hints of discoveries to come arose from earth-based infrared observations taken in the 1970s. an anomalously high thermal flux, compared to the other galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while io was in jupiter's shadow. at the time, this heat flux was attributed to the surface having a much higher thermal inertia than europa and ganymede. these results were considerably different from measurements taken at wavelengths of 20 μm, which suggested that io had similar surface properties to the other galilean satellites. robert nelson and bruce hapke attempted to explain these features in io's spectrum by suggesting fumarolic activity as a mechanism for producing short-chain sulphur allotropes on io's surface. it has since been determined that the greater flux at shorter wavelengths was due to the combined flux from io's volcanoes and solar heating, whereas solar heating provides a much greater fraction of the flux at longer wavelengths. a sharp increase in io's thermal emission at 5 μm was observed on february 20, 1978 by witteborn, "et al." the group considered volcanic activity at the time, in which case the data was fit into a region on io in size at . however, the authors considered that hypothesis unlikely, and instead focused on emission from io's interaction with jupiter's magnetosphere.