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]

YARR!! It still be there.

Here is a quick round up of amateur astronomer images of Jupiter and Io from this week:

• Paul Maxson caught a nice view of Io transit across the disc of Jupiter on September 15
• Anthony Wesley imaged Io's shadow on Jupiter on September 16

Icarus October 2008 Issue

The table of contents for the October 2008 issue of Icarus is now online. Included in this issue is "Galileo observations of volcanic plumes on Io" by Paul Geissler and Michelle McMillan. Geissler and McMillan describe the observations of Io's volcanic plumes as observed by Galileo, and look into the differences in appearance between the gas plumes observed in eclipse and the dust component observed in daylight. The paper was posted online in about four months, and yes, I still intend on doing a full post on that paper.

Link: Science Direct - Icarus, Volume 197, Issue 2, Pages 377-640 [www.sciencedirect.com]

Io at DPS

The abstracts for next month's Division of Planetary Sciences Meeting are now online. Several talks and posters are about everyone's favorite moon. The abstracts for DPS (and the upcoming AGU meeting in December) are much shorter than those submitted for LPSC, which would be better described as mini-papers. So there is less to say about the abstracts themselves. A few abstracts do stand out:

• Julie Rathbun and John Spencer have a talk scheduled for the morning of October 15 titled, "Io Eclipse Observations: Determining the History of Loki’s Flux at Multiple Wavelengths." The abstract just summarizes Loki's importance to Io's total heat flux and Rathbun and Spencer's theory for Loki's observed activity. The talk will cover their work on separating Loki's contribution to the observed heat flux from the other volcanoes on the sub-Jovian hemisphere. They will then discuss Loki's extracted brightness at three wavelengths in the near-infrared.
• Erinna Chen et al. will present a poster covering her group's summer school Team X project, a New Frontiers-class Io mission concept. The mission, which they call Argus, would use a high-inclination orbit around Jupiter and would flyby Io at least 40 times. The mission would use Advanced Stirling Radioisotope Generators (ASRG) as a power source, pushing the mission to after Discovery 2013. The payload included in the study would consist of a narrow-angle camera, a thermal imager, an NIR spectrometer, a UV spectrometer, and an INMS-like instrument, similar to the IVO Discovery mission concept, but with more advanced instrumentation and 4-10 times the number of Io flybys, so presumably the increased cost between the Discovery mission concept and this New Frontiers mission would be taken up in increased payload size and radiation shielding.
  

While I will not be at the conference, I am a co-author on three abstracts: "Evidence for Past Lake-Level Change in Titan's Ontario Lacus" by Jason Barnes et al., "Tiger Stripes and Cassini ISS High-Resolution Imaging of Enceladus" by Paul Helfenstein et al., and "Cassini Imaging Observations of Titan’s High-Latitude Lakes" by Elizabeth Turtle et al. Of these, I spent the most time working on the last one, and includes the coolest discovery we have made at Titan from Cassini images:

Differences between the two ISS observations may be due to changes on the surface as a result of precipitation from a large cloud system observed in Fall 2004 (Schaller et al., 2006), although diffuse clouds or atmospheric scattering could also play a role.

The meeting will occur between October 10-15 in Ithaca, New York on the Cornell University campus.

Link: Division of Planetary Sciences Meeting 2008 [dps08.astro.cornell.edu]

Restart - News since May

I am sorry for my long absence here on this blog. I am sure many of you are wondering what has been going on with Io lately so I thought I would finally give this blog a big of a kick-start. Rather than post long posts covering what has happened since May, I thought I would just provide a quick summary:

