OVERVIEW OF A HALF-CENTURY OF VENUS GEOLOGICAL EXPLORATION

Venus is similar to Earth in radius, gravity, bulk density and solar system position but differs in geology/geological evolution. Earth-based radio telescopes, Pioneer-Venus and the USSR Venera orbiter missions imaged continent-sized highlands and linear mountains but Venus’ unimodel hypsometry differs from Earth. Partial compositional data from USSR landers resembles Earth’s basalts. Magellan radar images show plains, ridges, complexly faulted regions, rifts, abundant volcanism but few impact craters and no evidence of Earth-style plate tectonics. Instead, studies propose global or regional resurfacing estimated ~750 ma. Unusual surface features include tessera, highly deformed and apparently old, and corona, large annular structures interpreted as plume deformation. Gravity correlated with surface topography indicates a strong lithosphere or possible convective upwelling. Plume-lithosphere interactions and plume-related volcanism have been studied as drivers of lithospheric heat-transfer and surface deformation.

Increasingly, studies suggest an active interior and surface geology. Quadrangle mapping (USGS planetary mapping program) has produced regional geologic histories.Episodic atmospheric change in sulfur content and possible lightning observed by Venera, Akatsuki and Venus Express may indicate active volcanism. Recently, ice block-style tectonics, with segments separated by ridge belts, was proposed. Disputed atmospheric phosphine may have implications for either microbial life or active volcanism.

Many geologic questions remain: what is its interior structure; how does Venus lose heat; are resurfacing mechanisms global or regional; the level of volcanic/tectonic activity over the last billion years; is there current volcanism; has Venus always had stagnant-lid tectonics; composition/age of the radar reflective mountain terrain, tessera and plains; global stratigraphy; evidence of past water/oceans; effects of surface-atmosphere exchange? Critically, many of these require rock samples, absolute age dates and seismic data.

What can Venus tell us about terrestrial planet evolution? The Venus Exploration Analysis Group developed a priority list/roadmap to answer many of these questions. Key areas are surface composition/morphology and interior structure/dynamics.