2015 Io Volcano Observer Proposal

The deadline for NASA's Discovery proposals was this week. 28 proposals were submitted with targets ranging from Phobos and Deimos, to Venus, to asteroids like metallic 16 Psyche, and even Enceladus.  Of course, the one that Ionians will be pulling for is the Io Volcano Observer (IVO), proposed by a team led by the University of Arizona's Alfred McEwen and managed by John Hopkins' Applied Physics Lab, which is making its second go around after also being submitted for the 2010 Discovery Announcement of Opportunity (AO).  The Martian lander, InSight, was ultimately chosen after that AO over a great set of proposed missions, which included a boat that would've sailed around Titan's great Kraken Mare.

IVO, if selected this time around, would launch in late May 2021 with an arrival at Jupiter in February 2026 after a 510 km altitude flyby of Io.  IVO would remain in an elliptical, inclined orbit around Jupiter, flying by Io eight more times during the nominal mission between August 2026 and late December 2027.  An extended mission with nearly year-long orbits is possible, providing high-resolution, imaging coverage of Io's leading hemisphere in daylight as well as collaboration with the JUICE spacecraft.  Unlike the 2010 proposal, this year's would make use of advanced, roll-out solar panels to power the probe and its five instruments, instead of the Advanced Sterling Radioisotope Generators that are unavailable to proposal teams this time around.  These instruments include: two cameras, one narrow-angle and the other wide-angle (NAC and WAC); a Thermal Mapper (TMAP); Dual Fluxgate Magnetometers (DMAG); and a suite of particle instruments (PEPI), which includes an ion and neutral mass spectrometer (INMS) and a Plasma Ion Analyzer (PIA). There is also the potential for an add-on, student collaboration instrument, a wide-angle, near-infrared camera named HOTMAP.  While the WAC, PEPI, and HOTMAP will be bolted to the spacecraft, the NAC and TMAP will be on a ± 90° pivot, allowing for off-nadir pointing of those instruments without turning the entire spacecraft.

The mission's main goals include mapping Io's active volcanism on a more global scale than Galileo and Voyager were able to obtain, measuring Io's induced magnetic field at different points in its orbit around Jupiter to provide a better estimate for the thickness, distribution, and melt percentage of its magma ocean, mapping Io's topography including its numerous mountains, and measuring the composition of the volcanic gases that are released from Io's interior.  During two of its flybys (I0 and I2), IVO will acquire gravity science using 2-Way Doppler tracking, which combined with the gravity data acquired during a few of Galileo's encounters with Io, will constrain our knowledge of mantle rigidity.  The mission will also act as a technology demonstration for Deep Space Optical Communications, which could substantial increase the data return of future missions.

On each orbit, IVO will spend a week acquiring images of Io, allowing it to map Io so changes at its many volcanoes can be observed and to monitor hot spots and auroral emissions during four different eclipses.  IVO would also use this time to help look for Europa's elusive plumes in support of Europa Clipper, which should arrive at Jupiter shortly after IVO.  During the 24 hours around closest approach, while IVO approaches and departs from Io over its polar regions, IVO will acquire several NAC mosaics of Io along with TMAP images to map heat flow and monitor volcanism.  The NAC will also be used to acquire movies of active plumes like Pele and Marduk.  Finally, right at closest approach, the spacecraft will acquire WAC, NAC, TMAP, and maybe HOTMAP imaging swaths along with INMS mass spectra and DMAG/PIA measurements as IVO sweeps north across Io.  At least 20 Gb of data (100x the Io data returned by Galileo) would be acquired during each encounter and will be played back during the apojove part of each orbit (distant monitoring observations will also be acquired to help watch for new major eruptions).

More information about this exciting mission can be found in an abstract submitted to next month's Lunar and Planetary Sciences Conference.  A fact sheet with even more details about Io Volcano Observer is also available.  NASA expects to select three (or so) finalists for Phase A studies in September with a final selection from those sometime next year.

Link: The Io Volcano Observer (IVO) for Discovery 2015 [www.hou.usra.edu]
Link: Io Volcano Observer Public Fact Sheet [pirlwww.lpl.arizona.edu]

Four years later...

Has it really been four years since I last updated this blog?  It's hard for me to believe, but yes, it has been quite a while.  As I mentioned in that last post, the blog really started to mess with my work/life balance quite a bit when I started doing the "Io Volcano of the Week" feature.  It was a neat idea for generating fresh content for this site, but I ended up spending a ridiculous amount of time on those posts, to the detriment of the rest of my free time.

I think with Discovery proposal season wrapping up and all the warm, over-optimistic feels that generates, I think now is a good time to revive The Gish Bar Times blog, but I really want to go back to focusing on new papers, missions, and data rather than trying to generate a lot of fresh content that took up way to much of my free time and quickly left me worn out.  I think I had this feeling that I always had to come up with more posts when honestly, the focus of this blog is not generating much news right now, and that's okay.

That being said, I do have a long backlog of papers that I haven't discussed here in the last four years, so I should run out of things to talk about here for a while.  My favorite part about doing this blog is that it really forced me to read the current literature and writing articles about them really helped to reinforce what I read.

In the meantime, I wanted to point out this neat site which presents planetary maps for children, including Io, Europa, Mars, Venus, and Titan. The maps were created by a group of graphic artists for the ICA Commission on Planetary Cartography.  The Io map (a portion of which is shown at the top of the post) was created by Dóri Sirály.  I kinda wish the Titan map had more of a medieval map art style (like I keep saying I want to make myself...) but I think these are all well done.

Link: Planetary Maps for Children [childrensmaps.wordpress.com]
Link: Planetary Map Series for Children (LPSC abstract) [http://www.hou.usra.edu]

Carnival of Space #143 @ Next Big Future

The 143rd edition of the Carnival of Space, a weekly series highlighting the best in the astronomy and space blogosphere, is now online at Next Big Future.  You know the drill.  Some great posts on 3D visualization of Martian dust avalanches based on a shape-from-shading DEM from HiRISE data, bad movie science, umbraphiles, and Russia now funding its nuclear programs.

In other news, LPSC started today.  While I am here in Tucson, I've been able to follow the results that have been presented at the conference using Twitter.  You can too using the hash topic, #LPSC.  Today there have been some great tweets from NASA Night when people from headquarters discussed the new NASA budget, getting most of their questions about the change in focus for the Exploration Directorate.  A few people to follow include: @elakdawalla (blogger from the Planetary Society), @jhjones (who set up the Outer Planets missions display), @aggieastronaut, @DAstronomer, @barbylon, @starstryder, @The_Stargazer, @WomenPlanetSci, @astraea_sophia.

Finally, Callisto is today's (March 2) Wikipedia Featured Article. This makes it the last of the four Galilean satellites to be featured on the popular online encyclopedia.

Link: Carnival of Space #143 [nextbigfuture.com]
Link: LPSC news on Twitter [twitter.com]
Link: Callisto (moon) [en.wikipedia.org]

Lunar and Planetary Science Conference Starting Tomorrow

Over the last month and a half, we have been taking an early look at some of the Io research that will be presented at the Lunar and Planetary Science Conference, which starts tomorrow in Houston, Texas.  I will not be there in Houston for the conference, but I will be in spirit.  That's not really the same...

