Volcanic Outbursts on Io from Ground-based Observations 2016–2021

While both spacecraft and telescopic observations have taught us much about Io’s frequent and diverse forms of volcanism, the stochastic nature of the moon’s energic volcanic outbursts remains a poorly constrained phenomenon. Outbursts are defined as events with brightness exceeding 80 GW/sr/micron in the L-band above Io’s background brightness, and have been observed as outliers in many Io observation campaigns since the 1980s (Veeder 1994; Stansberry 1997; Howell, de Kleer 2016). Herein we present the results of a modern observation campaign and combine them with the context of past observations to derive statistics of volcanic outbursts and other large events. We define outbursts as a brightness greater than 80 GW/sr/micron above Io’s background brightness in L-band (3.5 micron center). Of 140 observations acquired with NASA’s Infrared Telescope Facility (IRFT) from 2016-2021, three definitive outbursts were identified on May 10, 2018; May 28, 2019; and June 25, 2019. On May 28 and June 25 of 2019, we observed Io in eclipse and throughout its occultation. On May 10, 2018, we observed Io in reflected sunlight using the K, L’, and M bands. While the emission includes all of Io, we expect the outburst to dominate at these shorter infrared wavelengths. Fitting a single temperature blackbody to the measured emission suggests that observed outbursts are composed of localized silicate lava-flows with temperatures greater than 1,100 K over areas less than 30 km2. The locations of these three outbursts are consistent with several known active Ionian volcanoes, although the low spatial resolution does not allow us to determine the exact source. Images from Juno’s instrument are used to constrain the afterglow location for the June 2019 outburst.

Large scale volcanism on Io, particularly from large outbursts, is a primary explanation for variations in Jupiter’s magnetosphere and aurora. If outbursts are a major source of changes in Jupiter’s magnetosphere and auroras, our observations suggest such changes would have occurred in mid-2018 and mid-2019.

We also present the time-series of Io’s total eclipse brightness and Loki’s brightness for the duration of this observation campaign. At the moment, the outburst times do not correlate with Loki’s periodic brightening events or any other pattern discernable in the time-series.