Io’s stealthy volcanoes
A volcano on Jupiter’s moon Io erupts in this image captured by the Galileo spacecraft in 1997.
NASA/JPL/DLR
After observing Io when it passed into Jupiter’s shadow in September 1999, Imke de Pater at the University of California, Berkeley, and her team detected a mysterious emission line from the Galilean moon’s atmosphere. The 1.7 µ line, they eventually determined, comes from a forbidden transition of sulfur monoxide gas that is excited at 1500 K before getting ejected from volcanic vents. The SO molecules then cool in the atmosphere and consequently drop to the ground state.
Because of the gas’s volcanic source, the researchers had attributed the emissions to Loki Patera, one of Io’s most active volcanoes. But a new analysis by de Pater and colleagues overturns that conclusion. The researchers now attribute much of Io’s SO emission to extremely hot, vapor-dominated eruptions that have long evaded detection.
In 2010 and 2015, the researchers conducted observations of Io with the OSIRIS near-IR field integral spectrograph at Keck Observatory in Hawaii. For each pixel, the instrument produced a spectrum, which allowed the researchers to pin down sites of SO emission across the moon. They compared the spectroscopic data with near-IR images that map out the thermal emissions of major volcanic activity. To the researchers’ surprise, the two data sets were not well correlated; as shown in the figure below, most hot spots of SO emission could not be traced to any known volcanoes.
Most large areas of peak SO emission on Io, measured in 2010 (left) and 2015 (right), are not well correlated with known volcanoes (x marks).
I. de Pater, K. de Kleer, M. Ádámkovics, Planet. Sci. J. 1, 29 (2020)
Despite the lack of obvious volcanic sources, the spectroscopic data supported a volcanic origin for the gas, which had temperatures exceeding 1500 K. That led de Pater and her team to conclude that Io’s surface has widespread stealth volcanism . Whereas eruptions from volcanoes such as Loki Patera spew reflective material, including dust and condensates, stealth eruptions occur at such extreme temperatures that all the material is in the vapor state. The plumes are essentially pure gas, so they are transparent to sunlight and observable only when Io is eclipsed by Jupiter’s shadow, at which point the faint gas emissions are no longer drowned out by the brightness of sunlight reflecting off Io’s surface.
Looking ahead, de Pater is a co-principal investigator on one of the 13 proposals approved for the James Webb Space Telescope‘s Early Release Science program. One of the telescope’s early missions will be to seek out Io’s stealthy volcanic activity. (I. de Pater, K. de Kleer, M. Ádámkovics, Planet. Sci. J. 1, 29, 2020 .)