Anyways, if you are going to the conference and you start getting sick and tired of same ol' Mars and Moon talks and posters, and you start asking yourself, "Is there nothing here that's cool and different, and not covered in hematite concretions?", here is your Io itinerary:

Tuesday evening, March 2, 6:30–9:30 pm: Poster Session I

Mission Plans and Concepts

• Science Rationale for an Io Volcano Observer (IVO) Mission by Alfred McEwen et al.  My summary.

Wednesday Afternoon, March 3, 1:30 pm: Planetary Atmospheres

• 4:15 pm - Modeling the Sublimation-Driven Atmosphere of Io with DSMC by Andrew Walker et al.  My summary.
• 4:30 pm - Io's UV-V Eclipse Emission: Implications for Pele-type plumes by Chris Moore et al.  My summary.

Thursday evening, March 4, 6:30–9:30 pm: Poster Session II

Planetary Atmospheres

• DSMC Modeling of the Plume Pele on Io by William McDoniel et al.  My summary.

Satellites and their Planets

• Volcanism on Io: Results from Global Geologic Mapping by Dave Williams et al.  My summary.
• Paterae on Io: Insights from Slope Stability Analysis by Laszlo Keszthelyi et al.  My summary.
• Io: The Dark Paterae Component of Heat Flow by Glenn Veeder et al.  My summary.
• Distribution and Comparison of Io's Paterae: Areas, Effective Diameters, and Active Volcanism by Brandon Barth et al.  My summary.

Igneous and Volcanic Processes

• The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io by Ashley Davies et al.  My summary.
• The Geothermal Gradient of Io: Consequences for Lithosphere Structure and Volcanic Eruptive activity by Giovanni Leone et al.  My summary.

Also if you or someone you know will be blogging or twittering from the conference, send me a note and I will post a group of links to them in a post tomorrow.

Link: 41st Lunar and Planetary Science Conference [www.lpi.usra.edu]

LPSC 2010: Simulating Io's Auroral Emission in Eclipse

Yesterday, we talked about a model of Io's atmosphere using the Direct Simulation Monte Carlo (DSMC) method.  Today we look at another Monte Carlo (MC) model of Io's atmosphere, this time focusing on simulating the emission of Io's atmosphere during an eclipse.  Like the research discussed yesterday, both the abstract for next week's Lunar and Planetary Science Conference (LPSC) and a new paper in press in Icarus are available.  The LPSC abstract is titled, "Io's UV-V Eclipse Emission: Implications for Pele-type Plumes," by Chris Moore, David Goldstein, Philip Varghese, and Laurence Trafton.  This research will be presented as a talk next Wednesday afternoon, March 3 in the Planetary Atmospheres session.  The Icarus paper in press is titled, "Monte Carlo Modeling of Io’s [OI] 6300 Å and [SII] 6716 Å Auroral Emission in Eclipse," by Chris Moore, K. Miki, David Goldstein, K. Stapelfeldt, Philip Varghese, Laurence Trafton, and R.W. Evans.  Both cover the topic of simulating Io's auroral emission when the satellites goes into eclipse, but under different regimes: the LPSC abstract discusses emissions in the mid-ultraviolet as the result of SO₂ and S₂, while the Icarus paper talks about emissions in the red portion of the visible spectrum from oxygen (from decomposed SO₂).

Last year, this same group published a paper on the dynamics of Io's atmosphere during an eclipse, which occurs each Ionian day when the satellite passes into the shadow of Jupiter.  Each eclipse lasts around 2 hours and 20 minutes.  During this time, no direct sunlight reaches Io surface, though Europa-shine and refracted sunlight from Jupiter's atmosphere can faintly illuminate the surface.  The authors found that Io's atmosphere doesn't completely collapse during an eclipse, as a diffusion layer of non-condensable atmospheric species like oxygen and sulfur monoxide forms near the surface, preventing sulfur dioxide above it from condensing out on to the surface.  With their model from last year's paper in hand, the authors further examined it, seeing how their model results would appear at different emission bands of the species they included in their model atmosphere (SO₂, O, SO, S, and O₂).  They also examined the emission from S₂ gas present in volcanic plumes like Surt and Pele, and the effects of volcanic activity on the other emission bands.

In their Icarus paper, Moore et al. focused on two emission bands, the prominent [OI] oxygen emission line at 630 nm and the much fainter [SII] band at 670 nm (both in the red portion of the visible spectrum).  Thus in the image above, the emission examined would be colored in red, so that covers much of the limb glow.  This limb glow shifts between the north and south polar region of Io, as seen by Galileo and Cassini.  Observations by Trauger et al. 1997 also revealed a high altitude bright spot in the [OI] line over the leading hemisphere, which is actually on the wake side of Io since the magnetosphere of Jupiter spins faster than Io revolves around Jupiter.  Moore's simulation of Io's atmosphere in eclipse was able to match the observed position of the bright wake spot, but not its intensity.  The model suggests that the position of the wake bright spot (which can be seen either above or below the equator) and the polar limb glows are related to the depletion of electrons from the Io-Jupiter flux tube, which effects the 630 nm emission by Io's position above or below the plasma torus, volcanic plumes (particularly large polar plumes), the density of the polar atmosphere, and Io's effect on Jovian magnetic field lines.  The authors also found that the [SII] emission is much weaker than the [OI] oxygen line, in part because of the lower S⁺ (remember, the ratio of oxygen to sulfur in Io's atmosphere is roughly 2 to 1).

The LPSC abstract examines emission bands by SO₂ and S₂ in the mid-ultraviolet to the visible (250-600 nm) as Io ingresses into an eclipse.  The authors found that the distribution of this emission across Io's sub-jovian hemisphere (the area covered by Trauger et al. 1997 Hubble observations) is strongly effected by volcanic plume activity.  These plumes act as shields for the atmosphere south of them from the electrons from the Jupiter-Io flux tube, reducing the energy of these electrons.  For example, if both the Surt and Acala plumes are active, the northern Surt plume shows up bright due to S₂ emission above 300 nm, while Acala plume south of it appears fainter because molecules in its plume are not as excited by flux tube electrons.  The authors also examined the difference between the western and eastern halves of the sub-Jupiter side of Io at these wavelengths.  With the Surt and Pele plumes active, the emission from Io was much greater than if they were off.  The ratio between the eastern and western halves was greater than one when both are off because of emission from SO2, which as a greater density on the eastern half because before the eclipse, it had been in the afternoon (remember from the article yesterday that Io's sublimation atmosphere peaks in density where the frost temperature is greatest, around 2pm in the afternoon).  When both plumes are one, that ratio becomes less than one as the brightness of the S₂ emission from the Surt plume dominates the brightness on the western side.  With significant differences such as these, the authors suggest that even barely disk-resolved spectra, particularly around 300 nm, can be useful for determining the activity of volcanic plumes on Io from Earth-based data.

These two papers explore Io's auroral emission at various wavelengths from the mid-ultraviolet to the visible using a simulation to explain the observations we have on hand.  They show that the auroral glow of Io's atmosphere is affected by volcanic plume activity, such that observations from Earth can be used to determine the presence or absence of different plumes, Io's position in the magnetosphere, and the density of Io's atmosphere.  These simulations also explore the various chemical species in Io's atmosphere and how even minor constituents like oxygen, formed from the disassociation of sulfur dioxide, can have a strong effect on its auroral, so vividly seen when Io is in eclipse.

