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]

30th Anniversary of the Discovery of Volcanism on Io

Imagine you were a scientist on the Voyager Imaging Team 30 years ago today. Only days before, Voyager 1 had passed through the Jovian system, returning thousands of images of Jupiter and its moons. The Galilean moons Voyager revealed had strange and fascinating vistas: cratered Callisto, grooved Ganymede, streaked Europa. Perhaps the strangest of these four worlds was Io. It was clear from the images that were coming down in the days following Voyager's close pass that Io was a world that was constant renewing its surface. Instead of impact craters that were prevalent on most worlds, Io had none and was covered in apparent volcanic features. Given the lack of impact features on the surface, Io was likely even active today.

On Friday, March 8, 1979 at 13:28 UTC, Voyager 1 pointed its narrow-angle camera at Io, 4.5 million kilometers away, one last time. Thirty-eight minutes later, the image was received on Earth. The image was taken to help establish the position of Voyager in space. By comparing the position of Io in the image with known background stars also found in the image, the navigation team could determine if additional maneuvers were needed to keep the spacecraft on track for an encounter with Saturn in November 1980. This task was assigned to Linda Morabito, the cognizant engineer of the Optical Navigation Imaging Processing System at JPL. Among the first things she saw when looking at this image, just as you might have when you saw the same image above, was the crescent-shaped feature just off the limb of Io. Over the next day, she and other Voyager navigation engineers and scientists worked to eliminate the possibilities of what this feature could be. When the crescent, now thought of as a cloud was shown to be assocated with a possible volcanic feature (now known as Pele), this seemed to nail this down as a volcanic plume. There was even another plume (now known as Loki) just beyond the terminator, catching the first bits of morning sunlight.

A more detailed account than I can ever give of the discovery of volcanism on Io can be found on the Planetary Society website and was told by none other than Linda Morabito Kelly.

The discovery of active volcanism on Io was announced at a press conference on Monday, March 12, 1979. By then more plumes had been found as earlier images of Io were re-examined and areas of high thermal emission were being found by Voyager's Infrared Interferometer Spectrometer (IRIS) . For example, in the mosaic I posted earlier today (and if you haven't checked that out, I couldn't recommend to do so more!), two plumes are visible along the limb: Masubi and Pele. I've created a special version showing these plumes in the mosaic. Masubi is at lower left and Pele is at right. These two plumes show the two main types of plumes on Io. Masubi is a smaller, dust plume, also known as a Prometheus-type plume. It has an umbrella-like shape, with a dense central column (in this case TWO central columns) and a bright shock canopy. Pele is the archtype of the Pele-type or gas plumes. In this case, there is no central, dense, eruption column and the shock canopy has a filamentary structure.

I guess for me, the discovery of active volcanism on Io was perhaps more significant for me personally than the Apollo landings, or any of the other major events in the history of spaceflight. While I was not around for either event (I hate to make some of you feel any older, but yes, I was still 4.5 years away when Voyager 1 flew by Jupiter), that discovery, along with the Galileo and Cassini missions were most significant for me to decide to work in this field. Seriously, I could have been in law school right now. Thanks Voyager 1!

Later today, I will be working on a few more mosaics from Voyager 1, though these won't be anywhere nearly as big as the one I posted earlier today. Seriously, have you checked it out yet‽ One will be a reprocessing of the PPS support imaging strip and the other will be a higher resolution mosaic over Io's south polar region. I am trying to stick using the Voyager imaging teams different mosaic designs when deciding which images to use for these mosaics. That's why Loki is missing in the Southern Hemisphere mosaic; it wasn't covered in that mosaic design. It WAS covered in the lower resolution, 4-color northern hemisphere mosaic, but that one has a lot more missing images and smeared images, so it will take me a bit longer to figure out how to parse that one down.

And have you checked out that mosaic I did earlier ;)

Link: Discovery of Io's Volcanoes [members.fortunecity.com]

LPSC 2009: Simulated Plumes with Irregular Vents

Next up in our series on Io-abstracts at this year's LPSC is "DSMC Modeling of 3D Vent Geometries for Ionian Plumes" by William McDoniel, D. Goldstein, P. Varghese, L. Trafton, and B. Stewart. This abstract covers part of continuing research to study Io's atmospheric and volcanic plume dynamics using Direct Simulation Monte Carlo (DSMC) modeling. In this abstract, McDoniel et al. take a look at volcanic plumes with irregularly shaped plume vents.

