Bistatic radar observations have been made using thecombined VLA/Goldstone radar instrument at X-Band wavelength (near 8500MHz). This thesis contains a description of the instrument, observations, datareduction, and implications of some of the measurements. While the instrumenthas been used to probe many objects, discussion will be restricted to the data setsfor Mercury and Mars. This technique has provided the first unambiguous radarcross section maps of both planets, with surface resolutions as good as 150 km forMercury, and 100 km for Mars. The analysis of the radar cross section maps hasprovided a rich harvest of new information about the surface and near-surface ofboth planets.
Mars was observed twice during the opposition of 1988, and 3 times during theopposition of 1992/93. During the 1988 observations, the subearth latitude was∼ -24°, providing a good view into the south polar regions. The Martian seasonat the time was mid southern summer (L_s ∼ 295°), and thus the seasonal CO_2ice cap had sublimated away, exposing the residual south polar ice cap (RSPIC).The RSPIC was the area with the highest cross section on the planet in 1988,with a peak normalized cross section of 0.716. This is incredibly high, especiallyconsidering that it was at an incidence angle of ∼ 66° at the time. The RSPICalso exhibited the odd characteristic that throughout much of its extent, more echoenergy was received in the same sense circular (SS) polarization as that transmittedthan in the opposite sense (OS), a so-called polarization inversion. This is acharacteristic which has also been observed on the Galilean satellites, and on aportion of Greenland, and may be true for all cold, clear icy regions. This seemsto be a result of the radar wave penetrating into a relatively lossless medium containingmany volume scatterers. In the case of the RSPIC, the lossless medium isice, whether CO_2 or H_2O, and the scattering centers are most probably cracks andvoids in the ice. Simulations indicate that the radar wave penetrates down to 10'sof meters into the ice layer, implying that at the time the RSPIC was very clean,i.e., less than ∼ 1% volume fraction of contaminating dust, to that depth.
During the 1992/93 observations, the subearth latitude was ∼ +6° to ∼ +9°,providing a tolerable view into the north polar regions. The Martian season at thetime was early northern spring (L_s ∼20°), and thus much of the seasonal CO_2 capwas present, which covered the residual north polar ice cap (RNPIC). No regionswith enhanced cross section were found in the north polar regions, in stark contrastto the south. Fits to a sensible backscatter function provide an indication of slightcross section enhancements near the Chasma. Borealis, but the reliability of the fitsremains in question due to the restricted incidence angle range of the data. Thereare at least 3 possible reasons for the fact that the north polar regions show nocross section enhancements: 1 - there is some fundamental difference in the structureand/or composition (amount of dust contaminant) of the two residual caps,2 - the seasonal CO_2 cap which was present during the north polar experimentsabsorbed enough of the incoming radar energy to obscure the RNPIC, and 3 - thenorth polar regions were imaged with slightly poorer geometry. Some combinationof the three is most likely.
Many other regions with anomalous cross sections were found on the surface ofMars. The large volcanic provinces of Tharsis and Elysium have very high crosssections associated with them. These are most probably a result of the extremelyrough surfaces of the large volcanoes and their associated flows. One of the mostintriguing features in the Mars data set is a region which extends west from Tharsisfor over 2000 km, which displays no cross section distinguishable from the noisein either polarization, which we have termed "Stealth." The surface and near surface(to depths of meters) must be composed of very underdense material, withan absence of volume scatterers (rocks). The proximity of Stealth to Arsia andPavonis Montes suggests that it may be comprised of pyroclastic materials whichwere blown westward after eruptions from these two large shield volcanoes.
Mercury was observed twice during the conjunction in August of 1991, onceduring the conjunction in November of 1992, and twice during the conjunction inFebruary of 1994. The data in 1992 were compromised by transmitter problemsand will thus only be very briefly discussed. The 1994 data will be only brieflymentioned, as well, since the data reduction has not been fully completed, andthus all results are very preliminary. During the 1991 observations, the subearthlatitude was ∼ + 10°, providing nearly as good a view of the north polar regionsof Mercury as is obtainable via earth based remote sensing. The feature with thehighest SS cross section (.079) in either of the radar images was near the nominalpolar position. This feature also exhibits a polarization inversion throughout muchof its extent, similar to the RSPIC on Mars. This feature is probably the signatureof water ice deposits in permanently shadowed regions near the pole, whichexplains the reduced cross section when compared to the Martian RSPIC. The icemay be covered by a thin layer of dust, which would protect the ice from erosionfrom energetic sources as well as contributing to the reduced cross section. Otherregions with anomalous cross sections exist on the surface, most notably five largequasi-circular regions, which we refer to as "basins." It is clear from the 1992data that the Caloris basin has no such cross section enhancement in its interior,and so our "basins" are different from Caloris in some manner. During the 1994observations, the subearth latitude was ∼ -10°, providing nearly as good a viewof the south polar regions of Mercury as is obtainable from earth. Preliminaryresults indicate that there is a region of enhanced cross section near the southpole, similar to that near the north pole.
