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Moon mission: Can it solve the mystery of the glowing horizon?

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The mission fills a gap in recent US lunar-exploration missions. Since 2007, seven spacecraft have visited the moon to measure space weather around it; map its surface and the distribution of potential mineral resources, including water, there; and measure its gravity field to reveal the moon's interior structure.

The atmosphere is the only layer of this onion left largely unexplored.

The notion that the moon has an atmosphere runs counter to what most people learned in school about Earth's companion, noted Sarah Noble, the mission's program scientist, during a recent briefing at NASA headquarters.

But it does have one, she explained. "It's just really, really thin."

(Wait for it, wait for it....) How thin is it?

"It's so thin that the individual molecules that make up the atmosphere molecules are so few and far between that they don't interact with each other; they never collide," she said.

On Earth, a cubic centimeter of atmosphere at the surface hosts about 100 billion billion molecules. On the moon, the atmosphere – or, more properly, its surface "exosphere" – has perhaps 100,000 to 10 million molecules in each cubic centimeter. [Editor's note: This paragraph originally included the wrong atmospheric density for Earth and the moon.]

Those molecules come from several sources, Spudis explains. Gas escapes from the moon's interior through faults on the surface. Tiny micrometeoroids smack into tiny grains on the moon and vaporize them via the heat from the collision. This allows some atoms and molecules to escape as gas before the material chills and returns to solid form. And charged particles in the solar wind, which constantly stream from the sun, strike the lunar surface to form other gases.

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