Modeling radar scattering from icy lunar regoliths at 13 cm and 4 cm wavelengths

Abstract

[1] Two orbital synthetic aperture radars (SARs), the Chandrayaan-1 Mini-SAR (13 cm wavelength) and the Lunar Reconnaissance Orbiter (LRO) Mini-RF (13 and 4.2 cm wavelengths), have been imaging the lunar surface searching for ice deposits in the polar permanently shadowed areas. To understand the radar signatures of lunar polar ices, an empirical two-component model with parametric variations of the specular and diffuse components was developed and validated. This model estimates scattering differences associated with slopes, surface roughness, thin regolith over ice, and patches of ice. Lunar radar backscatter cross sections for the average surface for the Chandrayaan-1 and LRO instruments are estimated from the radar cross sections from the Moon at 3.8, 23, and 68 cm wavelengths measured in the 1960s at the Massachusetts Institute of Technology. This modeling predicts that enhanced diffuse scattering from near-surface ice can be separated from rocks if the scattering is characterized by both the high reflectivity and circular polarization ratios (CPRs) like those observed on Mercury, Mars, and the Galilean satellites. Scattering from near-surface ices covered by a thin regolith can be separated from rocks if the enhancement is twice the average or more. If, however, the lunar ice is dispersed throughout the regolith as ice-filling pores, then scattering differences might be too small to detect. Preliminary validation using LRO radar data for a few polar and midlatitude craters indicate that the observed CPRs are consistent with our models for different regolith ice and roughness conditions. Citation: Thompson, T. W., E. A. Ustinov, and E. Heggy (2011), Modeling radar scattering from icy lunar regoliths at 13 cm and 4 cm wavelengths,

Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radar

“…utilizes mixing of diffuse and specular components to describe the radar scattering behavior of ice-regolith mixtures [Thompson et al, 2011[Thompson et al, , 2012. In this model, the specular component results from surface and near-subsurface layers that are smooth to a tenth of a radar wavelength for large (>10 wavelengths) areas oriented perpendicular to the radar line-of-sight.…”

“…In each case, a specific pattern in the SC and OC signal strength accompanies the high CPR seen in the targets. This model uses the SC and OC echo enhancements, i.e., the ratios of observed powers for the crater interior and typical background terrain near these craters [Thompson et al, 2011[Thompson et al, , 2012. For each feature, the typical SC, OC, and CPR values are determined for the crater in question and for a patch of background terrain of the same size nearby with a precision of about 5%, as illustrated by their sizes in Figure 12.…”

“…These debris flows are very unusual; it is much more common for craters to become less blocky as they erode. Thompson et al [2011]). Here echo enhancements are the ratios of observed backscatter powers from the craters and their nearby backgrounds.…”

“…Table 2. Separation of Crater Classes Based Upon Scattering Parameters α (SC Enhancement), γ (OC Enhancement), the Ratio α/γ (a Proxy for CPR), and Total Power Enhancement (Σ of Thompson et al [2011Thompson et al [ , 2012 is that a subset of these multibounce craters contains nearly pure ice, which would enhance their ice-like backscatter properties.…”

“…[32] The scattering model of Thompson et al [2011Thompson et al [ , 2012 postulated three different surface conditions that might result in high CPR on the Moon (1) "rough," corresponding to typical fresh young crater wavelength-scale surface roughness (e.g., rock fields); (2) "icy," composed of a thin dry regolith covering substantial ice deposits and/or a regolith with ice patches partially filling the radar pixels; and (3) a "multibounce" category (see Table 2). In each case, a specific pattern in the SC and OC signal strength accompanies the high CPR seen in the targets.…”