Interpreting Radar Scattering: Circular-Polarization Perspectives from Three Terrestrial Planets

Citation

Haldemann, Albert Frank Christian (1997) Interpreting Radar Scattering: Circular-Polarization Perspectives from Three Terrestrial Planets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YQ6F-ZS42. https://resolver.caltech.edu/CaltechTHESIS:04222010-100328955

Abstract

Planetary radar astronomy has used circular polarization radar signals to probe the surfaces of many solar system targets. However the trend for terrestrial observations has been toward greater use of linearly polarized imaging radars. Fortunately the latest generation of imaging radars has been developed with a multi-polarization capability. This should allow a synergy of the two research communities to occur.

One of the unresolved debates on planetary radar astronomy is the nature of the scattering processes from cold planetary ices. This question recently received input from a terrestrial source: Greenland (Rignot et al. 1993). In this thesis a survey is made of high altitude sites to discover if the Greenland percolation zone scattering behavior is wide-spread on the Earth. The survey was carried out with the enormous, publicly available dataset from the 1994 missions of Shuttle Imaging Radar payload. This instrument (SIR-C) obtained full-polarization information with its linear-polarization system. These data allow reconstruction of circular polarizations for comparison to planetary results. The search proved fruitful. Hundreds of square kilometers in western Tibet's Kunlun Shan, and in the Central Andes at the latitude of Santiago display radar scattering behavior quite similar to that in Greenland where internal reflections of the radar waves within icy inclusions in the firn enhance scattering in the same sense of circular polarization.

A separate unresolved issue in the planetary radar astronomy is the question of the nature of the highlands of Venus that exhibit high radar reflectivity and low emissivity. These so-called anomalous radar behavior in these regions have alternately been ascribed to high-dielectric doping or low dielectric volume scatterinig. We present new dual circular-polarization radar maps of the western hemisphere of Venus. The results are from a 1993 experiment to image Venus with 3.5 cm radar. Maps of Venusian radar albedo were made for each of two days of observation in both OS (echo principally due to specular reflection) and SS (diffuse echo) channels. On both days, the sub-earth longitude was near 300E. The SS maps are dominated by a significant component of diffuse backscatter from the 285E longitude highlands: Beta, Phoebe, and Themis Regiones. Beta Regio includes previously observed radar-anomalous regions. The nature of these altitude-related electrical properties on Venus is one of the outstanding surface process questions that remain after the Magellan mission. Our experiment provides the first full-disk polarization ratio (µ_c) maps. The data show that different geology determines different radar scattering properties within Beta. Diffuse scattering is very important in Beta, and may be due to either surface or volume scattering. We find a strong correlation of the SS albedo σ_(SS) with altitude R_p (km) in Beta, σ_(SS) ∝0.3R_p. Also, σ_(OS) ∝0.7 R_p. The onset of this relationship is at the R_p~6054 km planetary radius contour. The nature and morphology of the highland radar anomalies in Beta is consistent with a diffuse scattering mechanism. In Beta Regio we find µ_c > 0.5 in general, with µ_c as high as 0.8 between Rhea and Theia Montes, to the west of Devana Chasma. These values are compatible with measurements of blocky terrestrial lava flows if surface scattering dominates. If volume scattering is important, the high RCP cross-sections may indicate an important decrease in embedded scatterer size with altitude, which could be related to enhanced weathering.

Finally, the techniques of planetary radar astronomy were used in an applied sense. Results are presented of 3.5-cm delay-Doppler and Doppler-only (continuous wave or CW) radar experiments to assess three potential Mars Pathfinder landing sites: Ares Vallis, Tritonis Lacus, and northwest (NW) Isidis. The regional relief at all of the landing sites is appropriate for a Pathfinder landing sequence: east-west slopes do not exceed 3° at any of the sites. We find that Ares Vallis has a Hagfors rms slope of θ_(rms)=4.8°±1.1° as measured by delay-Doppler radar, and θ_(rms)=6.4°±0.6° measured by CW radar. These values are similar to, or less than the previous measurements of the Viking Lander 1 region (θ_(rms)=6°, Tyler et al. 1976, Harmon 1997). The Tritonis Lacus landing site is rougher with delay- Doppler, θ_(rms)=5.6°±0.6°, while the NW Isidis landing site is very smooth, both in a regional sense (slopes < 0.7°) and in a Hagfors rms slope sense: θ_(rms)=1.8°±0.2°. Reflectivities at all of the sites should be sufficient to allow the radar altimeter on Pathfinder to function properly.

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