The more active the sun, the more often coronal mass ejections take place," said Richard Elphic, a planetary scientist at the NASA Ames Research Center in California who was not involved in the study.
"You can have at the height of solar maximum a [large] CME maybe every week or so, and in some cases every few days. And that might be followed by a couple weeks of quiet."
Right now the sun is ramping up toward the next predicted solar maximum in 2013—which might aid research that uses sputtering to reveal clues about lunar chemistry.
A new moon orbiter scheduled to launch in 2013, called the Lunar Atmosphere And Dust Environment Explorer, or LADEE, could test the new model's predictions.
If the simulation is correct, then CME sputtering should loft lunar surface atoms to LADEE's orbital altitude, around 12 to 30 miles (20 to 50 kilometers).
"As the LADEE project scientist," Elphic said, "my excitement is in using these CME 'scavenging events' as active probes of the sputtering process, to learn what [types of atoms] are liberated and how long they stick around before equilibrium returns."
Moon Landing Footprints in No Danger
While the new model hints that the amount of lunar material stripped off by sputtering is more than scientists had thought, the amount of lost material is still very small compared to the total mass of the moon.
Also, the loss of lunar surface material is more or less balanced by incoming particles from micrometeorites, meteors, and the solar wind itself.
That means Earth's only natural satellite—and the features on it—are in no danger of being eroded away anytime soon. (Also see "The Moon Has Shrunk, and May Still Be Contracting.")
"The astronaut's footprints will still be there in recognizable form after a million years—if we're still around to see them," Elphic said.
The lunar-sandblasting effect will be described in an upcoming issue of the Journal of Geophysical Research—Planets.
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