Brief synopsis of recent evidence for ice on the moon

Evidence from the Lunar Prospector spacecraft

September 6, 1999

The Lunar Prospector spacecraft carried an instrument, the neutron spectrometer, which detected many slow neutrons coming from the moon's surface. The simplest explanation is that neutrons in the solar wind bounce off atoms in the lunar soil and are reflected back into space. The neutron spectrometer's results indicate that there is a substantial amount of material with very small atomic weight at some locations in the soil: it could be hydrogen. Hydrogen, in turn, might be bound together with oxygen to form water compounds in the soil.

Visit this site for more information on the Lunar Prospector results, pictures, and links to other information:

NASA crashed the Lunar Prospector spacecraft into a crater near the Moon's South Pole on July 31, 1999, hoping that the impact might kick up a plume of material that could reveal the presence of ice in the permanently-shadowed region of the crater. Several telescopes, on the ground and in orbit, watched the impact site carefully, but so far there's no evidence for the detection of material from the impact. Rats. You can read the whole story at

The main Lunar Prospector WWW site is

Evidence from the Clementine spacecraft

Michael Richmond
December 4, 1996

Here's a very brief explanation of the recent announcements that there is ice on the moon. You can read the full story in the paper published in the Nov 29, 1996 issue of Science (to hit the newstands any minute now), or at

The spacecraft Clementine performed a series of experiments in which it pointed its antenna at the Moon (instead of an Earth station), and transmitted at 13-cm with Right Circular Polarization (RCP). The waves bounced off the Moon, and were picked up by a radio antenna here on Earth. The idea was to look for special characteristics in the signal, which might indicate properties of the lunar surface.

For example, some materials, with high reflectivity to radio waves and lots of small cracks or crannies, would give rise to an especially strong return signal when the radio waves were reflected exactly back whence they came; that is, when the spacecraft was EXACTLY between the reflecting lunar surface and the Earth. One material with these characteristics is water ice ... but there are others.

On one of its orbits around the Moon, the Clementine spacecraft was positioned exactly between the Earth and the south lunar pole (where there are areas perpetually shaded from the Sun). It had to bounce the radio signals off the surface at a very oblique angle -- that is, it pointed its antenna at the "edge" of the Moon, not the portion directly beneath it. This large angle of incidence can make the results harder to interpret.

The reflection from THIS orbit and area yielded strong backscattering, and the received radio waves had much stronger Right Circular Polarization than Left Circular Polarization. Reflections from other areas _near_ the south lunar pole didn't show such properties, nor did reflections from the north lunar pole (which has a smaller area in permanent shadow).

Conclusion: there _may_ be ice deposits at or near the surface of permanently-shadowed craters near the south lunar pole. However, the data could be produced by other materials with similar radar-reflecting properties.

Lunar Prospector, to be launched in Oct 1997, will orbit over this area (and others), and carries a set of neutron-measuring instruments which may provide additional evidence that the material in this part of the moon is truly ice. Until then, we have to wait.

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