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The polar contribution to the heat flow of Io
Icarus 169 (2004) 264–270 www.elsevier.com/locate/icarus
The polar contribution to the heat flow of Io Glenn J. Veeder,∗ Dennis L. Matson, Torrence V. Johnson, Ashley G. Davies, and Diana L. Blaney Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA Received 30 January 2003; revised 24 October 2003
Abstract Polar brightness temperatures on Io are higher than expected for any passive surface heated by absorbed sunlight. This discrepancy implies large scale volcanic activity from which we derive a new component of Io’s heat flow. We present a ‘Three Component’ thermal background, infrared emission model for Io that includes active polar regions. The widespread polar activity contributes an additional ∼ 0.6 W m−2 to Io’s heat flow budget above the ∼ 2.5 W m−2 from thermal anomalies. Our estimate for Io’s global average heat flow increases to ∼ 3 ± 1 W m−2 and ∼ 1.3 ± 0.4 × 1014 watts total. 2003 Published by Elsevier Inc. Keywords: Io; Volcanism; Thermal; Infrared
1. Introduction Io’s heat flow results from tidal and orbital interactions with Jupiter and Europa (Peale et al., 1979; Yoder and Peale, 1981). Extracted orbital energy powers Io’s volcanic activity. This volcanic activity, together with solar insolation, produces the observed thermal radiation. Lower bounds for Io’s heat flow have been estimated, by summing thermal anomalies, to be in the range of 1.7–3 W m−2 (Matson et al., 1980, 1981; Morrison and Telesco, 1980; Sinton, 1981; Johnson et al., 1984; McEwen et al., 1992, 1996; Veeder et al., 1994; Spencer et al., 2000a, 2000b, 2002, 2004). Veeder et al. (1994) present the largest available data base of multiple infrared observations of Io spanning nine apparitions. The weighted average heat flow from thermal anomalies of ∼ 2.5 W m−2 is well above that predicted by most theories of tidal dissipation in Jupiter and Io. Matson et al. (2001) demonstrated an upper bound on Io’s heat flow of 13.5 W m−2 by considering cooling lava flows covering the entire surface area of Io as an extreme case. Veeder et al. (1994) developed a thermal emission model of Io with passive surface regions and active source areas in order to derive the heat flow from observations of its global infrared flux. Ten thermal anomalies (‘hot spots’ * Corresponding author.
E-mail address: [email protected] (G.J. Veeder). 0019-1035/$ – see front matter 2003 Published by Elsevier Inc. doi:10.1016/j.icarus.2003.11.016
at five locations) on a two-component passive background surface were sufficient to fit a decade-long series of IRTF (Infrared Telescope Facility) lightcurves at three infrared wavelengths (4.8, 8.7, and 20 microns) and eclipse observations. The model thermal anomalies cover a relatively small percentage of Io’s surface area. Some were located at large volcanic centers discovered by Voyager and others at longitudes not well observed by Voyager. A few nonVoyager model sources were set on the equator to produce a conservatively low derived value for the heat flow. High resolution Voyager and Galileo spacecraft data, as well as ground-based short-wavelength infrared observations, have identified many more volcanic centers on Io (e.g., Hanel et al., 1979; Pearl and Sinton, 1982; Spencer et al., 1990, 2000a, 2000b; McEwen et al., 1998; Lopes-Gautier et al., 1999, 2000; Lopes et al., 2001; Marchis et al., 2002; de Pater et al., 2004; Macintosh et al., 2003; Blaney et al., 2003). All of the known stronger and persistent sources, including the unique Loki caldera, are located at low latitudes (|φ| < 30 ◦ ). This paper focuses on recent observational results that necessitate an update of our thermal emission model for Io. Simonelli et al. (2001) have derived an improved albedo map for Io. In particular, the average bolometric albedo (A) is about 0.52 and relatively independent of latitude. This new albedo result produces small numeric changes within the Veeder et al. (1994) model, but does not change any of their conclusions. Furthermore, this uniform average bolometric
Io’s polar heat flow
albedo now also eliminates the need to consider secondorder parametric functions for the background model albedo. Spencer et al. (2000a, 2000b) report Galileo radiometry that shows minimum night brightness temperatures in the range of 90–95 K for ‘non-active’ regions on the surface of Io. This is generally consistent with the model of Veeder et al. (1994) near the equator, but unexpectedly, the minimum brightness temperatures between localized volcanic sources at low latitudes reported by Spencer et al. (2000a, 2000b) appear to be relatively independent of both latitude and longitude (i.e., time of night). At higher latitudes (60◦ –70◦ ), Rathbun et al. (2001, 2002b) show that minimum night brightness temperatures remain typically 90–95 K. This result is not consistent with previous passive model components for Io (cf., Sinton and Kaminski, 1988; Spencer et al., 2004; Matson et al., 2002a, 2002b). All passive background models trend towards 0 K at the poles which suggests consideration of a new thermally active polar region.
