Volcanic gases are important to the scientific community for many reasons: the Earth's secondary atmosphere originated through volcanic degassing, and volcanic gases still play a crucial role in the Earth system. Occasional massive eruptions pump such large quantities of acid gas into the stratosphere that the resulting aerosols modify global climate for months or years. Also, volcanic gases escaping from magma bodies act as “messengers” that warn of impending eruptions and convey insight into magma chamber processes.

Volcanologists are interested in both the fluxes and the compositions of exhaled gases, but collecting in situ measurements on active volcanoes is hazardous. Several volcanologists were killed by an unexpected eruption of Galeras volcano in Colombia in January 1993, including our colleague Geoff Brown. This tragedy emphasized the need for remote methods for studying volcanoes. Two approaches are being explored: satellite remote sensing and ground-based techniques.