Two New Brightening Events at Io's Loki Patera

Loki Patera is one of the most dramatically time-variable volcanic features on Io, exhibiting periodic brightening events every 1-2 years that constitute over 15% of Io's global heat flow when active. We obtained new near-infrared (2-5 μm) observations of Loki Patera on 37 nights during and after two such brightening events using adaptive optics at the Keck and Gemini North telescopes in 2013-2015. We modify the Matson et al. (2006) model for Loki Patera as an overturning basaltic magma sea to model our observations, and find an overturn front propagation velocity of 0.85-0.95 km/day. The 445±45 day interval between the two events is 100 days shorter than the 540-day period calculated by Rathbun et al. (2002) for events prior to 2001. The overturn front appears to propagate around the patera in the clockwise direction, opposite to what has been inferred for these past brightening events, and may include irregular propagation patterns and multiple simultaneous fronts. Based on the anomalously-low intensities when Loki Patera is viewed at high emission angle, we find evidence for a topographic barrier to the west of the patera, which may be a raised region or the edge of a depression in which the magma sea resides.