Radar Scattering of Venus Terrains: Characterising Surface Roughness in Preparation for the Decade of Venus

The Magellan radar space mission [1] produced the largest, highest-resolution, most accurate survey of the surface of Venus to date. During the first cycle of the mission, its unique latitude-varying radar observation geometry allowed for the revisit of morphologically-equivalent terrains with a wide array of incidence angles, both in synthetic aperture radar (SAR) mode and in altimetry mode [2,3,4,5]. With the appropriate constraints and processing steps, this enables the reconstruction of the full scattering behaviour (a scattering curve) of distinct rock units [6] across approx. 0-50 degrees of local incidence, offering a unique opportunity to measure the mean radiometric signature of different terrain formations on Venus and, for the first time, map and characterise wavelength-scale surface roughness across the entire planet. Additionally, the derived scattering curves represent the reference scenarios that will enable radiometric comparison of Magellan SAR images with SAR data from upcoming Venus orbiters, even if the acquisition geometry between sensors for a specific target differs significantly [7,8].

To perform this study, we have built a comprehensive method that blends SAR and altimetry backscatter, postprocessed Magellan topography, Magellan radiometry, and geological data from Venus; the method incorporates a physically-constrained stochastic model of uncertainty for each individual backscatter measurement, and provides the framework for scattering models and constraints to appropriately fit the data. Our scattering curve results have been extensively tested against different cycle 1 and cycle 2 SAR and altimetry data to investigate the heterogeneity and/or isotropy of the measured surfaces, and to internally validate the radiometric consistency (absolute and relative) of the observed backscatter estimates.

In this work, we present insights into our findings regarding surface roughness properties for each terrain formation. We discuss the geological interpretation of the results, linking the scale of the retrieved mean roughness with the most-likely surface modification processes. In particular, we demonstrate the separability of the scattering characteristics of rift zones and Tessera terrain from the several plain units across the planet. Additionally, we globally constrain the predicted backscatter values for the forthcoming SAR missions to Venus. This is a critical step towards robust mission-to-mission SAR image intercomparison, enabling inter-mission SAR change detection from a radiometric perspective [9,10], and a key input to consolidate the on-going radar design, operation and data processing for these missions.

Figure 1. Example summary of diffuse scattering results from Magellan SAR backscatter data of Bell Formation (lava flow) units and Tessera terrain.

 

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