The Chemical Composition of Io

From the comments on the 2009 Fall AGU meeting abstracts post back in October, I think it might be useful to have an overview post on what is known about Io's composition and the volatile chemistry that takes place at Io's volcanoes and in its atmosphere.  Much of the information I will present here is based on models of the kinds of chemical reactions that are thought to occur, but a few key measurements do underlie this discussion.  This first is Io's bulk density, derived from measurements of Io's size and Io's effect on passing spacecraft and its fellow Jovian satellites.  The latter measurement allows for an estimate of Io's mass.  Second, spectroscopic measurements of Io's surface and atmosphere provide details on the sulfurous volatiles that are common at Io's volcanoes and cover the bulk of Io's surface.  Finally, in situ and spectroscopic measurements of the composition of the Io Plasma Torus, a belt of charged particles co-orbital with Io, provide hints to the atomic breakdown of compounds that escape from Io and its atmosphere.

Io's Interior Composition

Io has the highest bulk density (3.53 g/cm³) of any object in the outer solar system.  This high density suggests that Io is composed primarily of silicates with a metallic iron or iron sulfide core.  Unlike nearly all the other moons in the outer solar system, very little water exists on the surface as Io formed inside the Jovian "snow line", where relatively little water condensed compared to those moons outside the snowline like Ganymede and Callisto.  What little water Io did retain was later lost as Io's became a more active body.  Thus, sulfurous compounds became the dominant volatiles on Io as the original metal sulfides in Io's interior became oxidized.

Based on Io's density and moment of inertia measurements (which allow for estimates of the size of Io's core), Io's bulk composition is thought to match that of ordinary L- and LL-chondrites, based on modeling work by Kuskov and Kronrod 2001.  This suggest a low metallic iron content, with most of the iron and other metallic elements (like magnesium, aluminum, and titanium) tied up in oxides.  Model runs by Keszthelyi et al. 2007 assumed a refractory composition of 36% SiO₂/30% FeO/25% MgO bulk composition with the majority of the iron tied up in the core.  The rest of the bulk refractory composition was taken up by additional oxides with potassium, calcium, sodium, and aluminum.

Io's core consists primarily of iron with some unknown percentage of iron sulfide (up to 37% by weight for the iron/sulfur eutectic.  Because the amount of sulfur in the core is not known, the size of Io's core is only known as a range of sizes from 37% (assuming pure iron) to 52% (assuming an Fe-FeS eutectic mixture) of Io's radius.


Silicate Lavas

Based on temperature estimates from Galileo and ground-based observations of active volcanoes and near-infrared imaging by Galileo, Io's dark lava flows, diffuse pyroclastic deposits, and lava lakes are thought to be mafic to ultramafic in composition, high in magnesium and iron oxides and low in silica.  Minerals typically found in mafic basalt flows include plagioclase feldspar, olivine, and pyroxene.  The identification of Io's lava flows with basalt (rather than sulfur, as presumed following Voyager) is based in part on the high temperatures measured by the SSI camera on Galileo.  Initial temperature estimates in McEwen et al. 1998, based on the ratio of the observed brightness between the clear and 989 nm filters of Pillan during the summer 1997 eruption and other volcanic centers like Pele and Kanehekili, suggested ultramafic compositions for at least some of Io's lavas.  However the lower limit of 1600°C was found to be an overestimate, as new cooling models taking lava fountains into account and reprocessing of the Galileo data, suggested lava temperatures between 1250 and 1350°C, more in line with models of Io's mantle and tidal heating and with ordinary mafic compositions.  However, these estimates may underestimate the eruption temperature as the observed temperatures may be several hundred kelvin cooler after only a few seconds of cooling, so ultramafic compositions (less iron and more magnesium than regular mafic magmas) are not completely ruled out.  In addition, the eruption temperature may not be reflective of the liquidus temperature of the magma due to super-heating of the magma as it ascends to the surface.

