Io's volcanic control of Jupiter's extended neutral clouds

Abstract

Dramatic changes in the brightness and shape of Jupiter's extended sodium nebula are found to be correlated with the infrared emission brightness of Io. Previous imaging and modeling studies have shown that varying appearances of the nebula correspond to changes in the rate and the type of loss mechanism for atmospheric escape from Io. Similarly, previous IR observational studies have assumed that enhancements in infrared emissions from Io correspond to increased levels of volcanic (lava flow) activity. In linking these processes observationally and statistically, we conclude that silicate volcanism on Io controls both the rate and the means by which sodium escapes from Io's atmosphere. During active periods, molecules containing sodium become an important transient in Io's upper atmosphere, and subsequent photochemistry and molecular-ion driven dynamics enhance the high speed sodium population, leading to the brightest nebulas observed. This is not the case during volcanically quiet times when omni-present atmospheric sputtering ejects sodium to form a modest, base-level nebula. Sodium's role as a “trace gas” of the more abundant species of sulfur (S) and oxygen (O) is less certain during volcanic episodes. While we suggest that volcanism must also affect the escape rates of S and O, and consequently their extended neutral clouds, the different roles played by lava and plume sources for non-sodium species are far too uncertain to make definitive comparisons at this time.

Introduction

Jupiter's magnetosphere offers one of the richest blends of geological, atmospheric, and space plasma physics phenomena in the Solar System. The heavy ions of oxygen and sulfur which dominate the mass and energy budget of the magnetosphere have their origin in the volcanoes of Jupiter's remarkable moon, Io. From the volcanoes, the sulfur and oxygen move by steps into Io's atmosphere and onto its surface, and then into large extended neutral clouds around Io and Jupiter via atmospheric escape. By subsequent ionization and pickup they populate the plasma torus, and finally they exit the jovian system altogether by outward diffusion, charge exchange reactions, or by precipitation into Jupiter's auroral regions.

Several spacecraft encounters with Jupiter—Pioneer in 1973–1974, Voyager in 1979, and Ulysses in 1992—have provided snapshots of the jovian magnetospheric environment. These remarkable datasets provided insights into a complex system of volcanism, surface geology, atmospheres, charged particles and fields. On a much longer multi-year timescale, the Galileo satellite has just completed a detailed in-situ study of variations of Io, the plasma torus, and the magnetosphere as a whole. Complementing these spacecraft missions to Jupiter has been an ongoing series of observations conducted from the ground and from low-Earth-orbit. We describe here a decade long view of changes in the jovian sodium nebula and contemporaneous observations of volcanic activity on Io.

The sodium clouds generated by Jupiter's satellite Io provide a window through which to examine a complex chain of processes that appear to dominate a magnetosphere in a way not found elsewhere in the Solar System. While a consensus model is far from complete, its potential elements have been described in several review articles Schneider et al., 1989, Spencer and Schneider, 1996, Thomas, 1997, Bagenal, 2004. Briefly, volcanic eruptions of lava and geyser-like plumes populate its surface and atmosphere with several gases (SO₂, SO, S, O); a minor species is sodium, important for its easy detection at visible wavelengths and thus as a “tracer” of the more plentiful elements. Energetic ions trapped in Jupiter's magnetic field bombard the atmosphere and the surface of Io, “sputtering” some of the Na atoms off of Io and into orbits around Jupiter. Some of the atmospheric gases still bound to Io and others in near-Io orbits about Jupiter become ionized by the impact of energetic plasmas and solar EUV radiation. These low energy plasmas are subject to Jupiter's strong, corotating magnetic field, and thus magnetic capture (or “pick-up”) leads to the maintenance of a plasma torus surrounding the planet at approximately the radial distance of Io (5.9R_J), as well as an ionosphere on Io.

Recycling of plasma back into the neutral gas state involves an equally remarkable series of steps linked to the fact that three distinct populations co-exist near Io: torus plasma traveling at co-rotational speeds (∼75 km/sec), together with neutrals and ionospheric plasma moving with Io's orbital speed (∼17 km/sec). Neutralizations of the atomic and molecular ions near Io or in the plasma torus result in the release of neutral sodium at high speeds ($\text{Na}{}^{\mathrm{\ast }}$) sufficient to escape from the magnetosphere, essentially unaffected by any other process, to form a great nebula. Within this giant structure, the slower speed, sputtered sodium (Na) forms the elongated cloud that orbits Jupiter with Io (the “banana” cloud).

There is a rich literature relating to the individual sodium clouds composed of Na near Io and $\text{Na}{}^{\mathrm{\ast }}$ forming jets, streams and nebulas. Using images of the Io sodium clouds at multiple fields of view (±7R_J, ±30R_J, ±500R_J) from 1990–1996, (Wilson et al., 2002) formulated a working model of the overall jovian sodium budget. They determined that various combinations of two basic atmospheric escape processes—atmospheric sputtering of Na, and ionospheric escape of an unidentified molecular sodium ion (NaX⁺)—are needed to explain essentially all manifestations of the sodium clouds. The pick-up of NaX⁺ and its subsequent destruction in the torus to form a “stream” or distributed source of $\text{Na}{}^{\mathrm{\ast }}$ produces a bright and distinctly rectangular-shaped sodium nebula when it is the dominant process. Atmospheric sputtering may also contribute to the nebula, but its efficiency in doing so depends strongly on the velocity distribution of the escaping neutrals that result. Given a sufficient fraction of higher-speed ejections, atmospheric sputtering was shown to produce a faint and somewhat diamond-shaped nebula around Jupiter during times when it is not competing with NaX⁺ escape. Simple charge exchange, once thought to be the dominant escape mechanism, is now understood to be only a minor exospheric process because it is unable alone to account for the appearances seen simultaneously in the three fields-of-view datasets. Thus, we are now at the stage where imaging observations of the sodium nebula provide a quantitative measure of both the rates and means of atmospheric escape from Io. The next step is to investigate how the coupled neutral–plasma system at ∼6R_J responds to changes in Io's volcanism.