• NASA has pushed the downselection of the next Outer Planets Flagship Mission to February 2009 from this November. This will provide an opportunity for the community to digest the mission studies due from the two teams on November 3. In June, NASA removed the $2.1 billion cost cap in favor of a "sweet spot" strategy, allowing the study teams to determine a price that would provide the best science per dollar. This has increased the cost of both missions to around $3 billion.
• With respect to Io, "sweet spot" science for the Jupiter Europa Orbiter would include 3-5 Io flybys during a 24-33 month long Jupiter orbital phase. According to the above presentation, these missions would provide the first direct sampling of Io's volcanic plumes, which would indicate that they would try to target one, presumably Pele or Tvashtar. However, the "sweet spot" mission would not include a dust detector or an INMS-like instrument, which would provide crucial information on this regard. It would include a particle instrument, however.
• The next Outer Planet Assessment Group (OPAG) meeting will take place November 6-7 in Tempe, Arizona. Obviously, the Flagship missions will be an important part of the discussion, along with the Discovery & Scout Mission Capability Expansion program.
• The following papers have been published: The variation of Io's auroral footprint brightness with the location of Io in the plasma torus by Serio and Clarke in Icarus' September 2008 issue; Geologic mapping of the Zal region of Io by Bunte, Williams, and Greeley in Icarus; September 2008 issue; Spatially resolved SO₂ ice on Io, observed in the near IR by Laver and de Pater in Icarus' June 2008 issue; and Galileo observations of volcanic plumes on Io by Geissler and McMillan is in press in Icarus (eventually, I need to do a post on this article).

Mercury mosaics from MESSENGER

I have finished up my mosaics of Mercury and posted them on Unmannedspaceflight.com. These include a medium resolution global mosaic and a high resolution mosaic covering the northern hemisphere.

• 87-frame Global Mosaic - This global mosaic of Mercury consists of 87 MDIS Narrow-angle Camera images acquired during the MESSENGER spacecraft's January 14, 2008 flyby of Mercury. These images were acquired when MESSENGER was between 17,484 and 20,790 km from Mercury or between 53 and 63 minutes after closest approach. This mosaic has a resolution of 550 m/pixel. This mosaic uses calibrated MDIS images and the brightness was adjusted to improve the visibility of surface features. The features seen in this mosaic cover a significant portion of the terrain not seen by Mariner 10 during its three flybys in the 1970s. [JPG PNG]
• Northern Hemisphere Mosaic - This regional mosaic showcasing Mercury's northern hemisphere consists of 96 MDIS Narrow-angle Camera images acquired during the MESSENGER spacecraft's January 14, 2008 flyby of Mercury. These images were acquired when MESSENGER was between 8,148 and 11,516 km from Mercury or between 27.5 and 37 minutes after closest approach. This mosaic has a resolution of 309 m/pixel. This mosaic uses calibrated MDIS images and the brightness was adjusted to improve the visibility of surface features. The features seen in this mosaic cover a significant portion of the terrain not seen by Mariner 10 during its three flybys in the 1970s including a number of features that have been recently named. The Caloris impact, the largest and most recent large impact basin on Mercury, is visible as a circular brightish region along the eastern portion of the mosaic. Another large (and relatively young) impact basin, Raditladi, can be seen to the west of Caloris basin. Both basin are unique because they are host to Mercury's only known extensional tectonic features, represented by concentric and radial grabens (troughs). One such extensional feature, Pantheon Fossae, is a prominent feature in the central part of Caloris basin and consists of a radial pattern of cracks. [JPG PNG]
• High-Resolution Equatorial Mosaic - This regional mosaic showcasing Mercury's equatorial region consists of 68 MDIS Narrow-angle Camera images acquired during the MESSENGER spacecraft's January 14, 2008 flyby of Mercury. These images were acquired when MESSENGER was between 4,110 and 6,322 km from Mercury or between 16 and 22.5 minutes after closest approach. This mosaic has a resolution of 125 m/pixel. This mosaic uses uncalibrated MDIS images and the brightness was adjusted to improve the visibility of surface features. This is MESSENGER's highest resolution mosaic acquired during the January 2008 flyby. This mosaic is centered on 4.5 South Latitude, 128 East Longitude. [JPG]
• Inbound Crescent Mosaic - This global mosaic of Mercury's crescent consists of 29 MDIS Narrow-angle Camera images acquired during the MESSENGER spacecraft's January 14, 2008 flyby of Mercury. These images were acquired when MESSENGER was between 19,189 and 17,853 km from Mercury or between 58 and 54 minutes before closest approach. This mosaic has a resolution of 513 m/pixel. This mosaic uses uncalibrated MDIS images and the brightness was adjusted to improve the visibility of surface features. This is MESSENGER's highest resolution mosaic acquired during the January 2008 flyby. The features seen in this mosaic were observed by Mariner 10 in the 1970s, but the high-phase angle allows scientists better study the topographic structures in this region. [PNG]

