Io Volcano Observer (IVO)
Io is one of the most fascinating objects in the Solar System. Tidal flexing resulting from the Laplace orbital resonance (between Io, Europa, and Ganymede) drives active volcanism at a remarkable level. This produces volcanic plumes which reach up to 300 km above the surface. The driving volatiles contribute to a highly heterogeneous SO2-dominated atmosphere. Some of this material escapes (by processes which are not yet fully clear) and forms a neutral cloud which accompanies Io in its orbit about Jupiter. This material is ultimately ionized and fills Jupiter’s magnetosphere leading to other remarkable phenomena such as Jupiter’s aurorae.
Io is perhaps the ideal target for studying how planets work since it is a world that evolves over human timescales. The tidal heating that drives Io’s activity is also a key process in controlling the habitable zone in the Jovian system (and presumably other planetary systems throughout the universe). Paradoxically, Io may also provide clues to understanding the first billion years of Solar System history since major volcanic processes last seen billions of years ago on the Earth, Moon, and Mars are active on Io today. Discerning how Io became devoid of water and carbon but retained sulphur can instruct us about the evolution of volatiles essential for life as we know it.
Io’s Volcanic Activity
Over 150 sites actively erupt silicate lavas; many are hotter than active terrestrial lavas. Active lava lakes >200 km in diameter and lava flows >300 km long dwarf their terrestrial counterparts. Plumes rich in sulphur compounds are associated with many of these eruptive sites. About 150 mountains, many taller than Mt. Everest, have been thrust up by faulting.
Io’s Interaction with the Magnetosphere of Jupiter
Around 1 ton/s of material is removed from Io by the interaction between its atmosphere and Jupiter's magnetosphere. Some of the material is removed as neutral atoms and molecules (mostly comprising oxygen and sulfur atoms and their compounds). These neutrals accompany Io in its orbit about Jupiter (5.91 Jovian radii, RJ, from the center of the planet) until they are ionized through electron impact and charge exchange. They are then accelerated to the nearly corotational flow of the ambient plasma, which passes Io at a relative velocity of 57 km/s, to form a torus of ions (the Io plasma torus or IPT) completely surrounding Jupiter. This source of material in the IPT is probably enhanced by the direct pick-up of ions from Io's exosphere.
The first observational evidence of these processes was provided by Brown (1972) who detected sodium D-line emission (at 589.0 and 589.6 nm) from a “cloud” in the vicinity of Io. It was quickly established that resonant scattering of sunlight by the atoms in this neutral cloud was producing the observed emission. Brown and Yung (1976) gave a detailed description of the emission process. It was also suggested that the neutrals were being removed from Io by charged particle sputtering of its surface and/or atmosphere (Matson et al., 1975; Haff et al., 1980). Following the Voyager 1 observations of Io, which revealed an SO2 atmosphere (Pearl et al., 1979) and a sulphur-dominated surface (Sagan, 1979), it became widely assumed that Na was merely a trace element in a neutral cloud dominated by sulphur and oxygen atoms.
The PIG team has been involved in observations of the magnetospheric emissions for many years.
The Io Volcano Observer is a Discovery-class mission proposal which is planned to launch in the 2020-2022 timeframe.
An Io Observer mission was the top choice for a medium-size mission by the Decadal Survey large satellites group, but a Flagship Europa mission (later to become EJSM) was the top priority of the large satellites group. Consequently, the Io mission did not make the list of top five initial candidates for the medium-class New Frontiers missions.
At the time of the Decadal Survey, a dedicated Io mission was not considered to be feasible within the Discovery cost cap. Io missions have been proposed for Discovery, without radioisotope power sources, but never selected for Phase A studies. A solar-powered mission to Jupiter is feasible (e.g., Juno), but requires very large solar arrays and hence a large spacecraft with enough fuel to decelerate and be captured into Jupiter orbit. The Advanced Sterling Radioisotope Generators (ASRGs) serve to reduce greatly the cost and mass of an Io mission. Another recently-proven technology of great importance to IVO is Ka-band downlink, which returns more data for less mass and power than X-band downlink. With these innovations, it is believed that the Decadal Survey goals for a medium class Io Observer can be achieved within the cost cap of the small-mission Discovery program.
In FY2008 NASA selected Io Volcano Observer (IVO) as one of the Discovery and Scout Mission Capability Expansion (DSMCE) studies for missions that would be uniquely enabled by use of 2 government-furnished Advanced Stirling Radioisotope Generators (ASRGs). Each ASRG can provide ~140 W electric power from just 0.8 kg Pu238, as well as ~100 W thermal power that can be redistributed to heat spacecraft (S/C) components. The ASRGs enable a low-cost mission in Jupiter orbit to include significant imaging science by providing pointing flexibility and a high data rate.
It is not clear whether the forthcoming Discovery round will foresee use of ASRGs - our information at the present time is not. But nonetheless, an Io-mission might still be feasible.
IVO and Discovery
Following successful completion of the DSMCE study, IVO will now be proposed in the next selection round for NASA Discovery missions. Watch this space!
Adams, E., K. Hibbard, E. Turtle, E. Reynolds, B. Anderson, C. Paranicas, G. Rogers, J. McAdams, D. Roth, P. Christensen, A. McEwen, M. Wieser, N. Thomas, P. Wurz, and J. Janesick, (2012), Io volcano observer (IVO) integrated approach to optimizing system design for radiation challenges, Aerospace Conference, 2012 IEEE , page 1-13, 3-10 March 2012, doi: 10.1109/AERO.2012.6187177.