BISTATIC RADAR EXPERIMENT

I. Background

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 Clementine, the experiment consisted of having the spacecraft transmit an S-band radio signal through its high gain antenna towards a lunar target.The signals reflected off the Moon and were received by a 70 meter Deep Space Network (DSN) antenna on the Earth.

Frozen volatiles such as water ice are much more reflective to S-band radio waves than lunar rocks so that waves have different characteristics when reflected off ice than off silicate rock A silicate surface tends to scatter radio waves in all directions. So, some of the energy does not travel back to the receiving antenna. 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, such as ice.

This peculiar effect arises in part due to a property of light termed coherent backscatter. Two photons that enter the ice in phase, ricocheting along the same complex path once inside but traveling in opposite directions, will combine coherently and amplify the energy reflected toward the receiver. For this reason, radar echoes from icy Europa are 30 times stronger than those from the slightly larger but soil-covered Moon. Coherent backscatter alone would not raise the "ice is here" flag, because the same physics is at work in the rough lunar regolith and helps explain why the full Moon looks brighter than expected based on geometry alone (S&T: April 1993, page 14).

An analysis of the signals returned from orbit 234 showed reflection characteristics suggestive of water ice for the permanently shadowed areas near the south pole. Reflections from regions which are not permanently shadowed do not show these characteristics. It is possible that other scattering mechanisms could be responsible for this result, but the interpretation of the radio returns and the fact that they are associated only with the permanently shadowed regions seem to indicate that water ice is the most likely possibility. However, Arecibo radio telescope studies using the same radio frequency as Clementine showed similar reflection patterns from areas which are not permanently shadowed. These reflections have been interpreted as being due to rough surfaces, suggesting that the Clementine results may be due to roughness, rather than water ice, as well.

II. Orbit 234

Bistatic Radar Experiment Parameters
9-10 April 1994

Transmission .......... S-Band 2.273 GHz (13.19 cm)
Polarization ............ Right Circular (RCP)
Signal Power .......... 6 Watts
Axial Tilt ............... 4.5 to 5.5 degrees (Moon to Earth)
Orbits Used ........... 234 and 235

For a circularly polarized radar beam, the sense of polarization reverses during a mirror-like bounce off a rocky surface, Steven J. Ostro (Jet Propulsion Laboratory) says, but when passing through ice the beam emerges with its initial polarization largely intact. Greenland's ice sheet, the icy Galilean satellites, and the polar caps of Mars and Mercury all share this property. Clementine's first foray into radar astronomy failed in March 1994, but during a second trial in April one radar track swept directly over the south pole. "We lucked out immensely," Nozette says. Not only did the echo prove stronger than expected, but the all-important polarization ratio also showed a modest peak precisely when the shadowed areas were within the radio beam. The echo from the next orbit, which passed about 200 km from the pole, showed no enhancement, nor did a pair of scans over the north pole.

The most tantalizing radar echoes occurred when the phase angle (beta) was near 0°. The evidence for lunar ice hinges on how the radar beam's polarization changed when it struck the lunar surface. For pure ice, the echo would have been at least as strong in the original polarization (same sense, or SS) as in the opposite sense (OS). When SS/OS is less than 1, as here, the detection is less certain. However, radar specialists are encouraged that the ratio peaked during orbit 234--when the lunar south pole was within the radar beam.

This graph shows the power of reflected radio energy for four orbits by the Clementine spacecraft plotted against the angle made by lines from the spacecraft to Earth and from the spacecraft to the target on the Moon. Orbits 301 and 302 were data taken over the north pole of the Moon (where very little permanently dark area is seen), while data from orbits 234 and 235 were taken in the vicinity of the south pole. If ice is present, the power should have a distinct "peak" around beta = 0 (the "bicycle reflector" effect), when spacecraft, target, and Earth all are aligned.

Note that a peak is seen only on orbit 234, which is the orbit directly over the dark areas near the south pole of the Moon. This graph is our evidence for ice at the south pole of the Moon.

The lack of a peak is expected for all but the dark regions on the Moon because they are all illuminated by the Sun during the course of the lunar day. The total area of permanent darkness near the south pole exceeds 15,500 km2, about twice the areal extent of the island of Puerto Rico.The total amount of water is difficult to determine, but using the amount of permanently-shadowed area and the strength of the radar signal, we estimate that the total volume of ice is about 1 km3, an amount of water equivalent to that of a sizable lake.

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