Io: Heat flow from small volcanic features
Introduction
This is the fourth paper in our series to determine the magnitude and distribution of volcanic global heat flow on Io from its numerous thermal sources. We have previously analyzed large dark volcanic flow fields, dark paterae, and additional thermal sources (Veeder et al., 2009, Veeder et al., 2011, Veeder et al., 2012). We use a value of 1.05 ± 0.21 × 1014 W for Io’s thermal output. Veeder et al. (2012) describe how this number was derived from infrared observations with the IRTF (Veeder et al., 1994). This work has quantitatively accounted for more than half of Io’s total global heat flow and confirmed that the largest component originates from dark paterae. In addition, the longitudinal distribution of heat flow is different for volcanic flow fields and dark paterae as well as offset from any existing model of interior heating (Veeder et al., 1994, Veeder et al., 2012).
Strong hot spots are well correlated with dark paterae as well as some large dark flow fields (e.g., Radebaugh, 2005, Williams et al., 2011b, Veeder et al., 2012, Hamilton et al., 2013). All large dark volcanic features contain hot spots. Only a few bright features contain hot spots. Many small dark features have not yet been detected in the infrared. Galileo NIMS (Near Infrared Mapping Spectrometer) and PPR (Photo-Polarimeter Radiometer) covered complementary hemispheres of Io (e.g., Lopes et al., 2001, Lopes et al., 2004, Spencer et al., 2000, Rathbun et al., 2004). Some strong sources such as Prometheus, Zamama, Culann, and Tupan Paterae, were detected repeatedly throughout the Galileo mission in low spatial resolution NIMS observations beginning with orbit G1 (e.g., Davies et al., 2006, Davies and Ennis, 2011). Additional references are noted in the tables. Detailed references to Voyager IRIS (Infrared Interferometer Spectrometer) and New Horizons LEISA (Linear Etalon Infrared Spectral Array) space-craft and ground-based IRTF (Infrared Telescope Facility) and KO (Keck Observatory) as well as other observational data may be found in general overviews (e.g., Geissler, 2003, Davies, 2007, Lopes et al., 2007).
Blaney et al. (2000) proposed the concept of ‘myriads’ of small thermal anomalies on Io implied by an excess infrared spectral slope seen for some low spatial resolution {∼135 km} NIMS pixels within a sample of the earliest Galileo data of limited surface coverage during orbit C3 in November 1996. The density of candidate ‘myriads’ within this limited area (at ∼160–280W longitude) implied that there were numerous faint thermal sources widespread over Io’s surface that were not necessarily correlated with volcanic features. That is, only a few of these candidate ‘myriads’ can be associated with paterae (Blaney et al., 1999).
Lopes et al., 2001, Lopes et al., 2004 confirmed that additional faint thermal sources are revealed in NIMS 4.7 μm intensity images of Io at a spatial resolution of ∼25 km/pixel (from later Galileo NIMS data) associated with paterae and a few flows. Veeder et al. (2012) discussed the possible contribution of many faint ‘myriads’ to the total global heat flow of Io. They estimated the average properties of faint thermal sources and extrapolated the surface density trend of small dark paterae to the resolution limit of Galileo SSI (Solid State Imaging camera). A preliminary conclusion by Veeder et al. (2012) was that the total contribution of possible ‘myriads’ is only a small percentage of Io’s global heat flow, but with an uncertainty that had not yet been well constrained.
In this paper we follow a suggestion by Davies et al. (2007) to extend our analysis to relatively high spatial resolution NIMS data. Thus, we have developed and optimized an infrared color ratio technique (Fig. 1, Fig. 2, Fig. 3, Fig. 4) that enables the identification of new faint thermal sources on Io and quantification of their volcanic thermal emission via the highest available resolution (order of ∼17 km/pixel) NIMS data acquired late in the Galileo mission. This technique is significantly more sensitive than intensity images at only a single wavelength. We concentrate our search for new faint small thermal sources (‘myriads’) in the NIMS database because it has a higher spatial resolution than PPR (by a factor of ∼5) (see also Table 3 in Veeder et al., 2012 for many additional instrumental details).
Our focus is on a detailed analysis of the smallest identifiable dark thermal sources on Io with the goal of a robust determination for this heat flow component and its uncertainty. That is, we address the issue of whether there are even smaller dark volcanic features numerous and hot enough to significantly impact the global heat flow of Io. Our associations of NIMS infrared sources with SSI visual volcanic features utilize catalogs and maps compiled by Radebaugh (2005) and Williams et al., 2011a, Williams et al., 2011b. This detailed tracking exploits the synergy between the NIMS thermal sensitivity at near infrared wavelengths (i.e., 3–4.7 μm) and the higher SSI spatial resolution (a factor of ≈10). We also utilize available Voyager (e.g., Hanel et al., 1979) and complementary Galileo PPR data (Spencer et al., 2000, Rathbun et al., 2004) (see notes in the Appendix, Table 3 in Veeder et al., 2012; and the cited overviews).
