Last week I talked about a paper in press in the journal Icarus on the Formation of Io's Mountains. More specifically, the paper described the sorts of stresses that could result in the formation of thrust faults that are the leading theory for how individual chunks of Io's crust are pushed upward to form some of the tallest mountains in the solar system. This week I thought I would talk about how the other major landform type on Io is formed: paterae.
Much of the following discussion is developed from two sources: Keszthelyi et al. 2004, "A post-Galileo view of Io's interior," and Ashley Davies's 2007 book, "Volcanism on Io: A Comparison with Earth."
The term patera is the name given by the International Astronomical Union for "irregular craters, or complex ones with scalloped edges." Patera (plural=paterae) comes from the latin for "saucer." On most planetary bodies, the term patera is generally used for volcanic pits, like Garland Patera on Venus, Leviathan Patera on Triton, or Uranius Patera on Mars. On Io, this is also the convention used. Radebaugh et al. 2001 counted and measured more than 400 paterae across Io's surface from Voyager and Galileo imagery and found that they average 41 km in diameter, larger than similar volcanic depressions on Earth, Venus, and Mars. While these features bare a resemblance to terrestrial calderas, there are issues with their formation being similar. Calderas on Earth (and other planets like Mars) are formed when freshly-emptied magma chamber collapses after an eruption and the ground above fills the void, leaving a depression on the surface. In the case of mafic-rich volcanoes, like Kilauea, a caldera forms from subsidence as a result of flows mostly outside the volcanic pit. For Io, Keszthelyi et al. point out that there is no evidence for major voluminous eruptions with flows outside of patera with enough lava to explain the scale of many of the patera on Io. Plus, the composition of magma on Io is not likely to be silicic like those found in eruptions that form large, Yellowstone-type terrestrial calderas. Tectonic extension may result in the formation of some of Io's patera, like Hi'iaka and Monan, but this is not thought to be the dominant process.
Keszthelyi et al. 2004 put forward another hypothesis for the formation of patera. This model is the result of the authors' examination of the post-Galileo view of Io's lithosphere and high-resolution images of paterae from Galileo and Voyager. In the post-Galileo view of Io's lithosphere, the upper layers are cold, composed of interbedded sulfur and sulfur dioxide ice and frost mixed with cooled silicate lava flows and pyroclastic deposits. The temperature increases dramatically as you near the base of the lithosphere, but for much of its thickness, it is fairly close to the surface temperature. This is the result of the heat-pipe advection theory that Io's internal heat is release almost entirely by volcanic activity, first proposed by O'Reilly and Davies 1981 and more extensively discussed in my post about Kirchoff et al. 2009. So there is very little convection of heat from the asthenosphere in the lithosphere. Anyways, what this means is that a crust with interbedded sulfur and silicate rich materials is actually remarkably stable and can form topography at least 10 km in height. The lower you get in the lithosphere, the more volatiles are driven out, and the more silicate-rich it becomes. This allows magma to ascend through this volatile-poor region. When it reaches the volatile-rich upper lithosphere, the magma eventually can find it difficult to ascend further as it becomes neutrally buoyant. The magma can then stall out and form a sill, an intrusive magma body that forms parallel to the pre-existing crustal layers.
Looking at high-resolution images of Io, particularly between Chaac and Camaxtli Paterae on the satellite's anti-jovian hemisphere, Keszthelyi et al. 2004 came up with a model for how these sills can develop into paterae. As magma continues to be injected into the forming sill, the lithosphere nearby becomes heated, particularly the volatiles in the immediate vicinity. Over a period of hundreds to a few tens of thousands of years, these volatiles (like sulfur and sulfur dioxide) melt and rise to the surface, forming sulfurous flows (along with some silicate magma that may also reach the surface), as well as move laterally through the sub-surface away from the growing sill. This maybe what is going on now at Sobo Fluctus. This movement of volatiles causes a partial collapse above the sill, forming a shallow patera similar to Grannos Patera. As the sill grows and more sulfur melts, the patera gets deeper, down to level of the partially molten S/SO2 near the sill. This causes the kind of sulfur flows you see at Balder Patera or Ababinili Patera. More basaltic magma reach the patera floor as well, forming silicate flows on the floor of the patera, like at Camaxtli Patera.
Eventually, more and more ground is "eaten" away above the sill so that it becomes "unroofed", with the top of the sill becoming the new floor of the patera. So now you see things like silicate lava lakes, for example, like you see at Loki Patera or Tupan Patera (see top of post). "Islands" in these lava lake paterae may be the result of sufficient cooling on the roof of the sill during its formation to create a thick crust near its middle (or at least the part right over the conduit at the bottom of the sill). As paterae remain active, they not only become deeper, until they reach the level of the underlying sill, but they also grow laterally as the silicate flows on the floor and the growth of the sill undermine the sulfur-rich walls of the patera. Keszthelyi et al. suggests that this would explain steep wall as this would remain an active process as long as the volcano itself remains active. We've even seen some paterae degrade mountains in this process, like at Gish Bar Patera, which is being "eaten" by Gish Bar Patera and Estan Patera.
The patera formation model presented in Keszthelyi et al. 2004 seems to explain not only the general morphology of paterae on Io, but also variations in this morphology from volcano to volcano, which seem to be the result of magma supply and age. The model also explains why no new paterae have formed since the beginning of spacecraft observations in 1979 as their formation is a much more gradual process than the 30-year time period of observations.
Link: A post-Galileo view of Io's interior [dx.doi.org]