Link: Io's UV-V Eclipse Emission: Implications for Pele-type Plumes [www.lpi.usra.edu]
Link: Monte Carlo Modeling of Io’s [OI] 6300 Å and [SII] 6716 Å Auroral Emission in Eclipse [dx.doi.org]

LPSC 2010: Modeling Io's Atmosphere in Three Dimensions

Okay, it is about time I finished up my coverage of the abstracts covering Io science for next week's Lunar and Planetary Science Conference.  Today I am going to talk about "Modeling the Sublimation-Driven Atmosphere of Io with DSMC" by Andrew Walker, Sergey Gratiy, Deborah Levin, David Goldstein, Philip Varghese, Laurence Trafton, Chris Moore, and Benedicte Stewart.  A paper in press in Icarus was published last month by the group covering this topic, "A comprehensive numerical simulation of Io’s sublimation-driven atmosphere".  This post will act as a summary of both the LPSC abstract and the paper.  The LPSC paper will be presented as a talk next Wednesday afternoon, March 3 in the Planetary Atmospheres session.

In their model, Walker et al. used the Direct Simulation Monte Carlo (DSMC) method for simulated Io's rarefied atmosphere in three dimensions.  Previous modelers explored Io's atmosphere as a single dimension, looking at how column density and temperature changes over the course of a day in response to changes in surface temperature, or as a two dimensional model that looked at how these parameters changed across a single latitude, axi-symmetric with the sub-solar point.  With a three dimensional model, the authors were able to explore the effects on Io's atmosphere from volcanic plume activity at known volcanoes like Pele and Prometheus, plasma bombardment heating from above, planetary rotation, sub-solar temperature (115-120 K), the residence time of fine-grained sulfur dioxide frost on bare rock, and variations in frost temperature and areal coverage.  The DSMC method models individual sulfur dioxide molecules (usually representative of the total number of molecules), which is useful when the atmosphere has such low density that the mean free path of sulfur dioxide molecules exceed that the length over which many gas properties propagate.  Similar modeling was performed by Austin and Goldstein 2000, though this new model includes the inhomogeneous frost coverage mapped by Galileo NIMS. This also allows the authors to graph variations in the translational, vibrational, and rotational temperatures (related to the different emission bands of sulfur dioxide based on motions of the S-O bonds), density and column density (number of sulfur dioxide molecules per cubic centimeter or over a square centimeter of Io's surface, respectively), and flow rate (expressed in the article as mach number).

Since the authors primarily modeled the sublimation component of Io's atmosphere, the column density and many of the other properties of the lower atmosphere were related to the temperature and areal coverage of sulfur dioxide frost on the surface as this part of the atmosphere would be in vapor-pressure equilibrium with that frost.  Because of the difference between the position of the peak frost temperature and the sub-solar point, ~30° to the east or 2pm local time, the column density near the surface peaks to the east of the sub-solar point.  This lag in peak frost temperatures results from the thermal inertia of SO₂ frost.  Changing the sub-solar peak temperature from 115 K to 120 K causes a five-fold increase in the peak atmospheric column density from 4.7×10¹⁶ cm^–2 to 2.7×10¹⁷ cm^–2.  This brackets the lower and upper bounds for the atmospheric column density measured by earlier observers of Io's atmosphere.  Compare this to the column density to the Earth's, which is ~3×10²⁵ cm^–2.

I should point out at this point that this group published a companion paper (Gratiy et al.) that actually showed up in the Icarus in press page first, and was discussed here last month.  This paper compared their model of Io's atmosphere to actual observations taken a ultraviolet, infrared, and millimeter wavelengths.  One note that Walker et al. does make is that the variation in column density with latitude doesn't seem to match the Hubble Lyman-α observations, which showed a sharp drop-off in atmospheric density poleward of ±45°.  They suggest that this could be because of differences in frost temperature from the assumed cos^1/4(ψ) latitudinal variation.

In other results, the authors found that heating from the Io plasma torus inflates the upper atmosphere of Io and keeps the nightside atmosphere from completely freezing out.  Plasma from Jupiter's magnetosphere only penetrates down to an altitude of 1 km at the point of peak frost temperature, and the altitude decreases the further you get from that point, reach the surface at the poles and on the nightside.  This actually means that the low altitude translational temperature of the SO₂ in the atmosphere is higher on the nightside (where plasma reaches all the way to the surface due the lower atmospheric density) and particularly along the terminator.  At the terminator, the higher density dayside atmosphere interacts with the low density nightside atmosphere, leading to supersonic gas flow just past the dusk terminator and near the poles.

Tomorrow we will take a look at another LPSC abstract and Icarus paper by this group on modeling Io's auroral emission.

Link: Modeling the Sublimation-Driven Atmosphere of Io with DSMC [www.lpi.usra.edu]
Link: A comprehensive numerical simulation of Io’s sublimation-driven atmosphere [dx.doi.org]

LPSC 2010: Science Rationale for the Io Volcano Observer

For the last couple of weeks, we have been examining the Io-abstracts submitted for next month's Lunar and Planetary Science Conference.  Today we take a look at a paper submitted by Alfred McEwen and a host of co-authors from the IVO team titled, "Science Rationale for an Io Volcano Observer (IVO) Mission." For this paper, McEwen discusses the technology and science goals and objectives for the proposed Io Discovery mission.  McEwen also touches on how IVO could expand on our knowledge of Io beyond what Galileo obtained in the late 1990s and early 2000s and what the Jupiter Europa Orbiter (JEO) will get in the late 2020s.  This research will be presented as a poster at the Mission Plans and Concepts session on Tuesday, March 2.

We've discussed this proposed mission a number of times in the past. The Io Volcano Observer mission concept was first developed as part of NASA's Discovery & Scout Mission Capability Extension (DSMCE) program.  In this study, NASA hoped to get better grasp on what could be done within the Discovery/Mars Scout program cost cap if the missions were provided two, government-provided radioisotope power sources.  NASA's goal is to test the new Advanced Stirling Radioisotope Generator (ASRG) power source on one of these low-cost missions.  The ASRGs are a much more efficient power source than current RTGs, making them a smarter choice given the limited amount of plutonium available going forward.  The Io Volcano Observer (IVO) was one of nine mission concepts that were selected for further study for the DSMCE program; that study was completed back in February 2009.