Another piece of evidence toward the composition of Io's lavas is the presence of an absorption band at 0.9 μm associated with dark regions on Io found in SSI images taken with the 889 nm filter (identified in Geissler et al. 1999).  This absorption band has been associated with orthopyroxene, either the magnesium end member mineral enstatite (Mg₂Si₂O₆) or the magnesium/iron mixture mineral (what used to be known as hypersthene).  Either mineral is consistent with a mafic or ultramafic composition for Io's primary lavas.  The model Io lithosphere used by Keszthelyi et al. 2007 (which would consist primarily of cooled lava flows) is similar in composition to terrestrial tholeiitic basalt, but with less silica (SiO₂) and titanium oxide and more magnesium oxides.

Volatiles

The predominate volatiles, i.e. chemicals that can be sublimated or condensed at normal Io temperatures, are sulfur and sulfur dioxide (SO₂).  In fact, SO₂, the SO₂ photolysis product sulfur monoxide, and the various allotropes of sulfur are the only volatiles that have been definitely identified on Io's surface and in its atmosphere and volcanic plumes.  These two are also largely responsible for Io's colorful appearance.  Course grained sulfur dioxide is responsible for the white-gray regions seen across Io's surface, including the large Colchis and Bosphorus regions seen in the color C21 mosaic.  Finer grained sulfur dioxide is more transparent at visible wavelengths, but can be identified using near-infrared absorption bands.  Band depth and width maps using near-infrared spectral data from Galileo has been used to create maps of SO₂ abundance and grain size across Io in paper such as Doute et al. 2001. Sulfur dioxide is also the dominant chemical species in Io's plumes (from re-volatilized surface frost) and atmosphere and the deposition of which can produced bright regions surrounding volcanic plume vents.  Finally, sulfur dioxide maybe a primary lava in some areas, such as the bright floor of Balder Patera, during the early stages of patera formation as terrain above a sill starts to melt.

Sulfur in various forms can be seen across Io's surface as red, red-brown, orange, and yellow region across its surface.  Diatomic sulfur (S₂) is outgassed from Io's interior during volcanic eruptions, in some cases forming large plumes (along with condensing sulfur dioxide) such as those at Pele or Tvashtar.  S₂ is quickly reorganized into reddish S₄, by photolysis, when it is deposited on the surface, helping to create the large red rings seen around some active volcanoes on Io.  Over time, continued photolysis builds sulfur into the stable cyclic S₈ form, which is yellowish in color.  This is why plume deposits from briefly active volcanoes eventually fade back to the earlier appearance from before the eruption (like at Grian Patera).  At Io's poles, where charged particles can more easily reach the surface, cyclic sulfur can be broken back down into S₄ form, producing Io's dark, reddish-brown polar regions.

Additional volatiles have been suggested for Io based on models of Io's volcanic gas chemistry, tentative identification of absorption bands in near-infrared spectra of Io's surface, and spectra of the neutral cloud that surrounds Io.  For example, additional sulfur oxides are likely in Io's atmosphere, such as sulfur monoxide (SO) and polysulfur oxide (S_xO) based on models of Io's gas chemistry.  Additional thermochemical models of Io's volcanic gases suggest that sodium chloride would be a dominant salt in Io's plumes, and this is support by the identification of Na+ and Cl- in the Io Plasma Torus and NaCl in the dust streams that radiate out from Jupiter and have been associated with Io.  Potassium chloride is also likely.  Sulfuryl chloride (Cl₂SO₂) was tentatively identified at 3.92 μm within the reddish plume deposit at Marduk by Schmitt and Rodriguez 2003.  Those authors also suggested that Cl₂S might be the cause for the red color of the deposit, though how this fits with the ability for other reddish deposits to fade rather quickly is not certain.  Kargel et al. 1999 attributed Io's reddish material to impurities in Io's volcanogenic sulfur, such arsenic and selenium, which can drastically change the color of sulfur even at very low concentrations (~1%).  They also suggested that the green color of some paterae on Io, like Chaac Patera, may result from the interaction between sulfur and cooling, iron-rich lavas, forming pyrite. 

