North‐south roughness anisotropy on Venus from the Magellan Radar Altimeter: Correlation with geology

Abstract

[1] Analysis of the Doppler spectrum of Magellan Radar Altimeter echoes has revealed the spatially coherent deflection of the echo center from the nadir point (Tyler et al., 1992). We present and analyze a global map of the Doppler centroid of the radar altimeter echo, which, for the plains of Venus, characterizes the degree of anisotropy of the surface roughness in the north-south direction at scales from decimeters to kilometers. We found a global hemispherical trend of roughness anisotropy: on average, equator-facing slopes are steeper than pole-facing slopes. We searched globally for correlations between contrasts in roughness anisotropy and surface geology. We found that sharp contrasts in anisotropy often coincide with boundaries of geological units, although not consistently. We found a number of examples where radar-bright (rough) lava flows have higher asymmetry than their surroundings. Diffuse radar-dark crater-related parabolic features and halos often have a symmetric roughness signature, while typical regional plains away from these diffuse features often have an anisotropic roughness. There is not, however, any direct and uniform correlation between the roughness anisotropy signature and geomorphologic terrain type or appearance in the radar images. We believe that asymmetric topography of deposits of windblown material, e.g., microdunes or smallscale wind streaks in the wind shadow of obstacles, is the main contributor to the observed roughness anisotropy.

Crater‐associated dark diffuse features on Venus: Properties of surficial deposits and their evolution

“…The observed bands of increased asymmetry at the periphery of the crater‐related deposit indicate wind reworking of the deposit material in the areas where deposits are thin. This asymmetry coincides with microdune shapes formed by equatorward surface winds prevalent in the northern hemisphere of Venus [ Bondarenko et al , 2006]. The map of the rms slopes for the crater Boleyn area is shown in Figure 8c.…”

“…Three areas of very strong E‐W asymmetry (the observed difference is up to 8 dB), and interpreted as microdune fields [ Weitz et al , 1994], are localized in the western parts of parabolas near craters Stowe, Eudocia and Guan Daosheng. Widespread weaker scattering anisotropy over the plains of Venus has been reported by Kreslavsky and Vdovichenko [1999] and Bondarenko et al [2006]. These have been interpreted to represent the widespread occurrence of eolian features, perhaps, microdunes.…”

“…This often leads to formation of aeolian features which have an asymmetric shape (area C, Figure 6), such as dunes, ripples, accumulation of material in wind shadows (area D, Figure 6), etc. Unresolved asymmetric features on Venus have been recognized in the radar data on the basis of the difference between the strengths of the radar echo when probing from opposite sides at similar incidence angles [e.g., Weitz et al , 1994; Tyler et al , 1992; Bondarenko et al , 2006]. For example, if a steep dune slope is normal to incident radiation θ R = 0 for an observation from the right (area C, Figure 6), this leads to a much higher radar echo in comparison with the observation from the left.…”

“…However, eight dark parabolas, including the three roughest parabolas (craters Stowe, Glaspell, and Eudocia), exhibit a distinctive scattering asymmetry in the N‐S direction at the decameter scale. Such scattering asymmetry can be caused by small‐scale roughness asymmetry of the surface [ Bondarenko et al , 2006]. Additionally, craters Stuart, Caldwell, Annia, Faustina, and Stowe are characterized by a similar N‐S scattering asymmetry for both radar‐dark and typical surfaces.…”

Surface evolution of Venus

“…The apparent lack of dunes could, however, be partially an observational effect. Analysis of radiophysical properties in several regions of Venus by Bondarenko et al [2006] revealed a noticeable east-west asymmetry in the radar returns which was interpreted as possible dunes/ripples of meter to decameter size range.…”