The University of Texas research team led by Dr. David Goldstein have been modeling the plumes and atmosphere of Io for a number of years using a model called the Direct Simulation Monte Carlo method. From the Wikipedia article, "DSMC is a numerical method for modeling rarefied gas flows, in which the mean free path of a molecule is of the same order (or greater) than a representative physical length scale (i.e. the Knudsen number Kn is greater than 1)." I quoted that because, while I understand the individual words in that sentence, I'm not sure I would have summarized that properly. So while I may not have a perfect understanding of the method the group uses, I can't argue with results as they have managed, in prior work, to create a proper looking Pele-type plume in Zhang et al. 2003 and dust plume in Zhang et al. 2004. More recently, they have used the DSMC method to look at Io's atmospheric collapse when the satellite enters Jupiter's shadow.

In the research group's previous work with plumes, they assumed a disk-shaped source vent for the plume. First, assume a spherical cow... :-D Well, in McDoniel et al., Goldstein's group decided to look at the effect of a non-circular source vent. This would better match the source vents observed on Io. Prometheus's plume is thought to be generated as an advancing lava flow front cover over pre-existing sulfur dioxide surface frost, causing the ice to become heated and sent skyward to become part of the plume. Tvashtar's plume is thought to be formed at a somewhat linear lava curtain. In short, neither plume, nor other plumes on Io, seems to have a "disk-shaped" source vent. So the authors performed their DSMC simulation, with a Io temperature and atmospheric conditions, using a half-annular vent and compared the resulting plume with a previous simulation using similar conditions but with a disk-shaped source.

The authors found that even with an asymmetric source vent, the resulting plume shape is very similar to the axisymmetric case. All the differences in particle density that are apparent near the vent become smeared out before the particles even reach the top of the plume, or the shock canopy. The plume fallout zone is roughly axisymmetric around the half-annular source vent. This model is supported by observation. At Prometheus, there appears to be an irregular-shaped source region (rather than a specific vent crater) but the plume fallout pattern is circular around that source. A similar case can be seen at Pele. What is apparent from the simulation is that high-resolution images of a plume would be required to observe differences in the plume's shape (particularly the particle density near the vent) as a result of the vent shape.

It is nice to see that the DSMC model bear out what has been observed at Galileo once again. However, if anyone on the UT research group is reading this, please, please, perform a simulation with two plumes near each other. We have now observed a number of cases of interaction between two volcanic plumes at Io (Pele/Pillan in 1997, the two Masubi plumes in 2007, the two Kanehekili plumes in 1997, and the two Loki plumes in 1979). We have also seen similar interactions between a dominant dust plume and a much smaller sulfur-rich plume based on surface fallout patterns (like at Marduk and Prometheus). It would be interesting to see what these would look like modeled.

Link: DSMC Modeling of 3D Vent Geometries for Ionian Plumes [www.lpi.usra.edu]

The Giant Plumes of Io

Today I thought I would write a post on Io's class of giant, volcanic plumes. These shield-shaped clouds of gas and dust are generated at sites of vigorous volcanic activity reaching upwards of 500 km above Io's surface. These plumes are one of two basic classes of volcanic debris clouds observed at Io. The smaller, Prometheus-type plumes are generated as lava flows burn off volatiles such sulfur dioxide.

While Prometheus-type plumes are comparatively common, Three volcanoes have had observed giant plumes: Pele, Tvashtar Paterae, and Grian Patera. These three plumes share a number of characteristics. The first obviously is their large size. They reach altitudes in excess of 350 km above the surface and can even reach altitudes as high as 500 km. The observed plumes are generally 1100 km in width, or 550 km from the source vent. Secondly, these plumes rarely exhibit a central column, like Prometheus-type plumes. So the plumes appear more like a shield, rather than an umbrella. In Spencer et al. 2007, this difference in shape was interpreted as being caused by different sources for the particulates in these plumes. In Prometheus-type plumes, the dust is ejected along with the gas from the source vent(s). In giant plumes, the dust is condensed from the gas when it reaches the shock canopy, the point when the ejected material stops ascending and begins to fall back to the surface. As you can seen in the movie above of Tvashtar's plume, this dust can form large clumps, likely due to static electricity (resulting from the Io-Jupiter flux tube). High-resolution images of Pele's plume from Voyager also show these clumps at that site. The amount of condensed dust is generally a function of the amount of gas in the plume, which can vary. Less dense plumes, like Pele and Grian, can make these plumes appear quite faint at visible wavelengths, to the point where the plume is considered stealthy. Observations at high phase angles or at ultraviolet wavelengths enhance our ability to see these types of plumes. A third characteristic of these plumes is their sulfur-enriched composition. These plumes have much higher S₂ contents compared to Prometheus-type plumes, whose gaseous composition is almost entirely sulfur dioxide. Spencer et al. 2000 showed that the plume at Pele is composed of about seven parts SO₂ and one part S₂. However, the S₂ to SO₂ ratio can vary at these plumes from between 0.01 to 0.3. Tvashtar in 2007, for example, had a ratio closer to 0.01-0.02.