Link: Mercury mosaics from January 2008 Flyby [unnmannedspaceflight.com]

More MESSENGER at Mercury mosaics

Still working on mosaics of images from the MESSENGER mission's January flyby of Mercury. Here are a few more I've worked on. WARNING: these mosaics are quite large, so it may be best to right-click, save to hard drive, then view it on a your favorite image viewer.

• Mosaic covering the double ring crater Raditladi and the ejecta surrounding the crater (PNG, 3006x3341, 5.36 MB)
• Central part of Caloris Basin including Apollodorus, Cunningham, and Atget craters (PNG, 3912x2744, 4.86 MB)
• 3x5 mosaic covering Eminescu and Xiao Zhao craters (PNG, 4872x2936, 7.36 MB)

I am currently trying to generate a 8000x8000 pixel full-disk mosaic of Mercury. Not sure how well it will go. Photoshop hates these REALLY large mosaics... That's why I cut up the northern hemisphere mosaics into several, smaller chunks. Here is what I have so far showing the southern polar region.

MESSENGER at Mercury

Not Io related, but still interesting. There is a new special issue in Science on results from the MESSENGER flyby of Mercury this past January. One of the most interesting results were from FIPS instrument which suggested the presence of water ions in Mercury's rarefied atmosphere.

In the last day, I've been playing around the with the images of Mercury taken by MESSENGER last January. Here are a selection:

• Inbound crescent mosaic (PNG, 4122x7563, 9.22 MB)
• Outbound global mosaic (PNG, 4586x5606, 11.5 MB)
• High-resolution mosaic of Mercury's equator (JPG, 15402x3664, 7.26 MB)
  

Back and Ready to Go

Sorry I haven't managed to post in a while, I guess I have been on pseudo-vacation for the month of June. Maybe not so much a vacation, I still worked on my Cassini projects, but I've been trying, during June, to not do work during the evenings and such when I would usually work on blog posts or other extra-curricular activities.

Now that I am back, I will try to get a few blog posts out the door over the next couple of days. First, there is a paper in press in Icarus on Io's plumes as seen by Galileo. Second, I want to talk about some of the updates with respect to the flagship missions. I might post the next update as a podcast as a way to make it easier to get this rather long update out to you faster. If any of you have some good audio capture and editing software you can recommend, let me know.

Good Luck to Phoenix

Just wanted to wish the Phoenix mission good luck and I hope their landing on the Martian northern plains goes off without a hitch.

Update: Phoenix landed! WOOT!

Io Volcano Observer at LPLC

Yesterday was the second and final day of the Lunar and Planetary Laboratory Conference in Tucson, Arizona. This is a small conference where researchers at the University of Arizona's Lunar and Planetary Laboratory can share their current work with their colleagues here. Many of the talk in the morning where dedicated to dynamics and space physics. I am amazed I stayed awake...

The afternoon sessions included Titan and other satellites in the Solar System. I gave a talk on Cassini images of Titan, particularly those of Titan's polar regions. Alfred McEwen, my advisor, gave a talk on the Io Volcano Observer (IVO, pronounced eye-voh), a mission concept in the Discovery & Scout Mission Capability Expansion (DSCME) program. I have reported on this concept study previously and on the abstract McEwen submitted to the conference.