2. Discussion Passive thermal models were originally developed for airless bodies such as the Moon, satellites and asteroids (e.g., Morrison and Lebofsky, 1979; Lebofsky and Spencer, 1989; Veeder et al., 1989). Two end member cases have proven especially useful. One extreme represents a zero thermal inertia unit (also referred to in the literature as the ‘standard,’ ‘non-rotating’ or ‘dust’ model). Each surface element is in instantaneous equilibrium with absorbed sunlight. Another extreme case represents an infinite thermal inertia unit (cf., ‘fast-rotating’ or ‘rock’ model). Here, each surface element is in equilibrium with the whole diurnal cycle. Such homogeneous thermal models are not able to match both the observed day and eclipse radiometry of Io (e.g., Sinton and Kaminski, 1988; Veeder et al., 1994). For instance, a global zero thermal inertia model with the average albedo of Io fails because it produces noon temperatures that are too high to match day observations. By definition, this model cools rapidly upon crossing the terminator to very low temperatures that are well below observed eclipse and nightside temperatures. The infinite thermal inertia model has the advantage of mitigating high day temperatures by transporting some of the heat from solar insolation to the night-side. However, a global infinite thermal inertia model with the average albedo of Io still fails to simultaneously fit all available infrared color temperatures day and night as well as in eclipse. To resolve this, Sinton and Kaminski (1988) and Veeder et al. (1994) proposed spatially heterogeneous models with components of different thermal inertias. Our previous solution to these difficulties is discussed in detail by Veeder et al. (1994). An areal combination of both zero and infinite thermal inertia components produces a global, passive, compound, thermal background model for Io. A small, dark, zero thermal inertia unit and a large, bright, infinite thermal inertia unit generate effective tem-
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peratures for day, night and in eclipse as well as an average albedo. This two-component background model plus ten thermal anomalies (at five locations) fits fluxes for our day and eclipse IRTF observations at three infrared wavelengths. This thermal emission model predates the Galileo spacecraft era, but is nonetheless relatively consistent with Galileo PPR (Photo-Polarimeter–Radiometer) night observations of Io. On the equator, the model ‘thermal reservoir’ unit has a temperature of 109 K and 80% coverage. This results in an effective background temperature of ∼ 103 K. All previous passive models fail to reproduce the observed elevated minimum polar night brightness temperatures on Io. Due to low Sun angles near the poles, there is not enough solar insolation available to raise night temperatures significantly. This appears true for those models which include surface roughness, arbitrary (even infinite) thermal inertia or multiple layers (Matson et al., 2002b). Thus, we have considered active background models including the addition of various components with temperatures elevated by regional volcanism. However, addition of a uniform active source to either a zero thermal inertia or infinite thermal inertia global background model is not satisfactory. An active zero thermal inertia model is much too hot on the equator at noon. An active infinite thermal inertia model remains too warm at night (relative to during the day). That is, the strongest background model constraints are the low daytime 20 µm flux, the day/night flux ratios at 4.8 and 20 µm and the elevated minimum polar night brightness temperatures of Io. Our recent work on possible ‘self-consistent’ and ‘four-parameter’ models is discussed briefly in Veeder et al. (2002a, 2002b). 2.1. Three component model The current thermal background model of Io includes one active and two passive components. This conservative model improves on global uniform background models without becoming excessively complicated. A satisfactory choice preserves analogs of both passive components of Veeder et al. (1994) at low latitudes. The analog of the zero thermal inertia component of Veeder et al. (1994) is also retained at high latitudes. However, a new active module replaces the infinite thermal inertia component at high latitudes. Elevated minimum polar brightness temperatures at night suggest the presence of some extended volcanic region (or widespread unit) near the poles. A volcanic active background at low latitudes is possible but not necessary. This approach results in the least perturbation during our adaptation of the background model from Veeder et al. (1994). Our new ‘Three Component’ thermal background model is summarized in Table 1. We now describe construction of the ‘Three Component’ background model of Io. The functionality of each passive model unit in Table 1 is chosen so as to account for the solar energy absorbed within the infrared energy radiated. The ‘Zero γ ’ unit has zero thermal inertia and a low albedo (A).
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Table 1 Three component model Unit Zero γ Infinite γ Polar
Albedo A
Inertia γ
Latitude φ
Longitude λ
Characteristic
Low High High
0.0 ∞ n/a
0–90 0–60 60–90
Day Day and night Day and night
Passive hot subsolar region Passive warm equator Active volcanic
It is a global unit that covers a relatively small fraction of Io’s surface. Since the ‘Zero γ ’ unit is in instantaneous equilibrium with solar insolation, it contributes to infrared radiation only during the day. This passive unit continues to provide a moderately hot, sub-solar region as its temperature distribution is only a function of zenith angle (∝ cos1/4 ζ ). The ‘Infinite γ ’ unit has infinite thermal inertia and a high albedo. It is constrained to cover a relatively large fraction of surface area at low latitudes. Since the ‘Infinite γ ’ unit equilibrates to constant temperature bands that are not a function of longitude (i.e., local time), this unit always radiates the same flux, night and day. Thus, the ‘Infinite γ ’ unit anchors the infrared color temperatures at night as well as during eclipse. This passive unit continues to provide a relatively warm equatorial region since its temperature distribution is only a function of latitude (∝ cos1/4 φ). The active ‘Polar’ unit is the key to understanding this ‘Three Component’ background model. The functionality of this active model unit in Table 1 is also chosen so as to account for the solar energy absorbed within the infrared energy radiated. It is constrained to cover a relatively large fraction of surface area at high latitudes. This new active volcanic unit contributes to radiation both night and day in a new manner. The ‘Polar’ unit has a uniform temperature (TP ) at night due to the release of heat from an internal source. It has somewhat higher temperatures during the day due to additional input from solar insolation (FI ). [T 4 ∝ TP4 + FI (1 − A) cos φ, see Veeder et al. (1994) for a discussion of the pedestal effect with