For the last two weeks, we've been looking at the Io-related abstracts for presentations at this year's Lunar and Planetary Science Conference. Today we will look at a presentation titled "The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io" by Ashley Davies, Laszlo Keszthelyi, and Andrew Harris. This paper will be presented as a poster at the Igneous and Volcanic Processes session on Thursday, March 4.
In this poster, Davies et al. will present a method for determining the eruption style of a volcano based on the near-infrared spectrum of the volcano taken from space, either a satellite observing the Earth or spacecraft or ground-based observations of Io. This is done by examining the shape of near-infrared spectra taken of a volcano between two and five microns, the slope of this spectra between these two wavelengths, and how the shape and slope of the spectra change with time. This methodology allowed the research to differentiate between insulated flows and lava lakes on Earth. Extended to Io, the authors were able to predict based on spectra with a low-spatial resolution the eruption styles at a number of volcanoes such as Pele and Prometheus that was also confirmed, this time using higher-resolution observations later in the Galileo mission.
In the case of active lava lakes like Pele, they found that the ratio between the power output at 2 microns and 5 microns to be near unity. As active lava lakes this makes sense as there is a fixed amount of terrain (remember, the lava lakes are bound by the patera margin) covered with cooling lava that is renewed with fresh lava on a regular basis. So the power output at the two wavelengths is about the same. For insulated flows, there are generally small areas where fluid lava is exposed at the surface both at the source vent and at the active flow lobe. The vast majority of the insulated flow is covered by cooling lava whose thermal emission is concentrated at longer wavelengths like 5 microns, compared to the hotter vent and flow front, whose thermal emission will be concentrated at shorter wavelengths, like 2 microns. So, for insulated flows like Prometheus and Amirani, the power output at 5 microns is greater than at 2 microns. The ratio between 2 microns and 5 microns decreases as eruption intensity decreases. Interestingly, many paterae have a similar signature, but it is not known if this due to these paterae, like Tupan, Malik, and Hi'iaka, being more quiescent lava lakes than Pele or due to insulated flows on the floor of these volcanoes.
Eruption style can also be determined by monitoring how the spectra of these features can change with time. The authors in the abstract highlight research into the Pillan eruption. In that case, the first near-IR observations of Pillan revealed a 2μm/5μm ratio near unity, suggesting a rigorous eruption with lava fountaining and open channel flows. As the eruption progressed, its total power decreased as did the 2μm/5μm, show the shutting down of the fountaining and an increased area of the cooling flow. A similar eruption progress was seen at an outburst in 1990, though in that case, along with other outburst eruptions like Surt in 2001, the 2μm/5μm ratio was actually greater than 1 initially, suggesting an initial phase of very vigorous lava fountaining with only a small area of emplaced lava at that point in the eruption.
Finally, the authors suggest ways to apply this methodology on a future mission to Io. They suggest including filters at two and five microns in a thermal mapper in order to directly apply their method. They also suggest two at longer wavelengths such as 8 and 12 microns to further constraining the amount of cooler lava flows that are older than those that would detected at 5 microns. They also suggest that observation often enough to detect changes in eruption style or thermal emission that would further constrain that style, like a vigorous outburst eruption that starts out with high thermal emission at short wavelengths, then cools down as the eruption shuts down or changes to an insulated flow style, like Thor. The authors do point out that this type of analysis can be done at low-spatial resolutions, so regular observations would not need to be conducted just during Io flybys.
Link: The Thermal Signature of Volcanic Eruptions on Io and Earth - Implications for a Future Mission to Io [www.lpi.usra.edu]