Solar storms 'sandblasting' the moon
Solar storms and Coronal Mass Ejections (CMEs) are powerful enough to erode the surface of the moon, NASA simulations indicate.
And they could be one of the main ways in which planets like Mars, whcih lack a global magnetic field, lose their atmosphere.
A strong CME can contain around a billion tons of plasma - electrically charged particles - moving at up to a million miles per hour in a cloud many times the size of Earth. And because the moon has only the barest wisp of an atmosphere to protect it, when plasma from CMEs impacts the surface, atoms are ejected in a process called 'sputtering'.
"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," says William Farrell, DREAM team lead at NASA Goddard.
"The model predicts 100 to 200 tons of lunar material – the equivalent of 10 dump truck loads – could be stripped off the lunar surface during the typical two-day passage of a CME."
Sputtering had already been identified as one of the top five processes creating the moon's exosphere under normal conditions. However, the new research shows that, during a CME, it becomes the dominant method by far, with up to 50 times the effect of the other methods.
While plasma from CMEs might seem a tenuous substance to be having such a great effect, it's effective at removing lunar material not only because it's denser and faster than the normal solar wind, but also because it's enriched in highly charged, heavy ions, says the team.
The normal solar wind is dominated by lightweight hydrogen ions, or protons. However, a heavier helium ion with more electrons removed, and hence a greater electric charge, can sputter tens of times more atoms from the lunar surface than a hydrogen ion.
The team used data from satellite observations that revealed this enrichment as input to their model.
"The computer models isolate the contributions from sputtering and other processes," says Dana Hurley, a co-author on the paper at the Johns Hopkins University Applied Physics Laboratory.
"Comparing model predictions through a range of solar wind conditions allows us to predict the conditions when sputtering should dominate over the other processes. Those predictions can later be compared to data during a solar storm."
The researchers believe that NASA's Lunar Atmosphere And Dust Environment Explorer (LADEE) - a lunar orbiter mission scheduled to launch in 2013 - will be able to test their predictions. The strong sputtering effect should kick lunar surface atoms to LADEE's orbital altitude, around about 12.4 to 31 miles up.
"This huge CME sputtering effect will make LADEE almost like a surface mineralogy explorer, not because LADEE is on the surface, but because during solar storms surface atoms are blasted up to LADEE," says Farrell.
The team also believes that planets such as Mars can be affected by CMEs. Because Mars does not have a magnetic field surrounding the entire planet, CME gases have a direct path to sputter and erode its upper atmosphere.
In late 2013, NASA will launch the Mars Atmosphere and Volatile Evolution (MAVEN) mission that will orbit the Red Planet to investigate exactly how solar activity, including CMEs, removes the atmosphere.