"Rejected" Protons Offer a New View of the Moon

October 16, 2009

Solar particles "rejected" by the lunar surface are helping astronomers better understand how the sun affects the moon, a new study says.

The moon has virtually no atmosphere, so its surface is constantly being bombarded by solar wind, the charged particles flowing outward in all directions from the sun.

Some researchers think the solar wind could be what creates water on the moon.

"Conceptually, protons which come from the solar wind get absorbed by the surface [of the moon], and they may interact with oxygen, which is in minerals there" to produce water and hydroxyl, said Stas Barabash, of the Swedish Institute of Space Physics in Kiruna.

"Much more research is needed to prove or disprove this idea," he cautioned. (See pictures of moon exploration.)

But in an unexpected twist, data from India's Chandrayaan-1 moon probe show that one out of every five protons that impacts the moon gets bounced straight back into space. Previously astronomers had thought that virtually all solar particles that reach the moon are absorbed by the lunar soil.

(Related: "India's First Moon Probe Lost, But Data May Yield Finds.")

Like taking photographs by capturing light bouncing off an object, Barabash and colleagues hope to make a new class of images by mapping where these rejected protons are bouncing off the moon.

"This method provides a way to really see what happens when the solar wind impacts the surface," Barabash said.

Moon Bounce

When a proton bounces off the moon, it picks up an electron from the lunar surface and becomes a neutral hydrogen atom.

"Because they are neutral, they are not affected by any electromagnetic forces … and the [moon's] gravitational force affects them so little that they continue moving straight forward," Barabash said.

Since the atoms are such straight shooters, they should provide a highly accurate picture of which places on the moon are being most affected by solar wind.

For example, some pockets of the moon have local magnetic fields that shield them from solar wind. These areas wouldn't absorb or reflect protons and so would show up in the new maps as dark spots.

The method should also apply to other solar system bodies that have thin atmospheres, Barabash added.

A similar instrument to the one Chandrayaan-1 used to record the bouncing protons is on the European Space Agency's BepiColombo spacecraft, due to set off toward Mercury in 2013.

"With this technique," Barabash said, "we can directly see areas which are directly accessible by the solar wind on Mercury."

Findings published online September 20 in the journal Planetary and Space Science.

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