Io Volcano of the Week: Isum

I apologize for my absence the last couple of weeks.  When you write a blog in your spare time, it exists at the pleasure of my other obligations, my health, and other demands on my spare time.  So when a busy period with work, a nasty cold, and Halo: Reach all hit in the same week, well, unfortunately this blog takes a bit of a back seat.  This week I am feeling much better, work is a bit quieter (wait, there is a Titan flyby on Friday, lalalalalalalalala, I can't hear you), and I have grown wary of Halo: Reach, so I can come back to my weekly series on Io's volcanoes.  Today, we are discussing Isum Patera, one of Io's more active volcanic centers and the likely source of the largest lava flow field on Io, Lei-Kung Fluctus.

First off, let's get the basics out of the way.  There really isn't a polite way to describe the shape of the volcanic depression that is Isum Patera; it looks like a sperm cell.  Isum is located at 29.82° North Latitude, 208.46° West Longitude. The head of the Isum "spermatozoa" is 62 kilometers (39 miles) in length and 43 kilometers (27 miles) in width.  The southern end of the patera appears to have a greater depth than the rest of the volcano, which can often be indicative of multiple collapses (if formed like terrestrial calderas) or sills embedded in different layers, however the low resolution of our best images of the region (1.3 kilometers or 0.8 miles per pixel), poor phase coverage, and the complex albedo patterns in the area precludes a clear analysis of the topography in this region.  A small mountain may lie along the eastern margin of Isum Patera, though this is difficult to confirm from available imagery.  The "tail" of Isum extends to the northeast from the northern end of Isum Patera.  The tail measures 184 kilometers (114 miles) long and 11 kilometers (7 miles) wide.  The floor of Isum Patera is generally dark green in color, similar to Chaac Patera, suggestive of chemically-altered basaltic lava, though a few spots along the tail of Isum Patera are much darker, more indicative of recent activity.

Isum lies at the center of a multi-colored region along the northern margin of Colchis Regio on Io's anti-Jupiter and trailing hemispheres.  The background color of the area is reddish-brown, typical for Io's plains at this latitude, but it might be enhanced by sulfur deposits from activity at Isum.  Green deposits dominate the terrain to the south and east of the head of Isum Patera, as well as on either side of its tail.  The margins of these deposits are digitate, or finger-like, which is more suggestive of a pyroclasic deposit that a lava flow field, which typically have lobate margins (see Lei-Kung Fluctus to the north of Isum in the image at left, for example).  The most intense of these dark pyroclastic deposits surround the tail of Isum Patera.  Their lack of chemical alteration that results from the interaction between sulfur and the iron in the pyroclastic material suggests they were laid down most recently.  More likely though, they are being covered by fresh material on a regular basis, as their dark albedo has been a constant since the Voyager encounters in 1979.

Volcanic activity has been detected at Isum Patera over a period of 31 years, since it was first observed in 1979 to as recently an adaptive optics observations at Keck Telescope on June 28, 2010.  The first detection of a thermal hotspot at Isum, indicative of on-going volcanic activity, came from the IRIS (Infrared Radiometer, Interferometer, and Spectrometer) instrument on Voyager 1.  It was detected again as a group of hotspots by Galileo's SSI camera when Io was in the shadow of Jupiter in June 1996, June 1997, and November 1997, every time the geometry was appropriate during one of the spacecraft's eclipse observations.  In each case two or three hotspots were detected: at the head of Isum Patera, in the tail, and in the southern portion of Lei-Kung Fluctus.  Galileo's Near-Infrared Mapping Spectrometer (NIMS) also detected a thermal hotspot at Isum Patera during every viewing opportunity during the Galileo Nominal Mission, between September 1996 and September 1997.  It was also seen at high resolution by NIMS in August 2001 (show at right) during a flyby of Io.  NIMS found a line of thermal emission within the middle portions of Isum's tail section.  The intensity of the emission was so great that the NIMS detectors saturated at most of the wavelengths the instrument looked at except the shortest (1.313 and 1.593 μm).  This suggests that both high-temperature volcanism and that large percentages of each pixel that covered the tail region were hot at the time of the observation.

Taken all together, what does the morphology of Isum Patera and its surrounding terrain and its history of persistent, high temperature volcanism with multiple hotspots tell us about the style of volcanic activity going on at Isum?  The distribution of dark pyroclastic material external to Isum and bright and dark patera within the tail region are most similar to Pele, a persistently and vigorously active lava lake.  The thermal emission history is also roughly similar.  In this case, the tail of Isum is a large lava lake whose crust is continuously overturned due to fresh material being brought into the lake from below.  This overturning, which can involve short lasting lava fountains, also permit the release of sulfurous gases and pyroclastic material.  This latter material can then be laid down as dark deposits on either side of Isum's tail.  The tail of Isum Patera may be a fissure that has opened up in Io's crust, allow magma to reach the surface and resupply the lava lake at Isum.  This magma could have also formed a sill at one end of the fissure, which was then later unroofed to form Isum Patera proper.  Another patera may also be located at the northeast end of the fissure, but it isn't clear.

However, some of the evidence can be deceiving.  The global scale images from Galileo that are available of this volcano reveal a curved dark line connecting the northern end of Isum Patera to the southern end of the massive Lei-Kung Fluctus, a large compound lava flow field more than 125,000 sq. km (48,000 sq. mi.) in size.  We know from SSI and PPR measurements from the Galileo spacecraft that at least the southern end of it was still active as of 2002.  A similar relationship between an active patera and a nearby active lava flow field, with a curved dark line between the two, has been noted at other Ionian volcanoes, most importantly at Amirani.  This suggests that Isum Patera may be the source of the largest lava flow field on Io.  In this case, the dark curved line is a lava tube that channels lava from its source in the tail of Isum Patera north to active flow lobes across Lei-Kung Fluctus.  I should point out that that given the huge extent of Lei-Kung, multiple sources can't be ruled out, and given the pattern of thermal emission seen by Galileo's Photopolarimeter-Radiometer (PPR), that's probably likely.

Today, we have looked at one of the most persistently active volcanoes on Io, Isum Patera.  Isum has a rather unique shape for an Ionian volcano.  Regardless, it is the site of rigorous but consistent activity that is suggestive of a large lava lake within the tail end of Isum Patera.  That doesn't preclude the possibility that Isum is also the source (or one of the sources anyway) of Lei-Kung Fluctus, which may act as a kind of release valve for the lava lake, where the overflow from the lake is deposited.

This article is making up for the one I intended to write last Monday so I still need to catch up.  Later this week we'll look at Maasaw Patera, a small volcano seen up close by Voyager 1.

References:
Radebaugh, J. (2005). "Formation and Evolution of Paterae on Jupiter's Moon Io". Ph.D. Dissertation.  University of Arizona.
Lopes-Gautier, R.; et al. (1999). "Active Volcanism on Io: Global Distribution and Variations in Activity". Icarus 140: 243–264.