The Io Volcano Observer (IVO): Investigating the Interior of a Tidally Heated World

PPS01-18 JpGU-AGU Joint Meeting 2020

*Christopher W. Hamilton1, Alfred S. McEwen1, Xianzhe Jia2, and the IVO Science Team

1. University of Arizona, 2. University of Michigan

NASA’s Discovery program recently selected the Io Volcano Observer (IVO) as one of four missions to advance to a Phase A study. Io’s proximity to the Jupiter and its Laplace orbital resonances with Europa and Ganymede mean the interior of the moon is flexed by gravitational processes leading to heating and volcanic activity at its surface. This makes Io the quintessential location for investigating processes of tidal heating within the Solar System. The mission will provide fundamental new constraints on Io’s interior by assessing whether or not it has an extant magma ocean (i.e., a layer, on the surface or in the interior of a body, that has such a high melt fraction that it behaves like a fluid). One of IVO’s prime objectives is therefore to “follow the heat” and determine the degree and distribution of melt within Io’s mantle and test whether or not tidal heating maintains a magma ocean inside present day Io.

Magnetometer measurements made by the Galileo spacecraft have provided an important constraint on this problem. Galileo observed an induced magnetic field within Io, which can be accounted for if the moon’s upper mantle contains at least 20% melt. However, plasma interactions with an inhomogeneous atmosphere could generate similar magnetic signatures. IVO will address this issue by better constraining Io’s plasma environment and conducting multiple flybys that are optimized to the best times and places for measuring variations in Io’s magnetic field. Independent measurements will also be used to assess the rigidity of Io’s lithosphere and its coupling to the deeper mantle. Specifically, if Io has a detached lithosphere over a globally continuous partial melt layer, then two independent measurements would provide definitive evidence of a magma ocean. One of these measurements is the ratio of imposed gravitational potential from Jupiter (which is well known) and the induced gravitational potential from the

deformation of Io (to be measured by IVO), which is known as k2. The second measurement is libration amplitude of Io.

Combining these lines of evidence, a weak induced magnetic field, low k2, and low libration amplitude

would imply that Io does not include a magma ocean. A strong magnetic induction signal, high k2, and high libration amplitude would support the existence of a subsurface magma ocean. The accompanying figure summarizes these two end-member scenarios as well as important variations. For instance, dissipation within the solid interior of Io could occur preferentially in the asthenosphere or in deep mantle. Alternatively, melt within Io’s interior could either form a magma ocean or a globally connected “magmatic sponge” layer with a lower, but still interconnected melt fraction. Global maps of heat flow, as well as volcanic and tectonic features, would also enable new measurements of lava temperatures and compositions that would provide additional means to discriminate between these interior models, and better understand mechanisms of "heat pipe" magma transport.

IVO will orbit Jupiter at an inclination of ~45°, designed so that the radiation dose per flyby is ~20 krad. IVO’s total radiation dose over ten orbits of its nominal mission phase will therefore be less than one tenth that of the Europa Clipper spacecraft. Science instruments will include a narrow-angle camera nearly identical to that of the Europa Imaging System, the Plasma Instrument for Magnetic Sounding, dual fluxgate magnetometers with multi-mission heritage, a thermal mapper with heritage from the BepiColombo spacecraft, and a neutral mass spectrometer in development for JUpiter ICy moons Explorer

(JUICE) spacecraft. In Phase A, we will propose an additional wide-angle stereo camera as a student collaboration instrument. The geometry and timing of each Io encounter are carefully designed to accomplish our science objectives, and will enable IVO to revolutionize our understanding of Io and tidally heated worlds in the Solar System and beyond.

Keywords: Io, Juipter, Tidal, Heating, NASA, Discovery