Discovery of currently active extraterrestrial volcanism

Two volcanic plumes were discovered on an image of Io taken as part of the Voyager optical navigation effort. This is the first evidence of active volcanism on any body in the solar system other than Earth.     

Io volcanism seen by new horizons: a major eruption of the Tvashtar volcano

Jupiter's moon Io is known to host active volcanoes. In February and March 2007, the New Horizons spacecraft obtained a global snapshot of Io's volcanism. A 350-kilometer-high volcanic plume was seen to emanate from the Tvashtar volcano (62 degrees N, 122 degrees W), and its motion was observed. The plume's morphology and dynamics support nonballistic models of large Io plumes and also suggest that most visible plume particles condensed within the plume rather than being ejected from the source. In images taken in Jupiter eclipse, nonthermal visible-wavelength emission was seen from individual volcanoes near Io's sub-Jupiter and anti-Jupiter points. Near-infrared emission from the brightest volcanoes indicates minimum magma temperatures in the 1150- to 1335-kelvin range, consistent with basaltic composition.     

Galileo at Io: results from high-resolution imaging

During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper approximately 1 kilometer are rich in volatiles.     

The jupiter-io connection: an alfven engine in space

Much has been learned about the electromagnetic interaction between Jupiter and its satellite Io from in situ observations. Io, in its motion through the Io plasma torus at Jupiter, continuously generates an Alfvén wing that carries two billion kilowatts of power into the jovian ionosphere. Concurrently, Io is acted upon by a J x B force tending to propel it out of the jovian system. The energy source for these processes is the rotation of Jupiter. This unusual planet-satellite coupling serves as an archetype for the interaction of a large moving conductor with a magnetized plasma, a problem of general space and astrophysical interest.     

Io: ground-based observations of hot spots

Observations of Io in eclipse demonstrate conclusively that Io emits substantial amounts of radiation at 4.8 and 3.8 micrometers and a measurable amount at 2.2 micrometers. Color temperatures derived from the observations fit blackbody emission at 560 K. The required source area to yield the observed 4.8-micrometer flux is approximately 5 x 10(-5) of the disk of Io and is most likely comprised of small hot spots in the vicinity of the volcanoes.     

Pioneer 10 observations of the ultraviolet glow in the vicinity of jupiter

A two-channel ultraviolet photometer aboard Pioneer 10 has made several observations of the ultraviolet glow in the wavelength range from 170 to 1400 angstroms in the vicinity of Jupiter. Preliminary results indicate a Jovian hydrogen (1216 angstrom) glow with a brightness of about 1000 rayleighs and a helium (584 angstrom) glow with a brightness of about 10 to 20 rayleighs. In addition, Jupiter appears to have an extensive hydrogen torus surrounding it in the orbital plane of Io. The mean diameter of the torus is about equal to the diameter of the orbit of Io. Several observations of the Galilean satellites have also occurred but only a rather striking Io observation has been analyzed to date. If the observed Io glow is predominantly that of Lyman-alpha, the surface brightness is about 10,000 rayleighs.     

Volcanic hotspots on io: stability and longitudinal distribution

We report the first results of a program to determine the longitudinal distribution of volcanic activity on Jupiter's satellite Io. Infrared measurements at 8.7, 10, and 20 micrometers have been taken at a variety of orbital longitudes: strong variation in the 8.7- and 10-micrometer flux with longitude demonstrates that infrared emission arising from volcanic hotspots on Io is strongly concentrated in a few locations. Analysis of these data suggests that the active volcanic regions observed by the Voyager experimenters are still active, particularly the region around the feature known as Loki. Another source of flux, although of somewhat smaller magnitude, is indicated on the opposite hemisphere. If these sources are the only major volcanic centers on Io, then current global heat flow estimates must be revised downward. However, heat flow from as yet unobserved longitudes, hotspots at high latitudes, and conducted heat flow must still be measured.     