For this poster, McEwen will focus on the science objectives and goals for an IVO mission, now a possible proposal for the next Discovery AO.  Some of these goals were discussed here back in September:

1. A1 - Understanding Io's current active volcanism by understanding how its active lavas and plumes are emplaced and generated.  The team plans to acquired repeated imagery of the same volcanic sites at global scales and at high-resolution (< 10 meters per pixel) in order to monitor changes at these volcanoes.  They also plan to take movies of dynamic phenomena like plumes as well as make in situ mass spectra of plume material and Io's atmosphere.
2. A2 - Understand Io's internal structure and tidal heating mechanisms.  The IVO team will use electromagnetic sounding of Io's induced magnetic field and lava temperature measurements to measure the amount of partial melting in Io's asthenosphere.  Thermal mapping in the mid-infrared (~15-20 microns) will allow the group to map Io's heat flow.  The distribution of thermal sources on Io could help distinguish the region of tidal heating in Io's mantle, whether it is in the asthenosphere or in the deep mantle close to the core.
3. B1 - Investigate the processes that form Io’s mountains and paterae and the implications for tectonics under high-heat-flow conditions that may have existed early in the history of other planets.  This will be accomplished through high resolution stereo mapping of large portions of Io's surface, with particular emphasis on areas where we already have at least medium resolution imagery, in order to look for topographic changes on a time-scale of decades (42 years between Voyager 1 and the arrival of IVO in 2021).
4. B2 - Understand how Io affects the Jupiter system.  They plan to accomplish this through in situ measurements of the composition of Ionian volcanic products, Io's atmosphere, and the plasma and neutrals in near-Io space. They also plan to study how Ionian material is lost to Jupiter's magnetosphere.  Finally, they will remotely monitor Io's sulfur dioxide atmosphere and Na-D and OI emissions.
5. B3 - Search for evidence for activity in Io's core and deep mantle by looking for an internal magnetic field in addition to the induced field discovered last year.  Resolving the conundrum of why Io can be so active and not have an intrinsic field might help us better understand how planetary magnetosphere are created.  They also plan to investigate the neutral and plasma densities and energy flows in the Io plasma torus, plus their variations over time, and characterize the ionic radiation belts in the vicinity of Io and their influence on the surface.

In addition to these science goals, a major technology goal for this mission is to study the effectiveness of the new ASRGs. One way to accomplish this is to extend the life of the mission past its primary mission of 7-8 years (including cruise to Jupiter and Io).  They could either expand IVO's orbital period to 1 year to test the ASRGs for their entire nominal lifetimes (~14 years), or they could tighten the orbits to test how well the ASRGs handle the Jovian radiation environment.  ASRGs are needed for an Io mission as the rapid flybys require fast turn times, which a non-gimbaled solar panel wouldn't support and a gimbaled solar panel may not be stable enough and too expensive for a Discovery-class mission.  The inclined orbits of IVO would result in low doses per flyby (10 krads compared to 85 krads for the average JEO flyby), so that actually wouldn't be the limiting factor for a solar-paneled Io mission, the high turn rates and high data rates during the encounters would be.

Finally, the IVO team compare the possible science generated by the Io Volcano Observer and other missions to Io: Galileo and the Europa/Jupiter System Mission.  Galileo's instruments were designed before the discovery of volcanism on Io so the camera and near-infrared spectrometer were not optimized to take advantage of this discovery, and the limited downlink bandwidth brought on by the high-gain antenna failure didn't help.  Compared to the Jupiter Europa Orbiter, IVO would fly over Io's polar regions, mapping the heat flow in those areas and performing sounding of Io's induced magnetic field.  The instruments can also be designed to specifically perform measurements needed for Io that might not be possible with those on the Jupiter Europa Orbiter, such as the near-simultaneous color imaging needed for color measurements.  JEO would accomplish some Io science that would be complementary to that of IVO, such as ground-penetrating radar and laser altimetry.  One interesting possibility is a simultaneous close-up observation with both IVO and JEO.  If IVO's mission at Jupiter and Io is extended with year-long orbits, its extended mission could overlap with the Europa/Jupiter System Mission.

Link: Science Rationale for an Io Volcano Observer (IVO) Mission [www.lpi.usra.edu]

LPSC 2010: Modeling the Volcanic Plume of Pele

For the last couple of weeks, we have been examining the Io-abstracts submitted for next month's Lunar and Planetary Science Conference.  Today we take a look at a paper submitted by William McDoniel, David Goldstein, Philip Varghese, Laurence M. Trafton, and Benedicte Stewart titled, "DSMC Modeling of the Plume Pele on Io." For this paper, the authors modeled the plume of Pele using a curvilinear source region.  The goal is to replicate not only the extreme height of Pele's large plume, but also the elliptical shape of the red deposit it leaves on Io's surface.  This research will be presented as a poster at the Planetary Atmospheres session on Thursday, March 4.

During last year's Lunar and Planetary Science Conference, McDoniel and his colleagues reported on their computer modeling of volcanic plumes that erupt from irregularly-shaped vents, such as those that would be expected from the flow front of a lava flow.  The group uses the Direct Simulation Monte Carlo (DSMC) method for simulating the motion and properties of gas molecules within a rarefied flow like an Ionian volcanic plume.  The method was previously used with great success in 2-D space, which is appropriate for replicating the height, width, and appearance when projected against black space, by Zheng et al. 2003 for a Pele-type plume and Zhang et al. 2004 for a Prometheus-type plume.  DSMC modeling is also being used to simulate the entire atmosphere of Io as generated by sublimation and volcanism and what can happen to the atmosphere when Io is in the shadow of Jupiter. The work done by McDoniel and his colleagues at the University of Texas in the last few years has been to extend the earlier plume modeling into the third dimension in order to match the 3D shape of Ionian plumes and their non-circular deposits on the surface.  As mentioned, last year the authors examined a half-annular vent region, as opposed to the circular vent region assumed by Zheng et al.  This was done to simulate the Prometheus plume, which is generated by the interaction between warm silicate lava and surficial sulfur dioxide frost at a broad, half-circular lava flow front.  They found that such as vent geometry generated a plume that was roughly similar to one generated from a circular vent source, but with prominent jets forming along the inner concave portion of the half-annulus vent area and at the ends of convex side.  These jets had some effect on the strength of the canopy shock (the upper bright region of a plume), but the plume deposit was still roughly circular, albeit shifted in the direction of the convex side of the half-annulus source.

This year, McDoniel et al. have shifted their attention to the larger Pele plume.  The Pele plume is one of the largest on Io and was seen off and on by Voyager 1, Galileo, and the Hubble Space Telescope.  Fallout from this plume produces a large red ring that encircles the Pele volcano at a distance of 400-600 kilometers.  Two of the major features of this deposit are its elliptical shape where it is more elongated in the north-south direction and its subtle changes in shape and localized intensity over time.  McDoniel et al. used a DSMC model of the eruption conditions of Pele (sulfur and sulfur dioxide-rich gas desolving from lava fountaining at an active lava lake) and a Galileo image from the I24 encounter in October 1999 of a portion of the lava lake as a representation of the vent geometry in order to replicate the shape of the Pele plume deposit.  There simulations showed an elliptical plume deposit, with the elongation roughly perpendicular to the curvilinear line of thermal hotspots assumed to be vent sources.  Closer to the vents, the research found that jets should be created generated by the concave portions of the curvilinear line of small gas vents, including the ends of the line of vents.  The jets are associated with areas of greater deposition in the fallout region far from the vent.

What I think is most significant about this research is that it suggests that a roughly linear Pele-type plume source, like this curved line of hotspots, should produce an elliptical plume deposit.  The elongation of this deposit should be perpendicular to the trend of linear source.  Variations in intensity of the deposit are related to stronger jets resulting from deviations from the linearity of the plume source, like the concave and convex sections of the source used in the simulation.  Now, the exact source they used for their simulation may not be the source of the Pele plume, as there is a much more intense section of the Pele lava lake to its east, as seen during the I32 encounter.  Interestingly enough, this intense portion is an elongated depression that is perpendicular to the elongation of the Pele plume deposit, which is what you would expect if it is the source of the plume based on this simulation.  Variations in the deposit may result from different portions of this active region turn on or shutting off, producing jets in the plume.

