Back in October, I pointed out the then-recently released abstracts for December's Fall Meeting of the American Geophysical Union related to Io. Out of all the planned talks, the one I was most excited about was one by Krishan Khurana et al. titled, "Evidence of a Global Magma Ocean in Io Revealed by Electromagnetic Induction." Unfortunately, I was not at the AGU meeting so I couldn't report on this talk here on the blog at the time. However, Richard Kerr of Science Magazine was, and in Friday's issue of the journal, he delivers several reports on presentations given at the meeting, including Khurana's talk.
The question of a magnetic field on Io had been a vexing one for Galileo scientists on the magnetometer team. On December 7, 1995, Galileo performed its only Io flyby of its primary mission as the spacecraft approached its Jupiter Orbital Insertion maneuver. While remote-sensing instruments like the camera were turned off for the pass, the gravity data and fields-and-particles instruments provide a wealth of data. This data showed that Io had an iron- or iron-sulfide core that was 36-52% of Io's radius in size. While magnetometer data from the encounter was originally thought to be consistent with an intrinsic magnetic field at Io, however the low resolution of the data acquired precluded scientists from distinguishing between an intrinsic field, an induced magnetic field (precluded by Kivelson et al. 1996 due to the lack of free iron in Io's mantle), or interaction between Jupiter's magnetosphere and Io's extended ionosphere. The two polar flybys during the Galileo Millennium Mission (I31 and I32) were used to distinguish between these alternatives. This data showed that there was no intrinsic magnetic field at Io, perhaps resulting from an iron core that has no convection currents to generate an internal dynamo (at least a dipole anyway, higher-order fields were NOT ruled out).
Fast forward to last month's conference. According to Richard Kerr, Khurana looked back at Galileo magnetometer data and used a magneto-hydrodynamic model of the interaction between Jupiter's magnetosphere and the material surrounding Io in order to remove that interaction's signature in the magnetometer data. When they did this for the data from one Galileo's encounters with Io, what appeared to be the signature of an induced magnetic field remained. When they took a look at data from another Io flyby, they found that the poles of the magnetic field had flipped, as would be expected if there was an induced magnetic field at Io.
So what could create an induced magnetic field at Io? Induced magnetic fields are created when a time-variable magnetic field sweeps through an electrically-conductive material, like the briny water oceans of Europa, Ganymede, and Callisto. Jupiter's magnetic field is tilted with respect to Io's orbital plane, so at times Io is above or below the normal plane of Jupiter's magnetic field. The time-variable magnetic field produces electrical currents within the conductive material, which produce a magnetic field through induction. The direction of this current changes twice each Jovian day (remember, the magnetosphere is co-rotational with Jupiter, even at the distance of Io), causing the poles of the induced field to switch twice each Jovian day.
For the icy Galilean satellites, the conductive material is salty water, but what about Io? Khurana in his talk said that the data was consistent with a silicate magma ocean 50 kilometers beneath the surface. In order for an induced magnetic field to be produced, this ocean must be global or nearly so, though David Stevenson points out in a quote in the article that the depth and level of partial melting of this ocean don't seem to be uniquely determinable in the current data set. To do so would require a new mission to Io. Keep in mind that Khurana is also the head of the magnetometer group in the Io Volcano Observer proposal. A magma ocean (at least a mushy one with a large crystal fraction) was suggested by Laszlo Keszthelyi, Alfred McEwen, and G. J. Taylor in 1999 based on the very high temperatures thought to exist at some of Io's volcanoes, such as Pillan and Kanehekili, based on Galileo SSI eclipse results. Their models suggested 25-65% partial melting in Io's upper mantle in order to support the eruption temperatures observed (think of slushy magma). However, a re-evaluation of this same SSI camera data in Keszthelyi et al. 2007 reduced the estimates of the eruption temperature of Io's lavas, reducing the amount of partial melting required to 20-30% liquid in a mechanically weak asthenosphere, consistent with tidal heating models. Kerr's article does not mention whether this new magnetometer analysis is consistent with that lesser amount of partial melting or if more would be required.
Certainly a very exciting result, and I look forward to the paper to see what Khurana et al. has to say about what limits their analysis places on the amount of partial melting would be required to produce this magnetic field. This result also shows that a magnetometer on a future Io mission is a requirement, not just for understanding the near-Io environment of Jupiter's magnetosphere, but also to understand Io's interior structure and to provide another data point for the amount of partial melting is needed in Io's asthenosphere beyond tidal heating models and eruption temperatures.
Link: Magnetics Point to Magma 'Ocean' at Io [dx.doi.org]
EDIT: Just a minor correction of an error pointed out by Jon Eckberg. That shouldn't be inductance... changed that to induction