THE PUZZLE OF RADAR-BRIGHT HIGHLANDS ON VENUS: A HIGH-SPATIAL RESOLUTION STUDY IN OVDA REGIO

Venus’ highland areas appear brighter than its lowlands in reflected radar (i.e., Magellan Synthetic Aperture Radar = SAR). The cause of this variation has been debated for >20 years and is still not understood. The transition is hypothesized to relate to chemical weathering or to precipitation from the atmosphere, although the chemical compounds involved are not known. Without new data about Venus’ highlands, it is important to re-examine old data in new ways to understand the chemical processes at Venus’ surface.

Our objective is to refine the relationship between radar properties and elevation for Venus highlands at high spatial resolution, using stereo topography (600m footprint, from R. Herrick) in place of radar altimetry (8x12km footprint) and using SAR reflectance (75m footprint) in place of emissivity (15x23km footprint). We studied two areas of ~150-200km extents in the Ovda Regio highlands. In these areas, we collected average elevations and radar backscatter brightness, as SAR DN, for ~1000 polygons of ~4km extent each. SAR DNs were converted to radar backscatter coefficients. The studied areas all showed the same relationship between elevation and radar backscatter coefficient (i.e. brightness): low backscatter at the lowest elevations (~0.04 at 2400m above the datum), a rapid rise to high backscatter at high elevation (~0.8 at 4500m), and a precipitous drop in backscatter coefficient at the highest elevations (~0.1 at 4700m).

Our results are consistent with (but less dispersed than) those of Klose et al. (1992), Arvidson et al. (1994) and Shepard et al. (1994), who worked with radar altimetry and emissivity. Our new data confirm their finding of low radar backscatter (high emissivity) at the highest elevations in Ovda; they had only a few such low-backscatter areas, and we have several hundred. So, our data supports the interpretation of Shepard et al. that the radar properties of Ovda could be explained by the presence of ferroelectric substance at its surface. The identity of this compound remains elusive.

In the future, we will refine elevations for portions of Ovda using stereo altimetry, which should further constrain the mineralogy of the highlands surface. We also will extend this method to other highlands areas on Venus, to see if our results are applicable across the whole planet.