Giant plumes are often associated with outburst volcanic eruptions. These eruptions have vigorous outpourings of lava and usually have a large lava fountain or curtain at the vent. Giant plumes are thought to form from gas dissolved from these lava fountains. This was particularly the case for Tvashtar in 2000 and 2007 and at Grian in 1999. Pele is an exception to this, however. The source for Pele's plume is a vigorously overturning lava lake. The lava crust is broken up in a number of places at any given time and gas is dissolved from the lava from these cracks.

A final characteristic of Io's giant plumes is the large, red plume deposits they form. Unlike the deposits surrounding Prometheus-type plumes, these giant red rings are often oval in shape, with greater north-south radii. The plume deposit from Grian Patera can be seen as a red-orange ring near the limb in the image at right from near the tail end of that eruption. Their distinctive appearance also allows scientists to identify additional giant plume sites even if the plume itself was not observed. This includes Aten Patera and Surt by Voyager 2 in 1979 and Dazhbog Patera, Surt, and a deposit near 70 North, 55 West by Galileo.

A factor in the small number of detection is their transience (though again, because of the difference in eruption style, Pele is the exception). The deposit at Grian is a perfect example. The 1999 eruption of Grian Patera began shortly after June 8 of that year (as reported in Howell et al. 2001). It reached a peak brightness on June 22 and was observed by Galileo both in daylight and in eclipse on July 3. It was during these observations that Galileo observed a hotspot at Grian, as well as a giant plume and a red ring deposit surrounding Grian. A dark deposit also surrounded the source patera. During the next perijove in mid-August, the deposit had faded completely as the S₂ in the deposit had recombined into the more stable, S₈ form of sulfur.

I hope you all enjoyed this little discussion of giant plumes on Io. I should probably write more blog posts like this.

Galileo observations of volcanic plumes on Io

The last major paper on Io out of the Galileo mission was published in the October 2008 issue of the journal Icarus. "Galileo observations of volcanic plumes on Io," by USGS researcher Paul Geissler and NAU grad student Melissa McMillan, describes the observations acquired of Io's plumes during the entire Galileo mission. The abstract can be found at the link above, but the article itself is available to subscribers (individual or institution) only.

In this paper, the authors searched the Galileo SSI data set for volcanic plumes on Io, then used those images to determine particle sizes, column densities, and plume masses.

In all, the authors found plumes at 13 sites spread out across Io. These include optically bright dust plumes (like the one shown at left), faint dust plumes, and gas plumes, with the latter seen in eclipse observations by Galileo. In some cases, the authors found several of these types at a single volcanic center. For example, on I31 in August 2001, a Prometheus-type dust plume was observed at Thor, which was vigorously erupting at the time. A faint outer halo was also observed in the high phase angle images from that orbit. Pele-type plumes, large, faint plumes generated from gas emitted from lava fountains, went largely unseen by Galileo as SSI had poor sensitivity in the ultraviolet where these types of plumes are best seen. Pele's plume was observed on two occasions, during E4 and G29. Another Pele-type plume was also observed at Grian, resulting in a transient large plume deposit. Several Pele-type deposits were also observed, at Dazhbog and Tvashtar, while additional changes at Pele suggest that additional large plumes existed during the Galileo mission.

The authors then examined the visible-light spectrum of various dust plumes to determine the mean particle size and total plume mass. The authors determined that the small, optically-dense plumes seen at such volcanoes as Zamama, Pillan, and Prometheus consist of course-grained "ash" particles. Combined with the presence of a central dense column in these plumes, and the authors suggest that these particles erupt with the gas in the plume. Typical mass for these plumes was found to be around 10⁶ to 10⁷ kg. The authors also examined the faint outer halo at Thor. They found that the faintness of the plume is not because there is very little dust, but because they are made of much finer particles, 10 nm versus the 80-120 nm found in the brighter core of the plume. With particles that small, the plume would be more easily visible at ultraviolet wavelengths. Strangely enough, the faint outer halo has 10-100 times more mass than the inner, "dense" core. Keep in mind, as well, that even accounting for that inner core, the dust makes up only 10% of mass of these plumes, with the gas making up for the rest of it. The particles in the faint outer halo at Thor (and seen at Loki during the Voyager 1 flyby) are thought to condense directly from the gas in the plume, forming small, sulfur "snowflakes."

The authors also compared the observed plumes to the surface changes found by Galileo. They determined that the surfaces changes were caused by the dense, dust plumes, and not by the fainter gas/snowflake plumes, except in the case of the Pele-type plumes, which produce large, red rings. The deposits made by the gas plumes likely take the form of SO2 frost that is transparent at visible wavelengths. The mass of the plumes suggests that dust fallout makes up only a small fraction of the overall resurfacing on Io.

Link: Galileo observations of volcanic plumes on Io [dx.doi.org]

Galileo E11 Reprocessed Images

Yesterday, I posted on one of the images I had worked on from the Galileo's E11 orbit, that last of that spacecraft's primary mission. This evening, I finished processing the rest of the images (except for the eclipse images) from that orbit.

