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Radar as an ice detectorActive sensing technique that “sees” into the
permanently dark areas near the polesMini-SAR and Mini-RF are hybrid
architecture polarimetric radars, S-band(12.5 cm) that provide Stokesparameters for diffuse backscatter
Mapped both poles of the Moon (> 80° lat.)at optimum viewing angles (~40°)
Characterize the physical nature of the polarregolith and surface
SAR mapping of about 2/3 of remaininglunar surface - image a variety ofterrains of varying ages to providecomparative data base
Mini-RF bistatic experiment - illuminateMoon with Arecibo transmitted S-bandradar and receive on LRO MRFinstrument
Radar detects surface roughness and RFtransparent media. All factorsaffecting response in real geologicalsettings are not fully understood
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Circular Polarization Ratio (CPR)Ratio of received power in both
right and left sensesNormal rocky planet surfaces =
polarization inversion(receive opposite sense fromthat transmitted)
“Same sense” received indicatessomething unusual:double- or even-multiple-
bounce reflectionsvolume scattering from RF-
transparent material(interferrometric additionof same sense waves -CBOE)
High CPR (enhanced “samesense” reception) is commonfor fresh, rough (atwavelength scale) targetsand water ice
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Distribution of Anomalous CratersHigher density of anomalouscraters in polar regions thanrest of Moon (44 north, 28south vs. 80 over remainingentire Moon)
Strong correlation ofanomalous craters with areasof permanent darkness
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Mini-RF: Bistatic Experiment
The Mini-RF transmitter ceased operatingin December of 2010
Mini-RF collected data in bistatic mode,using the Arecibo Observatory as atransmitter
Provides information on scatteringproperties of lunar materials as afunction of beta (phase) angle
Data of nearside and polar targets hasbeen successfully collected on 23occasions
Goal is to determine if high CPR in polardark craters is caused by ice orsurface roughness
Ice shows strong dependence on betaangle whereas rock does not
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Bistatic ResultsTheory vs. Observation
Bistatic data from Cabeus floorshows increasing CPR withdecreasing β angle
In accord with theoreticalpattern for ice deposits
Background deposits show “flat”pattern with respect to beta, similarto rock targets
Conclude: Significant water icein floor of Cabeus
Clementine bistatic experiment(1994) - found “peak” in CPRsymmetric around β=0
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SummaryWe have successfully mapped more than 98% of both lunar polar areas
with imaging radarAreas of high CPR have been identified:
Some high CPR is clearly associated with surface roughness (e.g.,Main L ejecta blanket)
Some deposits (e.g., near north pole on floor of Peary) show highCPR and are restricted to the interior of craters; these features arein permanent darkness.
Statistical analysis suggests that these features constitute a distinctpopulation from normal, fresh crater high-CPR features
Modeling using new diffuse scattering model suggests anomalouscraters are ice-rich
Anomalous craters are found at both poles and correlate with Pixon modelreconstructions of LP neutron data and areas of low surfacetemperature revealed by DIVINER
If these anomalous deposits are water ice, over 600 million m3 are presentin vicinity of north pole
Bistatic radar observations of Cabeus crater indicate peak at β=0,scattering behavior consistent with the presence of water ice
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