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Solar storms have great impact on lunar surface

Dr Amanda Doyle, Feature writer
Dec 10, 2011, 8:00 UTC

Sen— - NASA scientists have been investigating the complex interplay between high energy solar particles and the lunar surface.

While our Moon is not massive enough to retain an Earth-like atmosphere, it does have a tenuous atmosphere, known as an exosphere. The Moon is also lacking in a protective magnetic field, so when high energy particles expelled from the Sun in a Coronal Mass Ejection (CME) pay a visit, they impact directly with the lunar surface. This collision causes atoms from the surface to be ejected in a process known as sputtering. The exact nature of these reactions is not fully understood, and it is the aim of NASA’s Dynamic Response of the Environment At the Moon (DREAM) team to fill in the blanks, particularly as the information will be useful for future human exploration.

Our Sun does not sit quietly in the sky. As well as a constant stream of particles known as the solar wind, it also occasionally exhibits flares and CMEs. A flare is a local release of energy from the Sun, while CMEs occur on a much greater scale, with the size of the eruption sometimes being larger than the Sun itself. CMEs can expel billions of tonnes of plasma, which is an ionised gas, that travels through the Solar System at a speed of over 1.5 million kilometres per hour. At the Earth CMEs produce the aurora borealis, but they can also have negative effects on electronic equipment and satellites.

The ordinary solar wind is comprised mainly of lightweight hydrogen ions. Ions are atoms that either have electrons removed or added, which gives the ion an electrical charge. A CME contains five times more helium ions than the solar wind, and these have a greater charge than hydrogen ions as they have more electrons removed. This means that they pack more of a punch on the lunar surface, causing increased sputtering. A CME also moves much faster than the solar wind and the particles are more densely packed, meaning it has a more pronounced effect with anything it impacts.

“The observations that originally led us to suggest that the solar wind is important in ejecting material into the exosphere were the ground based observations of the sodium exosphere of Mercury which show occasional high latitude enhancements,” explains Rosemary Killen, researcher with the DREAM team. “The Moon is a simpler case than Mercury since Mercury has a magnetosphere that prevents the solar wind from impinging onto the surface except in the open cusps (sort of like the auroral regions on Earth). The Moon is fully exposed to the solar wind, so that the CME would affect the entire dayside surface.”

According to the DREAM computer models, in the typical two day duration of a CME several hundred tonnes of volatile lunar material are blasted from the surface. There are several causes of the lunar exosphere, but during a CME sputtering becomes the main culprit.

“When a micro-meteoroid hits the moon, it releases vapour that contributes to the exosphere,” team leader William Farrell tells SEN. “Also, for atoms loosely bound to the surface (volatiles), sunlight alone can provide enough energy to break their bond and release the volatile atom into the exosphere. This process has a formal name called 'photon stimulated desorption' ...for sodium and other loosely bound volatile atoms, this process can release the gas, especially at dawn. So in normal conditions, there are a number of processes to consider...we like to think of the list as the 'usual suspects' ...but when a CME passes, sputtering becomes dominant in creating a bulked up exosphere.”

Sputtering effects should be detectable by NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE), due to launch in 2013, as surface particles rise to meet the orbiter. In addition, the lunar exosphere should be observable from the Earth. “We have proposed to build a remotely operated telescope with a coronagraph to observe the moon on every clear moonlit night,” says Killen. “In this way we will have observations whenever a CME hits the moon, and we will be able to measure its effect. The sodium exosphere can easily be observed from the ground.  We have observed the sodium exosphere of the moon with ground based telescopes but not on a regular basis.”

Mars, like the Moon, also has no magnetic shielding leaving it vulnerable to CMEs. However in this case the solar particles collide with Mars’ upper atmosphere, causing the gases to be lost to space. The Mars Atmosphere and Volatile Evolution (MAVEN) mission will investigate these effects further, upon being launched by NASA in 2013.