Composition and location of volatiles at Loki Patera, Io.
R. R. Howell, C. E. Landis, R. M. C. Lopes
Icarus, 229: 328-339, Feb. 2014
Volatiles play a critical role in determining the nature of volcanic activity on Earth, but their role in the volcanism on Io is less clear. To help determine that role we analyze Voyager and Galileo images of Loki Patera. Loki is the largest caldera in the Solar System and Io’s most powerful volcano, however its eruptive behavior is still not understood. It appears to be relatively volatile poor, in comparison to other sites like Pele where volatiles drive a 350 km high plume. A resurfacing wave, either from spreading flows or from foundering of a lava lake crust, periodically sweeps around Loki Patera. Photometry from Voyager I and II violet and blue images shows that most of the features in and around Loki have colors well matched by macroscopic mixing of sulfur and basalt. The dark western portion of the patera has the color of bare basalt. Assuming such a macroscopic sulfur–basalt mix, in the Voyager I images most of the rest of the patera appears to be covered with a background of 13–38% sulfur. We infer this background sulfur was deposited from the nearby plume observed by Voyager I and II. The surface of the patera is also dotted by numerous small bright areas which have colloquially been called “bergs”. We find that they are also composed of sulfur, with coverage ranging up to 100%. Darker regions adjacent to the patera, such as the “southwest overflow”, are a mixture of intermediate amounts of sulfur and basalt. The “bathtub ring” at the edge of the overflow is again roughly 100% sulfur, perhaps with significant amounts of SO2 included. Colors seen during the Voyager II flyby are also consistent with this general pattern, but most of the patera has a sulfur abundance higher than that seen in Voyager I, while the then-dark southern portion is again close to the reflectance of bare basalt. We have also analyzed the spatial distribution of the bergs. They clearly avoid the inner and outer margins of the patera, and they also avoid each other. While a simple explanation of the above patterns could be that the bergs are fumarolic sulfur deposited on a periodically resurfaced lava lake crust, other observations seem inconsistent with that simple model. Careful comparison of Voyager I and Galileo data shows that the largest bergs in the southern patera have survived the intervening 22 years. This requires that at least those larger bergs represent some more permanent feature, perhaps higher standing kipuka, which avoid inundation by the lava resurfacing wave.