In February 2001, an eruption from the Surt volcano on the hellscape of Jupiter’s moon, Io, the volcanic epi-center of our solar system, exploded with an estimated output of an almost incomprehensible 78,000 gigawatts. By comparison, the 1992 eruption of Mount Etna, Sicily, was estimated at 12 gigawatts. During its peak, observed by the WM Keck II Telescope on Hawaii, its output almost matched the eruptive power of all of Io’s active volcanoes combined.
Io’s eruptions are thought to eject material into space, wafting plasma through the entire Jupiter system, where it swirls along magnetic field lines, some falling back on the surfaces of other moons like the ocean-moon, Europa, a prime candidate in the search for life.
On hundreds of clear nights over the last five years, giant telescopes on a dormant, sacred volcano in Hawaii have trained their gaze across space toward Io’s active volcanoes. “You just see so many volcanoes. It’s incredible,” said Katherine de Kleer, a planetary scientist at Caltech who has led the effort to understand the underlying rhythms of this eerie world.
Last week, Dr. de Kleer’s team released their full five-year record of Io’s volcanic activity in The Astronomical Journal. Their data, reports Joshua Sokol in the New York Times, “show a pimple-ridden surface roiling with eruptions. Some hot spots glow continuously, while other areas flare up, then die back down. The researchers’ hope is that other planetary scientists may be able to glimpse or dig into the underlying rhythms of this world, the most volcanically active body in the solar system.”
In 2016, using near-infrared adaptive optics on two of the world’s largest telescopes — the 10-meter Keck II and the 8-meter Gemini North, both located near the summit of the dormant volcano Maunakea in Hawaii — UC Berkeley astronomers tracked 48 volcanic hot spots on the surface over a period of 29 months from August 2013 through the end of 2015.
“On a given night, we may see half a dozen or more different hot spots,” said de Kleer in 2016, then a UC Berkeley graduate student. “Of Io’s hundreds of active volcanoes, we have been able to track the 50 that were the most powerful over the past few years.”
De Kleer and Imke de Pater, a UC Berkeley professor of astronomy and of earth and planetary science, observed the heat coming off of active eruptions as well as cooling lava flows and were able to determine the temperature and total power output of individual volcanic eruptions.They tracked their evolution over days, weeks and sometimes even years.
High-resolution image of Io, showing hot spots — Loki Patera and Amaterasu Patera — visible from Earth only with adaptive optics on the planet’s largest telescopes, Keck and Gemini.
Interestingly, some of the eruptions appeared to progress across the surface over time, as if one triggered another 500 kilometers away. “While it stretches the imagination to devise a mechanism that could operate over distances of 500 kilometers, Io’s volcanism is far more extreme than anything we have on Earth and continues to amaze and baffle us,” said de Kleer.
Io’s intense volcanic activity is powered by tidal heating — heating from friction generated in Io’s interior as Jupiter’s intense gravitational pull changes by small amounts along Io’s orbit. Models for how this heating occurs predict that most of Io’s total volcanic power should be emitted either near the poles or near the equator, depending on the model, and that the pattern should be symmetric between the forward- and backward-facing hemispheres in Io’s orbit.
Over the observational period, August 2013 through December 2015, the team obtained images of Io on 100 nights. Though they saw a surprising number of short-lived but intense eruptions that appeared suddenly and subsided in a matter of days, every single one took place on the trailing face of Io rather than the leading face, and at higher latitudes than more typical eruptions.
“The distribution of the eruptions is a poor match to the model predictions,” de Kleer said, “but future observations will tell us whether this is just because the sample size is too small, or because the models are too simplified. Or perhaps we’ll learn that local geological factors play a much greater role in determining where and when the volcanoes erupt than the physics of tidal heating do.”
One key target of interest was Io’s most powerful persistent volcano, Loki Patera shown toward bottom of image above, which brightens by more than a factor of 10 every 1-2 years. A patera is an irregular crater, usually volcanic. It brightens and fades about every 460 or 480 days, according to an analysis published by Dr. de Kleer and colleagues in the Geophysical Research Letters in May 2019.
If Loki Patera continues to wax and wane into the next few years as predicted, reports Sokol, that time frame would match other cyclical variations in how Io orbits Jupiter — providing a suggestive link between changing tides exerted by Jupiter and the ebbs and flows of surface volcanoes
Many scientists believe that Loki Patera is a massive lava lake, and that these bright episodes represent its overturning crust, like that seen in lava lakes on Earth. In fact, the heat emissions from Loki Patera appear to travel around the lake during each event, as if from a wave moving around a lake triggering the destabilization and sinking of portions of crust. Prior to 2002, this front seemed to travel around the cool island in the center of the lake in a counter-clockwise direction.
All hot spots detected are shown on a map of Io. Each circle represents a new detection; the size of the circle corresponds logarithmically to the intensity, and more opaque regions are where a hot spot was detected multiple times. The color and symbol indicate the type of eruption, following the legend. Loki Patera is at 310 West, 10 North and Kurdalagon Patera is at 220 West, 50 South.
After an apparent cessation of brightening events after 2002, de Pater observed renewed activity in 2009. “With the renewed activity, the waves traveled clockwise around the lava lake,” she noted.
Another volcano, Kurdalagon Patera, produced unusually hot eruptions twice in the spring of 2015, coinciding with the brightening of an extended cloud of neutral material that orbits Jupiter. This provides circumstantial evidence that eruptions on the surface are the source of variability in this neutral cloud, though it’s unclear why other eruptions were not also associated with brightening, de Kleer said.
De Kleer noted that the Keck and Gemini telescopes, both atop the dormant volcano Maunakea, complement one another. Gemini North’s queue scheduling allowed more frequent observations – often several a week – while Keck’s instruments are sensitive also to longer wavelengths (5 microns), showing cooler features such as older lava flows that are invisible in the Gemini observations.
The astronomers are continuing their frequent observations of Io, providing a long-term database of high spatial resolution images that not even Galileo, which orbited Jupiter for eight years, was able to achieve.
Imager credits: NASA/JPL