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Heating of Jupiter’s upper atmosphere above the Great Red Spot


Letter
J.?O’Donoghue1, L.?Moore1, T.?S.?Stallard2 & H.?Melin2

doi:10.1038/nature18940

Heating of Jupiter’s upper atmosphere above the Great Red Spot
The temperatures of giant-planet upper atmospheres at mid- to low latitudes are measured to be hundreds of degrees warmer than simulations based on solar heating alone can explain1–4. Modelling studies that focus on additional sources of heating have been unable to resolve this major discrepancy. Equatorward transport of energy from the hot auroral regions was expected to heat the low latitudes, but models have demonstrated that auroral energy is trapped at high latitudes, a consequence of the strong Coriolis forces on rapidly rotating planets3–5. Wave heating, driven from below, represents another potential source of upper-atmospheric heating, though initial calculations have proven inconclusive for Jupiter, largely owing to a lack of observational constraints on wave parameters6,7. Here we report that the upper atmosphere above Jupiter’s Great Red Spot—the largest storm in the Solar System—is hundreds of degrees hotter than anywhere else on the planet. This hotspot, by process of elimination, must be heated from below, and this detection is therefore strong evidence for coupling between Jupiter’s lower and upper atmospheres, probably the result of upwardly propagating acoustic or gravity waves. On 4 December 2012 (Coordinated Universal Time, utc) we observed Jupiter for 9?h using the SpeX spectrometer8 on the NASA Infrared Telescope Facility. The spectrometer slit was aligned along the rotational axis in the north–south direction at local noon on the planet. This arrangement is illustrated in Fig. 1a, which contains a slit-jaw image showing bright auroral emissions at the poles as well as a localized Great Red Spot (GRS) emission enhancement at mid-latitudes. Exposures from the instrument in this setup give wavelength and intensity information as a function of latitude as shown in Fig. 1b. By exposing continuously throughout the night, we obtained longitudinal information for most of the planet (a Jovian day is 9?h?56?min long).
a
89 60 50 40 30 20 10 0 –10 –20 –30 –40 –50 –60 –87 3.38 3.40 3.42 Wavelength (μm) 3.44 3.46

The spectrum in Fig. 1b shows strong emission features at six wavelengths, which appear prominently in the auroral regions and wane towards the equator. These are discrete ro-vibrational emission lines from H3+, a major ion in Jupiter’s ionosphere, the charged (plasma) component of the upper atmosphere. The colour contours highlight the weaker emissions from this ion across the body of the planet. Far from a uniform intensity at low latitudes, there is a substantial intensity enhancement in all of the emission lines within the ??13° to ??27° planetocentric latitude range occupied by the GRS9. As seen in the coloured contours of Fig. 1b, the H3+ emissions are isolated in wavelength, indicating that there is no continuum reflection of sunlight at the latitudes of the GRS. The ratio between two or more emission lines can be used to derive the temperature of the emitting ions10,11. With the observing geometry used here, such temperatures are altitudinally averaged ‘column temperatures’ of H3+, where the majority of H3+ at Jupiter has been observed to be located at altitudes between 600?km and 1,000?km above the 1-bar pressure level12. H3+ has been demonstrated to be in quasi-local thermodynamic equilibrium throughout the majority of Jupiter’s upper atmosphere, meaning that derived temperatures are representative of the co-located ionosphere and (the mostly H2) thermosphere13. In the Methods section we detail the data reduction techniques and temperature model fitting procedures, and in Fig. 2 we show two example model fits; only the strongest, outermost lines are used to fit temperatures, because the central H3+ lines are contaminated by telluric absorption. Note that, even though the H3+ peak intensities at the GRS (Fig. 2a) are lower than those at 45° latitude, this is a result of lower column-integrated H3+ densities at lower latitude. Derived temperatures remain unaffected by the density differences because they are based entirely on H3+ line ratios.

b
10–4.0 10–4.5

Planetocentric latitude (°)

Intensity (W m–2)

10–5.0

10–5.5 10–6.0

Figure 1 | The acquisition of Jovian spectra. a, Jupiter as observed by the SpeX slit-jaw imager and L-filter (3.13–3.53?μ? m), on 4 December 2012. Bright regions at the poles result from auroral emissions; the contrast at low and mid-latitudes has been enhanced for visibility. The vertical beige line in
1

the middle of the image indicates the position of the spectrometer slit, which was aligned along the rotational axis. b, The co-added spectrum of seven GRS-containing exposures; dotted horizontal lines indicate the latitudinal range of the GRS. Further details are given in the Methods section.

