Two new, rare, high-effusion outburst eruptions at Rarog and Heno Paterae on Io

•Two outburst-class eruptions were detected on Io in August 2013.•Rarog Patera (T=1040K, area=120km2); Heno Patera (T=720K, area=300km2).•Total thermal emission at Rarog and Heno Paterae was 8TW and 5–6TW, resp.•Exposed surfaces are very young: 5min at Rarog, a few hrs at Heno Paterae.•Peak effusion rates at each site were between 5 and 10×104m3/s. Observations obtained with the near-infrared camera NIRC2, coupled to the adaptive optics system on the 10-m W.M. Keck II telescope on Mauna Kea, Hawaii, on 15 August 2013 at ∼15:30UT revealed two large “Outburst”-class volcanic eruptions on Io. Follow-up observations five days later showed that both eruptions had substantially faded. The most energetic eruption was at Rarog Patera, at a location near 305°W, 42°S; a smaller one occurred further south at ∼310°W and ∼57°S, close to Heno Patera. Total radiant fluxes at Rarog Patera on August 15 were of order ∼500GW/sr/μm at wavelengths between 2 and 4μm, and close to 250GW/sr/μm at 1.6μm, indicative of an effective temperature of ∼1040K over an area of ∼120km2, and a total thermal emission of nearly 8TW. At Heno Patera the 4-μm flux measured ∼250GW/sr/μm, and ∼90GW/sr/μm at 2.2μm, suggestive of an effective temperature of ∼720K over an area of more than 300km2, and a total thermal emission of ∼5–6TW. Fits of the Davies (Davies, A.G. [1996]. Icarus 124(1), 45–61) Io Flow Model indicate that these two eruptions are vigorous and the exposed surfaces are mostly very young, no older than 4–5min at Rarog Patera and a few hours at Heno Patera. The model fits suggest that in both locations lava fountaining is taking place, a highly-energetic style of volcanism. Using follow-up observations taken between August 20 and September 7 we estimate peak effusion rates between 5×104 and 105m3/s.