Very intriguing research, IMHO!  Certainly it provides an explanation for the shape of Pele's plume and provides a possible explanation for changes in the shape and intensity of its deposit on the surface.  A more linear source would be expected from a fissure-like eruption, which is what you would expect from an explosive eruption that produces a short-term giant plume, like Grian, Tvashtar, Surt, or Dazhbog.  This may explain why at least Grian also produced an elliptical plume deposit.

Link: DSMC Modeling of the Plume Pele on Io [www.lpi.usra.edu]

LPSC 2010: Re-examining the Iothermal Gradient

For the last couple of weeks, we have been examining the Io-abstracts submitted for next month's Lunar and Planetary Science Conference.  Today we take a look at a paper submitted by Giovanni Leone, Lionel Wilson, and Ashley Davies titled, "The Geothermal Gradient of Io: Consequences for Lithosphere Structure and Volcanic Eruptive activity." For this paper, the authors modeled the structure of Io's lithosphere by calculating how its temperature varies with depth.  This research will be presented as a poster at the Igneous and Volcanic Processes session on Thursday, March 4.

Io's internal heat, generated by tidal stresses on Io's mantle, is released through volcanic activity in a process called advection.  As opposed to convection or conduction, with advection, heat is transported from a system through a warm liquid, in this case, liquid hot magma.  The model used by Leone and his colleagues was first developed in O'Reilly and Davies 1981 in order to explain how Io's lithosphere could be releasing so much heat (2.4 Watts per m²) yet still hold up Io's steep paterae walls and tall mountains.  A conducting crust would be far too warm at shallow depths and too thin to hold up these structures.  Thus, thanks to advection, all of the internal heat from the asthenosphere is released through volcanic eruptions and the lithosphere stays pretty cool except for the lower two to three kilometers of the 30-kilometer thick lithosphere, preventing viscous relaxation of Io's topography (see the craters of Saturn's moon Enceladus to see how viscous relaxation can distort topography).

For their model, Leone et al. used two equations from O'Reilly and Davies 1981 as well as improved knowledge about the chemistry and properties of Io's lithosphere to calculate the geothermal (or iothermal, if you will) gradient within the lithosphere, from the cold (100 K) surface to the lithosphere/asthenosphere interface at a depth of 30 kilometers and a temperature of 1500 K. Their inputs include estimates for the porosity of the lithosphere as a function of depth, the density of the magma, the globally-averaged, advected heat flux, radiogenic heatign rate, the magma specific heat, latent heat of crystallization, and thermal diffusivity. From these equations, the authors derived the lithospheric density, pressure, and temperature at different depths in Io's lithosphere. As expected, the lithosphere remains below the melting point of sulfur dioxide from the surface down to a depth of 21 kilometers. It remains below the melting point of sulfur until a depth of 26 kilometers. Much of the lithospheric heating takes place in the bottom few kilometers of the lithosphere.

The iothermal gradient generated by Leone's model does support the transport of magma all the way to the surface. Without any entrained volatiles, magma from the asthenosphere can rise to a depth of 23 kilometers before becoming negatively buoyant and forming magma reservoirs, assuming a pore-space fraction of 30% at the top of the lithosphere. As mentioned above, this is within the depth range of the melting point of the dominant volatiles on Io, sulfur and sulfur dioxide. These may then become entrained in the lava, allowing the magma to rise further to the surface. Leone et al. conclude that with volatile contents as low as 5% by mass, magma should be able to reach the surface using reasonable values for lithospheric porosity. With even more volatiles, such as the 10-30% suggested at some plume sites like Tvashtar, the modeled iothermal gradient would support the kinds of high eruption speeds observed at that volcano. They conclude "that there should be a positive correlation between mass eruption rate and volatile content." So it should not come as a surprise that major eruption on Io, like Tvashtar 1999/2001/2007, Thor 2001, Grian 1999, and Pillan 1997 all had volcanic plumes.  Finally, they also place a limit on the porosity of Io's lithosphere at the surface at 38% as magma could not ascend into the lithosphere above that level, the crust would be too light.

Another factor that the authors examined was the effect that changes in the advected heat flow would have have on the lithosphere.  Just as Kirchoff and McKinnon found last year, a decrease in volcanic activity but not a decrease in the amount of heat generated in the mantle (i.e. the temperature remains the same) would leading to a heating of the lower to mid-lithosphere, possibly leading to some melting.  In Leone's model, the gradient changed from a steep curve at 2.4 W/m² (remaining relatively cool until close to the lithosphere/asthenosphere boundary) to a much shallower one at 0.5 W/m².

With this model of Io's geothermal gradient, Leone and his co-authors have placed limits on the amount of pore spaces are possible in Io's lithosphere.  Their model is supported by their ability to replicate the ascent of magma to the surface, which is readily visible on Io's surface.  Their model also helps support the argument that the volatiles in Io's lava are incorporated into its magma within reservoirs in the lithosphere.  I would be interested to see how this model fits with the view that the upper 2-3 kilometers of Io's lithosphere maybe dominated by volatiles with silicates being predominant deeper into Io.

Link: The Geothermal Gradient of Io: Consequences for Lithosphere Structure and Volcanic Eruptive activity [www.lpi.usra.edu]

LPSC 2010: Comparing the Distribution of Io's Paterae

For the last couple of weeks, we have been examining the Io-abstracts submitted for next month's Lunar and Planetary Science Conference.  Today we take a look at a paper submitted by Brandon Barth, Jani Radebaugh, and Adam McKean titled, "Distribution and Comparison of Io's Paterae: Areas, Effective Diameters, and Active Volcanism".  In this paper, the authors discuss measurements of volcanic depressions on Io's surface, known as paterae, and dark paterae floor material.  The authors then studied the size-distribution of paterae across different quadrants of Io's surface.  This research will be presented as a poster at the Satellites and their Planets session on Thursday, March 4.  Last year, this group presented their research on the distribution of paterae (both in general and those with dark material on their floors) at last year's LPSC.  This year, they add the size of the volcanoes found into the mix.

For her PhD dissertation, co-author Jani Radebaugh created a database of paterae back in 2003 and 2004 based on measurements from Galileo and Voyager images, along with parallel MySQL databases of thermal hotspots and mountains.  I still remember the pizza involved, yes, even 7±1 years later... The measurements of the length and area for this database were made assuming that these patera were ellipses, however many paterae as you can see in the image above have complex shapes.  For example, Yaw Patera is shaped like a gasoline nozzle, as seen in the I27 CAMAXT01 mosaic where Yaw is the dark patera in the lower right corner.  Now, one of her students, Brandon Barth, has measured 426 paterae (minus 30 or so in the polar regions still to be measured in time for the poster session) using ArcGIS™.  This allowed Barth and the other co-authors to calculate the size and areas of these oddly-shaped volcanoes more accurately as they are able to mark the boundaries of the patera and ArcGIS does the work in calculating the area of the marked region.

From these measurements, the average effective diameter for paterae on Io is 56.8 kilometers.  Effective diameter is the size of a circle with the same area as the paterae, which aids in comparing the sizes of paterae to one another by normalizing them.  The average effective diameter found is quite a bit larger than Radebaugh et al. 2001, where it was found to be 41 km.  The authors attribute this discrepancy to the measurement techniques mentioned above employed in the different works.  This new method ensures that the entire volcano is captured in the area measurement.

