Radar Properties and Block Abundance of Impact Craters on the Moon

Maria Arias de Saavedra BenitezDuke University

LPI Visiting Summer Undergraduate Student

Advisors: P.D. Spudis and S.M. Baloga

Project Goals

• Studying the features of a number of craters with an anomalous radar signal identified by previous work, creating a detailed database

• Contribute to our understanding of how block fields are created in the Moon and how they evolve

• Determine how these craters differ from polar, permanently shadowed craters with similar radar signal but which are candidates for ice

Objectives

• Understand the mechanisms of diffuse backscatter in radar images of lunar craters

• Determine the density of decimeter-scale (~10 cm) rocks in relation to impact craters

• Determine how such rock densities correlate (or not) with Mini-RF measurements of circular polarization ratio

• Use these results to distinguish high-CPR ice deposits from blocky impact ejecta in radar images of the poles

Data Used

• LRO-MRF (S band λ=12.6 cm, 30 m/pixel, 48 incidence) for measuring CPR

• The Lunar Reconnaissance Orbiter Narrow Angle Camera images(0.5-1.6 m/pixel) for counting blocks

Mini-RF Imaging Radar on Chandrayaan-1 and LRO

Mini-RF is a two frequency (S-band (12.6 cm) and X-band (4.5 cm) imaging radar with hybrid polarity architecture

Map both polar regions at 30 m/pixel, 48 incidence

Transmit LCP, receive H and V linear, coherently

Use Stokes parameters and derived “daughter” products to describe backscattered field

Map locations and extent of anomalous radar reflectivity

Cross-correlate with other data sets (topography, thermal, neutron)

Circular Polarization Ratio (CPR)

Ratio of received power in both right and left senses

Normal rocky planet surfaces = polarization inversion (receive opposite sense from that transmitted)

“Same sense” received indicates something unusual:

double- or even-multiple-bounce reflections

Volume scattering from RF-transparent material

High CPR (enhanced “same sense” reception) is common for fresh, rough (at wavelength scale) targets and water ice

Radar Data Collection Procedure

• Identify and collect Mini-RF images from Planetary Database System (PDS)

• Convert images to raw files via USGS “ISIS” imaging software

• Analyze images with NIH “ImageJ” • Record mean and σ for each distribution

CPR Values:Results for Gardner crater

All Floor

Wall Exterior

Rock Count Data Collection Procedure

• LROC and NAC images obtained from Quickmap (LROC image browser)

• Images orthographic map projected with ISIS

• Analyze with feature function of ArcGIS, taking long dimensions of blocks and counting at least ~200 rocks per crater.

• Areas coincide with CPR areas as closely as possible

Gardner Crater Rock Counting

Block abundance in Gardner

Pixel Height: 0.88m; Pixel Width: 0.81m

Data Processing (ongoing)

• Cumulative rock count plotted as a function of diameter• Data trimmed where rollover due to resolution occurs

So far, identified 3 classes of rock distributions (post-impact processes?)

Departure from power law

Ongoing Research

• Modeling fits for extrapolation to smaller rock sizes (wavelength-scale)

• Correlation of decimeter-scale surface roughness with CPR values

Future Results useful for:

• Understanding uneven processes of erosion (high block abundance inside crater, low in the

exterior)

• Understanding radar properties of possible ice deposits in permanently shadowed craters in the poles