The volcanic activity on Io primarily takes two forms: molten lava eruptions that are similar to the volcanoes on Hawaii; and great plumes of gas vented from fissures in the surface, like giant versions of the geysers in Yellowstone National Park. Geysers on Earth are made of superheated water, but the plumes of Io are composed of sulfur dioxide and other molecules of sulfur. Unlike Earth, where a dense, gravitationally bound atmosphere absorbs the gasses emitted from volcanoes, Io's lower gravity and meager atmosphere barely manage to keep the gas plumes from escaping directly to space. In fact, what little atmosphere Io does possess is formed in large part by the plumes themselves.
But, alas, Io is ultimately unable to retain these plume gasses for very long. Io resides in a kind of radiation belt, called the plasma torus, which encircles Jupiter. Ions and electrons in this plasma torus constantly bombard Io and it's fragile atmosphere, and this causes the molecules in the atmosphere to be ejected entirely from Io. The Earth resides in a similar environment of ions and electrons that are constantly streaming away from the sun (the solar wind); but the Earth has a magnetic field which protects the atmosphere from these particles, preventing its atmosphere from being lost to space. Io has no such magnetic field, so it is at the mercy of the environment around Jupiter in which it resides. Thus, Io's atmosphere is in a constant state of flux; gases are constantly being produced by volcanic activity and constantly lost to the space around Jupiter.
Some of the gasses that escape from Io are ionized and become part of the plasma torus that scavenges Io. The remainder of the escaped gasses form huge, nebulous "clouds" in space near and around Jupiter. These clouds are composed mostly of sulfur and oxygen, but there is also a small percentage of sodium atoms within the clouds. These sodium atoms are uniquely responsive to sunlight, which causes them to glow brightly at the yellow-orange color of sodium streetlights. The image to the left was taken by the Galileo spacecraft with a camera filter that is sensitive to the light emitted by sodium atoms; Matthew Burger has processed the image to enhance the visibility of the sodium (Burger et al. 1999).
The glowing sodium atoms are visible to telescopes on Earth, meaning we can study the clouds, and ultimately, Io's atmosphere and volcanic activity, from the Earth. The image to the right was taken by Nick Schneider and John Trauger using the Catalina Observatory 1.5-meter Telescope (adapted from Spencer and Schneider, 1996). As it turns out, the sodium clouds coming from Io have different shapes and behaviors that depend on exactly how the atoms escape from Io. There are several different ways that the ions and electrons in the plasma torus can interact with Io's atmosphere to eject sodium, sulfur, and oxygen to form the clouds, and it is these ejection processes that my colleagues and I work to identify and characterize.
The relationship between an atmospheric ejection process and the cloud that it ultimately forms is not a simple one, so we use computers to simulate the escape of gasses from Io and the clouds that result. When one of our computer simulations produces a cloud that is similar in appearance to an actual cloud observed around Jupiter, we can conclude that the ejection process in the simulation is more or less similar to the ejection process actually happening at Io.
Io's sodium clouds were first observed in the mid 1970's, before we even knew about Io's volcanic activity. Since that time, many different types of observations of the sodium clouds have been made, and several types of ejection processes have been proposed to explain them. More recently, my collegues and I have developed a model using just two types of escape processes, atmospheric sputtering and pickup ion neutralization, to explain essentially all of the sodium cloud features. Consensus was reached some time ago that atmospheric sputtering produces the cloud we call the "banana." (This cloud has the approximate shape and yellow color of a banana!) We identified pickup ion neutralization more recently from observations of peculiar time-variable cloud features near Io. We now recognize that pickup ion neutralization can account for all of the cloud features not produced by sputtering, meaning that atmospheric escape from Io appears to be dominated by just two processes. This successful "decoding" of Io's sodium clouds not only advances our understanding of Io's atmospheric escape, but will provide a tool for studying Io's atmosphere and volcanic activity in general, as well as atmospheric escape from other solar system bodies.