DPS Meeting so far

Despite still being in Tucson, for the last few days I have been remotely attending the DPS meeting in Ithaca, New York. The DPS meeting (otherwise known as the Meeting of the Division of Planetary Science of the American Astronomical Society... *phew*) is one of the big three American planetary science meetings, which also includes LPSC (generally in March), the AGU Spring Meeting (generally in May), and the AGU Fall Meeting (generally in December). I attended the DPS meeting last year, and even gave a talk on the Cassini camera's observations of Titan's trailing hemisphere.

So this year, I am watching the DPS meeting online via Cornell University's Live Webcasting system. No Io-related results in the talks presented so far, but there were quite a few sessions I was interested in. First and foremost for me were the Titan sessions. In fact, it seems like Titan has replaced Mars in importance at DPS with three Titan oral sessions versus just one Mars surface session.

Perhaps the talks that had most relevence to Io in these Titan sessions were the "Subsurface" talks given yesterday morning. Many of these talks related to surface processes generated by internal activity, such as topography, mountains, and cryovolcanism. The cryovolcanism talks, given by Rosaly Lopes, Bob Nelson, and Ashley Davies, often sounded like Io talks from a few years ago. Lopes discussed RADAR results at a feature known as Hotei Arcus, seen above in ISS data from July's T45 flyby. The RADAR team inteprets some of the features in the region as cryolava flows, composed of water and ammonia. The flows appear to be part of a compound flow field, similar to Amirani and Prometheus on Io. The main body of the flow field appears bright in RADAR, indicating a rough surface. On top of this flow field are several large flow lobes that are darker, or smoother, than the rest of the flow field.

Bob Nelson gave a talk on the photometry of Hotei Arcus and a region in western Xanadu. These two regions correspond to areas where RADAR has seen evidence for compound flow fields. Based on a comparison of several observations acquired by VIMS, Nelson suggests that brightness changes have been observed at these regions, possibly due to fumerolic activity at these sites. With the addition of the RADAR results, Nelson's finding are becoming a lot more convincing. Finally, Ashley Davies presenting a cooling model for cryolava flows on the surface of Titan, similar to cooling models he presented for Io in recent years.

The Galilean Satellites session takes place Wednesday morning starting at 8:30 am EDT (5:30 am Tucson time). You can catch it live or archived following the talk by following this link. The only Io surface-related talk to be given tomorrow is by Julie Rathbun. She will talk about ground-based observations of Loki at multiple wavelengths. I will present a summary on the blog tomorrow afternoon. Also, tonight, the NASA Night session will be presented online at 7:30 pm EDT (4:30 pm Tucson time). These are always pretty interesting. I am not sure if the Outer Planets flagship mission will be discussed, but you can presume the recent headaches over the Mars Science Laboratory will.

Link: DPS Meeting main link [dps08.astro.cornell.edu]

Lava lakes on Io: New perspectives from modeling

Tracy Gregg and Rosaly Lopes have a new paper in the March issue of Icarus titled, "Lava lakes on Io: New perspectives from modeling." The paper provides a possible model for the volcanism observed at Loki Patera (shown at left from images taken by Galileo's SSI and NIMS instruments). Previous models by Davies et al. and Rathbun et al. suggested that the episodic activity was the result of lava flows spreading out from a fissure and an overturning lava lake, respectively. The lava lake hypothesis has gained particular currency among the Iophile community over the last few years, particularly its ability to explain the distribution of thermal sources as seen by NIMS.

Gregg and Lopes, in their paper, point out a few problems with both models. In the first, wherein lava flows spread out from a fissure, the lack of overflowing lava despite repeated eruption episodes over the last 20 years is a concern. In the second, the difference in scale between terrestrial lava lakes (most are on the order of 100 meters across) and Loki (approximately 200 km across) is an issue. The authors point out several other issues, including the scale of magma reservoir needed and the thermally patchy nature of the patera floor.

The authors instead propose the following model for Loki Patera: 1) Magma is fed into a thin (10s-100s meters thick) chamber from a tidally heated source deep beneath Loki; 2) When this chamber is filled, magma travels up a conduit a few km long (assuming relatively low surface porosity) and into a fissure along the southwestern margin of the patera; 3) The lava then flows out from the fissure into pre-existing lava tubes or covered-over lava channels, travelling out from the fissure across the rest of the patera; 4) The lava ends up being intruded into the country rock of the patera floor (rather than flowing along the surface as lava flows) or fills lava ponds along the patera margin; 5) The eruption episode ends when the magma chamber is emptied and the lava in the fissure trench drains back down. The authors suggest that the eruption style at Loki is roughly analogous to eruptions along the East Pacific Rise.

This model is consistent with observations obtained of Loki. The thermal wave seen by ground-based observers and NIMS would be produced by heat conducted up from the lava tubes to the patera floor. The hotspot along the southeastern margin of the patera represents the location of the fissure. The other hotspots along the patera margin and along the margin of the "island" on the floor of Loki likely represent lava ponds where lava has collected at a topographic obstacle. The corresponding darkening wave at visible wavelengths seen along the patera floor by Voyager 1 and 2, rather than being the result of lava flowing across the surface or new crust in an overturning lava lake, is the result of volatiles being driven off a surface that is heated from below by lava flowing through lava tubes.

This is certainly a very interesting model for Loki as it explains the lower temperature measured by NIMS and PPR compared to eruptions at other volcanoes on Io and the lack of lava overflowing the patera margin. I think the lava lake model works best for smaller volcanoes like Pele and the southeastern portion of Gish Bar Patera, where the difference in size between terrestrial and Ionian lava lakes wouldn't be so great. It would be interesting to see how well this model might work for other inter-patera flows, like the main floor of Gish Bar or Emakong Patera.

Link: Lava lakes on Io: New perspectives from modeling [dx.doi.org]