respect to thermal anomalies.] In the present paper, the absorbed solar power is redistributed uniformly over the dayside of each high latitude band (to avoid excessive night temperatures). In contrast to the ‘Infinite γ ’ unit (now limited to low latitudes), the ‘Polar’ unit does not carry any solar power across the terminator. This results in bands of uniform temperatures that are not a function of longitude except for the important difference between the day and night sides. Thus, there is a small step in temperature at the terminator for the volcanic unit. In this sense, the diurnal response of the ‘Polar’ unit is intermediate between the ‘Zero γ ’ and ‘Infinite γ ’ units. The defined ‘Polar’ unit provides support for elevated temperatures; but must be confined to high latitudes (|φ| > 60 ◦ ) to simultaneously model the several IRTF color temperatures (between different wavelength pairs) both day and night as well as the PPR minimum polar night brightness temperatures. For computational convenience only, this ‘Polar’ unit is given the same albedo as the ‘Infinite γ ’ and occupies the same fractional surface area at the poles as the ‘Infinite γ ’ unit does at the equator. Thus, the albedo and
fractional areas of the model ‘Polar’ and ‘Infinite γ ’ units are adjusted together to converge on a fit to the observational constraints. We compared several variations of possible ‘Three Component’ background models by first selecting a TP which was expected to approximate the floor of PPR polar night brightness temperatures and then examining the flux residual differences from the Veeder et al. (1994) background model while trading the area and albedo of the ‘Zero γ ’ unit against the area and albedo of the ‘Polar’ and ‘Infinite γ ’ units. Finally, the effective polar night temperature is calculated from TP and the fractional area of the ‘Polar’ unit itself. The further complication of trading TP against the latitude of the break between the ‘Infinite γ ’ and ‘Polar’ units (|φ| ∼ 60◦ ) is not pursued beyond the following. 2.2. Three component example A quantitative example of the ‘Three Component’ model of Io is summarized in Table 2. This background model reproduces the sum of the infrared flux contributions from both passive components of Veeder et al. (1994) at each of 4.8, 8.7, and 20 microns during day, night, and eclipse. For the present example, we take Io at a nominal heliocentric distance (R = 5 AU) with a nominal emissivity (ε = 0.9). We also assume that the local albedo (A) of mixed components everywhere is identical to the global average (A ≡ 0.52) and that the albedo and filling factor (fraction f ) of the active ‘Polar’ unit are identical to those of the passive ‘Infinite γ ’ unit. Other characteristic model parameters include the sub-solar, anti-solar and polar component temperatures (TSS , TAS , and TP ). The filling factor describes the proportion of units within regions. Local effective temperatures for this composite model are listed in the last column of Table 2. These result from areal mixing of the ‘Zero γ ’ and ‘Infinite γ ’ units in the equatorial zone and areal mixing of the ‘Zero γ ’ and ‘Polar’ units in the polar regions. The ‘Zero γ ’ unit is dark (A = 0.3) with modest areal coverage (f = 25%) but globally distributed. This passive Table 2 Example model Parameter
Zero γ
Infinite γ
Polar
Teff
Albedo A Fraction f TSS TAS TP
0.300 0.250 166 0 0
0.585 0.750 109 109 n/a
0.585 0.750 n/a n/a 100
n/a n/a 131 102 93
Io’s polar heat flow
unit is characterized by a relatively hot sub-solar temperature (TSS ∼ 166 K) and zero temperatures at the anti-solar and polar points (TAS ≡ TP ≡ 0 K); i.e., everywhere at or beyond the terminator. The ranges of the other two (regional) units are defined by latitudinal zones above and below |φP | of 60◦ . The ‘Infinite γ ’ unit is bright (A = 0.585) with a greater areal coverage (f = 75%) in the equatorial region. This passive unit is characterized by a relatively cool subsolar temperature (TSS ∼ 109 K) which is identical to the anti-solar temperature (TAS ≡ TSS ∼ 109 K). The ‘Polar’ unit covers most of the polar region (f = 75%) with bright material (A = 0.585). This active unit is characterized by a relatively warm polar temperature (TP ≡ 100 K) at night. The ‘Three Component’ model summarized in Table 2 improves on the passive background units of Veeder et al. (1994) with minimum change. It accommodates the minimum night and polar brightness temperatures implied by the PPR observations while reproducing the fluxes predicted by
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the Veeder et al. (1994) background model. These fluxes plus contributions from the model thermal anomalies are a reasonable fit to the IRTF fluxes observed for Io as a function of longitude. The most severe observational constraints remain the fluxes at 20 microns during the day and 4.8 microns at night where this background model predicts some excess (order of ∼ 10%). This situation is similar to that detailed in Veeder et al. (1994). Figure 1 shows the noon temperatures of the current passive units as a function of latitude. The temperatures of both these units (and other possibilities with intermediate thermal inertia) fall rapidly to zero near the pole. Figure 2 shows the noon temperatures of the active volcanic unit as a function of latitude. This unit remains at TP of 100 K throughout the nightside of the polar cap. Figures 3 and 4 show the day and night (local effective) temperatures of the ‘Three Component’ model as a function of latitude. Note that the background day temperatures are a slow function of latitude (except for the boundary at
Fig. 1. Temperature vs. latitude at noon for the passive components of Io. The ‘Zero γ ’ unit temperature distribution is a function of zenith angle (ζ ) and albedo (A = 0.3). The ‘Infinite γ ’ unit temperature distribution is a function of latitude (φ) and albedo (A = 0.585), but is uniform with longitude (λ) [R = 5 AU and ε = 0.9].
Fig. 2. Temperature vs. latitude at noon for the active component of Io. The ‘Polar’ unit day temperature distribution is a function of latitude (φ), albedo (A = 0.585), and polar temperature (TP = 100 K); but is uniform with longitude (λ). In contrast, its night temperature (T ≡ TP ) is constant everywhere [R = 5 AU and ε = 0.9].
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Fig. 3. Temperature vs. latitude at noon for the ‘Three Component’ model of Io. The composite background temperature (local Teff ) distribution is a function of latitude (φ), longitude (λ), albedos (A), filling factors (f ), and polar temperature TP . The boundary between the passive ‘Infinite γ ’ and active ‘Polar’ units is at latitude |φP | = 60◦ .
Fig. 4. Temperature vs. latitude at night for the ‘Three Component’ model of Io. The composite background temperature (local Teff ) distribution is a function of latitude (φ), albedos (A), filling factors (f ), and polar temperature TP ; but uniform with longitude (λ). The boundary between the passive ‘Infinite γ ’ and active ‘Polar’ unit is at latitude |φP | = 60◦ .