These images were acquired on November 7-8, 1997 and for the most part show Io at high phase angles. These observations were designed to look at the state of Io's plume activity at the end of Galileo's primary mission. In these images, a total of five plumes were observed: Prometheus, Zamama, Marduk, Pillan, and Kanehekili. The Pillan and Kanehekili plumes were only observed twice by Galileo, while the other three were observed at various times throughout the Galileo mission.

The Kanehekili plume stood out to me while looking through these images. The plume is visible in the 11ISKANEHI01 observation. Enhancing the contrast of the VIOLET filter reveals something rather interesting about the Kanehekili plume: there are two eruption columns separated by about 70 km. This would suggest that there are two plumes at Kanehekili (or at least two plume sources within the flow field). Both were formed as different flow lobes from Kanehekili 1997 eruption overtook an area of sulfur dioxide frost and launched the frost skyward. Similar behavior was seen at the Masubi flow field last year by New Horizons, though the separation between plume sources was greater in the latter case. The Kanehekili plume was also observed in May 1997. Only one eruption column was seen at the time.

Link: Galileo E11 Images [pirlwww.lpl.arizona.edu]

Sulfur dioxide and Sulfur in Tvashtar's Plume

There is a nice blog post by Ted Stryk, guest blogging for the Planetary Society at the Lunar and Planetary Sciences Conference, on some of the presentations from last night's poster sessions and yesterday afternoon's lunar session. One of posters Ted highlights is the one by Kandis Lea Jessup and John Spencer titled, "Detailed Analysis of the Tvashtar Plume Spectral Behavior." I highlighted the abstract for this poster last week.

Ted reports that the authors have determined that the S₂ to SO₂ ratio in the Tvashtar plume was between 0.01 and 0.02. In other words, in their measurements based on images taken by Hubble last February, the plume consisted of significantly more Sulfur dioxide than diatomic Sulfur. This value would seem to be in conflict with the presence of a red ring plume deposit, which is thought to be caused by the deposition of sulfur from Io's plumes. However, measurements of the Pele plume suggest that the Sulfur to Sulfur dioxide in these larger plumes can vary between 0.01 and 0.3, so the low ratio at Tvashtar may not be representative of the average.

Another consequence of the low S₂/SO₂ ratio is that the high optical depth seen in the Hubble WFPC2's F255W filter (an ultraviolet filter that senses gases in Io's plumes, rather than the dust seen in visible wavelength images like the LORRI images above) was not due to absorption by S₂, but instead is due to SO₂.

An interesting result. Definitely indicates that Tvashtar's plume was quite gas rich.

LPSC 2008: Detailed Analysis of the Tvashtar Plume Spectral Behavior

In the second LPSC abstract highlighted on this blog, Kandis Lea Jessup and John Spencer present the work they have done on Hubble images of Io taken during last year's New Horizons encounter. In particular, they are using the images they acquired at different wavelengths with Hubble's Wide Field and Planetary Camera 2 to study the spectral behavior of the Tvashtar plume.

While the images acquired by Hubble have a lower spatial resolution than those taken by New Horizons' LORRI camera (180 km per pixel for the WFPC2 versus at top resolution of 11.2 km per pixel for LORRI), the WFPC2 has a higher spectral resolution than New Horizons' MVIC instrument, particularly at ultraviolet wavelengths which is particularly important for identifying gases within Io's plumes. Jessup and Spencer observed Io and the Tvashtar plume on multiple occassions last February, allowing the authors to examine the plume's reflectance spectra (i.e. looking at how much light reflects off the plume, which can depend on composition, particle size, and phase angle) and absorption spectra (i.e. looking at how much light from the background Jupiter passes through the plume to Hubble).

The authors found the plume to be most noticeable in both sets of observation in the ultraviolet F255W filter, indicative of S2 gas in the plume. The authors had a similar result at Pele in 2000. They do note that the Tvashtar plume has a much higher optical depth in the F255W filter than Pele.

Interesting work. They do promise to present more work on how optical depth varies by wavelength for both Pele and Tvashtar in their poster. It is so interesting to see just how similar the Pele and Tvashtar plumes despite the apparent difference in volcanic styles: Pele being a vigorously erupting lava lake and Tvashtar being a fissure eruption. Must have to do with the magmas at both locations having a higher volatile content, allowing the formation of a bright lava fountain at Tvashtar and a constantly overturning lava lake at Pele. Note the fact that as far as I know, Pele and Tvashtar are the only two volcanoes where using relatively short exposures, it is easy to see their hotspots in the daylight, at wavelengths less than 1 micron, and at relatively low resolutions.

Link: Detailed Analysis of the Tvashtar Plume Spectral Behavior [www.lpi.usra.edu]