Liquid Hot Magma Ocean Resolved in Galileo Magnetometer Data

I'm sorry I haven't posted on this blog in eight months.  I'm not sure why I stopped writing.  I think it just became more difficult for me to push past writer's block when you have to be the one to generate new content for a blog as opposed to using a flow of new information to help me to populate new articles.  But that's where I was last year, and it didn't help that I was interested in starting a new history blog which took up even more of my free time than I suspected.  It didn't help that in the middle of trying to write about this news for you, Blogger went down for more than 20 hours.  But as of five minutes ago, Blogger is back online and I can bring this exciting news to you.

Anyways, at least for today, that is over now as there is a fresh report out today in Science Express providing further evidence for a magma ocean beneath the surface of Io!  I know!  Big news!  This is a paper I've been looking forward to seeing for more than year and half.  Scientists have long suspected that Io has a mushy magma ocean based on tidal heating models where much of it is dissipated in the asthenosphere (otherwise known as the upper mantle) and on eruption temperature estimates from Galileo data.  This new paper provides another method, electromagnetic induction sounding, to look for a magma ocean inside Io.

I reported on this new work in October 2009 when it was first presented at a Division of Planetary Sciences meeting and again in January 2010 when Richard Kerr wrote about that presentation, given by Krishan Khurana, in Science magazine. These results are now online as an article in press at the Science Express website (meaning the paper has been approved for publication but has not been published in the print version of Science magazine).  For those outside the Science mag paywall, check out the JPL press release.

Khurana and his co-authors, Xianzhe Jia, Margaret Kivelson, Francis Nimmo, Gerald Schubert, and Christopher Russell re-examined Galileo Magnetometer data acquired during two of the spacecraft's encounters with Io in October 1999 and February 2000.  Data was acquired on two other encounters, however they were polar passes and weren't nearly as useful for detecting a magma ocean.  The magnetometer measured the absolute magnitude of the magnetic field surrounding the spacecraft and its magnitude in the three spatial components (B_x, B_y, and B_z).  Near Io, the spacecraft mostly measured how Jupiter's magnetic field was perturbed by Io's atmosphere.  In the atmosphere, plasma from Jupiter's magnetosphere is slowed as it takes on more mass and as charge is exchanged with these new particles.  The magnetic field lines are also affected by interactions with Io's conducting ionosphere.  Alfven wings which couple the Ionian and Jovian ionosphere in an electrical current called the Io flux tube further affect the local magnetic field vectors (this charged particle interaction produces the aurorae seen at Io as well as the auroral footprint in Jupiter's atmosphere).  The big breakthrough in sorting out these interactions have been new magnetohydrodynamic (MHD) models developed in the last decade.  With these interactions removed, Khurana and his colleagues were able to look at the residual magnetic field, with a field strength of greater than 500 nanoteslas.  This residual magnetic field could either be intrinsic, generated by convection with Io's molten iron core, or from induction, within a conducive layer within Io.

The authors looked at an inducted magnetic field on Io first.  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.  Additional induced responses are created using the greater rotational harmonics of Jupiter's internal dynamo.

In order to determine the best fit to the available Galileo data, Khurana and his group created a model of Io's interior using multiple shells, each layer with a conductivity based on its expected composition, temperature, and physical state, to measure the induced response to Jupiter's magnetic field.  They were only able to test the three strongest rotational harmonics (13, 5.6, and 5 hours) given the limited data set that included two flybys, however these just by chance happened to fit these harmonics and were outside the densest part of the Jupiter plasma sheet.  These harmonics are excited by the dipolar, quadrupolar, and octupolar terms of Jupiter's internal dynamo.  The interior model for Io used a bulk chondritic composition divided into a solid, cold, silicate crust 50 kilometers thick and with zero conductivity, a molten iron core 900-1000 kilometers in radius, and a mantle in between consisting of 44% SiO₂, 32% MgO, and 14% FeO.  The mantle composition is similar to lherzolite, a ultramafic igneous rock found in Spizbergen, Sweden and in the French Pyrenees.  The research team used the conductivity of that rock at various temperatures to simulate Io's mantle.