|φP | of 60◦ , such that the day pole value is the same as the entire night cap. That is, this background model predicts some decrease of day temperature with latitude above |φP | of 60◦ , but a constant polar temperature at night. The small step in temperature across the terminator within the polar regions decreases with increasing latitude and disappears at the poles. Smoothing of the model at |φP | of 60◦ and across the terminator above |φP | of 60◦ are possible refinements, but are not required and would introduce more model parameters. The sensitivity of the ‘Three Component’ background model to variations of the albedos and fractional areas of the units as well as TP was checked by calculating the square root of the sum of the squares of the residuals for three wavelengths day and night (for a total of six differences from the Veeder et al. (1994), background model). The residual flux differences are near the 15% level (n.b., the revision of the global average albedo downward produces an increase in model flux). Variations of the ‘Infinite γ ’ unit albedo (A) by ±0.05, the ‘Infinite γ ’ unit fractional area (f ) by ±0.05, TP by ±10 K and φP by ±10 ◦ were explored. Nominal values for albedo and fractional area appear to be near a local minimum in the sum of the squares of the residuals. In contrast, the trends for the polar temperature and the latitude bound-
ary are monotonic because either decreasing TP or shrinking the polar zones tends to make the active component disappear and thus transitions into an analog of the previous Veeder et al. (1994) model for Io. 2.3. Implications The preferred albedo of the active ‘Polar’ unit of the ‘Three Component’ model remains an open issue. Young calderas and flows are obviously dark in visual images, but larger and older volcanic features have a somewhat higher albedo. This is almost certainly due to the condensation of sulfur-rich compounds on surfaces, once temperatures have dropped sufficiently low (see Davies, 1996, and Davies et al., 2003, for cooling rates of lava bodies both unbuffered and buffered by release of latent heat). The ‘Polar’ unit of the ‘Three Component’ model has a high albedo identical to that of the passive ‘Infinite γ ’ unit which counters the correlation between albedo and temperature demonstrated by McEwen et al. (1985). On the other hand, Simonelli et al. (2001) indicate that the poles of Io are not darker on the average. The regional ‘Polar’ unit does not have a simple geologic interpretation. Moreover, although the model boundary
Io’s polar heat flow
of the ‘Polar’ unit (at |φP | of 60◦ ) is not necessarily sharp, any physical distinctions between high and low latitudes on Io are subtle. Clearly, there are caldera and other volcanic features at all latitudes on Io as well as a lack of impact craters. However, the known stronger and persistent sources are located at low latitudes (|φ| < 30◦ ). This suggests that the surface of the poles of Io is somewhat older than the average for Io. The time scale for lava flows on Io to cool to ∼ 100 K is tens of thousands of years (Davies, 1996). The relative importance of lava lakes, surface flows and subsurface intrusions has yet to be determined and it is plausible that the poles are a regional volcanic complex with a mixture of surface ages. The ‘Three Component’ background model can be combined with a few localized hot sources to generate the multiwavelength infrared lightcurves observed for Io. The active ‘Polar’ background helps to provide a floor under several sinusoid contributions from individual thermal anomalies. Loki is the most important. The dimensions of the Loki caldera are now well determined from spacecraft observations, but its effective temperature, and therefore power output, continue to vary (Davies et al., 2000b; Rathbun et al., 2002a; Davies, 2003). Veeder et al. (1994) suggested four particular sources with temperatures 130–180 K that cover ∼ 4% of the surface area of Io. At least six other smaller, hotter model sources are needed to synthesize reasonable lightcurves. These ten model hot spots are located at Loki (3), Pele (2), the Isum flow field (north of Colchis Regio), and the longitudes of Culann (2) and Hi’iaka (2) and all are variable over time. The Galileo NIMS maps reveal many more small volcanic features and thermal sources as well as some large subtle ones (Davies et al., 2000a; Lopes et al., 2001). Consideration of an active background model unit provides a constraint on the total power contribution of the possibly very large number of sub-resolution thermal sources (Blaney et al., 2003).
3. Heat flow A widespread warm volcanic region is consistent with the observed elevated minimum night polar brightness temperatures on Io. The ‘Three Component’ model presented is one example of a class consisting of a mosaic of low and high albedo, zero and infinite thermal inertia, and, passive and active units (introduced by Veeder et al., 2003). All models in this class can add a similar amount of additional (polar) heat flow due to volcanic activity while retaining both day and night temperatures at high and low latitudes which are more or less reasonable. The value of the polar heat flow is derived from the values adopted for TP and the areal coverage of the ‘Polar’ unit. The nominal active component has a temperature (TP ) of 100 K with a filling factor (f ) of 75% within the polar regions above |φP | of 60◦ . Thus, the background effective temperature at the poles and night caps is ∼ 93 K. The caps cover ∼ 16% of Io’s surface and there-
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fore the active component covers ∼ 12%. This results in an additional ∼ 0.6 W m−2 to Io’s heat flow budget above the ∼ 2.5 W m−2 from thermal anomalies. Our estimate for Io’s global average heat flow increases to ∼ 3 ± 1 W m−2 and ∼ 1.3 ± 0.4 × 1014 watts total.
Acknowledgments This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. We appreciate the stimulation received from reviewers Julie Rathbun and Larry Lebofsky. The authors are supported by grants from the NASA Planetary Geology and Geophysics program.