Io: evidence for silicate volcanism in 1986

Infrared observations of Io during the 1986 apparition of Jupiter indicate that a large eruptive event occurred on the leading side of Io on 7 August 1986, Universal Time. 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 approximately 900 Kelvin. Together with previously reported events, 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 Kelvin) and thus constitutes strong evidence for active silicate volcanism on the surface of Io.     

A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer

During the inbound pass of the Galileo spacecraft, the magnetometer acquired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by Io. A field decrease, of nearly 40 percent of the background jovian field at closest approach to Io, was recorded. Plasma sources alone appear incapable of generating perturbations as large as those observed and an induced source for the observed moment implies an amount of free iron in the mantle much greater than expected. On the other hand, an intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations. It seems plausible that Io, like Earth and Mercury, is a magnetized solid planet.     

Evidence of a global magma ocean in Io's interior

Extensive volcanism and high-temperature lavas hint at a global magma reservoir in Io, but no direct evidence has been available. We exploited Jupiter's rotating magnetic field as a sounding signal and show that the magnetometer data collected by the Galileo spacecraft near Io provide evidence of electromagnetic induction from a global conducting layer. We demonstrate that a completely solid mantle provides insufficient response to explain the magnetometer observations, but a global subsurface magma layer with a thickness of over 50 kilometers and a rock melt fraction of 20% or more is fully consistent with the observations. We also place a stronger upper limit of about 110 nanoteslas (surface equatorial field) on the dynamo dipolar field generated inside Io.     

Atomic clouds as distributed sources for the io plasma torus

Several recent developments have implications for the neutral particle environment of Jupiter. Very hot sulfur ions have been detected in the Io torus with gyrospeeds comparable to the corotation speed, a phenomenon that would result from a neutral sulfur cloud. Current evidence supports the hypothesis that extensive neutral clouds of oxygen and sulfur exist in the Jupiter magnetosphere and that they are important sources of ions and energy for the Io torus.     

Finite-frequency tomography reveals a variety of plumes in the mantle

We present tomographic evidence for the existence of deep-mantle thermal convection plumes. P-wave velocity images show at least six well-resolved plumes that extend into the lowermost mantle: Ascension, Azores, Canary, Easter, Samoa, and Tahiti. Other less well-resolved plumes, including Hawaii, may also reach the lowermost mantle. We also see several plumes that are mostly confined to the upper mantle, suggesting that convection may be partially separated into two depth regimes. All of the observed plumes have diameters of several hundred kilometers, indicating that plumes convey a substantial fraction of the internal heat escaping from Earth.     

Sulfur volcanoes on io

Widespread volcanism on Jupiter's satellite Io, if it occurred over the age of the solar system, would quickly reduce the inventory of most common volatiles needed to drive such volcanism. One exception is the volatile element sulfur. It is therefore postulated that sulfur is the driving volatile for Ionian volcanism. Its presence is consistent with a carbonaceous-chondrite-like bulk composition for the original material that formed Io 4.5 billion years ago. The ubiquity of sulfur on Io today demonstrates the importance of this element in the processes that formed its surface.     

Images of Io's Sodium Cloud

The first direct images of Io's sodium cloud are reported and analyzed. The observed cloud extends for more than 10(5) kilometers along Io's orbit and is a somewhat "banana-shaped" partial toroid. More sodium atoms precede Io than follow it. A model based on the escape of sodium from a specific localized area on Io provides a reasonable fit to the observed intensity distribution whereas isotropic escape does not.     

Electrical origin of the outbursts on io

The outbursts seen on Jupiter's satellite Io have been described as volcanic eruptions. They may instead be the result of large electric currents flowing through hot spots on Io and causing evaporation of surface materials. A strictly periodic behavior would then be expected.     

