Earth-based observations since Galileo

Since 2001, Imke de Pater and other researchers, including Franck Marchis, Conor Laver, Katherine de Kleer, and Clayton Cantrall, have used adaptive optics images acquired at the Keck II and Gemini North telescopes atop Mauna Kea in Hawaii to monitor volcanic activity on Io. While the images don’t have the detail we can see from spacecraft flying past Jupiter, they do have sufficient resolution to distinguish different volcanoes across Io’s surface. The telescopes can also measure the intensity of each volcano’s glow at different wavelengths. Armed with these measurements, the astronomers estimated the heat put out from individual volcanic centers, and can watch how it changes year over year.

Clayton Cantrall and his co-authors, in a paper published in the 15 September 2018 issue of the journal Icarus, consolidates the de Pater group data with two other data sets: some unpublished data acquired between 2003 and 2009, and observations taken as New Horizons flew by Jupiter in 2007. The combined data set included 156 separate nights of Earth-based observations, spanning 15 years and covering all longitudes of Io.

Cantrall et al. identified 88 distinct hot spots, of which 24 had not been detected previously by Galileo (though one of the 24 had been detected as a hot spot by Voyager). Of the 88 hot spots, 82 were detected more than once. The astronomers classified 18 as "persistent," meaning they were seen in more than half of all the observations in which they might have been visible. Many of these, like Pele, Marduk, Isum, and Janus, were considered persistent during the Galileo mission. A few are new since Galileo, like Uta Patera and another near Chalybes Regio in Io’s north polar region. Apparently Io's volcanoes can be dormant for a long time, but also become active for a period spanning decades.

The authors also identified five outbursts, eruptions so powerful that Io’s brightness as seen at an infrared wavelength of 5 microns more than doubled, but which only lasted a few days at that level. They detected 15 “mini-outbursts” from 11 different volcanoes. It was one of these outbursts, at Tvashtar, that New Horizons observed during its encounter. The massive volcanic eruption included lava fountains so powerful and bright that New Horizons saw them in sunlit imaging of Tvashtar and a 400-kilometer-tall dust-rich plume. The outbursts are not spaced out evenly in time, with three outbursts detected in August 2013 and only two others across the rest of the 15-year timespan.

With 15 years of data to work with, the authors of this paper were able to note that despite the occasional outburst or new eruptions coming and going, Io’s total heat output remained steady, which is important for understanding how evolution of Io’s orbit and the subtle changes in tidal heating in its interior might affect heat output. They were also able to chart how activity at individual hot spots varies with time, leading to a rather intriguing finding.

In a few cases, activity at nearby volcanoes tended to mirror each other. For example, heat output from Janus Patera decreased after a mini-outburst at the nearby Kanehekili lava field in 2010, before returning to normal in 2013. They suggest that these similarities in thermal emission across hundreds of kilometers might arise from very large regional magma sources (instead of each volcano being powered by a distinct hot spot), and that these regional magma sources can be depleted by outburst-scale eruptions.