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Rathbun, J., Spencer, J.R., Davies, A.G., Howell, R.R., Wilson, L., 2002a. Loki: a predictable volcano? Geophys. Res. Lett. 29, 84–88. Rathbun, J.A., Spencer, J.R., Tamppari, L.K., Martin, T.Z., Barnard, L., Travis, L.D., 2002b. Recent results from Galileo PPR at Io: orbits I31 and I32. In: Proc. Lunar Planet. Sci. Conf. 33rd. Abstract 1371 [CDROM]. Simonelli, D.P., Dodd, C., Veverka, J., 2001. Regolith variations on Io: implications for bolometric albedos. J. Geophys. Res. 106 (E12), 33241– 33252. Sinton, W.M., 1981. The thermal emission spectrum of Io and a determination of the heat flux from its hot spots. J. Geophys. Res. 86, 3122–3128. Sinton, W.M., Kaminski, C., 1988. Infrared observations of eclipses of Io, its thermophysical parameters and the thermal radiation of the Loki volcano and environs. Icarus 75, 207–232. Spencer, J.R., Shure, M.A., Ressler, M.E., Goguen, J.D., Sinton, W.M., Toomey, D.W., Denault, A., Westfall, J., 1990. Discovery of hot spots on Io using disk resolved infrared imaging. Nature 348, 618–621. Spencer, J.R., Rathbun, J.A., Travis, L.D., Tamppari, L.K., Barnard, L., Martin, T.Z., 2000a. High-resolution observations of Io’s thermal emission from the Galileo photopolarimeter–radiometer. EOS Trans. AGU Spring Suppl. 81, 288–289. Spencer, J.R., Rathbun, J.A., Travis, L., Tamppari, L.K., Barnard, L., Martin, T.Z., McEwen, A.S., 2000b. Io’s thermal emission from the Galileo photopolarimeter–radiometer. Science 288, 1198–1201. Spencer, J.R., Rathbun, J.A., McEwen, A.S., Pearl, J.C., Bastos, A., Andrade, J., Correia, M., Barros, S., 2002. A new determination of Io’s heat flow using diurnal heat balance constraints. In: Proc. Lunar Planet. Sci. Conf. 33rd. Abstract 1831 [CD-ROM]. Spencer, J.R., McEwen, A.S., Rathbun, J.A., 2004. Upper limits to Io’s background heat flow from diurnal heat balance considerations. In: Bagenal, F. (Ed.), Jupiter—The Planet, Satellites and Magnetosphere, Boulder, CO. In press. Veeder, G.J., Hanner, M.S., Matson, D.L., Tedesco, E.F., Lebofsky, L.A., Tokunaga, A.T., 1989. Radiometry of near-Earth asteroids. Astron. J. 97, 1211–1219. Veeder, G.J., Matson, D.L., Johnson, T.V., Blaney, D.L., Goguen, J.D., 1994. Io’s heat flow from infrared radiometry: 1983–1993. J. Geophys. Res. 99 (E8), 17095–17162. Veeder, G.J., Matson, D.L., Johnson, T.V., Davies, A.G., Blaney, D.L., 2002a. A self-consistent thermal emission model for Io. EOS Trans. AGU Spring Suppl. 83, 214 [CD-ROM]. Veeder, G.J., Matson, D.L., Johnson, T.V., Davies, A.G., Blaney, D.L., 2002b. Io’s heat flow: a model including “warm” polar regions. EOS Trans. AGU Fall Suppl. 83, 840 [CD-ROM]. Veeder, G.J., Matson, D.L., Johnson, T.V., Davies, A.G., Blaney, D.L., 2003. Polar heat flow on Io. In: Proc. Lunar Planet. Sci. Conf. 34th. Abstract 1448 [CD-ROM]. Yoder, C.F., Peale, S.J., 1981. The tides of Io. Icarus 47, 1–35.
The polar contribution to the heat flow of Io Glenn J. Veeder,∗ Dennis L. Matson, Torrence V. Johnson, Ashley G. Davies, and Diana L. Blaney Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA Received 30 January 2003; revised 24 October 2003
Abstract Polar brightness temperatures on Io are higher than expected for any passive surface heated by absorbed sunlight. This discrepancy implies large scale volcanic activity from which we derive a new component of Io’s heat flow. We present a ‘Three Component’ thermal background, infrared emission model for Io that includes active polar regions. The widespread polar activity contributes an additional ∼ 0.6 W m−2 to Io’s heat flow budget above the ∼ 2.5 W m−2 from thermal anomalies. Our estimate for Io’s global average heat flow increases to ∼ 3 ± 1 W m−2 and ∼ 1.3 ± 0.4 × 1014 watts total. 2003 Published by Elsevier Inc. Keywords: Io; Volcanism; Thermal; Infrared
1. Introduction Io’s heat flow results from tidal and orbital interactions with Jupiter and Europa (Peale et al., 1979; Yoder and Peale, 1981). Extracted orbital energy powers Io’s volcanic activity. This volcanic activity, together with solar insolation, produces the observed thermal radiation. Lower bounds for Io’s heat flow have been estimated, by summing thermal anomalies, to be in the range of 1.7–3 W m−2 (Matson et al., 1980, 1981; Morrison and Telesco, 1980; Sinton, 1981; Johnson et al., 1984; McEwen et al., 1992, 1996; Veeder et al., 1994; Spencer et al., 2000a, 2000b, 2002, 2004). Veeder et al. (1994) present the largest available data base of multiple infrared observations of Io spanning nine apparitions. The weighted average heat flow from thermal anomalies of ∼ 2.5 W m−2 is well above that predicted by most theories of tidal dissipation in Jupiter and Io. Matson et al. (2001) demonstrated an upper bound on Io’s heat flow of 13.5 W m−2 by considering cooling lava flows covering the entire surface area of Io as an extreme case. Veeder et al. (1994) developed a thermal emission model of Io with passive surface regions and active source areas in order to derive the heat flow from observations of its global infrared flux. Ten thermal anomalies (‘hot spots’ * Corresponding author.
E-mail address: [email protected] (G.J. Veeder). 0019-1035/$ – see front matter 2003 Published by Elsevier Inc. doi:10.1016/j.icarus.2003.11.016
at five locations) on a two-component passive background surface were sufficient to fit a decade-long series of IRTF (Infrared Telescope Facility) lightcurves at three infrared wavelengths (4.8, 8.7, and 20 microns) and eclipse observations. The model thermal anomalies cover a relatively small percentage of Io’s surface area. Some were located at large volcanic centers discovered by Voyager and others at longitudes not well observed by Voyager. A few nonVoyager model sources were set on the equator to produce a conservatively low derived value for the heat flow. High resolution Voyager and Galileo spacecraft data, as well as ground-based short-wavelength infrared observations, have identified many more volcanic centers on Io (e.g., Hanel et al., 1979; Pearl and Sinton, 1982; Spencer et al., 1990, 2000a, 2000b; McEwen et al., 1998; Lopes-Gautier et al., 1999, 2000; Lopes et al., 2001; Marchis et al., 2002; de Pater et al., 2004; Macintosh et al., 2003; Blaney et al., 2003). All of the known stronger and persistent sources, including the unique Loki caldera, are located at low latitudes (|φ| < 30 ◦ ). This paper focuses on recent observational results that necessitate an update of our thermal emission model for Io. Simonelli et al. (2001) have derived an improved albedo map for Io. In particular, the average bolometric albedo (A) is about 0.52 and relatively independent of latitude. This new albedo result produces small numeric changes within the Veeder et al. (1994) model, but does not change any of their conclusions. Furthermore, this uniform average bolometric
Io’s polar heat flow
albedo now also eliminates the need to consider secondorder parametric functions for the background model albedo. Spencer et al. (2000a, 2000b) report Galileo radiometry that shows minimum night brightness temperatures in the range of 90–95 K for ‘non-active’ regions on the surface of Io. This is generally consistent with the model of Veeder et al. (1994) near the equator, but unexpectedly, the minimum brightness temperatures between localized volcanic sources at low latitudes reported by Spencer et al. (2000a, 2000b) appear to be relatively independent of both latitude and longitude (i.e., time of night). At higher latitudes (60◦ –70◦ ), Rathbun et al. (2001, 2002b) show that minimum night brightness temperatures remain typically 90–95 K. This result is not consistent with previous passive model components for Io (cf., Sinton and Kaminski, 1988; Spencer et al., 2004; Matson et al., 2002a, 2002b). All passive background models trend towards 0 K at the poles which suggests consideration of a new thermally active polar region.
