A Tale of Two Hot Spots: Charting Thermal Output Variations at Prometheus and Amirani from Galileo NIMS Data

Galileo Near Infrared Mapping Spectrometer (NIMS) data show that the active ionian volcanoes Prometheus and Amirani have significant thermal emission in excess of non-volcanic background emission in every geometrically appropriate NIMS observation. The 5 μm brightness of these volcanoes shows considerable variation from orbit to orbit. Like the flank eruptions on Kilauea, Hawai'i, which began in 1983, these ionian volcanoes exhibit periods of elevated activity. A study of the 5 μm thermal emission in all low-spatial resolution NIMS observations (both night and day-time) from June 1996 (Orbit G1) to May 2001 (Orbit C30) shows that the Prometheus thermal output (uncorrected for emission angle, e) ranges from 4 to 21 GW/μm, and Amirani shows greater variations, from 4 to 31 GW/μm. Correcting all thermal outputs for observations where e< 60° yields an average Prometheus 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 Davies et al., 2000, Icarus, 148, 212-225) and Amirani showed its greatest thermal emission during May 1997 (orbit G8), nearly 100 GW/μm, nearly five times that seen during orbit G1. Prometheus and Amirani spectra obtained at night show that the overall spectral shape of the thermal emission from 2 to 5 μm does not greatly change. The style of eruption is not resulting in disproportionately large areas at very high temperatures in relation to cooler crustal areas, such as seen at Pillan in 1997 and at Pele, indicating that the style of eruption is not changing, just the areal extent of activity. Scaling magma eruption rates derived from G1 NIMS data yields maximum and average volumetric eruption rates of 337 and 154 m³ s^-1 for Amirani, and 128 and 52 m³ s^-1 for Prometheus. The style and behavior of eruptions at Prometheus and Amirani are apparently very like current Kilauea flank activity, a more-or-less continuous eruption with emplacement of mostly insulated surface flows, punctuated with periods of increased activity. Prometheus and Amirani are on a much greater areal scale, however. These eruptions are generally effusive and non-explosive: that there is no large-scale fire-fountaining observed indicates that the magma is relatively low in volatiles. Magma, fed from a deep source, may be stored in near-surface magma chambers where degassing takes place, before erupting at the surface. Alternatively, the magma may already be low in dissolved volatiles, or is of such low viscosity that explosive release of gas is not taking place. This work was performed at JPL-Caltech under contract to NASA. AGD is supported by NASA PG&G grant 344-30-23-09.