The authors looked at how the size-distribution of all paterae and active paterae (those with at least some dark material on their floors) varies across Io's surface.  They determined that the anti- and sub-Jupiter quadrants of Io have more paterae than the leading and trailing quadrants.  They define these quadrants as the 90 degrees of longitude surrounding the sub-Jupiter (0°W), anti-Jupiter (180°W), leading (90°W), and trailing (270°W) points.  However, they also found that the average effective diameter for the leading and trailing quadrants was larger (63.5 km) than those found on the anti- and sub-Jupiter quadrants (52 km). A similar trend was seen in active paterae, with active paterae being larger than inactive ones on the trailing and leading quadrants and vice versa for the sub- and anti-Jupiter quadrants.  This distribution may have consequences for how Io's releases its heat since paterae are the dominant contributor to Io's total heat flow.

I am curious how much image resolution plays into this.  The resolution of images of Io from the Voyager and Galileo missions is best on the sub- and anti-Jupiter quadrants, respectively.  Better resolution would allow for better identification of paterae margins and would allow smaller paterae to be resolved.  This issue may be most acute on parts of the leading quadrant where the best images have a pixel scale of around 9 kilometers per pixel.  Such low-resolution data may cause an over-estimate of paterae margins as they are confused with nearby bright or dark flows.  I know that Shamshu Patera looks larger than it really is in the global basemap as a result of a bright flow to the south of the volcano.  So more paterae (from smaller paterae being detected) on the sub- and anti-Jupiter quadrants and larger paterae (from mis-identification of margins due to the low resolution of the available data) on the trailing and leading quadrants would be an expected result from resolution effects.  That being said, the trailing and leading quadrants do have some of the largest paterae I can think of like Loki, Dazhbog, Shamshu, and Zal so maybe there is something to this hemispheric difference in average paterae size after all.  Just looking at the poster in front of me, I can only quickly see west Tvashtar and Mentu in that ballpark on the anti-Jupiter side.

This isn't meant as a criticism of their work.  Their methodology is sound; using ArcGIS is an excellent way to measure areas of irregular surface features.  Every day conclusions are made from less than ideal data; science doesn't stop just because the effective resolution of a basemap isn't uniform.  You make do with what you have and just attempt to remain humble when in future years better images are acquired and your conclusions have to be changed.  Look at Titan - a world full of less than ideal data.  When a cryovolcano turns into just another patch of bright material surrounded by dunes, you suck it up and move on.  But you don't let methane windows, low spatial resolution data, or ambiguous terrain stop you from making interpretations and measurements.

Link: Distribution and Comparison of Io's Paterae: Areas, Effective Diameters, and Active Volcanism [www.lpi.usra.edu]

LPSC 2010: Determining Volcano Eruption Styles from Infrared Spectra

For the last two weeks, we've been looking at the Io-related abstracts for presentations at this year's Lunar and Planetary Science Conference.  Today we will look at a presentation titled "The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io" by Ashley Davies, Laszlo  Keszthelyi, and Andrew Harris.  This paper will be presented as a poster at the Igneous and Volcanic Processes session on Thursday, March 4.

In this poster, Davies et al. will present a method for determining the eruption style of a volcano based on the near-infrared spectrum of the volcano taken from space, either a satellite observing the Earth or spacecraft or ground-based observations of Io.  This is done by examining the shape of near-infrared spectra taken of a volcano between two and five microns, the slope of this spectra between these two wavelengths, and how the shape and slope of the spectra change with time.  This methodology allowed the research to differentiate between insulated flows and lava lakes on Earth.  Extended to Io, the authors were able to predict based on spectra with a low-spatial resolution the eruption styles at a number of volcanoes such as Pele and Prometheus that was also confirmed, this time using higher-resolution observations later in the Galileo mission.

In the case of active lava lakes like Pele, they found that the ratio between the power output at 2 microns and 5 microns to be near unity.  As active lava lakes this makes sense as there is a fixed amount of terrain (remember, the lava lakes are bound by the patera margin) covered with cooling lava that is renewed with fresh lava on a regular basis.  So the power output at the two wavelengths is about the same.  For insulated flows, there are generally small areas where fluid lava is exposed at the surface both at the source vent and at the active flow lobe.  The vast majority of the insulated flow is covered by cooling lava whose thermal emission is concentrated at longer wavelengths like 5 microns, compared to the hotter vent and flow front, whose thermal emission will be concentrated at shorter wavelengths, like 2 microns.  So, for insulated flows like Prometheus and Amirani, the power output at 5 microns is greater than at 2 microns.  The ratio between 2 microns and 5 microns decreases as eruption intensity decreases.  Interestingly, many paterae have a similar signature, but it is not known if this due to these paterae, like Tupan, Malik, and Hi'iaka, being more quiescent lava lakes than Pele or due to insulated flows on the floor of these volcanoes.

Eruption style can also be determined by monitoring how the spectra of these features can change with time.  The authors in the abstract highlight research into the Pillan eruption.  In that case, the first near-IR observations of Pillan revealed a 2μm/5μm ratio near unity, suggesting a rigorous eruption with lava fountaining and open channel flows.  As the eruption progressed, its total power decreased as did the 2μm/5μm, show the shutting down of the fountaining and an increased area of the cooling flow.  A similar eruption progress was seen at an outburst in 1990, though in that case, along with other outburst eruptions like Surt in 2001, the 2μm/5μm ratio was actually greater than 1 initially, suggesting an initial phase of very vigorous lava fountaining with only a small area of emplaced lava at that point in the eruption.

Finally, the authors suggest ways to apply this methodology on a future mission to Io.  They suggest including filters at two and five microns in a thermal mapper in order to directly apply their method.  They also suggest two at longer wavelengths such as 8 and 12 microns to further constraining the amount of cooler lava flows that are older than those that would detected at 5 microns.  They also suggest that observation often enough to detect changes in eruption style or thermal emission that would further constrain that style, like a vigorous outburst eruption that starts out with high thermal emission at short wavelengths, then cools down as the eruption shuts down or changes to an insulated flow style, like Thor.  The authors do point out that this type of analysis can be done at low-spatial resolutions, so regular observations would not need to be conducted just during Io flybys.

Link: The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io [www.lpi.usra.edu]

LPSC 2010: Heat Flow from Dark Paterae Floors

Last Wednesday, the abstracts for this year's Lunar and Planetary Science Conference were posted online and since then I have been discussing a few of these abstracts here on this blog, including ones on the new global geologic map to the stability of patera margin slopes.  Today we are going to take a quick look at an abstract by Glenn Veeder, Ashley Davies, Dennis Matson, Torrence Johnson, Dave Williams, and Jani Radebaugh titled, "Io: The Dark Paterae Component of Heat Flow".  In this abstract, the authors discuss new thermal modeling work based Galileo NIMS, SSI, and PPR infrared data and the USGS global basemap.  This modeling was done to determine the contribution to Io's heat flow made by dark material on the floor of volcanic depressions known as paterae.