2. Discussion Passive thermal models were originally developed for airless bodies such as the Moon, satellites and asteroids (e.g., Morrison and Lebofsky, 1979; Lebofsky and Spencer, 1989; Veeder et al., 1989). Two end member cases have proven especially useful. One extreme represents a zero thermal inertia unit (also referred to in the literature as the ‘standard,’ ‘non-rotating’ or ‘dust’ model). Each surface element is in instantaneous equilibrium with absorbed sunlight. Another extreme case represents an infinite thermal inertia unit (cf., ‘fast-rotating’ or ‘rock’ model). Here, each surface element is in equilibrium with the whole diurnal cycle. Such homogeneous thermal models are not able to match both the observed day and eclipse radiometry of Io (e.g., Sinton and Kaminski, 1988; Veeder et al., 1994). For instance, a global zero thermal inertia model with the average albedo of Io fails because it produces noon temperatures that are too high to match day observations. By definition, this model cools rapidly upon crossing the terminator to very low temperatures that are well below observed eclipse and nightside temperatures. The infinite thermal inertia model has the advantage of mitigating high day temperatures by transporting some of the heat from solar insolation to the night-side. However, a global infinite thermal inertia model with the average albedo of Io still fails to simultaneously fit all available infrared color temperatures day and night as well as in eclipse. To resolve this, Sinton and Kaminski (1988) and Veeder et al. (1994) proposed spatially heterogeneous models with components of different thermal inertias. Our previous solution to these difficulties is discussed in detail by Veeder et al. (1994). An areal combination of both zero and infinite thermal inertia components produces a global, passive, compound, thermal background model for Io. A small, dark, zero thermal inertia unit and a large, bright, infinite thermal inertia unit generate effective tem-
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peratures for day, night and in eclipse as well as an average albedo. This two-component background model plus ten thermal anomalies (at five locations) fits fluxes for our day and eclipse IRTF observations at three infrared wavelengths. This thermal emission model predates the Galileo spacecraft era, but is nonetheless relatively consistent with Galileo PPR (Photo-Polarimeter–Radiometer) night observations of Io. On the equator, the model ‘thermal reservoir’ unit has a temperature of 109 K and 80% coverage. This results in an effective background temperature of ∼ 103 K. All previous passive models fail to reproduce the observed elevated minimum polar night brightness temperatures on Io. Due to low Sun angles near the poles, there is not enough solar insolation available to raise night temperatures significantly. This appears true for those models which include surface roughness, arbitrary (even infinite) thermal inertia or multiple layers (Matson et al., 2002b). Thus, we have considered active background models including the addition of various components with temperatures elevated by regional volcanism. However, addition of a uniform active source to either a zero thermal inertia or infinite thermal inertia global background model is not satisfactory. An active zero thermal inertia model is much too hot on the equator at noon. An active infinite thermal inertia model remains too warm at night (relative to during the day). That is, the strongest background model constraints are the low daytime 20 µm flux, the day/night flux ratios at 4.8 and 20 µm and the elevated minimum polar night brightness temperatures of Io. Our recent work on possible ‘self-consistent’ and ‘four-parameter’ models is discussed briefly in Veeder et al. (2002a, 2002b). 2.1. Three component model The current thermal background model of Io includes one active and two passive components. This conservative model improves on global uniform background models without becoming excessively complicated. A satisfactory choice preserves analogs of both passive components of Veeder et al. (1994) at low latitudes. The analog of the zero thermal inertia component of Veeder et al. (1994) is also retained at high latitudes. However, a new active module replaces the infinite thermal inertia component at high latitudes. Elevated minimum polar brightness temperatures at night suggest the presence of some extended volcanic region (or widespread unit) near the poles. A volcanic active background at low latitudes is possible but not necessary. This approach results in the least perturbation during our adaptation of the background model from Veeder et al. (1994). Our new ‘Three Component’ thermal background model is summarized in Table 1. We now describe construction of the ‘Three Component’ background model of Io. The functionality of each passive model unit in Table 1 is chosen so as to account for the solar energy absorbed within the infrared energy radiated. The ‘Zero γ ’ unit has zero thermal inertia and a low albedo (A).
