Roughness Asymmetry as a Clue to the Evolution of Crater-associated Dark Diffuse Features on Venus

Introduction. About 65% of large (>30 km) craters on Venus have associated radar-dark diffuse features (DDFs) [1], including well-expressed radar-dark parabolas [2]. DDFs were interpreted as surficial deposits of loose material lifted during crater formation [2, 3]. Craters with parabolas are thought to be the youngest [2]. The morphological sequence of the DDFs from parabolas to halos to faint radar-dark patches has been used as a degradation sequence of DDFs [1]. Here we study possible mechanisms of parabola degradation, which is as yet not well understood. We consider two end-members: (1) removal of the loose material by wind and (2) induration and subsequent roughening by eolian erosion, which diminishes the radar signature of the deposits. We analyze DDFs using several Magellan data sets in order to understand the relative role of these mechanisms. North-South roughness asymmetry-Doppler centroid maps. Radar echo Doppler centroid derived from the Magellan radar altimeter data is a measure of surface scattering asymmetry in the north-south (N-S) direction [4]. The Doppler centroid measurements are a part of GVDR and SCVDR data sets from PDS. To a first order, the Doppler centroid represents the deflection of the strongest surface echo from the nadir. The effective spatial resolution of Doppler centroid is rather poor: ~ 50 km in the 20°S – 40°N latitude zone (where we used these data) and allows the study of large DDFs only. In general, observations have shown that the strongest surface echo systematically deflects from the nadir in many areas [4]. In the plains, the Doppler centroid is a measure of the N-S asymmetry of surface roughness at scales from centimeters to hundreds of meters [5]. We studied about 45 large craters (>30 km) with associated DDFs localized in 20°S – 40°N latitude zone. We also picked up several smaller DDF craters clearly recognized in the Doppler centroid map. We found that radar-dark parabolas and well-expressed halos are usually associated with areas of zero Doppler centroid (N-S symmetric roughness). As examples, dark parabola crater Bassi (19°S 64.7°E, 35 km) and dark halo crater Elena (18.4°S, 73.4°E, 17 km) are shown in Fig. 1. Crater-related DDFs here have zero Doppler centroid, while the volcanic plains over which they are super-posed have a pronounced N-S slope asymmetry. Decameter-scale surface roughness derived from Magellan radar altimeter measurements (both ARCDR [6] and SCVDR [4] data sets) is usually lower for the parabolas and halos than for the surroundings. …