March 6, 2010

Water Ice on the Moon

Image: ISRO

With the help of ISRO's Chandrayaan-1 spacecraft and NASA's Mini-RF Synthetic Aperture Radar (SAR) instrument it carried onboard, more than 40 craters have been detected on the Moon to contain water ice. These craters are small and their sizes vary from 2 to 15 km (1 to 9 miles) in diameter. In total, at least 600 million metric tons (1.3 trillion pounds) of water ice are estimated to be in the craters.

I wrote about the Mini-RF SAR instrument last year. The instrument was designed to study craters permanently in the dark. These craters are always hidden from the Earth and never exposed to the Sun. Ice deposits were hoped to be present in these craters, where they would lay protected from the heat of the Sun.

The Mini-RF SAR is basically a radar, utilizing the fact that reflected radio waves can be polarized depending on the reflecting surface's properties. The Mini-RF SAR sends out left-circular polarized radar pulses, and when reflected by typical planetary surfaces, the polarization is reversed so that the pulses are right-circular polarized. Dividing the power of the pulses received in the same polarization as transmitted (left-circular) by the power of the pulses in the opposite polarization (right-circular) gives a ratio called the circular polarization ratio (CPR). Most areas of the Moon have low CPR, since they exhibit the typical behavior of planetary surfaces with the reversal of polarization. The Mini-RF SAR studies the places with high CPR, such as rough and fresh surfaces (which scatter the pulses) and ice (which is transparent to the pulses and also scatters them). But how can the two different types of places (rough vs. ice) be distinguished?

When studying craters, the fresh craters, or those with rough surfaces, were found to have roughness both on the outside and on the inside of the crater rim. The roughness was distributed throughout the whole crater area, inside the craters and in their surroundings, giving high CPR in the whole area. You can see an example of this in the picture below.
Overall High CPR
Image: NASA

In addition to these craters, those with low CPR outside and high CPR inside were also found by the Mini-RF SAR. Therefore, the craters' high CPR just found on the inside is probably not due to roughness, but materials like water ice. The scientists working on this determined that the ice must be relatively pure and at least a few meters thick. This characteristics can be seen in the image below.
Inside, High CPR; Outside, Low CPR
Image: NASA

So what is important about finding water ice on the Moon? It makes the Moon a lot more interesting, and leads to more questions about what led to the presence of water. Adding up the previous discoveries by the Chandrayaan-1 and LCROSS missions makes the presence of water on the Moon not rare but abundant. It also may help us learn more about the formation and composition of the Solar System. Others have analyzed the practical uses for the water as a resource for future Moon missions. The finding will almost definitely impact future lunar research and just how look at the Moon itself.

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