Io's total heat flow, ~9.5×10¹³ W, has been measured from disk-integrated, ground-based IRTF data along with incomplete global data from Voyager IRIS and Galileo PPR.  Galileo's SSI camera and NIMS near-infrared spectrometer acquired more complete global data (except over the polar regions), providing information on current or recently active volcanoes, but most of Io's heat flow is released at much longer infrared wavelengths from cooling lava flows, wavelengths IRTF, IRIS, and PPR were sensitive to.  To inventory how Io's internal heat is released, the authors created a thermal model to estimate the amount of total energy released by volcanoes that are either outside of the terrain covered by IRIS or PPR, or were too small for those instruments to detect.  In 2008 and 2009, this same group examined the contribution to Io's total heat flow from dark lava flows on the plains of Io (e.g. Amirani), both at LPSC in March 2008 and in a paper published in November 2009.

For this research, the authors mapped the distribution of dark paterae floor materials across Io's surface and measured their areas.  In total they found a total of 148,000 square kilometers, which is about 0.4% of Io's total surface area or a little less than the "dark patera floor" unit mapped by Williams et al.  The authors found that the distribution of dark patera floor material has a similar bimodal distribution in longitude (with peaks near 130° W and 315° W) as paterae in general.

Of the total mapped area, 30,500 km² are composed of a combination of Io's two largest dark floored paterae, Loki Patera (shown above) and Dazhbog Patera.  The authors then took the areas of this dark patera floor material and using an effective temperature of that material, estimated their total power output.  The total average power output of these two volcanoes was modeled to be 9.6×10¹² W and 4.0×10¹² W, respectively.  In the case of Loki, this is 10% of Io's total heat flow.  Both modeled power outputs are close to the measurements made by PPR.  The other areas of dark paterae floor material account for six times that seen at Loki Patera, Io's most powerful volcano.

The abstract is part of research into how Io's internal heat is released, i.e. what heat sources make up Io's global heat flow.  Assuming the same effective temperature for all these materials as Loki, dark paterae would account for 70% of Io's total heat flow (compared to 5% for dark flow fields on the Ionian plains like Masubi or Amirani).  This makes dark paterae floor materials the most significant contributors to Io's heat flow. 

Link: Io: The Dark Paterae Component of Heat Flow [www.lpi.usra.edu]

LPSC 2010: Analysis of the new Io Global Geologic Map

The next abstract we will take a look at is "Volcanism on Io: Results from Global Geologic Mapping" by Dave Williams, Laszlo Keszthelyi, Dave Crown, Paul Geissler, Paul Schenk, Jessica Yff, W.L. Jaeger. The authors completed a global geologic map of Io over the last few years in ArcGIS™ based on a basemap containing Voyager and Galileo created by the USGS.  Their map has gone through peer-review, but has not been published online as far as I know (I'd love to be proved wrong though).  Geomorphologic maps such as this global one can be used to determine the distribution of different terrain types across a planetary surface.  Progress on this project was reported earlier at the last few LPSCs, including 2008 and 2009, as well as other other conferences like EPSC.  This year's paper will be presented as a poster at the "Satellites and their Planets" session on Thursday, March 4.

This year, the authors of the global geologic map will present statistical analyses of the units in the map, which include: plains, lava flows, paterae, mountains, and diffuse deposits.  This involves looking at where certain units and sub-units are most concentrated and how different sub-units are correlated.  As an example, they found that bright flow fields outnumber dark flow fields 3 to 2.  These are considered the youngest flows on Io, composed of sulfur compounds and silicate basalt respectively.  They note a concentration of bright flows at 45-75° N, 60-120° W, covered in the area shown at right.  Williams et al. argue that this region might be indicative of extensive sulfur volcanism here in the past.  An alternative explanation, particular given what happened at Thor (bottom center in the image at right, far right in the Thor link), would be that these flows are older silicate flows that have been coated with sulfur in falling from nearby plumes.  While these flows are old enough to be completely coated and turn a bright shade of yellow, they are not old enough to have been converted to red-brown sulfur (S₄) through radiation damage.  So these flows stand out against the background plains, while at the equator, they might not have been as noticeable.  Thus the process of a lava flow aging and becoming indistinguishable from the background might take longer at the poles than at the equator.

Comparing the distribution of Io thermal hotspots (indicative of volcanic activity) to terrain type, the authors found that 20.3% of hotspots are associated with dark flow fields, 9.3% with undivided flows (older mapped flows, like the ones at lower left in the color image above), 45.3% with dark patera floor material, 1.7% with bright flows, and 18.6% with other patera floor units.  This matches with intuition that recent active volcanism on Io is dominated by silicates.  One difficulty that would be interesting to see how they address are surface changes on Io over the course of the Voyager and Galileo missions, and between them.  For example, hotspots associated with undivided or bright flows may well come from fresh dark flows that formed after the images used in the basemap were taken, as would be the case for Thor.  Activity from Pillan in 1997 would also not be represented in the map since they don't appear in the basemap.

In additional analysis, the authors found that lineated mountains tend to be taller than other mountain that show signs of mass wasting.  This is as expected as mountains are thought to be uplifted crustal blocks that almost immediately begin to waste away.  When looking at diffuse deposits, they found that these materials are dominated by condensed gas deposits from volcanic plumes as opposed to pyroclastics, the latter normally associated with transient outburst-class eruptions, like Pillan or Thor, though some pyroclastic deposits seem to be more permanent around some volcanoes like Pele and Babbar.  Finally, they note that white plains, composed of sulfur dioxide ice fields, are mostly concentrated along the equator on the anti-Jupiter hemisphere in regions such as Colchis Regio, Bosphorus Regio, and Bulicame Regio.  They suggest that this region might be a colder region of Io's surface, possibly due to differences in magma sources, delivery mechanisms, and crustal thickness, but I wonder if runaway thermal segregation, the kind seen on Iapetus, might be a possibility.

In this abstract, Williams et al. once again take a look at the global geologic map the group has created over the last few years, attempting to use the map to determine correlations between different surface units and other Io data, such as mountain height and volcanic hotspots.  Hopefully, sometime in the next year, the map will be officially published, like the Ganymede map was last year.

LPSC 2010: The Stability of Io's Paterae Walls

The abstracts for this year's Lunar and Planetary Science Conference are now online. LPSC is one of the largest planetary science conferences of the year, along with DPS in the fall and AGU in May and December. This year's Io presentations will be a diverse bunch with mission concepts, geologic mapping, and plume modeling, and statistical analyses of Ionian landforms.

The first LPSC abstract I want to discuss here is titled "Paterae on Io: Insights from Slope Stability Analysis" by Laszlo Keszthelyi, Windy Jaeger, and Chris Okubo.  Keszthelyi will present this paper as a poster on Thursday, March 4 at the Satellites and Their Planets session.  In this presentation, the authors uses the slopes of Io's volcano-tectonic depressions, also known as paterae, to probe the properties of the upper 2-3 kilometers of Io's lithosphere.  A nice example imaged by Galileo is Tupan Patera, shown at left. Keszthelyi et al. use slope stability analysis to constrain the potential composition of the upper part of the lithosphere by using observed slopes on Ionian paterae, which is related to the strength of that material.

While Io's lithosphere is dominated by cooled lava flows that have stacked one on top of another, the upper few kilometers are thought to be a mix of sulfur and sulfur dioxide ices intermixed and layered with layers of basalt.  These ices are volatilized by shallow magma chambers and magma conduits and mixed with ascending magma below those first few kilometers.  Generally though, the upper 2-3 kilometers, according to Jaeger and Davies 2006, would be dominated by sulfur and sulfur dioxide ices.  This process leads to a density gradient in Io's lithosphere, with the least dense material near the surface with density increasing with depth (just how much it increases depends on the resurfacing rate).

