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Radar Data and Lunar Polar Volatiles

Paul D. SpudisLunar and Planetary Institute

18 November 2015

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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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Peary floor

76 km diameter

CPR imageMRF S1

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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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Polar Lighting, Neutron, Radar Data

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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