Venus’ radar-bright highlands: Different signatures and materials on Ovda Regio and on Maxwell Montes

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Venus’ highlands appear much brighter than its lowland plains in reflected radar, which has been explained by several conflicting hypotheses. We study this transition at higher spatial and elevation resolution than previously possible by combining Magellan synthetic aperture radar (SAR) images with Magellan SAR stereo elevations. We confirm that SAR backscatter over Ovda Regio (5°N to 15°S) grades from low to high as elevation increases (2–4.5km above the datum), and then drops precipitously above ∼4.5km (T= ∼702K). This pattern is consistent with presence of a substance that undergoes a phase transition from ferroelectric to normal dielectric at ∼700K; the mineral chlorapatite is a likely candidate. This pattern is seen across Ovda, on other near-equatorial highlands, and on some shield volcanoes like the Tepev Montes. We also confirm that Maxwell Montes (60–68°N) shows a different pattern; its surface transitions abruptly from low backscatter to high backscatter at ∼4.5km above the datum, and remains so to nearly its highest elevations (∼10km). This pattern is consistent with the presence of a semiconductor material either precipitated from the atmosphere (e.g., a frost) or produced by atmosphere–surface interaction. If a ferroelectric substance were in the rock at Maxwell (as at Ovda), it could be invisible beneath the coating of semiconductor material. However, the absence of a semiconductor material on Ovda requires either that [1] the atmosphere compositions at Maxwell and Ovda are substantially different, or [2] that the semiconductor at Maxwell forms by atmosphere–surface reaction (not as an atmospheric precipitate) and that the surface materials at Ovda and Maxwell are substantially different.