The Oxygen Neutral Cloud Surrounding Jupiter’s Volcanic Moon

Lakes of lava and hundreds of volcanoes dot the surface of Jupiter’s moon Io, some spewing lava dozens of kilometers into the air. Only slightly larger than our own planet’s moon, Io is the most volcanically active place in the solar system. Its thin atmosphere is made up largely of sulfur oxides. As Io orbits, neutral gas particles escape its atmosphere and collide with electrons, giving rise to a donut-shaped cloud of ionized particles around Jupiter, known as the Io plasma torus.

Exactly how those neutral gases escape Io’s atmosphere is not well understood, however. Previous studies have shown that most atomic oxygen and sulfur escape Io’s atmosphere by colliding with energetic particles, such as torus ions, which bump the particles out of the atmosphere in a process known as atomic sputtering. Some of the particles escape from Io’s gravity and form clouds of neutral sulfur and oxygen. Here Koga et al. provide new insights into the role of the neutral cloud in the Io plasma torus.

The team took advantage of data collected by Japan’s Hisaki satellite, which launched in 2013 and became the first space telescope to observe planets like Mars and Jupiter from Earth’s orbit. The researchers used spectrographic data from the Extreme Ultraviolet Spectroscope for Exospheric Dynamic (EXCEED) instrument aboard the satellite to measure atomic emissions at 130.4 nanometers around Io’s orbit. The measurements were collected over 35 days between November and December 2014, a relatively calm volcanic period for the moon.

The authors found that Io’s oxygen cloud has two distinct regions: a dense area that spreads inside Io’s orbit, called the “banana cloud,” and a more diffuse region, which spreads all the way out to 7.6 Jovian radii (RJ). The team plugged the satellite observations into an emissions model to estimate the atomic oxygen number density. They found more oxygen inside Io’s orbit than previously thought, with a peak density of 80 atoms per cubic centimeter at a distance of 5.7 RJ. The team also calculated a source rate of 410 kilograms per second, which is consistent with previous estimates.

This study provides the first good look at Io’s neutral cloud, which has historically been too dim to measure. Neutral particles from Io’s atmosphere are one of the primary sources for charged particles in Jupiter’s massive magnetosphere. Ultimately, the authors note, a better understanding of the neutral cloud will provide important insights into the gas giant’s magnetosphere. (Journal of Geophysical Research: Space Physics,, 2018)

—Kate Wheeling, Freelance Writer

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