Finally, water or at least hydroxyl may have been identified on Io by way of a 3.15 μm absorption band and a broad one found at 3 μm in the low spectral resolution NIMS data from the flybys. The 3.15 μm band was initially found in ground-based data by Salama et al. 1990 and identified with either H₂O or H₂S.  However the lack of a corresponding 2.97 μm feature suggests another culprit for this absorption band, perhaps HCl.  The 3 μm band is observed in high-spatial, but low-spectral, resolution data at several mountain structures, such as Gish Bar Mons, Tvashtar Mensae, and Tohil Mons.  One possible explanation is that these features maybe the result of water ice or hydrous minerals deposited on Io by small cometary impacts in the last million years that have been brought back to the surface by the uplift of these mountains.  However the lack of other water ice absorption bands at 1.48 and 2.0 μm led Granahan in 2004 to look for another compound that might create the observed band at 3 μm.  He identified pyrite (FeS) or pyrrhotite as possible compounds responsible for the absorption bands. 

Of course many of the chemical identifications on Io (save sulfur and sulfur dioxide) are either tentative or are based on chemical models of Io's volcanic gases or photolysis of gases in its atmosphere and plumes.  Additional spectroscopic studies with much higher spectral resolution than what was obtained by Galileo during its Io flybys will be needed to settle many of our questions about Io's surface composition.  Information on the eruption temperature of Io's lavas, in situ mass spectroscopy of its atmosphere and plumes, and gravity estimates of its interior structure will also be needed to refine our knowledge of Io's bulk composition.  The measurements will hopefully await us in the 2020s with IVO and JEO.

Model Projects More Oxygen in Europa Ocean than Previously Expected

The Galilean satellites session at this year's DPS meeting was held today in Fajardo, Puerto Rico.  I am not at the meeting, but you can check out my thoughts on the Io-related abstracts for this meeting that I posted a few weeks ago.  While I haven't heard word on what was presented at the Io talks, there is a new press release today covering one of the Europa talks, "Vertical Transport through Europa’s Crust: Implications for Oxidant Delivery and Habitability," by Richard Greenberg.

At this talk, Greenberg presented results on the production of oxygen through radiolysis and photolysis of water.  During these processes, some water molecules on Europa surface are broken down into their oxygen and hydrogen components by high-energy particles in Jupiter's magnetosphere and photons from the Sun.  Greenberg combined this research with estimates of Europa's resurfacing rate to determine how much oxygen is delivered to the satellite's sub-surface ocean.  He found that given this resurfacing rate, the concentration of oxygen in Europa's ocean would exceed those of the Earth, making possible not only microbial like, but the kinds of multi-cellular aquatic like we are more familiar with.  Greenberg also notes that an initial, 2-billion year delay in this process would prevent the premature oxidation of organic compounds that would have prevented the development of life.

So for those who dream of eating Europa calamari, you just got a big boost today.  Now we just need to find organic compounds at Europa... otherwise, all you have is a quite oxygenated, but sterile, ocean.

Link: Press Release - Vertical Transport through Europa’s Crust: Implications for Oxidant Delivery and Habitability [dps.aas.org]

Carnival of Space #122 @ Cumbrian Sky

The latest edition of the Carnival of Space, the weekly roundup of the best in the Space and Astronomy blogosphere, is now online over at Cumbrian Sky.  Not surprisingly, many of the submitted posts were related to the discovery of water molecules on the Moon, mine included.

Included in the list of blog posts is one explaining some of the details and caveats of the Lunar water discovery over at the Planetary Society blog.  Definitely worth checking out.

Link: Carnival of Space #122 [cumbriansky.wordpress.com]

Water on Dry Worlds

Updated 09/24/2009 12:03 PM MST based on info from this morning's press conference:

The internet is abuzz this evening regarding the possible discovery of wide-spread "water" or hydroxyl molecules of the surface of Earth's moon, a discovery made by spectrometers on three different spacecraft: M³ on Chandrayaan 1, VIMS on Cassini, and Deep Impact.  The papers, if I understand correctly, will be published later today in this week's issue of the journal Science and I have not had a chance to look at them.  There will also be a press conference later today at 2pm EDT (11am MST) discussing these results.

