Known Parameters • Surface atmosphere, pressure Venus Volcanism – Average 480°C (decreases with increasing elevation) – Pressure = 92 bars at mean planetary radius • Uncompressed density = 5250 kg/m3 GLY 424/524 • Gravity = 8.82 m/s2 March 20, 2002 • Surface composition: Venera landers Magellan Radar image Composition: Gamma-ray Composition Spectroscopy Venera Venera Venera Vega Vega 8 9 10 1 2 Venera Venera Vega 2 Earth Earth 13 14 th. bslt. alk. bslt. SiO 45.1 ± 3.0 48.7 ± 3.6 45.6 ± 3.2 49.7 45.1 K2O 4.8 0.6 0.4 0.54 0.48 2 Al2O3 15.8 ± 3.0 17.9 ± 2.6 16.0 ± 1.8 16.5 13.4 wt.% ± 1.4 ± 0.1 ± 0.2 ± 0.26 ± 0.24 FeO 9.3 ± 2.2 8.8 ± 1.8 7.7 ± 3.7 8.3 12.7 MgO 11.4 ± 6.2 8.1 ± 3.3 11.5 ± 3.7 9.0 11.5 U 2.2 0.6 0.5 0.64 0.68 CaO 7.1 ± 0.6 10.3 ± 1.2 7.5 ± 0.7 13.8 10.4 K2O 4.0 ± 0.6 0.2 ± 0.1 0.1 ± 0.1 0.1 0.7 TiO 1.6 ± 0.5 1.2 ± 0.4 0.2 ± 0.1 0.7 2.0 ppm ± 0.7 ± 0.2 ± 0.3 ± 0.47 ± 0.38 2 NaO 0.2 ± 0.1 0.2 ± 0.1 0.1 ± 0.1 - - SO 1.6 ± 1.0 0.9 ± 0.8 4.7 ± 1.5 - - Th 6.5 3.7 0.7 1.5 2.0 3 Cl 0.3 0.4 <0.3 - - ppm ± 0.2 ± 0.4 ± 0.3 ± 1.2 ± 1.0 Na2O - - - 1.9 2.9 Total 96.4 96.7 93.7 100 98.7 1 Venera 8 & 13 Sites • More alkalic compositions • Evolved lavas? – Trachytes? – Syenites? • Venera 8 landed on “pancake dome”? Volcanic Features on Venus Large Shield Volcanoes • Shield volcanoes • Similar to terrestrial, martian shields • “Paterae” (not identical to martian paterae) • Hot-spot volcanism • Lava plains • Locally may be associated with • Coronae – Rift zones • “Pancake domes” – coronae • Canali, valley networks • Lava flow fields • Shield fields (a.k.a. “shielded plains”) Location of volcanoes >100 km diameter 2 Sif and Gula Mons Sif Mons ~2 km high ~300 m across 700 km apart Gula = 3.2 km high Sapas Mons Ushas Mons ~120 km across ~500 km across ~1.5 km high ~2 km high Arrows indicate flooded Dikes? impact craters Thea Mons Small Shields and Shield Plains Largest volcano >4 km tall • Shields ~20 - 100 km across ~800 km across Found at juncture • Shield clusters (shields £20 km across) of 3 major rifts – Ubiquitous – 11,000 small shields per 106 km2 – Unique form of venusian volcanism 3 Patterns of Distribution • Small shields and shield fields tend to cluster around larger shield volcanoes • Small shields and shield fields found at mid- to low-latitudes – Real? – Or burial by lava plains? Shield Field Small Shield F.o.v. ~120 km ~40 km across Shield Field and Corona Shield Field and Flows Corona ~125 km Shields ~1 - 8 km Flows <200 km 4 Coronae • Large (~100 - 1000 km diameter) • Round or oval shaped • Ring of faults or ridges • Flat, raised or down-dropped center • Surrounded by moat or trough Small Vencor Corona Coronae F.o.v. ~100 km Coronae Evolution Large Coronae 5 Artemis Corona Lava Flow Fields ~2000 km Across ~700 km long Possible Subduction? Mylitta Lava Flow Fields Fluctus • Common • Volumes (thicknesses) unknown • Similar areas to terrestrial continental flood basalts • Typical lava flow lengths of ~1000 km – How? • Ambient conditions? • Unique lava compositions? • Unique emplacement parameters? Ozza Mons Flow Field 6.