Io's heat flow from infrared radiometry: 1983–1993

Abstract

We report the following results from a decade of infrared radiometry of Io: (1) The average global heat flow is more than ∼2.5 W m−2, (2) large warm (≤200 K) volcanic regions dominate the global heat flow, (3) small high‐temperature (≥300 K) “hotspots” contribute little to the average heat flow, (4) thermal anomalies on the leading hemisphere contribute about half of the heat flow, (5) a substantial amount of heat is radiated during Io's night, (6) high‐temperature (≥600 K) “outbursts” occurred during ∼4% of the nights we observed, (7) “Loki” is the brightest, persistent, infrared emission feature, and (8) some excess emission is always present at the longitude of Loki, but its intensity and other characteristics change between apparitions. Observations of Io at M (4.8 μm), 8.7 μm, N (10 μm), and Q (20 μm) with the Infrared Telescope Facility presented here were collected during nine apparitions between 1983 and 1993. These measurements provide full longitudinal coverage as well as an eclipse observation and the detection of two outbursts. Reflected sunlight, passive thermal emission, and radiation from thermal anomalies all contribute to the observed flux densities. We find that a new thermophysical model is required to match all the data. Two key elements of this model are (1) a “thermal reservoir” unit which lowers daytime temperatures, and (2) the “thermal pedestal effect” which shifts to shorter wave‐lengths the spectral emission due to the reradiation of solar energy absorbed by the thermal anomalies. The thermal anomalies are modeled with a total of 10 source components at five locations. Io's heat flow is the sum of the power from these components.