With a mission now planned for the Jupiter system in the 2020s, how will the Io Volcano Observer proposal be affected? Would a dedicated Io mission even be necessary?
The Io Volcano Observer and the Jupiter Europa Orbiter would conduct complimentary science. Both spacecraft have high-resolution cameras capable to studying Io's surface in fine detail during flybys as well as monitoring Io's global volcanic activity from a distance. Both can conduct mass spectroscopy of Io's atmosphere and plumes as well as observe Io's thermal inertia. The Jupiter Europa Orbiter would be capable of acquiring observations not currently in IVO's baseline payload such as near-infrared spectroscopy, ground-penetrating radar, laser altimetry, and particle and plasma analysis. So seemingly, the Io Volcano Observer would not be necessary. Not so fast.
The Jupiter Europa Orbiter's instruments are designed to study Europa, with bandpasses of the various instruments and their functionality driven by that requirement. Studying the other bodies in the Jupiter system, while a level 1 science requirement, really is just gravy for the mission. JEO's unique instrumentation, such as the Ground-penetrating radar, can answer quite a few questions that IVO can't. However, the design of the payload for IVO has been defined to specifically answer questions at Io. For example, the camera on IVO would be capable of observing volcanic activity with multiple filters with less than 0.1 seconds between color frames. This allows fairly accurate measurement of the lava temperatures at Io's volcanoes. This can constrain the amount of partial melting in the mantle needed to support the eruption temperature observed. The band passes on the Thermal Mapper, rather than being selected to search for warm spots on an icy world, will be selected to explore different volcanic processes on Io of different ages as well as looking at the silicate composition of these flows.
Also, don't forget that IVO will perform at least seven Io flybys during its 1.5-year primary mission (starting in early 2021), three more than the encounters planned for JEO. In addition, IVO has enough margin in its radiation shield to support seven more encounters, which could be spaced out by as much one year apart to help study the long-term life time of IVO's power source, the two Advanced Sterling Radioisotope Generators (ASRGs). This extended mission could help fill the gap between IVO's primary mission which ends in late 2022 and JEO's arrival in late 2025. This provides the potential for spacecraft monitoring of Io covering almost eight years, similar to Galileo's time at Jupiter.
While the Jupiter Europa Orbiter will perform quite a bit of science at Io, since the instruments are not optimized for Io science, there is still a need for a dedicated mission like Io Volcano Observer. Potentially JEO could allow IVO to trim some costs by reducing some of the redundancy, like the magnetometer instrument. However, the priority for an Io mission may go down in comparision to other potential Discovery-class missions with the EJSM arrrive only a five years later than IVO.