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.
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.
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.
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.