Bistatic Radar Observations Of The Moon Using The Arecibo Observatory & Mini-RF On LRO
Abstract
Introduction: The Mini-RF team is acquiring bi-static radar measurements that will test the hypothesis that permanently shadowed areas near the lunar poles contain water ice. These bistatic observations (where the Arecibo Observatory Planetary Radar (AO) trans-mits a 12.6 cm wavelength signal, which is reflected off of the lunar surface and received by the Mini-RF instrument on LRO) have produced the first lunar non beta-zero radar images ever collected. Rationale: Typically, orbital radar observations use the same antenna to both transmit and receive a signal. The angle between the transmitted and re-ceived signals (the bistatic, or beta angle) for these observations is therefore zero, and they are referred to as monostatic observations. By using the AO radar as the transmitter and Mini-RF as the receiver, we have the opportunity to collect data for the Moon with beta angles other than zero. These measurements provide a new and unique test of the water ice hypothesis for the Moon. A common science product produced using planetary radar is the Circular Polarization Ratio (CPR). CPR is the ratio of the powers of received sig-nal in the same sense transmitted divided by the oppo-site sense. Typical dry lunar surface has a CPR value less than unity. Higher CPR signals can result from multiple-bounce backscatter from rocky surfaces or from the combined volume scattering and coherent backscatter opposition effects (CBOE) from an ice/regolith mixture. The physics of radar scattering predict that high CPR caused by a rocky surface will be relatively insensitive to the beta angle, whilst high CPR caused by ice will be very sensitive to beta, with elevated CPR values dropping off abruptly at beta an-gles greater than about 1-2°. Mini-RF monostatic data shows many craters with high CPR values. Most of these features are associated with fresh, young craters and display elevated CPR both inside and outside their rims. Some permanently shadowed craters near both poles show elevated CPR inside the crater rims but low CPR outside the crater rim. This has been interpreted as being consistent with RF backscatter caused by surface roughness in the former case and water ice in the latter. Planned Observations: We are imaging both po-lar, and non-polar targets that have high monostatic CPR values. By acquiring non beta zero data of equa-torial high-CPR regions (which we can safely assume have high CPR due to the presence of surface rocks) we can confirm the hypothesis that high CPR caused by rocks is reasonably invariant to the beta angle. We are looking to see if monostatic high-CPR polar craters have high or low values in the bistatic data. If we find areas that become low only in the bistatic data then this provides strong supporting evidence that these are ice deposits. Conclusions: Using Arecibo and Mini-RF we are acquiring the first ever planetary bistatic radar images at non β=0 angles. These data provide a unique new piece of evidence to determine if the Moon's polar craters contain ice.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.P43B1919B
- Keywords:
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- 5462 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Polar regions;
- 6250 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Moon