Comparison of Voyager imaging data with Voyager 1 infrared observations (IRIS) and with ground-based telescopic infrared data demonstrates a strong correlation between hot spots and low-albedo features (reflectivity ≤0.3 in all visual wavelengths) on Io. Most of these low-albedo areas occur on the floors of volcanic calderas, and many are associated with active or recent volcanism. We use this correlation to identify additional hot spots in regions not observed by IRIS and to model their temperatures.

The major features discovered by this method are in a ringlike structure in the south-polar region (centered at 140¿W, 75¿S) and Amaterasu Patera (307¿W, 38¿N). Our model temperature for the south-polar feature is 280¿50 K. Taking account of the high latitudes of these features, the estimated global average heat flow from the hot spots becomes 1.8¿0.5 W m-2. Most of this heat emanates from just a few very large hot spots. The global distribution of hot spots is further evidence that Io is dimorphous body: the hemisphere centered on ~310¿W, 30¿S is much more volcanically active than the opposite hemisphere. The styles of heat transfer in Io (conductive versus convective) may be related to the global distribution of hot spots, large eruptive plumes, and large-scale topographic relief.

The correlation between hot spots and dark areas is due to (1) the absence of the bright materials, such as SO2 frost, which cover most of the surface, and (2) the presence of a low-albedo substance. Spectral properties of the low-albedo features, extracted from the Voyager multispectral data, are consistent with a number of candidate materials: liquid sulfur, silicate-sulfur mixtures, zinc sulfide, Fe sulfate, and Fe chloride. Quenched sulfur allotropes, on the other hand, are redder than the low-albedo features on Io and are incompatible with the elevated temperatures of the hot spots. Some of the dark caldera floor materials may be liquid sulfur and/or thermally altered (boiled) sulfur. Liquid sulfur would tend to collect in the topographic depressions and fractured bedrock of the calderas, and this would explain the concentration of low-albedo material in the calderas. The presence of liquid sulfur is also suggested by the hot spot temperature distribution (typically, 200--400 K) and by morphologies suggestive of lava lakes. However, the liquid/boiled sulfur model does not seem to work for Babbar and Amaterasu Paterae, and the evidence for liquid sulfur is not conclusive.