My thoughts on a Galilean Satellite Observer have prompted two thoughtful responses from readers that I'll post over the next couple of days. Before doing so, I wanted to share my intellectual basis for a Jovian icy moon observer. The National Academy of Sciences reviewed possible missions for the New Frontiers program and prioritized a Ganymede Observer as a candidate mission. (An Io observer was also prioritized, but it would ideally have instruments suited to a silicate world rather than an icy moon. So this post will concentrate on an icy moon observer.) The text below is copied from the working group's 2008 report, Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity.
My thinking is that if a Ganymede multiple flyby mission is a priority, then a simple extension of that mission to flyby multiple Jovian moons would be even more valuable (assuming that the Jupiter Europa Orbiter doesn't fly, which I hope it does but budget realities my decide otherwise). As you'll see, these two readers either only partially agree or completely disagree. I'll present all sides of the argument so you can form your own opinion.
From the report:
Large icy satellites may hold the key to answering many fundamental questions about the solar system, and Jupiter’s largest moon, Ganymede, is of particular interest because of its unique internal magnetic field and its interaction with Jupiter. Ganymede is the only icy body in the solar system known to generate its own magnetic field, thus providing a unique window into its interior and, moreover, shedding light on how internal magnetic fields are generated elsewhere
in the solar system. Ganymede also provides a laboratory for the study of plasma effects on satellite surfaces: the decadal survey notes that “Ganymede’s magnetic field is strong enough that it creates a mini-magnetosphere of its own in Jupiter’s magnetosphere, partially shielding the satellite from plasma bombardment. The interaction between Ganymede’s magnetosphere and Jupiter’s magnetosphere is similar to the interaction between Earth’s magnetosphere and the solar wind, where magnetic reconnection plays a key role.”
Ganymede also exhibits evidence for a subsurface ocean. In contrast to Europa, an ocean in Ganymede may be bounded both above and below by ice rather than rock; nonetheless, it is likely to illuminate processes that may produce habitable environments elsewhere in the solar system (or maybe on Ganymede itself). Ganymede’s surface suggests a complex geologic history (see Figure 2.12) with similarities to those of Miranda and Enceladus. Moreover, some of its geologic terrains may be analogous to terrestrial features, thereby providing a bridge between silicate and icy bodies that could well provide fundamental information regarding the behavior
of ice in geologic processes.
Ganymede’s geologic activity and magnetic field are probably powered by tidal heating. The decadal survey states that “Ganymede’s differentiated interior and actively convecting core (required to generate its magnetic field) may be a consequence of its passage into resonance, while Callisto has not experienced this history”. Thus, better understanding of Ganymede could provide information about the tidal history of the entire Jovian system.
A Ganymede Observer mission that addresses fundamental goals for solar system exploration may be possible and would also enable broader goals within the Jovian system. Consequently, such a mission should be included in the next New Frontiers announcement of opportunity. Because the Ganymede Observer was not described in significant detail in the decadal survey, the committee chose to list science objectives that such a mission could address, but stresses that this list should not be exclusive. In no case should these science questions be considered
to be mission requirements—they are merely options for such a mission. This list includes far more science than can be included in a single New Frontiers mission and the committee stresses that it fully expects those proposing such a mission to choose among these science objectives. It will be up to the proposers to make the case as to why some science objectives are more important than others. These objectives, which are not prioritized, include:
• Understand Ganymede’s intrinsic and induced magnetic fields and how they are generated, and characterize their interaction with Jupiter’s magnetic field.
• Determine Ganymede’s internal structure, especially the depths to and sizes or thicknesses of the probable metallic core and deep liquid water ocean, and the implications for current and past tidal heating and the evolution of the Galilean satellite system as well as ocean chemistry.
• Understand Ganymede’s endogenic geologic processes, e.g., the extent and role(s) of cryovolcanism, the driving mechanism for the formation of the younger, grooved terrain, and the extent to which Ganymede’s tectonic processes are analogs for tectonics on other planetary bodies (both icy and silicate).
• Document the non-ice materials on Ganymede’s surface and characterize in detail the connection between Ganymede’s magnetosphere and its surface composition (e.g., polar caps).
• Document the composition and structure of the atmosphere, identifying the sources and sinks of the atmospheric components and the extent of variability (spatial and/or temporal).
Under a New Frontiers budget it is likely that the most feasible way to address these objectives is by a Jupiter-orbiting spacecraft with multiple Ganymede flybys—in other words, the spacecraft may not have to enter Ganymede orbit. Even so, it is possible that such a mission may exceed the New Frontiers cost cap. Nevertheless, innovative approaches might be able to circumvent these problems and enable fundamental Ganymede science under New Frontiers constraints.