We proceed to analyze each source individually by fitting temperatures and areas to observed NIMS spectral data (3–4.7 μm). This enables direct comparison of results for new hot spots with previously known hot spots updated via the same methodology. Our heat flow analysis includes both detections of currently active hot spots and estimates for NIMS non-detections of small dark paterae that have probably been active recently. We do consider some small dark flows as well as small dark paterae since these may be hard to distinguish morphologically near the resolution limit of SSI. Although we do not revisit the data reduction for most large dark flows, large dark paterae, high albedo features and other strong thermal sources discussed elsewhere (e.g., Lei-Kung Fluctus, Loki Patera, Prometheus, Thor and Tvashtar Paterae), we present updated values for Amirani and Volund, and At’am, Emakong and Ra Paterae. The results for all thermal sources analyzed in our series are summarized and collated into Appendix A.
Section snippets
Search for new thermal sources
Our search for new small thermal sources on Io is concentrated on Galileo NIMS Io data with the highest available spatial resolution (i.e., 24i000, 25i003 [Fig. 4], 27i014, 31i001, 31i005 [Fig. 1], 32i007 [Fig. 2], 32i004 and 32i008 region 2). These near infrared images span latitudes between approximately ±60° and longitudes from 250 to 80°W. Observation 32i008 [Fig. 3] is especially useful for quantifying hot spot volcanic power because it scanned a swath of Io’s surface that was in darkness
Dark volcanic features on Io
While searching for new Galileo NIMS sources on Io, we also specifically checked the locations of previously known dark volcanic features in our ratio images. These include dark paterae, flows, P/PV and ‘PFd’, ‘PFu’ and ‘Fd’ (Patera Floor dark, Patera Floor undivided and Flow dark) units as well as known eruptive centers (Radebaugh, 2005, Williams et al., 2011a, Williams et al., 2011b). Many of the dark volcanic features detected in high spatial resolution NIMS data were previously analyzed by
Determination of thermal emission
Power outputs from the hot spots examined in this analysis are derived using different techniques, depending on the data available. We avoid the use of brightness temperatures in our analysis. The color temperature is a measure of the flux ratio between two wavelengths for a thermal emitter comparable to an ideal black body radiator. We find that it is useful to calculate Teff, the effective temperature, of a thermal source, especially when using multiple components at different temperatures to
Heat flow contributions
Table 5 summarizes our estimates for the heat flow components identified on Io (see footnotes). Individual values for 242 hot spots are listed in Table A2. These values come from Table 1 and Veeder et al., 2011, Veeder et al., 2012. These hot spots are located at or close to easily recognized volcanic features in Galileo SSI and Voyager images (e.g., Radebaugh, 2005, Williams et al., 2011a, Williams et al., 2011b).
The nine small volcanic features (Table 1, Table 5) with new detections in medium
Discussion
Table A2 collates our new and revised estimates for the volcanic thermal power of individual sources on Io from Table 1, Table 2 with our previous results (Veeder et al., 2009, Veeder et al., 2011, Veeder et al., 2012). Although not complete, as it does not include short-lived “outburst” eruptions (many of which are documented in Veeder et al., 2012), this listing attempts to systematically account for all other thermal sources analyzed in our series of papers on Io’s heat flow. We then give an
Implications
Near infrared ratio images from future high resolution observations of the unlit side of Io are expected to reveal additional very faint hot spots associated with the smallest dark volcanic features on Io. This is especially so since the smallest observed active areas of paterae on Io are in almost every case still much larger than terrestrial lava lakes (see Davies et al., 2010, Davies, 2011). Very faint hot spots are not likely to outnumber small visual volcanic features. Possibly, additional
Conclusions
Infrared color ratio images derived from the highest spatial resolution available NIMS data reveal nine new hot spots on Io. Every new NIMS hotspot is associated with a small, dark, patera. The volcanic power from these newly detected faint NIMS hot spots, ∼0.53 × 1012 W (or ∼0.5% of Io’s total heat flow), is comparable to our estimate of ∼0.3 × 1012 W (or ∼0.3% of Io’s total heat flow) for 24 small dark paterae scanned but not detected by NIMS at resolutions down to ∼17 km/pixel. These recently
Acknowledgments
We gratefully acknowledge the support of the NASA Outer Planets Research and NASA Planetary Geology and Geophysics Programs. Part of this work was performed at the Bear Fight Institute and the Jet Propulsion Laboratory – California Institute of Technology, under contract to NASA. We thank Diana Blaney for her analysis initiating the search for Io’s “myriad” small hot spots. We also thank Imke de Pater and an anonymous reviewer for their helpful comments.
Glossary
- AO
- Adaptive optics
- E.C.
- Eruptive Center
- F.
- Fluctus (extensive flow field dark unless otherwise noted)
- Fd
- ‘Flow, dark’ units from Williams et al., 2011a, Williams et al., 2011b
- FOV
- Instrument field of view
- IAU
- International Astronomical Union
- IRIS
- Voyager Infrared Interferometer Spectrometer
- IRTF
- Infrared Telescope Facility
- ISS
- Cassini Imaging Science Subsystem
- JPL
- Jet Propulsion Laboratory
- KO
- Keck Observatory
- LEISA
- New Horizons Linear Etalon Infrared Spectral Array
- NIMS
- Galileo Near Infrared Mapping Spectrometer
- NITED
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