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Table 1 Three component model Unit Zero γ Infinite γ Polar
Albedo A
Inertia γ
Latitude φ
Longitude λ
Characteristic
Low High High
0.0 ∞ n/a
0–90 0–60 60–90
Day Day and night Day and night
Passive hot subsolar region Passive warm equator Active volcanic
It is a global unit that covers a relatively small fraction of Io’s surface. Since the ‘Zero γ ’ unit is in instantaneous equilibrium with solar insolation, it contributes to infrared radiation only during the day. This passive unit continues to provide a moderately hot, sub-solar region as its temperature distribution is only a function of zenith angle (∝ cos1/4 ζ ). The ‘Infinite γ ’ unit has infinite thermal inertia and a high albedo. It is constrained to cover a relatively large fraction of surface area at low latitudes. Since the ‘Infinite γ ’ unit equilibrates to constant temperature bands that are not a function of longitude (i.e., local time), this unit always radiates the same flux, night and day. Thus, the ‘Infinite γ ’ unit anchors the infrared color temperatures at night as well as during eclipse. This passive unit continues to provide a relatively warm equatorial region since its temperature distribution is only a function of latitude (∝ cos1/4 φ). The active ‘Polar’ unit is the key to understanding this ‘Three Component’ background model. The functionality of this active model unit in Table 1 is also chosen so as to account for the solar energy absorbed within the infrared energy radiated. It is constrained to cover a relatively large fraction of surface area at high latitudes. This new active volcanic unit contributes to radiation both night and day in a new manner. The ‘Polar’ unit has a uniform temperature (TP ) at night due to the release of heat from an internal source. It has somewhat higher temperatures during the day due to additional input from solar insolation (FI ). [T 4 ∝ TP4 + FI (1 − A) cos φ, see Veeder et al. (1994) for a discussion of the pedestal effect with respect to thermal anomalies.] In the present paper, the absorbed solar power is redistributed uniformly over the dayside of each high latitude band (to avoid excessive night temperatures). In contrast to the ‘Infinite γ ’ unit (now limited to low latitudes), the ‘Polar’ unit does not carry any solar power across the terminator. This results in bands of uniform temperatures that are not a function of longitude except for the important difference between the day and night sides. Thus, there is a small step in temperature at the terminator for the volcanic unit. In this sense, the diurnal response of the ‘Polar’ unit is intermediate between the ‘Zero γ ’ and ‘Infinite γ ’ units. The defined ‘Polar’ unit provides support for elevated temperatures; but must be confined to high latitudes (|φ| > 60 ◦ ) to simultaneously model the several IRTF color temperatures (between different wavelength pairs) both day and night as well as the PPR minimum polar night brightness temperatures. For computational convenience only, this ‘Polar’ unit is given the same albedo as the ‘Infinite γ ’ and occupies the same fractional surface area at the poles as the ‘Infinite γ ’ unit does at the equator. Thus, the albedo and
fractional areas of the model ‘Polar’ and ‘Infinite γ ’ units are adjusted together to converge on a fit to the observational constraints. We compared several variations of possible ‘Three Component’ background models by first selecting a TP which was expected to approximate the floor of PPR polar night brightness temperatures and then examining the flux residual differences from the Veeder et al. (1994) background model while trading the area and albedo of the ‘Zero γ ’ unit against the area and albedo of the ‘Polar’ and ‘Infinite γ ’ units. Finally, the effective polar night temperature is calculated from TP and the fractional area of the ‘Polar’ unit itself. The further complication of trading TP against the latitude of the break between the ‘Infinite γ ’ and ‘Polar’ units (|φ| ∼ 60◦ ) is not pursued beyond the following. 2.2. Three component example A quantitative example of the ‘Three Component’ model of Io is summarized in Table 2. This background model reproduces the sum of the infrared flux contributions from both passive components of Veeder et al. (1994) at each of 4.8, 8.7, and 20 microns during day, night, and eclipse. For the present example, we take Io at a nominal heliocentric distance (R = 5 AU) with a nominal emissivity (ε = 0.9). We also assume that the local albedo (A) of mixed components everywhere is identical to the global average (A ≡ 0.52) and that the albedo and filling factor (fraction f ) of the active ‘Polar’ unit are identical to those of the passive ‘Infinite γ ’ unit. Other characteristic model parameters include the sub-solar, anti-solar and polar component temperatures (TSS , TAS , and TP ). The filling factor describes the proportion of units within regions. Local effective temperatures for this composite model are listed in the last column of Table 2. These result from areal mixing of the ‘Zero γ ’ and ‘Infinite γ ’ units in the equatorial zone and areal mixing of the ‘Zero γ ’ and ‘Polar’ units in the polar regions. The ‘Zero γ ’ unit is dark (A = 0.3) with modest areal coverage (f = 25%) but globally distributed. This passive Table 2 Example model Parameter
Zero γ
Infinite γ
Polar
Teff
Albedo A Fraction f TSS TAS TP
0.300 0.250 166 0 0
0.585 0.750 109 109 n/a
0.585 0.750 n/a n/a 100
n/a n/a 131 102 93
Io’s polar heat flow
unit is characterized by a relatively hot sub-solar temperature (TSS ∼ 166 K) and zero temperatures at the anti-solar and polar points (TAS ≡ TP ≡ 0 K); i.e., everywhere at or beyond the terminator. The ranges of the other two (regional) units are defined by latitudinal zones above and below |φP | of 60◦ . The ‘Infinite γ ’ unit is bright (A = 0.585) with a greater areal coverage (f = 75%) in the equatorial region. This passive unit is characterized by a relatively cool subsolar temperature (TSS ∼ 109 K) which is identical to the anti-solar temperature (TAS ≡ TSS ∼ 109 K). The ‘Polar’ unit covers most of the polar region (f = 75%) with bright material (A = 0.585). This active unit is characterized by a relatively warm polar temperature (TP ≡ 100 K) at night. The ‘Three Component’ model summarized in Table 2 improves on the passive background units of Veeder et al. (1994) with minimum change. It accommodates the minimum night and polar brightness temperatures implied by the PPR observations while reproducing the fluxes predicted by
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the Veeder et al. (1994) background model. These fluxes plus contributions from the model thermal anomalies are a reasonable fit to the IRTF fluxes observed for Io as a function of longitude. The most severe observational constraints remain the fluxes at 20 microns during the day and 4.8 microns at night where this background model predicts some excess (order of ∼ 10%). This situation is similar to that detailed in Veeder et al. (1994). Figure 1 shows the noon temperatures of the current passive units as a function of latitude. The temperatures of both these units (and other possibilities with intermediate thermal inertia) fall rapidly to zero near the pole. Figure 2 shows the noon temperatures of the active volcanic unit as a function of latitude. This unit remains at TP of 100 K throughout the nightside of the polar cap. Figures 3 and 4 show the day and night (local effective) temperatures of the ‘Three Component’ model as a function of latitude. Note that the background day temperatures are a slow function of latitude (except for the boundary at
Fig. 1. Temperature vs. latitude at noon for the passive components of Io. The ‘Zero γ ’ unit temperature distribution is a function of zenith angle (ζ ) and albedo (A = 0.3). The ‘Infinite γ ’ unit temperature distribution is a function of latitude (φ) and albedo (A = 0.585), but is uniform with longitude (λ) [R = 5 AU and ε = 0.9].
Fig. 2. Temperature vs. latitude at noon for the active component of Io. The ‘Polar’ unit day temperature distribution is a function of latitude (φ), albedo (A = 0.585), and polar temperature (TP = 100 K); but is uniform with longitude (λ). In contrast, its night temperature (T ≡ TP ) is constant everywhere [R = 5 AU and ε = 0.9].
