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Icarus

Volume 221, Issue 1, September–October 2012, Pages 466-470
Icarus

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Charting thermal emission variability at Pele, Janus Patera and Kanehekili Fluctus with the Galileo NIMS Io Thermal Emission Database (NITED)

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Abstract

Using the NIMS Io Thermal Emission Database (NITED), a collection of over 1000 measurements of radiant flux from Io’s volcanoes (Davies, A.G. et al. [2012]. Geophys. Res. Lett. 39, L01201. doi:10.1029/2011GL049999), we have examined the variability of thermal emission from three of Io’s volcanoes: Pele, Janus Patera and Kanehekili Fluctus. At Pele, the 5-μm thermal emission as derived from 28 night time observations is remarkably steady at 37 ± 10 GW μm−1, re-affirming previous analyses that suggested that Pele an active, rapidly overturning silicate lava lake. Janus Patera also exhibits relatively steady 5-μm thermal emission (≈20 ± 3 GW μm−1) in the four observations where Janus is resolved from nearby Kanehekili Fluctus. Janus Patera might contain a Pele-like lava lake with an effusion rate (QF) of ≈40–70 m3 s−1. It should be a prime target for a future mission to Io in order to obtain data to determine lava eruption temperature. Kanehekili Fluctus has a thermal emission spectrum that is indicative of the emplacement of lava flows with insulated crusts. Effusion rate at Kanehekili Fluctus dropped by an order of magnitude from ≈95 m3 s−1 in mid-1997 to ≈4 m3 s−1 in late 2001.

Introduction

The volcanoes of Io offer insight into the geophysical nature of this jovian satellite. As such, they are important observational targets for missions to the Jupiter system and also for ground-based telescopes (see summary in Davies (2007); see also de Pater et al., 2004, Lopes-Gautier et al., 1997, Marchis et al., 2005, McEwen et al., 1997, Veeder et al., 1994).

We are examining the variability of volcanic heat flow at local, regional and global scales using primarily Galileo Near Infrared Mapping Spectrometer (NIMS) data, adding to previous time-series analyses of Pillan and Pele (Davies et al., 2001), and the full NIMS datasets for Prometheus (Davies et al., 2006), Tupan Patera, Culann and Zamama (Davies and Ennis, 2011) and Loki Patera (Davies et al., 2012, Rathbun and Spencer, 2006). By examining thermal emission variability we better understand the role of volcanism in removing heat from the Io’s interior and the surface expressions of volcanic activity. In order to identify individual eruption episodes and style of activity we have quantified thermal emission at 5 μm (4.7 μm after 11 October 1997) for every hot spot observed by Galileo NIMS. Using the NIMS Io Thermal Emission Database (NITED) (Davies et al., 2012), we now describe the variability of thermal emission from three hot spots on Io that were observed multiple times: Pele and Janus Patera (possible silicate lava lakes) and Kanehekili Fluctus (possible silicate lava flows).

Section snippets

NIMS observations of Io

NIMS was an instrument well-suited to observing thermal emission from ongoing or recent volcanic activity (Davies, 2007, Davies et al., 2010). Volcanic hot spots were counted by Lopes et al. (2004). The acquisition and processing of NIMS data of volcanic thermal emission, and descriptions of “tube” and “cube” products, are described in detail in Davies (2007) and references therein, but can be summarized as follows. NIMS observations consisted of data at 8–408 wavelengths between 0.7 and 5.3 μm

NIMS Io Thermal Emission Database

We have used all appropriate NIMS observations of Pele, Janus Patera and Kanehekili Fluctus to examine style and variability of volcanic activity. Where necessary, cube products were used to obtain accurate locations of thermal sources. We calculated radiant flux at ≈5 μm wherever possible, and at 4.6967 μm for observations obtained on and after 11 October 1999. Data were corrected for incident sunlight, where necessary, and a cosine correction was used as a correction for emission angle

Pele

Pele (255.7°W, 18.4°S) is a feature of particular interest because it is the only volcano on Io that, in every usable NIMS observation, displays a thermal emission spectrum similar to those of terrestrial active, overturning lava lakes (Davies et al., 2008, Davies et al., 2010, Davies and Ennis, 2011). Pele has been identified as a hot spot in every appropriate observation by Voyager (Carr, 1986), Galileo NIMS (Davies et al., 2001) and SSI (e.g., Keszthelyi et al., 2001, McEwen et al., 1998,

Kanehekili Fluctus and Janus Patera

NITED has many examples of pairs of hot spots that are only rarely resolved due to the low spatial resolution of the NIMS data (although these observations still allow regional volcanic heat flow to be quantified). One example of such a hot-spot pair is Kanehekili Fluctus and Janus Patera. Kanehekili Fluctus (33.6°W, 18°S) is a lava flow field covering ≈34,500 km2 (Williams et al., 2011) with the darkest and presumably most recently-emplaced areas of the fluctus totaling ≈16,500 km2. Janus Patera

Data analysis

Most data were of the sunlit side of Io, but a few observations were obtained at night (Table 1). These spectra, unadulterated by incident sunlight, are fitted with a two-temperature, two-area thermal emission model (Davies et al., 1997) to calculate the total radiant thermal emission (Qrad) from the active or recently active areas of the volcano under scrutiny (e.g., Fig. 2).

Radiant flux as a function of wavelength is found by fitting the corrected NIMS spectra with the functionIλ=c1λ5A1(ec2/λT

Pele

NITED contains details of 55 observations of Pele. NITED data confirm the steadiness of thermal emission and, by inference, the steadiness of the ongoing volcanic activity. Activity at other volcanoes examined using the full NIMS database show more variability. This variability is very pronounced at Loki Patera (Davies et al., 2012, Rathbun and Spencer, 2006), but is also seen at Prometheus (a persistent volcano seen in every appropriate NIMS observation) and others, including Pillan (one of a

Discussion and conclusions

As part of a wide-ranging effort to quantify the magnitude and variability of all of Io’s volcanoes, we have measured the 5-μm (4.7 μm after 11 October 1999) thermal emission for Pele, Janus Patera and Kanehekili Fluctus and estimated effusion rates from available night time spectra. Although the latter two hot spots are only rarely resolved, the shapes of the thermal emission spectra yield important clues as to the nature of volcanic processes at these two locations. The inference from the

Acknowledgments

This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and is supported by the NASA Outer Planets Research Program. We thank Oleg Abramov and David Williams for their reviews. © 2012 Caltech. All rights reserved.

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