We present observations of Io's Loki Patera taken with the 10-m Keck telescopes between 1998 and 2016. Adding these data to those published by Rathbun and Spencer (2006) and the Gemini data of de Kleer and de Pater (2016a, 2017) results in a database of 3.5-3.8. μm emission from Loki Patera over almost 3 decades. Data presented here contain adaptive optics (AO) observations of Io's sunlit hemisphere at wavelengths between 1.6 and 5. μm, AO observations of Io in eclipse at 2-5. μm, and non-AO observations of Io in eclipse at 1.6-12. μm. The non-AO data were taken in September of 1999, during the early phase of a brightening event that was documented by Howell et al. (2001). Dual-component Io Flow model (IFM) fits to our 1999 observations show a mostly cool lava crust over almost the entire patera floor, with a relatively small hotter component making up less than 1% of the total area, consistent with previous observations. The 30-year timeline of Loki Patera revealed that, after an apparent cessation of, or change in, brightening events in 2002, Loki Patera became active again in 2009. The more recent activity may have a slightly shorter periodicity than observed by Rathbun et al. (2002), and the direction of flow propagation appears to have reversed. Since 2009 the flow direction is in the clockwise direction, starting in the north or north-east corner and propagating along the patera towards the south-west. During the Galileo era the propagation was in the counter-clockwise direction, starting in the south-west and propagating towards the east. Both the 30-year timeline and the 1.6-12. μm spectrum that was obtained during the brightening event in 1999 agree well with Matson et al.'s (2006) overturning lava lake model, as modified by de Kleer and de Pater (2017).

We report observations of four energetic volcanic eruptions on Io: at Tupan Patera on UT 8 March 2003; Tung Yo Patera on UT 28 May 2004; Sui Jen Patera on UT 30 May 2004; and south of Babbar Patera on UT 31 May 2005. The Tung Yo, Sui Jen and south of Babbar Paterae eruptions are in locations where no activity had been seen before. Our observations were obtained at near-infrared wavelengths (1.2-4.7μm) with the 10-m Keck telescope equipped with adaptive optics. We report single and two-temperature blackbody fits, as well as single-component and dual-component Io Flow Model (IFM) fits (Davies, 1996, Icarus, 124, 45-61) to all four eruptions where applicable. We use 2-μm and 5-μm radiant fluxes, the 2:5-μm radiant flux ratio, and radiant flux density of each thermal source to constrain the likely style of volcanic eruption. All eruptions are characterized by a high temperature IFM component (ranging from 1475 to ∼900K) from a relatively small area (2 to several tens of km²), and a lower temperature component with a more extensive surface area. The relationship of the areas at the highest temperatures to the cooler, more extensive area is of particular importance in deriving eruption style. Model fits to the Sui Jen Patera data are strongly suggestive of lava fountaining, although not at a level consistent with a large "outburst" eruption. Activity at Tupan Patera suggests that the entire floor of the patera may have been resurfaced with silicate lava in 2003.

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 14 August 2007 revealed an active and highly-energetic eruption at Pillan at 245.2±0.7°W and 8.5±0.5°S. A one-temperature blackbody fit to the data revealed a (blackbody) temperature of 840±40K over an area of 17km², with a total power output of ~500GW. Using Davies' (Davies, A.G. [1996]. Icarus 124(1), 45-61) Io Flow Model, we find that the oldest lava present is less than 1-2h old, having cooled down from the eruption temperature of >1400K to ~710K; this young hot lava suggests that an episode of lava fountaining was underway. In addition to an examination of this eruption, we present data of the Pele and Pillan volcanoes obtained with the same instrument and telescope from 2002 through 2015. These data reveal another eruption at Pillan on UT 28 June 2010. Model fits to this eruption yield a blackbody temperature of 600-700K over an area of ~60km², radiating over 600GW. On UT 18 February 2015 an energetic eruption was captured by the InfraRed Telescope Facility (IRTF) via mutual event occultations. The eruption took place at 242.7±1°W and 12.4±1°S, i.e., in the eastern part of Pillan Patera. Subsequent observations showed a gradual decrease in the intensity of the eruption. Images obtained with the Keck telescope on 31 March and 5 May 2015 revealed that the locations of the eruption had shifted by 120-160km to the NW.In contrast to the episodicity of Pillan, Pele has been persistent, observed in every appropriate 4.7μm observation. Pele was remarkably consistent in its thermal emission from the Galileo era through February 2002, when a blackbody temperature of 940±40K and an area of 6.5km² was measured. Since that time, however, the radiant flux from what is likely a apparently large, overturning lava lake has gradually subsided over the next decade by a factor of ~4, while the location of the thermal source was moving back and forth between areas roughly ~100km to the W of the 2002 location and an area roughly ~100km to the SE of the 2002 location.