The talk provided a few new details on this mission concept. The IVO team is aiming for a 2013 launch on an Atlas V with a back-up launch date in 2014. The spacecraft would be launched into a Venus-Earth gravity assist trajectory with an arrival at Jupiter in the 2019 timeframe. Once at Jupiter, IVO would flyby Io shortly before Jupiter orbit insertion, which would lower JOI delta-V and provide an insurance flyby in case any issues occur during the JOI burn. Once at Jupiter, IVO would be injected into a 200-day orbit around Jupiter with a 45 degree inclination.

Over the course of the 1-2 year mission, IVO would use 5-10 Io flybys to bring its orbital period down to 30 days. Each flyby would have a close-approach distance between 500 and 1000 km with 100 km flybys possible later in the mission. The high-inclination orbit would provide much needed polar coverage, which would be complementary to observations acquired by Galileo and the Europa orbiter mission. Flybys would occur at similar solar longitudes on Io to reduce the variables when comparing images between flybys.

The base payload for IVO includes a narrow-angle camera, a Thermal Imager, a Neutral Mass Spectrometer, and a Radiation detector for a total payload weight of 34 kg (44 kg with 30% margin). The nominal narrow angle camera planned would weigh 15 kg and would have a resolution of 10 microradians per pixel (the same as Galileo SSI). The goal for this camera is to observe Io's ever-changing surface by observing the same volcanic features during each flyby. The camera would also be used to measure Io's limb topography, which would help constrain Io's tidal bulge and thus provide constraints on Io's current tidal heating. The camera will make use of active pixel sensors, which have increased radiation hardness compared to traditional CCDs. These sensors will also allow the IVO team to acquire simultaneous multi-spectral imaging. This would make acquiring color mosaics a much simpler task as well as help support lava temperature measurements, since lava fountains can change on the order of seconds.

The Thermal Imager would be similar to the THEMIS instrument on the Mars Odyssey spacecraft. The IVO team is looking for an instrument that would acquire near- to mid-IR imaging at 3, 8, and 20 microns, to measure Io's heat flow. Assuming something akin to THEMIS is used (which is possible considering that the PI for THEMIS, Phil Christensen, is on the IVO team), the Thermal Imager would have a 4.6 deg. FOV with 250 microradians/pixel resolution (effectively 1 km/pixel per 4000 km).

Unfortunately, Alfred was nearing his alloted time by the time he finished discussing the Thermal Imager so he didn't spend time covering NMS, UVS, and the Radiation Detector. He pointed out that the "Powerpoint" mission might have a chance in Hell of flying, considering that Io is a world of fire and brimstone, there is a team member named Dante, and that Alfred will be 66 years old when IVO arrives at Jupiter (assuming that IVO flies on the back-up 2014 VEEGA trajectory).

Io Wallpaper

Wanderspace has posted two new Io wallpapers for your computer's desktop. These graphics are based on mosaics I put together a few months ago. One graphic uses a mosaic from I27 and covers Sobo Fluctus, a volcano between Chaac and Camaxtli on Io's anti-Jovian hemisphere (seen on the right side of the graphic at left). The other uses a mosaic of Tvashtar Patera from I32 that was then colorized by UMSF reader Ricardo Nunes.

Link: Volcanic Io Wallpapers [wanderingspace.net]

Awesome Images of Io transiting Jupiter

I haven't posted any links to images acquired by amateur astronomers lately, but trust me, Io is still there. Paul Haese acquired a couple of very nice views of Io transiting in front of Jupiter on May 10. The detail is good enough to easily make out the difference in brightness between the equatorial and polar regions. Christopher Go and Tomio Akutsu acquired several images of an Io transit on May 1.

Geologic Mapping of the Zal region of Io

There is a new article in press in the journal Icarus titled, "Geologic Mapping of the Zal region of Io," by Melissa Bunte, David Williams, and Ron Greeley. A summary of their results was presented in March at the Lunar and Planetary Sciences Conference and was reported on here. Now the full paper is available on the Icarus website (subscription required to view paper). This paper is based on imagery acquired during the I25 and I27 Galileo encounters with Io.

