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

Abstract

Abstract. "Stealth" is a region on Mars that has no distinguishable radar return. Stealth was discovered in 1988 by Muhteman e! at. [1991] material which exhibits Stealth properties must also be relatively young. If the Stealth radar feature is the result of a volcanic ash deposit, then it would imply that explosive volcanism not only occurred in the Tharsis region but that it occurred relatively late in Martian history. The purpose of this study is to examine the geographic and geologic context of Stealth and to ascertain the plausibility of the volcanic ash hypothesis. There is no way to prove that this hypothesis is true with the data presently available for the region. 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. Next, we examine the geologic context of Stealth by looking at the medium-scale (1:15,000,000) geologic map of Scott and Tanaka [1986] plus medium-and high-resolution Viking orbiter images of the region. In looking at the geologic context of Stealth, we (1) identify the geologic/geomorphic features and units that Stealth overlies; (2) determine if the proposed several meters thick Stealth deposit can be observed in highresolution (7-50 m/pixel) images; (3) seek landforms in medium-and high-resolution images that can be attributed to Stealth and/or be used to better understand its history and geologic context; and (4) search for plausible volcanic vents that could have contributed tephra to the region. Finally, we 21,545

Aeolian Dunes as Evidence for Explosive Volcanism in the Tharsis Region of Mars

“…The high-resolution thermal inertia work Amazonian explosive volcanism (Muhleman et al 1991, (Fig. 7) was completed under direction of J. R. Zimbelman when we were working at the Lunar and Planetary Institute (Houston, Texas) in Edgett et al 1997). Finally, the result is also consistent with 1986.…”

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

“…The Stealth component of the MFF was inferred to be at least 5 m thick, although the radar could potentially penetrate up to several tens of meters in low electrical-loss material (Harmon et al, 1999;Muhleman et al, 1995). The terrain with the lowest radar backscatter is centered south of Olympus Mons, but the total area showing reduced radar reflectivity stretches across the equator from 140 to 240 • E, and encompasses the entire longitudinal extent of the MFF (Edgett et al, 1997).…”

Arecibo radar imagery of Mars: The major volcanic provinces

“…Stealth overlaps the outer portion of the Arsia FSD, the Arsia flows farther west and northwest, and much of the Medusae Fossae Formation north of these Arsia flows. A tongue of Stealth also extends east between Arsia Mons and the minor shields Biblis Patera (7-L) and Ulysses Patera (7-M) (Muhleman et al, 1991;Edgett et al, 1997). This would explain the radar-darkness of this region (7-N), despite coverage by At 5 flows, as well as the fact that the bright flows surrounding the two paterae appear as if disconnected from their source.…”

“…This is not the case, as the flows remain mostly radar-dark west and northwest of the FSD. The most likely explanation is that the radar waves are being absorbed by the mantling deposits of the so-called ''Stealth'' feature, an equatorial radar-dark region that may be a volcanic ash deposit (Muhleman et al, 1991;Butler, 1994;Edgett et al, 1997;Edgett, 1997;Ivanov et al, 1998). Stealth overlaps the outer portion of the Arsia FSD, the Arsia flows farther west and northwest, and much of the Medusae Fossae Formation north of these Arsia flows.…”