Tesserae on Venus may preserve evidence of fluvial erosion

Fluvial erosion is absent on Venus at present, precluded by a high surface temperature of ~450°C. This interpretation is supported by the abundance of uneroded volcanic flows. Recent climate modeling, however, has suggested the possibility of Earth-like climatic conditions on Venus for much of its earlier history, prior to a catastrophic runaway greenhouse effect. We observe that within some tesserae—the stratigraphically oldest and most complex geological units—there are valleys that are morphologically similar to those resulting from fluvial erosion on Earth. Given poor topographic resolution we use an indirect technique to recognize these valleys, observing the pattern of lava flooding of the margins of tesserae by adjacent plains volcanism. Eight areas were studied (1 in Salus, 5 in Manatum and 2 in Ananke tesserae), and these reveal valley patterns that are morphologically similar to 5 of the 6 main types of terrestrial stream patterns. We attribute these patterns to a combination of the initial rock types (including possible layered sequences of sedimentary and volcanic rocks), affected by folding and faulting, followed by erosion by fluvial (and, to a lesser extent, wind) action. This proposed fluvial erosion in tesserae provides support for climate models that suggest cooler conditions prevailed on Venus in the past, before the onset of major effusive volcanism that gives the surface its relatively young average model age. High-resolution radar images and topographic data acquired by future missions to Venus will provide the basis for testing the hypothesis of fluvial reshaping of the enigmatic tesserae.