Observing Active Volcanic Eruptions Remotely

Well, I should finally get to writing up an article on a new paper out this month in the Journal of Volcanology and Geothermal Research on Io titled, "The thermal signature of volcanic eruptions on Io and Earth."  The authors for this paper are Ashley Davies (pictured at right with the nice manly man-beard), Laszlo Kestay (formerly Keszthelyi), and Andrew Harris.  Unfortunately, my delay in writing something up about this paper for the blog has meant that other bloggers have had time to scoop me.  Seriously, how was I to know that someone would write up a post about an Io paper before me?  This hasn't really happened before.  You can read Emily Lakdawalla's excellent discussion of Ionian volcanism and this paper's treatment of it on her Planetary Society Blog.  Davies emailed out copies of the paper to various Ionians and Emily, which as Emily states, is something other scientists should take note of.  However, I have to one up Emily just a little bit.  He told me about the paper personally last week :-p  So there!  Though maybe my parody ads for Io have had some effect after all?

Anyways, enough with my professional jealousies [humor using hyperbole alert!], let's get to the paper, shall we.  In the article, the authors investigate methods for identifying volcanic eruption styles using low spatial resolution, near-infrared data.  This is particularly useful for Io, as the only thermal observations available of Io have a resolution of at best a few kilometers per pixel but more typically have pixel sizes of tens to a couple hundred kilometers across.  The problem of spatial resolution is compounded in more recent data from the Keck telescope, where observations are typically acquired at only a few wavelengths between one and five microns.  Just last week, Franck Marchis showed off data on his blog showing Io at three such bandpasses from the Keck telescope at 2.1, 3.8, and 4.7 μm.  Davies and his two co-authors also examined satellite data of terrestrial volcanoes, which could then be compared with observational ground-truth.  This provided a way to test their method.

Davies and his co-authors determined that by examining the ratio between thermal output at two and five microns of a volcano in Galileo NIMS or ground-based data and tracking how that ratio changes with time, they could characterize the style of volcanic activity.  These different styles include open-channel or insulated lava flows (like those seen at Kilauea), lava fountains, lava lakes, lava domes, silicic lava flows, though the latter two, while important volcanic features on Earth, have not been identified on Io.  This works because the peak wavelength for the thermal emission of a lava flow or lava lake shifts to longer and longer wavelengths as it cools.  Basaltic lava that has only been cooling for one second has a peak thermal emission wavelength of two microns, while the thermal emission of lava that has been cooling for more than seven hours (or two hours on Earth) peaks around five microns.

So more vigorously active eruptions will have more fresh lava exposed than older, more quiescent eruptions, and thus will have greater 2 μm : 5 μm ratio.  More active volcanic eruptions include Tvashtar's back in 1999 and 2007, when lava curtains were observed at one of its constituent paterae.  The eruptions of Pillan in 1997 and Surt in 2001 also fit this model, with Surt having a 2 μm : 5 μm ratio of 2.  Also typical of these outburst eruptions is their short duration.  Over time, the 2 μm : 5 μm ratio at these eruptions decreases as the fire fountaining ceases and the thermal emission becomes dominated by large areas of cooling lava.  The volcano I profiled on Sunday maybe in this stage.  High 2 μm : 5 μm ratios can also be found at vigorous lava lakes such as Pele, where smaller lava fountains balance out the emission from the cooled lava crust that covers most of the lake.  Similar activity such as this can be seen at a much smaller scale at the Erta'Ale lava lake in Ethiopia, shown at night in the image at the top of this post and at above right.

Quiescent eruptions, such as those with insulated lava flows (where lava flows from the source to the flow front via lava tubes) or episodically overturning lava lakes, have much lower 2 μm : 5 μm ratios as their thermal emission is dominated by cooling lava with only small areas of recently emplaced lava.  Such activity can be seen at Io's large, persistent flow fields like Amirani, Zamama, and Prometheus or the multitude of volcano depressions like Altjirra Patera.

Combined with analysis of terrestrial data, the authors found that low 2 μm : 5 μm ratios typified volcanic eruptions with "older surfaces, increasing insulation [more lava flowing through lava tubes to breakouts], and quiescent emplacement". Eruptions with a high 2 μm : 5 μm ratio (> 0.5) suggest the presence of "younger [flow] surfaces, decreasing insulation, and more violent emplacement.  Eruptions with greater overall radiant fluxes have increasing effusion rates and lava with lower viscosity and less silicon dioxide (less silicic).  This ratio must also be combined with repeat observations for temporal coverage.  This allows for the disambiguation between vigorously active lava lakes, open-channel lava flows, and lava fountains, for example, which are active for different timescales.

Finally, the authors provide suggests for applying their method to data from future spacecraft to the Jupiter system and Io.  For example, they suggest that temporal resolution trumps spatial and spectral resolution for monitoring the progress of a volcanic eruption, particularly for understanding processes at different temperature regimes (though high spatial resolution observations are great for spotting small scale features like skylights over active lava tubes).  For example, observations with a temporal resolution on the order of 1-10 seconds, or less, are useful for obtaining temperatures from vigorously active lava bodies such as lava fountains.  Observations with scales on the order of a few minutes to hours are useful for monitoring changes in the flow rate at open-channel lava flows, while lava lakes and insulated lava flows can be observed on a daily to weekly basis.  They also suggest that thermal imagers on future spacecraft use a few, select wavelength windows such as 2 and 5 microns, and others in the thermal infrared between 8 and 12 microns for monitoring different volcanic eruption styles.

Link: The thermal signature of volcanic eruptions on Io and Earth [dx.doi.org]