Planetary Science Research DiscoveriesDecember, 1996

Graph showing results of the Clementine radar experiment with a discussion on radio energy immediately below.

energy graph

Radio energy waves were sent to the Moon from the Clementine spacecraft during four orbits. Each orbit passed over a different area. Orbits 301 and 302 passed over the north polar region. Orbits 234 and 235 passed over the south polar region. Note the striking peak in the power curve of Orbit 234. What does it mean?


In general, radio energy waves can be reflected or scattered by whatever target they encounter. If a receiving antenna is set up to collect the radio energy traveling back from a target, then the power of the returned energy can be used to interpret the characteristics of the target. In the case of the Clementine experiment, the radio waves were received on the large dish antennas of the Deep Space Network here on Earth!

A silicate (rocky) surface tends to scatter radio waves in all directions. So, some of the energy does not travel back to the receiving antenna; notice the data curves with lower energy (lower down on the graph). Higher energy curves mean that something is causing more radio energy to travel back in the direction of the receiving antenna. That "something" could be flat surfaces which act like mirrors, bouncing energy waves in a particular direction. In this case, the geometry of the target is very important to return the radio energy to the antenna. Or that "something" could be internal reflections that enhance the radio energy reflected and scattered from a target.

Ice is Nice

We know that ice is partly transparent to radio energy, so the radio waves penetrate the ice and scatter from internal reflections and impurities. An energy peak at beta=0, when the spacecraft, target, and receiving antenna on Earth are all aligned, is indicative of ice.

Orbit 234 data curve is interpreted to be due to ice.

Only orbit 234 passed over the permanently shadowed area of the Moon's south pole.

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