T. W. Thompson

Multipolarization Radar Images for Geologic Mapping and Vegetation Discrimination

The NASA/JPL airborne synthetic aperture radar system produces radar image data simultaneously in four linear polarizations (HH, VV, VH, HV) at 24.6-cm wavelength (L-band), with 10-m resolution, across a swath width of approximately 10 km. The signal data are recorded optically and digitally and annotated in each of the channels to facilitate a completely automated digital correlation. Both standard amplitude, and also phase difference images are produced in the correlation process. Individual polarization and range-dependent gain functions improve the effective dynamic range, but as yet do not permit absolute quantitative measurements of the scattering coefficients. However, comparison of the relative intensities of the different polarizations in individual black-and-white and color composite images provides discriminatory mapping information. In the Death Valley, California, area, rough surfaces of young alluvial deposits produce strong responses at all polarizations. Smoother surfaces of older alluvial deposits show significantly lower responses. Evaporite deposits of different types and moisture contents have distinct polarization signatures. In the Wind River Basin, Wyoming, sedimentary rock units show polarization responses that relate to differences in weathering. Local intensity variations in like-polarization images result from topographic effects; strong cross-polarization responses denote the effects of vegetation cover and, in some cases, possible scattering from the subsurface. In the Savannah River Plant, South Carolina, forest cover characteristics are discriminated by polarization responses that reflect the density and structure of the canopy, and the presence or absence of standing water beneath the canopy.

Regolith composition and structure in the lunar maria: Results of long‐wavelength radar studies

Radar measurements at 70-cm and 7.5-m wavelengths provide insight into the structure and composition of the upper 5–100 m of the lunar regolith and crust. We combine high-resolution (3–5 km) 70-cm radar data for the nearside with earlier calibrated full-disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These data are tested against models for echoes from a buried substrate and Mie scattering from surface and buried rocks. These mechanisms are expected to dominate the 70-cm radar echo, with their relative importance determined by the rock population, regolith depth, substrate roughness, and the loss tangent of the soil. Results indicate that the 70-cm radar echo for the maria comes largely from Mie scattering by rocks buried within the fine soil. Radar scattering from a buried substrate is not likely to greatly affect the observed return. We also compared the 70-cm and 7.5-m radar images to infrared eclipse temperature maps, crater-population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth-based telescopic and Clementine multispectral observations. These data imply that (1) the TiO2 (ilmenite) content of the regolith controls variations in 70-cm depolarized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70-cm return for a given TiO2 abundance between maria of different ages do occur, but uncertainties in the current radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects; (3) the 7.5-m radar echoes are controlled by the age of the mare basalt flows, with older deposits having a greater degree of fracturing and higher backscatter. Future mapping at 12.6-cm and 70-cm wavelengths will help to resolve some of the issues raised here.

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

[1]xa0Two 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.

Radar probing of planetary regoliths: An example from the northern rim of Imbrium basin

[1]xa0Imaging radar measurements at long wavelengths (e.g., >30 cm) allow deep (up to tens of meters) probing of the physical structure and dielectric properties of planetary regoliths. We illustrate a potential application for a Mars orbital synthetic aperture radar (SAR) using new Earth-based 70-cm wavelength radar data for the Moon. The terrae on the northern margin of Mare Imbrium, the Montes Jura region, have diffuse radar backscatter echoes that are 2–4 times weaker at 3.8-cm, 70-cm, and 7.5-m wavelengths than most other lunar nearside terrae. Possible geologic explanations are (1) a pyroclastic deposit associated with sinuous rilles in this region, (2) buried mare basalt or a zone of mixed highland/basaltic debris (cryptomaria), or (3) layers of ejecta associated with the Iridum and Plato impacts that have fewer meter-sized rocks than typical highlands regolith. While radar data at 3.8-cm to 7.5-m wavelengths suggest significant differences between the Montes Jura region and typical highlands, the surface geochemistry and rock abundance inferred from Clementine UV-VIS data and eclipse thermal images are consistent with other lunar terrae. There is no evidence for enhanced iron abundance, expected for basaltic pyroclastic deposits, near the source vents of the sinuous rilles radial to Plato. The regions of low 70-cm radar return are consistent with overlapping concentric “haloes” about Iridum and Plato and do not occur preferentially in topographically low areas, as is observed for radar-mapped cryptomaria. Thus we suggest that the extensive radar-dark area associated with the Montes Jura region is due to overlapping, rock-poor ejecta deposits from Iridum and Plato craters. Comparison of the radial extent of low-radar-return crater haloes with a model for ejecta thickness shows that these rock-poor layers are detected by 70-cm radar where they are on the order of 10 m and thicker. A SAR in orbit about Mars could use similar deep probing to reveal the nature of crater- and basin-related deposits.