Khurana's group found that using a solid mantle, even one with induction, didn't provide a good fit to the Galileo data.  They then added a conducting asthenosphere shell between the cold lithosphere and the solid lower mantle in their model.  They found that using a conductivity of 1 Siemen per meter for the asthenosphere, they were able to generate an induced field with a strength greater than 600 nanoteslas, closely matching the Galileo data.  It turns out that like the salt water in the sub-surface oceans within Europa, Ganymede, Callisto, and Titan, ultramafic rock melts are also conductive with conductivities in the range of 1-5 S/m at 1200-1400°C or, according to the paper, partial molten rocks with conductivities ranging from "10^-4 to 5 S/m depending on factors such as temperature. composition, melt fraction, and melt connectivity."  This approaches the conductivity of sea water, like the ocean found beneath Europa's crust.

The thickness of the conductive layer cannot be independently determined from the data other than it is thicker than 50 kilometers.  Beyond 200 kilometers in thickness, the induction response saturates.   They did find that the induced field strength is sensitive to melt fraction and the authors determined that Io's magma ocean would need to be at least 20% molten to replicate the Galileo data.  So think of it as more a slurry rather than the ocean you might envision beneath Europa's surface, which would be much less viscous.  Finally, the authors determined that in order to produce this induced magnetic field, the magma ocean would have to be global, rather than just a few patches near active volcanoes or just along the equator, though they don't rule out variations in asthenospheric thickness or melt fraction due to differences in tidal heating between the equator and the poles.

This discovery does help to put to rest the question of whether Io has a magma ocean beneath its surface.  You would think it had one considering the wide-spread nature of its extreme volcanism.  The idea was first proposed by M. H. Ross and Gerald Schubert in 1985 and it was revived in an Icarus note in 1999 by Laszlo Keszthelyi, Alfred McEwen, and G. Jeffrey Taylor [Taylor wrote an article for the University of Hawaii website on this model in case you are outside the Icarus paywall].  Keszthelyi et al. proposed that the then recent results from Galileo's Solid State Imager, suggesting eruption temperatures during 1997 eruption at Pillan reaching 2000 K, necessitated a large melt fraction within at least the upper portion of Io's mantle.  Their article presented a model where the melt fraction was approaching 35% near the boundary between the crust and the asthenosphere and decreased the deeper you got into Io until you hit 6% near the core-mantle boundary.  However, the high temperatures seen at volcanoes like Pillan would suggest melt fractions in some places as high as 70%.  A re-evaluation of the Pillan data in 2007 by Keszthelyi et al. reduced the eruption temperatures required at Pillan and conversely the melt fraction needed in Io's upper mantle to 20-30% with interconnected magma reaching down as far as 600 km below Io's surface.  The new results presented by Khurana confirm the presence of a magma ocean suggested by these authors and support Keszthelyi's current model of Io's interior.

The expected melt fraction, ≥ 20%, means that while this is a global ocean, it is more slushy hot magma, as opposed to liquid hot magma thanks to the 80% by volume suspended crystals in the ocean.  In fact it would be physically impossible to the melt fraction to get to close to 100% as tidal heating would become much less efficient and the asthenosphere would cool down, decreasing the melt fraction.  Conversely, the melt fraction can't be too low, or friction from tidal heating would become much more efficient that today, and the melt fraction would increase.

Finally, Khurana removed both the MHD and inductive response from the Galileo magnetometer data to look for evidence for an intrinsic magnetic field at Io, one that would be created by convection within Io's molten core.  They determined an upper boundary of 110 nT, making it a very weak magnetic field if it exists.

In order to better determine the melt fraction within this magma ocean and to determine its thickness, more data is needed in order to resolve fainter rotational harmonics from Jupiter's magnetosphere.  However, even just two flybys worth of data has been enough to provide a useful proof of concept for probing the interiors of bodies like Io using electromagnetic sounding.  By timing future encounters with Io to conicide with times where Io is not within the densest part of the Jupiter's plasma sheet, researchers would have an easier time picking out induced fields produced by weaker harmonics in Jupiter's magnetic field which maybe lost in the noise of the moon/plasma interaction.  If only there was a new spacecraft on its way to Io... another time perhaps.

Regardless, this is exciting news that Io's magma ocean has been independently confirmed by both eruption temperature data and models of Io's interior and by magnetic induction sounding.  It is always nice to see Io get some press after all these years since the New Horizons flyby in 2007.

Link: Evidence of a Global Magma Ocean in Io's Interior [sciencemag.org]
Link: Galileo Data Reveal Magma Ocean Under Jupiter Moon [jpl.nasa.gov]
Link: PIA14116 - Io's "Sounding Signal" [photojournal.jpl.nasa.gov]

[I want to thank Emily Lakdawalla for providing a home for this post during Blogger's down time.  She posted this article on her Planetary Society Blog this morning.]