Why Venus lacks plate tectonics: Insights from possible subduction zones Sue Smrekar (JPL) Anne Davaille (U. Paris Sud/ CNRS) Steve Tomlinson (UCLA) Conclusions Subduction on Venus was proposed early on, but ›  disputed Lab modeling (Davaille) demonstrates how plumes can ›  initiate subduction Details match many observations of largest coronae on ›  Venus Gravity agrees with subduction as seen in the lab ›  Lab experiment show a different type of ‘sluggish’ lid ›  convection when the lithosphere is weaker relative to regimes that develop full up plate tectonics Consistent with limited subduction, and associated limited ›  resurfacing on Venus Implies a wet lithosphere, not a dry lithosphere ›  Could be a good model for the initiation of subduction on ›  early Earth (or exoplanets) Why does Venus lack plate tectonics? And why do we care? Leading Hypotheses: ›  Too dry? (based on comets as a source of water?) ›  No Low viscosity zone to allow plate motion ›  Lithosphere too dry and thus too strong to break ›  No clays to allow fault sliding ›  Too hot? Faults anneal? ›  (see abstracts at Comparative Geodyn & Tectonics!) ›  The # of Earth-sized exoplanets is increasing! ›  Will they be Earth-like or Venus-like? ›  Plate tectonics is increasingly being linked to habitability ›  through volatile recycling and variable environments Does Venus have subduction? (or a case study in paradigm shift…) Same planform ✔ ›  Same mechanical deformation ✔ ›  No “paired” extension ✗ ›  Fractures cross subduction zone ✗ ›  Different mechanism (e.g. direction of subduction ›  changes along the length of a rift zone ✗ Subduction: Same planform ✔ Artemis Corona Aleutian Trench Sandwell and Schubert, 1992 Subduction: � Same mechanical deformation ✔� � � � � � Sandwell and Schubert, 1992 Subduction: Fractures cross subduction zone ✗ Hansen and Phillips, Tectonics and Volcanism of Eastern Aphrodite Terra, Venus: No Subduction , No Spreading, 1993 Problems for subduction: 1. Radial fractures extend across trenches 2. Radial fractures are typically the oldest structures at coronae 3. Consistent with upwelling plumes Subduction: Fractures cross subduction zone ✔ Hansen and Phillips, Tectonics and Volcanism of Eastern Aphrodite Terra, Venus: No Subduction , No Spreading, 1993 Problems for subduction: 1.  fractures extend across trenches *Also seen at terrestrial subduction zones *Rarely seen on Venus or Earth Subduction: Different mechanism ✗ Problems for subduction: 2. Radial fractures are typically the oldest structures at coronae Consistent with upwelling plumes Hansen and Phillips, Tectonics and Volcanism of Eastern Aphrodite Terra, Venus: No Subduction , No Spreading, 1993 Subduction: Different mechanism ✗ Problems for subduction: 3. Inferred subduction direction changes reverses