Like many of the regions mapped by the ASU previously, such as near Camaxtli Patera, Tohil Mons, Amirani, and Thor, the authors mapped 5 basic units in the region: mountains/plateaus, smooth/layered plains, patera floor material, flow material, and diffuse materials. The flow features in this area appear to be generated from a small patera lying near the western margin of South Zal Montes (they propose the name "Rustam Patera" for this volcano) or from a fissure that runs north from "Rustam" along the western margin of South Zal Montes and the eastern margin of North Zal Montes. The flows include bright flows (possibly of sulfurous composition) radiating out from "Rustam" and dark flows which flow east across part of Zal Patera from the northern part of the fissure. Additional flow features are also seen within Zal Patera, but these appear to be older in age based on their brighter appearance.

One interesting hypothesis made in this paper is that the various components of Zal Montes, which surround Zal Patera to the west, east, and south, were originally part of a single structure. This feature then broke-up due to strike-slip then extensional faulting, opening up Zal Patera. Similar plate tectonics-in-miniature is theorized for formation of Hi'iaka Patera. The paper goes on to describe the degradational processes that have occurred at the mountains in the region.

One feature I wished the paper expounded on further is a small volcano west of North Zal Montes, which they suggest the name "At'am Patera" for. What makes the volcano interesting is that it appears to be one of a very rare breed of explosive Ionian volcano. "At'am" erupted between late-June and mid-September 1997, producing a white, Sulfur dioxide-rich plume deposit and a dark-green pyroclastic deposit with a digitate margin. Some of both materials was deposited on North Zal Montes. The digitate appearance is due to the interaction between the pyroclastic flow and the arcuate margin of the western part of North Zal Montes. This morphology may provide clues on how these pyroclastic deposits are formed on Io. Oddly, for an Ionian eruption, no lava flows or thermal emission were observed at this volcano. Also, the central vent is among the smallest paterae found on Io. It is possible that the 1997 eruption could have been the result of an intrusive event, where magma ascends from a deeper chamber, but fails to reach the surface. However, volatiles and other materials, being more buoyant, do make it to the surface.

The paper does touch a bit on the plume seen at Zal last year by New Horizons. This plume is centered on Zal Patera (unlike the plume deposit seen by Galileo starting in Sept. 1997 which surrounds "At'am Patera"). Zal Patera is also the site of fresh surface changes, which include a new dark plume deposit and fresh dark lava flows.

Link: Geologic Mapping of the Zal region of Io [dx.doi.org] (subscription required to view paper)

More Details on Io Volcano Observer

As mentioned last month, NASA has commissioned several concept studies for Discovery-class mission that make use of the new Advanced Stirling Radioisotope Generator (ASRG) power source, a more efficient RTG that makes use of plutonium for power. One of these concepts is the Io Volcano Observer (IVO), a mission to study Io's volcanic activity.

The leader of the study, Alfred McEwen, submitted an abstract on the mission concept to the Lunar and Planetary Laboratory Conference (LPLC) to be held next week in Tucson. I will be there giving a talk on the state of Cassini Imaging, particularly imaging of Titan. The abstract makes public a few more details on the study. The IVO team envision a small spacecraft that orbits Jupiter in a highly inclined orbit, encountering Io at every periapse. The high-inclination orbit helps to reduce IVO's radiation exposure during periapse. Each orbit would last between 30 and 200 days, though shorter orbit would provide better distant monitoring.

Due to the low-cost of the mission, the payload has to be kept simplistic. The team has currently include a baseline payload of a narrow-angle camera (with some color imaging capability), a thermal imager (capable of mapping Io's thermal emission at 3, 8, and 15 microns, and at some silicate absorption bands), a Neutral Mass Spectrometer, and a Radiation detector. Other potential instruments include a Wide-angle camera, an ultra-stable oscillator (used for gravity measurements), a near-infrared spectrometer (I guess a toss-up between this and the thermal mapper), an ultraviolet spectrometer, and a magnetometer.

The team has also outlined the scientific goals for this mission which include understanding Io's volcanic processes, composition, heat flow, and environment.

Link: Mission Concept: Io Volcano Observer (IVO) [www.lpl.arizona.edu]