Radar observations of asteroid 1986 JK

Echoes from this near-Earth asteroid were obtained in May and June 1986, three weeks after its discovery, using the Goldstone 3.5-cm-wavelength radar. The asteroids minimum distance during the observations was less than 0.029 AU, only 11 times further than the Moon and closer than for any other asteroid or comet radar onbervation to date. 1986 JKs circular polarization ratio μc, of echo power received in the same sense of circular polarization as transmitted (the SC sense) to that in the opposite (OC) sense, averages 0.26 ± 0.02, indicating that single backscattering from smooth surface elements dominates the echoes, although there is a moderate degree of wavelength-scale, near-surface roughness. Variations in μc and in the shapes of the OC and SC echo spectra suggest that the surface is at least moderately heterogeneous at structural scales no smaller than the wavelength and probably much larger. The asteroids echo bandwidth provides the constraint Dmax ≥ P/5, where P is the apparent spin period, in hours, and Dmax, in kilometers, is the maximum width of the asteroids polar silhouette. Our estimate of 1986 JKs average OC radar cross section is 0.022 ± 0.007 km2. Combining that result with an indirect size constraint based on W.Z. Wisniewskis (1987, Icarus 70, 566–572) photometry yields an interval estimate for 1986 JKs radar albedo that overlaps values reported to date for comets and the radar-darkest asteroids. A “working model” of 1986 JK postulates a 1- to 2-km object whose shape is not extremely irregular, with little elongation but some polar flattening; the rotation period is not more than a few hours longer than 10 hr and the near-surface bulk density is within a factot of 2 of 0.9 g cm−3. The orbital and physical characteristics of 1986 JK are somewhat comet-like. However, the Earth passes within 0.005 AU of the asteroids orbit, and evidence for recent meteor shower activity associated with this object is lacking. Estimates of the asteroid echo Doppler frequencies (i.e., its radial velocities) were used in conjunction with the available optical astrometric data to provide refined orbital elements and ephemeris predictions. The radar astrometric data are extremely powerful for orbit improvement. At the next Earth close approach (0.12 AU in mid-2000), a search ephemeris based upon all optical and radar data will have a plane-of-sky, solid-angle uncertainty an order of magnitude smaller than that for an ephemeris based upon the optical data alone. A recovery attempt made on June 17, 2000, would have a plane-of-sky position uncertainty ≈20′, so prospects for recovering 1986 JK are good.

Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon

The southern highlands of the Moon comprise superposed ejecta layers, individually as thick as a few kilometers, from the major basins. Smaller (1–16-km-diameter) impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks. We observe a significant difference in the population of radar-bright craters, 1–16 km and larger in diameter, between regions of the southeastern near-side highlands north and south of ~lat 48°S. There are about one-third more radar-bright craters north of this line than to the south, broadly coincident with the mapped boundary between southern deposits mapped as pre-Nectarian age and those of Nectarian–Imbrian age to the north. The radar-bright crater population is consistent with a megaregolith thickness of ~1.5 km in the north and ~2.5 km in the south, a difference we attribute to South Pole–Aitken basin ejecta.