Tidal dissipation, surface heat flow, and figure of viscoelastic models of Io

We report the following results from a decade of infrared radiometry of Io: (1) The average global heat flow is more than approx. 2.5 W/sq.m, (2) large warm (less than or equal to 200 K) volcanic regions dominate the global heat flow, (3) smal high-temperature (greater than or = 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 (greater than or = 600 K) 'outbursts' occurred during approx. 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 micrometer), 8.7 micrometer, N(10 micrometer), and Q(20 micrometer) with the Infrared Telescope Facility presented here were collected during nine apparitions between 1983 and 1993. These measurements provide full longitudinal coveraged 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 wavelengths 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.

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

▪ Abstract Io, innermost of Jupiter's large moons, is one of the most unusual objects in the Solar System. Tidal heating of the interior produces a global heat flux 40 times the terrestrial value, producing intense volcanic activity and a global resurfacing rate averaging perhaps 1 cm yr−1. The volcanoes may erupt mostly silicate lavas, but the uppermost surface is dominated by sulfur compounds including SO2 frost. The volcanoes and frost support a thin, patchy SO2 atmosphere with peak pressure near 10−8 bars. Self-sustaining bombardment of the surface and atmosphere by Io-derived plasma trapped in Jupiter's magnetosphere causes escape of material from Io (predominantly sulfur, oxygen, and sodium atoms, ions, and molecules) at a rate of about 103 kg s−1. The resulting Jupiter-encircling torus of ionized sulfur and oxygen dominates the Jovian magnetosphere and, together with an extended cloud of neutral sodium, is readily observable from Earth.

Io on the eve of the galileo mission

The importance of the thermal-infrared spectra region for investigations of asteroids lies traditionally in the dependence of the thermal emission of an asteroid on its visual albedo and size. Knowledge of albedos and sizes is crucial in many areas of asteroid research, such as mineralogy and taxonomy, the sice-frequency distribution of families and populations of asteroids (e.g., near-Earth asteroids), and the relationship between asteroids in the outer solar system and comets. However, the rapid increase in the availability of computing power over the past decade has cleared the way for development of sophisticated thermophysical models of asteroids with interesting new areas of application. Recent progress in the thermal modeling of asteroids and observational work in the thermal infrared are described. The use of thermal models for the physical characterization of asteroids and other applications is discussed.

Asteroids in the Thermal Infrared

Infrared observations of Io during the 1986 apparition of Jupiter indicate that a large eruptive event occurred on the leading side of Io on August 7, 1986, UT. Measurements made at 4.8, 8.7, and 20 micrometers suggest that the source of the event was about 15 kilometers in radius with a model temperature of about 900 K. These measurements indicate that high-temperature volcanic activity on the leading side of Io may be more frequent than previously thought. The inferred temperature is significantly above the boiling point of sulfur in a vacuum (715 K) and thus constitutes strong evidence for active silicate volcanism on the surface of Io.

https://science.sciencemag.org/content/sci/242/4883/1280.full.pdf

Io: Evidence for Silicate Volcanism in 1986

The thermal emission from Io's volcanic hot spots is linearly polarized. Infrared measurements at 4.76 micrometers show disk-integrated polarization as large as 1.6 percent. The degree and position angle of linear polarization vary with Io's rotation in a manner characteristic of emission from a small number of hot spots. A model incorporating three hot spots best fits the data. The largest of these hot spots lies to the northeast of Loki Patera, as mapped from Voyager, and the other spot on the trailing hemisphere is near Ra Patera. The hot spot on the leading hemisphere corresponds to no named feature on the Voyager maps. The value determined for the index of refraction of the emitting surface is a lower bound; it is similar to that of terrestrial basalts and is somewhat less than that of sulfur.

https://science.sciencemag.org/content/sci/230/4721/65.full.pdf

Characterization of Io's volcanic activity by infrared polarimetry

This paper derives midtimes and relative satellite positions from nearly 200 light curves of mutual eclipses and occultations of the Galilean satellites that occurred in 1985. It is shown that, at least for observations of the highest quality, the standard errors on the midtimes are smaller than 1 s and close to 0.01 arcsec on the positional offsets in right ascension and declination. 12 refs.

An Analysis of the 1985 Observations of Mutual Phenomena of the Galilean Satellites

New polarization measurements of Io were made with two different polarimeters at 3.8 and 4.8 microns. The measurements, at phase angles of about 2 deg, detect the polarization of reflected sunlight at both wavelengths. Only upper limits can be determined for the volcanic activity in 1986, but these limits show that there has been a remarkable decrease in activity, particularly of the Loki volcano, at these short wavelengths. The 1984 data (Goguen and Sinton (1985) are reanalyzed with the new model, which includes polarization of reflected sunlight. The large amount of thermal emission from the Loki volcano in 1984 produced a polarization that was dominant over that of reflected sunlight.

Jay D. Goguen

Papers

Characterization of Io's volcanic activity by infrared polarimetry

An Analysis of the 1985 Observations of Mutual Phenomena of the Galilean Satellites

Infrared Polarization Measurements of Io in 1986