Why do I bring these results up here on this blog?  Well, according the few reports I have been able to find online, like this one here from Universe Today, from Bad Astronomy, and from NASA Watch, this discovery was made by finding a weak absorption band near 3 microns, associated with water and the hydroxyl ion (OH^-), concentrated mostly near the moon's high latitude.  The absorption band found on the Moon is very weak, suggesting a very low concentration of water or OH^- in the moon's soil.  The M³ instrument team suggests a concentration of as much as 770 ppm has been observed on the sunlit side of the Moon, according to the NASA Watch posting.  While the discovery isn't quite Moon-shattering, previously water ice (or hydrogen anyway) had only been observed within cold-traps in permanently-shadowed craters near the poles.

A similar absorption band was found on Io using ground-based spectroscopy (Salama et al. 1990) and Galileo NIMS (Carlson et al. 1997 and Cataldo 1999) observations.  In these measurements, a weak absorption band in Io's near-infrared spectrum at 3.15 microns was observed to be ubiquitous across its surface with a concentration of 4 ppm according to Carlson et al. 1997 and 1000 ppm according to Salama et al. 1990, on the order with what has been observed on the Moon.  This absorption band is associated with the O-H stretch transition.  Such an atomic bond between an oxygen and hydrogen atom would be found in water, hydrated minerals, or the hydroxyl ion (OH^-).  Small concentrations of this band have also been observed.  An absorption band near 3 microns, attributed to water ice crystals or hydrates mixed with sulfur dioxide frosts, was seen to the north and west of Gish Bar Patera by the Galileo NIMS instrument during the October 2001 I32 encounter (Douté et al. 2004).  The low spectral resolution of NIMS at the time (12 spectral measurements spread out between 1 and 5 microns) makes this result a bit tenuous, but if true would indicate that concentrations of possible water ice on top of the low background levels exist on the surface of Io.

So where does the "water" come from on these two, supposedly dry worlds?  For the Moon, two possible mechanisms are likely.  The first would be recent cometary impacts, which would bring their water to the Moon's surface near the site of these impacts.  Concentrations within the ejecta blankets of several small craters on the moon provide further evidence for this hypothesis, but the pattern of the hydroxyl absorption within the ejecta seems to be more consistent with material from the target body rather than material from the impactor.  The widespread distribution of water or hydroxyl ions across the moon's sunlit surface suggests another explanation.  In this scenario, charged particles, in the form of hydrogen ions and transported from the Sun by the solar wind, impact the Moon's unprotected surface (remember that the Moon is outside Earth's magnetic field most of the time).  These hydrogen ions split oxygen atoms from silicate molecules in the Moon's soil, and combine with newly freed oxygen ions to form hydroxyl ions or water.  As the day progresses and the Moon's surface heats up, these new molecules themselves split up, freeing the hydrogen to space and returning the oxygen to the soil.  The process of water formation from the combination of hydrogen from the solar wind and oxygen in the lunar soil kicks back up the surface starts to cool down in the late afternoon and evening.  Alternatively, the water molecules may become excited and be transported to Moon's polar regions, where they are deposited within those aforementioned cold-traps.

For Io, the solar wind can't reach its surface due to Jupiter's strong magnetic field.  So where does its water come from?  Again, oblique cometary impacts could be a source of water for Io.  The two recent cometary impacts on Jupiter in 1994 and again this year would suggest that Io could be hit by water-rich cometary bodies on a regular basis.  This could certainly be the source for the concentration found near Gish Bar Patera.  For the global ubiquitous concentrations of water or hydroxyl ions, another mechanism maybe necessary.  For example, low concentrations of water might be present in Io's magma, like here on Earth.  Water vapor would then be released during volcanic eruptions and water ice would be deposited on the surface, however, no water vapor has ever been observed within Io's plumes.  Another possibility could be that hydrogen ions from Jupiter's magnetic field break off oxygen from sulfur dioxide and silicate compounds on the surface then combine with them to form OH^- or water, akin to the preferred scenario for the Moon.

This discovery of OH molecules on the Moon is certainly interesting, and just goes to show everyone that water is quite common place in the solar system, even in the driest of places.

Graphic above by University of Maryland/F. Merlin/McREL.

Link: Water on the Moon...?  Yep. It's Real. [blogs.discovermagazine.com]