Radar Images of Mars

Abstract

Full disk images of Mars have been obtained with the use of the Very Large Array (VLA) to map the radar reflected flux density. The transmitter system was the 70-m antenna of the Deep Space Network at Goldstone, California. The surface of Mars was illuminated with continuous wave radiation at a wavelength of 3,5 cm. The reflected energy was mapped in individual 12-minute snapshots with the VLA in its largest configuration; fringe spacings as small as 67 km were obtained. The images reveal near-surface features including a region in the Tharsis volcano area, over 2000 km in east-west extent, that displayed no echo to the very low level of the radar system noise. The feature, called Stealth, is interpreted as a deposit of dust or ash with a density less than about 0.5 gram per cubic centimeter and free of rocks larger than 1 cm across. The deposit must be several meters thick and may be much deeper. The strongest reflecting geological feature was the south polar ice cap, which was reduced in size to the residual south polar ice cap at the season of observation. The cap image is interpreted as arising from nearly pure CO(2) or H(2)O ice with a small amount of martian dust (less than 2 percent by volume) and a depth greater than 2 to 5 m. Only one anomalous reflecting feature was identified outside of the Tharsis region, although the Elysium region was poorly sampled in this experiment and the north pole was not visible from Earth.

The unique radar properties of silicic lava domes

“…Double reflections (from ''dihedral'' corner reflectors), coherent backscatter effects [Hapke, 1990], and diffuse scattering can all contribute to the SC component, giving a m c > 0. Circular polarization ratios in excess of unity are observed on a variety of icy surfaces, including the icy Galilean satellites [Ostro et al, 1992], the polar regions of Mars [Muhleman et al, 1991] and Mercury [Slade et al, 1992], terrestrial ice sheets [Rignot et al, 1993;Rignot, 1995] and glaciers [Haldemann, 1997]. On rocky surfaces, m c values are typically less than unity, with several notable exceptions.…”

Geologic context of the Mars radar “Stealth” region in southwestern Tharsis

“…Stealth was discovered in bistatic radar observations obtained during the Mars opposition of 1988 [Muhleman et al, 1991]. In this experiment a circularly polarized electromagnetic wave with a frequency near 8.5 GHz (wavelength near 3.5 cm)…”

“…What we can do, however, is examine the available data and look for relationships that suggest the presence, nature, and origin of Stealth. We start with the assumption that Stealth is indeed caused by an underdense deposit of relatively finegrained (<<1 cm grains), nonlithified material that is exposed at the surface in southwestern Tharsis [Muhleman et al, 1991;Butler, 1994].Following a review of the Stealth radar observations, we present a rigorous redefinition of Stealth. The new definition differs from that of Muhlernan et al [1991] and Butler [1994] because it is based on an improved estimation of the uncertainty in the fit of the radar cross section at normal incidence.…”

“…What we can do, however, is examine the available data and look for relationships that suggest the presence, nature, and origin of Stealth. We start with the assumption that Stealth is indeed caused by an underdense deposit of relatively finegrained (<<1 cm grains), nonlithified material that is exposed at the surface in southwestern Tharsis [Muhleman et al, 1991;Butler, 1994].…”

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

“…Radar backscatter from the lunar surface is modeled as a mixture of these specular and diffuse components. Diffuse scattering from rocky areas associated with fresh craters is assumed to have CPR of about 1.0; and ice is assumed to have CPR of 2.0, as observed in the polar features of Mercury and Mars and the surfaces of the icy Galilean satellites of Jupiter [Muhleman et al, 1991;Harmon and Slade, 1992;Butler et al, 1993;Ostro, 2002]. The differences in appearance between the lunar and mercurian polar craters suggest that more ice is present on Mercury than on the Moon, probably a result of the higher cometary flux near Mercury compared with the Moon [e.g., Hartmann et al, 1981] and the fact that the higher surface gravity on Mercury (~0.3 g) means that body will retain more water on its surface.…”