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Fig. 3. Temperature vs. latitude at noon for the ‘Three Component’ model of Io. The composite background temperature (local Teff ) distribution is a function of latitude (φ), longitude (λ), albedos (A), filling factors (f ), and polar temperature TP . The boundary between the passive ‘Infinite γ ’ and active ‘Polar’ units is at latitude |φP | = 60◦ .
Fig. 4. Temperature vs. latitude at night for the ‘Three Component’ model of Io. The composite background temperature (local Teff ) distribution is a function of latitude (φ), albedos (A), filling factors (f ), and polar temperature TP ; but uniform with longitude (λ). The boundary between the passive ‘Infinite γ ’ and active ‘Polar’ unit is at latitude |φP | = 60◦ .
|φP | of 60◦ , such that the day pole value is the same as the entire night cap. That is, this background model predicts some decrease of day temperature with latitude above |φP | of 60◦ , but a constant polar temperature at night. The small step in temperature across the terminator within the polar regions decreases with increasing latitude and disappears at the poles. Smoothing of the model at |φP | of 60◦ and across the terminator above |φP | of 60◦ are possible refinements, but are not required and would introduce more model parameters. The sensitivity of the ‘Three Component’ background model to variations of the albedos and fractional areas of the units as well as TP was checked by calculating the square root of the sum of the squares of the residuals for three wavelengths day and night (for a total of six differences from the Veeder et al. (1994), background model). The residual flux differences are near the 15% level (n.b., the revision of the global average albedo downward produces an increase in model flux). Variations of the ‘Infinite γ ’ unit albedo (A) by ±0.05, the ‘Infinite γ ’ unit fractional area (f ) by ±0.05, TP by ±10 K and φP by ±10 ◦ were explored. Nominal values for albedo and fractional area appear to be near a local minimum in the sum of the squares of the residuals. In contrast, the trends for the polar temperature and the latitude bound-
ary are monotonic because either decreasing TP or shrinking the polar zones tends to make the active component disappear and thus transitions into an analog of the previous Veeder et al. (1994) model for Io. 2.3. Implications The preferred albedo of the active ‘Polar’ unit of the ‘Three Component’ model remains an open issue. Young calderas and flows are obviously dark in visual images, but larger and older volcanic features have a somewhat higher albedo. This is almost certainly due to the condensation of sulfur-rich compounds on surfaces, once temperatures have dropped sufficiently low (see Davies, 1996, and Davies et al., 2003, for cooling rates of lava bodies both unbuffered and buffered by release of latent heat). The ‘Polar’ unit of the ‘Three Component’ model has a high albedo identical to that of the passive ‘Infinite γ ’ unit which counters the correlation between albedo and temperature demonstrated by McEwen et al. (1985). On the other hand, Simonelli et al. (2001) indicate that the poles of Io are not darker on the average. The regional ‘Polar’ unit does not have a simple geologic interpretation. Moreover, although the model boundary
Io’s polar heat flow
of the ‘Polar’ unit (at |φP | of 60◦ ) is not necessarily sharp, any physical distinctions between high and low latitudes on Io are subtle. Clearly, there are caldera and other volcanic features at all latitudes on Io as well as a lack of impact craters. However, the known stronger and persistent sources are located at low latitudes (|φ| < 30◦ ). This suggests that the surface of the poles of Io is somewhat older than the average for Io. The time scale for lava flows on Io to cool to ∼ 100 K is tens of thousands of years (Davies, 1996). The relative importance of lava lakes, surface flows and subsurface intrusions has yet to be determined and it is plausible that the poles are a regional volcanic complex with a mixture of surface ages. The ‘Three Component’ background model can be combined with a few localized hot sources to generate the multiwavelength infrared lightcurves observed for Io. The active ‘Polar’ background helps to provide a floor under several sinusoid contributions from individual thermal anomalies. Loki is the most important. The dimensions of the Loki caldera are now well determined from spacecraft observations, but its effective temperature, and therefore power output, continue to vary (Davies et al., 2000b; Rathbun et al., 2002a; Davies, 2003). Veeder et al. (1994) suggested four particular sources with temperatures 130–180 K that cover ∼ 4% of the surface area of Io. At least six other smaller, hotter model sources are needed to synthesize reasonable lightcurves. These ten model hot spots are located at Loki (3), Pele (2), the Isum flow field (north of Colchis Regio), and the longitudes of Culann (2) and Hi’iaka (2) and all are variable over time. The Galileo NIMS maps reveal many more small volcanic features and thermal sources as well as some large subtle ones (Davies et al., 2000a; Lopes et al., 2001). Consideration of an active background model unit provides a constraint on the total power contribution of the possibly very large number of sub-resolution thermal sources (Blaney et al., 2003).
3. Heat flow A widespread warm volcanic region is consistent with the observed elevated minimum night polar brightness temperatures on Io. The ‘Three Component’ model presented is one example of a class consisting of a mosaic of low and high albedo, zero and infinite thermal inertia, and, passive and active units (introduced by Veeder et al., 2003). All models in this class can add a similar amount of additional (polar) heat flow due to volcanic activity while retaining both day and night temperatures at high and low latitudes which are more or less reasonable. The value of the polar heat flow is derived from the values adopted for TP and the areal coverage of the ‘Polar’ unit. The nominal active component has a temperature (TP ) of 100 K with a filling factor (f ) of 75% within the polar regions above |φP | of 60◦ . Thus, the background effective temperature at the poles and night caps is ∼ 93 K. The caps cover ∼ 16% of Io’s surface and there-
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fore the active component covers ∼ 12%. This results in an additional ∼ 0.6 W m−2 to Io’s heat flow budget above the ∼ 2.5 W m−2 from thermal anomalies. Our estimate for Io’s global average heat flow increases to ∼ 3 ± 1 W m−2 and ∼ 1.3 ± 0.4 × 1014 watts total.
Acknowledgments This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. We appreciate the stimulation received from reviewers Julie Rathbun and Larry Lebofsky. The authors are supported by grants from the NASA Planetary Geology and Geophysics program.
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