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]