Outstanding Science Questions at Io

Eric posted a few comments yesterday which provided a great suggestion for a blog post. What are the outstanding science questions that remain following Galileo and New Horizons? Can these questions be answered by the Jupiter Europa Orbiter (and Io Volcano Observer)? While better understanding Io's potential habitability by native lifeforms is certainly not one of them, there are at least five I can list here.

Before I do that, I want to point out Van Kane's post on his blog giving some of his closing thoughts on the Flagship mission selection. He pointed to my last post on how the selection of Europa as the target for that mission might effect the Io Volcano Observer and noted that the instruments could be more finally tuned to better answer questions at Io. That idea has crossed my mind. If the cost of IVO can't be brought in line with the Discovery mission cost cap, one potential alternative is to submit beefed up versions of some of IVO's instruments, particularly RCam and the Thermal Mapper, for the JEO Instrument Announcement of Opportunity. RCam's radiation-hard color push-broom camera could be just as effective at observing small-scale features on Europa as it could for determine eruption temperatures on Io. Additional bandpasses on Thermal Mapper compared to the Thermal Imager in JEO's model payload could also be useful for Jupiter on JEO. However, you do still lose the spatial coverage that would be provided by IVO, which would help it be more robust against variability in Io's volcanic activity. For example, what if Amirani was inactive in the mid-2020s?

So with that out of the way, here are five of the top (in my mind) outstanding science questions at Io:

1. What is the composition of Io's lava's?
2. What is the typical eruption temperature for Io's volcanoes? These first two questions are quite related. Following Galileo, there was consensus that Io's primary lavas were composed of basalt, a silicate lava rich in iron and magnesium and common on the terrestrial bodies in the solar system. However, the exact composition was very poorly constrained by the available data (dark at visible wavelengths, an absorption at 1 micron consistent with iron, and estimated lava temperatures in mafic to ultramafic range). The eruption temperature could be related to the amount of partial melting in Io's mantle. And of course temperature is related to the composition of Io's lavas. Generally the higher the magnesium level, the more mafic it is, the greater the liquidus temperature (and thus the eruption temperature). Understanding the effect of superheating during the ascent of the magma is also important. These questions can be answered through the measurement of Io's thermal emission in the 0.7-1 micron range (like from a near-infrared spectrometer or RCam on IVO) and by looking for absorption and emission features in the near-infrared that are consistent with materials in Io's lava, such as the Christiansen Feature.
3. Is tidal heating on Io steady-state or time-variable? One of the potential implications of measurements of Io's heat flow is that Io may be pumping out more heat from its interior than it currently receives from tidal heating as a result of its orbital resonances with Europa and Ganymede. This would indicate that Io is cooling down from a period of much greater tidal heating in its past. A potential way to test this is by high-resolution tracking of the position of Io and Europa over time. One of the mission goals for JEO is to provide this kind of tracking to better constrain the orbital evolutions of Io and Europa. IVO's polar orbits would allow for better thermal emission mapping of Io's polar regions, which are thought to be much warmer than they should be given the lower solar angles.
4. How are Io's two major structural landforms, paterae and mountains, formed? Various models following Voyager and again after Galileo have been produced to explain how paterae and mountains form on Io. Sub-surface radar sounding and high-resolution, low sun imaging could go a long way toward looking at the tectonic mechanisms behind how these intriguing features are formed. JEO would provide the sub-surface sounding, while IVO (and JEO to an extent, but longitude coverage would be limited) could provide imaging of a variety of mountains and paterae.
5. Does Io have an internal magnetic field? This question may have been answered as a no by Galileo, but closer encounters at different latitudes with a magnetometer-equipped spacecraft would help put this issue to bed. Understanding Io's magnetic environment would help with our understanding of Io's deep interior.

Obviously, there are certainly more than just these five. These five also reveal my geology bias as there are certainly a lot of remaining questions regarding Io's interaction with Jupiter's magnetosphere and about Io's atmosphere.