Formation and Evolution of Paterae on Jupiter's Moon Io

Abstract

Paterae (volcano-tectonic depressions) are among the most prominent topographic features on Io. They are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins, typical of terrestrial and Martian basalt shield calderas. However, they are much larger (2 km - 202 km diameter, mean 42 km 3 km) and typically lack obvious shields. They are often angular in shape or are found adjacent to mountains, suggesting tectonic influences on their formation. A preferential clustering of paterae at the equatorial sub- and anti-jovian regions is likely a surface expression of tidal massaging and convection in the asthenosphere. Paterae adjacent to mountains have a mean diameter 14 km 9 km larger than that for all paterae, which may indicate paterae grow larger in the fractured crust near mountains. Nightside and eclipse observations of Pele Patera by the Cassini and Galileo spacecraft reveal that much of Pele’s visible thermal emission comes from lava fountains within a topographically confined lava body, most likely a lava lake. Multiple filter images provided color temperatures of 1500 80 K from Cassini ISS data, and 1420 100 K from Galileo SSI data. Hotspots found within paterae (79% of all hotspots) exhibit a wide range of thermal behaviors in global eclipse images. Some hotspots are similar to Pele, consistently bright and confined; others, such as Loki, brighten or dim between observations and move to different locations within their patera. A model for patera formation begins with heating and convection within a high-temperature, low-viscosity asthenosphere. Magma rises through the cold, dense lithosphere either as diapirs [for thermal softening of the lithosphere and sufficiently large diapirs (20 km - 40 km diameter, >5 km thickness)] or through dikes. Magma reaches zones of neutral buoyancy and forms magma chambers that feed eruptions. Collapse over high-level chambers results in patera formation, filling of the patera with lava to create a lava lake, or lateral spreading of the magma chamber and subsequent enlargement of the patera by consuming crustal materials.