The Pulse of the Volcano: Discovery of Episodic Activity at Prometheus

Introduction: The temporal behaviour of thermal output from a volcano yields valuable clues to the processes taking place at and beneath the surface. Galileo Near Infrared Mapping Spectrometer (NIMS) data show that the ionian volcanoes Prometheus and Amirani have significant thermal emission in excess of nonvolcanic background emission in every geometrically appropriate NIMS observation. The 5 μm brightness of these volcanoes shows considerable variation from orbit to orbit. Prometheus in particular exhibits an episodicity that yields valuable constraints to the mechanisms of magma supply and eruption. This work is part of an on-going study to chart and quantify the thermal emission of Io’s volcanoes, determine mass eruption rates, and note eruption style. Prometheus and Amirani: background. Located at 154 W, 2 S the flows at Prometheus cover over 6000 km [1] and were emplaced in the years between Voyager (1979) and the first Galileo encounters in 1996. Flow morphology and thermal emission is consistent with insulated silicate lava flows, with resurfacing rates of 535 m/s [2, 3] and volumetric eruption rates estimated at ~35 m/s [3]. The flows are estimated to be ~1 m thick [3]. Prometheus is a volcanic plume source, active at least since the Voyager epoch. The surface flows appear to have overflowed from Prometheus Patera. Amirani is located at ~116 W, 23 N, on the anti-jovian hemisphere of Io (like Prometheus). The Amirani flow field is the longest active lava flow in the Solar System [2]. Insulated silicate lava flows are emplaced at areal coverage rates of 50-80 m/s. Mass eruption rates from G1 and I24-I27 Galileo data are estimated to be up to 100 m/s [2, 3]. At both Prometheus and Amirani, NIMS 4.5-5 μm data show elevated thermal emission along the entire flow [4]. The bulk of the thermal emission seen by NIMS and SSI comes from small fractions of each pixel. Thermal output variation: episodic Prometheus! Like the current eruption of Kilauea, Hawai’i [5], these two ionian volcanoes exhibit periods of elevated activity (Prometheus thermal output is shown in Figure 1). A study of the 5 μm thermal emission in all NIMS observations (both night and day-time) from June 1996 (Orbit G1) to May 2001 (Orbit C30) shows a Prometheus thermal output (corrected for emission angle, e, for cases where e < 60o) yields an average output of 13.3 GW/μm (standard deviation of 7.3 GW/μm) and a larger average Amirani thermal output of 44 GW/μm (standard deviation of 26 GW/μm). Prometheus showed its greatest e-corrected thermal emission during November 1997 (33 GW/μm), more than four times that seen in June 1996 (orbit G1; see [3]) and Amirani showed its greatest thermal emission (up to May 1998) during May 1997 (orbit G8), nearly 100 GW/μm, nearly five times that seen during orbit G1. Including observations where e > 60o, Amirani’s maximum observed 5 μm output is 291 GW/μm (May 1998), and the Prometheus maximum is 54 GW/μm (May 1998).