Molecular Origin of Io's Fast Sodium

Neutral sodium emissions encircling Jupiter exhibit an intricate and variable structure that is well matched by a simple loss process from Io's atmosphere. These observations imply that fast neutral sodium is created locally in the Io plasma torus, both near Io and as much as 8 hours downstream. Sodium-bearing molecules may be present in Io's upper atmosphere, where they are ionized by the plasma torus and swept downstream. The molecular ions dissociate and dissociatively recombine on a short time scale, releasing neutral fragments into escape trajectories from Jupiter. This theory explains a diverse set of sodium observations, and it implies that molecular reactions (particularly electron impact ionization and dissociation) are important at the top of Io's atmosphere.     

Io: longitudinal distribution of sulfur dioxide frost

Twenty spectra of Io (0.26 to 0.33 micrometer), acquired with the International Ultraviolet Explorer spacecraft, have been studied. There is a strong ultraviolet absorption shortward of 0.33 micrometer that is consistent with earlier ground-based spectrophotometry; its strength is strongly dependent on Io's rotational phase angle at the time of observation. This spectral feature and its variation are interpreted as indicative of a longitudinal variation in the distribution of sulfur dioxide frost on Io. The frost is most abundant at orbital longitudes 72 degrees to 137 degrees and least abundant at longitudes 250 degrees to 323 degrees . Variations in spectral reflectivity between 0.4 and 0.5 micrometer, reported in earlier ground-based spectral studies, correlate inversely with variations in reflectivity between 0.26 and 0.33 micrometer. It is concluded that this is because the Io surface component with the highest visible reflectivity (sulfur dioxide frost) has the lowest ultraviolet reflectivity. At least one other component is present and may be sulfur allotropes or alkali sulfides. This model is consistent with ground-based ultraviolet, visible, and infrared spectrophotometry. Comparison with Voyager color photographs indicates that the sulfur dioxide frost is in greatest concentration in the "white" areas on Io and the other sulfurous components are in greatest concentration in the "red" areas.     

Io: an intense brightening near 5 micrometers

Spectrophotometric observations of the jovian satellite Io on 20 and 21 February 1978 (Universal Time) were made from 1.2 to 5.4 micrometers. Io's brightness at 4.7 to 5.4 micrometers was found to be three to five times greater at an orbital phase angle of 68 degrees than at orbital phase angles of 23 degrees (5.5 hours before the brightening) and 240 degrees (20 hours after the brightening). Since the 5-micrometer albedo of Io is near unity under ordinary conditions, the observed transient phenomenon must have been the result of an emission mechanism. Although several such mechanisms were examined, the actual choice is not clear.     

Large-scale nitrogen oxide plumes in the tropopause region and implications for ozone

Continuous measurements of nitrogen oxide and ozone were performed from a commercial airliner during 1 year at cruising altitudes below and above the tropopause. The upper tropospheric nitrogen oxides distribution was found to be strongly influenced by large-scale plumes extending about 100 to 1300 kilometers along the flight track. The plumes were frequently observed downwind of thunderstorms and frontal systems, which most probably caused upward transport of polluted air from the continental boundary layer or nitrogen oxide production in lightning strokes, or both. Particularly in summer, average ozone concentrations in the plumes were enhanced compared to the tropospheric background levels.     

Detection of SO in Io's exosphere

The Galileo orbiter's close pass by Io in 1995 produced evidence for extensive mass loading of the plasma torus through the ionization of SO2. On 11 October 1999, Galileo passed even closer to Io, this time across the upstream side relative to the flow of magnetospheric plasma that corotates with Jupiter. On the first flyby, ion cyclotron waves gave direct evidence for the production of SO2+ ions. On the second flyby, ion cyclotron waves associated with SO+ were stronger and more persistent. Moreover, SO+ emissions were seen closer to Io than SO2+ emissions, suggesting that the exosphere was spatially inhomogeneous. The location of the waves suggests a fan-shaped region of ion pickup extending in the anti-Jupiter direction. Because the wave spectra were different even where the 1995 and 1999 trajectories crossed, we infer that Io's exosphere is temporally variable.