Keszthelyi et al. tested three possible compositions: basaltic rock, solid sulfur, and unwelded ash (as would be expected from a pyroclastic deposit).  The slope they tested in the program Slide was the 2.8-kilometer margin of Chaac Patera shown at right.  The right part of the cliff in this image has a slope of 70° based on measurements of the shadow at the cliff base.  Their slope stability analysis tested whether the above compositions can support this kind of steep slope.  They determined that the slope could not be supported by unconsolidated material, like unwelded ash, as these would collapse to the angle of repose (like the talus apron you see at the cliff base on the bottom left part of this image).

Such a steep cliff would be okay for cold sulfur or basalt.  The authors note that without more detailed slope profiles (see, we need the laser altimeter and radar instrument on JEO), they can not distinguish between the two compositions.  In general, based on the abstract's Figure 2, slopes composed of cold sulfur would have more concave profiles (steeper at the top than at the bottom) than those composed of basalt. 

One additional point that Keszthelyi et al. makes is that these slopes (regardless of composition) could not withstand Ioquakes greater than a moment magnitude of 4.  For comparison, last week's earthquake in Haiti had a moment magnitude of 7.  This goes against intuition a bit when you see the height of Io's mountains and how quickly these structures are built, not to mention Io's intense volcanic activity, which could cause tremors as well.  The authors note that this contradiction would be resolved if the stress on Io's faults is relieved by daily tidal flexing, rather than in massive earthquakes, as they are on Earth.  They also point out that Windy Jaeger's work on tectonics in Io's lithosphere in 2003 suggested that the "globally-averaged stress level in the crust is close to the sliding friction of the faults."  Put together, stress on Io's faults, rather than building up slowly over time until released on a sudden jolt, are relieved on a regular basis in much smaller events, small enough that they are not a major factor in the degradation of Ionian paterae wall slopes.

I want to return for a second to that image of the margin of Chaac Patera.  I have re-posted it here, this time with annotations.  I am definitely going to do another post on this thing, maybe later today, because it really is just freaking awesome to finally understand what is going on here (hint, I am now a firm believer of sulfur volcanism on Io...).  Anyways, let's stick to analysis specific to our discussion here, the stability of the Chaac Patera margin.  Now, the 70°-slope section can be seen on the right side, with very little loose material at the bottom of the slope.  In fact the patera floor at the bottom of that slope looks like asphalt pavement (not saying it is, just that it looks that smooth at this scale, ~7 meters per pixel).  On the bottom left and the far upper right ends, we see a change in slope, the result of talus — loose debris at the base of the cliff at about the angle of repose (we'll leave the two-tone talus discussion for tomorrow).  So what makes these sections different from the more stable middle?  It is a little more difficult to tell in the upper right corner because of the data dropouts and the edge of the frame, but to the lower left we see section of the wall that have broken away from the upper plateau and slumped down hill.  The likely cause of this slumping seems to be local faults that run parallel to the margin of the patera, best seen at the top of the image.  These suggest that Chaac Patera is continuing to expand due to continued collapsing along the patera margin.  We see examples of this kind of faulting at other Ionian Patera, such as Gish Bar Patera and Radegast Patera.  Regardless, while it would seem that some patera wall slopes eventually succumb to slope failure (note the terracing along some sections of the margin of Radegast Patera), many observed paterae slopes show no sign of mass wasting like talus aprons or terracing and thus require strong material like cold sulfur ice and/or basalt to hold up these steep slopes.

Keszthelyi et al. will present an interesting paper on the geology of Ionian volcanoes in March at LPSC.  Definitely check it out if you are going to that conference.

LPSC 2010 Abstracts Now Online

Abstracts for the 2010 Lunar and Planetary Sciences Conference (also known as LPSC) are now online. The final announcement for attendees has also been published.

This year's conference is at The Woodlands Waterway Marriott Hotel north of Houston, Texas. This year's conference has also been moved to one week earlier than usual, the week before spring break for many universities. The conference is scheduled for March 1–5, 2010.

Several Io-related abstracts have been submitted for the conference. Unlike previous years, talks and posters this year are generally organized by process, as opposed to specific Io or Galilean satellites sessions.  Also, there is definitely an increase in the percentage of Moon or Mars related sessions as opposed to meteorites or outer solar system topics.  As a consequence of the former, these talks and posters will be in different sessions.

By now I am sure you know the drill.  Over the next few days, I will post discussions of each abstract here on the blog.  The links below take you to the abstracts themselves.  I will add links to my discussion of them as they are posted in the bullet list below.

• Volcanism on Io: Results from Global Geologic Mapping by Dave Williams et al. This poster will present updates regarding the global geologic mapping project and research based on this new map.  A blog post about this abstract has been posted.
• Io: The Dark Paterae Component of Heat Flow by G. J. Veeder et al.  Last year, this same group published a paper on the contribution to Io's total heat flow from dark lava flows.  This poster appears to be a continuation of that work, not focused on dark paterae.  A blog post about this abstract has been posted.
• DSMC Modeling of the Plume Pele on Io by W. J. McDoniel et al.  Last year, this group presented results from modeling non-circular plume vents on Io, and their application to Prometheus.  This year's poster extends this modeling to the specific case of the Pele plume, with very encouraging results.  A blog post about this abstract has been posted.
• The Geothermal Gradient of Io: Consequences for Lithosphere Structure and Volcanic Eruptive activity by G. Leone et al.  A blog post about this abstract has been posted.
• Science Rationale for an Io Volcano Observer (IVO) Mission by A. S. McEwen et al.  A blog post about this abstract has been posted.
• Io's UV-V Eclipse Emission: Implications for Pele-type plumes by C.H. Moore et al.  A blog post about this abstract has been posted.
• Modeling the Sublimation-Driven Atmosphere of Io with DSMC by A. C. Walker et al.  This talk will cover the model that was examined and compared to real data in Gratiy et al. and discussed here last week.  A blog post about this abstract has been posted.
• Distribution and Comparison of Io's Paterae: Areas, Effective Diameters, and Active Volcanism by B. Barth et al.  A blog post about this abstract has been posted.
• Paterae on Io: Insights from Slope Stability Analysis by L. Keszthelyi et al.  I guess you can scratch "Ioquakes" off a list of natural hazards for future Ionian colonists.  A blog post about this abstract has been posted.
• The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io by A. Davies et al.  A blog post about this abstract has been posted.

Because these talks and posters are less centralized than they usually are in an Io session, it is possible that I might have missed one.  Let me know if I did.

Oh one other abstract I will be talking about here that isn't Io-related, but still cool and is Jupiter-related: A New Ring or Ring Arc of Jupiter? by A. F. Cheng.  Apparently, Phoebe isn't the only outer irregular moon with an associated dust ring.  Seriously, at this point, can we just say that small moons in the outer solar system, unless strongly gravitational effected by a much larger moon (so scratch Telesto or Helene at Saturn, yeah I know, I will come to eat those words come March 4), have dust rings associated with them.  Weaklings, can't even hold up to micrometeorite impacts...

Link: 41st Lunar and Planetary Science Conference (2010) [www.lpi.usra.edu]