Saturday, April 23, 2005

Infrared Moon

The Astronomy Picture of the Day brings us this view of our moon:

In September of 1996, the Midcourse Space Experiment (MSX) satellite had a spectacular view of a total lunar eclipse from Earth orbit. SPIRIT III, an on board infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - humans can not see it but feel it as heat. So, the bright spots correspond to the warm areas on the lunar surface, and dark areas are cooler. The brightest spot below and left of center is the crater Tycho, while the dark region at the upper right is the Mare Crisium. Of course, this Sunday's lunar eclipse will not be a total, or even a partial one. Instead, the Moon will glide through the subtle outer portion of the Earth's shadow in a penumbral eclipse of the Moon.

What they don't explain is why features like Tycho are warm, nor (in a brief Google search) could I find any reference that did. During an eclipse, it can't be solar reflectivity. My best guess is that the bright infrared areas are places where higher-than-average amounts of solar radiation is absorbed, thus warming the rocks. During an eclipse, these places would appear "hot" (or bright) in infrared...

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