Center for Space Physics, Boston University, Boston 02215, USA. 2Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK.

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Letter RESEARCH
3.0 2.5 Intensity (×10–6 W m–2) 2.0 1.5 1.0 0.5 0.0 3.38 3.40 3.42 Wavelength (μm) 3.44 3.46 3.38 3.40 3.42 Wavelength (μm) 3.44 3.46

a

GRS Data H3+ model t to data H3+ model Sky transmission

b

Non-GRS

Figure 2 | Model fit to observed H3+ intensity as a function of wavelength. a, The data in Fig. 1b plotted between ??13° and ??19° planetocentric latitude; b, as for a but plotted between ??40° and ??49° planetocentric latitude. The H3+ model fit to the data is shown as a solid red line: only m and 3.454?μ? m are included in the temperature the H3+ lines at 3.383?μ?

derivation (see Methods for the full list). Telluric absorption, normalized to show sky contamination, is shown in grey. The derived temperatures are 1,644?±??161?K (a) and 900?±??42?K (b) (±? standard errors of the mean). The H3+ model is extended to the central region (dotted red line) based on the temperatures and densities of the fits. Intensity errors are 1σ.

The difficulty in explaining the observed upper-atmospheric temperatures of the giant planets was realized more than 40?years ago1, and has since been termed the giant-planet “energy crisis”2,4. For Jupiter, only the observed temperatures within the auroral regions have been adequately explained, as the 1,000–1,400?K temperatures14 observed there result from auroral heating mechanisms that impart 200?GW of power per hemisphere through ion-neutral collisions and Joule heating15,16. The low to mid-latitudes do not have such a heat source, and yet are measured to be near 800?K, which is 600?K warmer than can be accounted for by solar heating15,17,18. If heating does not come from above (solar heating), and cannot be produced in situ via magnetospheric interactions, then a solution is likely to be found below. Gravity waves, generated in the lower atmosphere and breaking in the thermosphere, represent a potentially viable source of upperatmospheric heating. Previous modelling studies, however, have led to inconclusive results for Jupiter: while viscous dissipation of gravity waves in Jupiter’s upper atmosphere can lead to warming of the order of 10?K, sensible heat flux divergence can also lead to cooling by a similar amount, depending on the properties of the wave6,7. Recent re-analysis of Galileo Probe data has shown that gravity waves impart a negligible amount of heating vertically to the stratosphere (gravity-wave motion is primarily longitudinal and latitudinal) and that heating near the thermosphere is less than 1?K per Jovian day19. A more likely energy source is acoustic waves that heat from below (also via viscous dissipation); this form of heating requires vertical propagation of disturbances in the low-altitude atmosphere. Acoustic waves are produced above thunderstorms, and the subsequent waves have been modelled to heat the Jovian upper atmosphere by 10?K per day20 and on Earth have been observed to heat the thermosphere over the Andes mountains20,21. On Jupiter, acoustic-wave heating has been modelled to potentially impart hundreds of degrees of heating to the upper atmosphere22. However, to the best of our knowledge, no such coupling between the lower and upper atmosphere has been directly observed for the outer planets, so vertical coupling has not been seriously considered as a solution to the giant-planet energy crisis. Jupiter’s GRS is the largest storm in the Solar System, spanning 22,000?km by 12,000?km in longitude and latitude, respectively. The GRS lies within the troposphere, with cloud tops reaching altitudes of 50?km, around 800?km below the H3+ layer9. In Fig. 3 we show (red circles) that the pattern of H3+ intensity seen above the GRS, when fitted to our model, gives column-averaged H3+ temperatures of over 1,600?K, higher than anywhere else on the planet, even in the auroral region. We also fitted temperatures to a swath of longitudes away from the GRS in order to illustrate that the enhancement in temperature

occurs only within this longitude band. The latitudinal variation of temperatures away from the GRS is similar to the ranges previously observed17, indicating that the high temperature above the GRS is localized in both latitude and longitude. The high temperature in the northern part of the GRS provides direct observational evidence of a localized heating process. We interpret the cause of this heating to be storm-enhanced atmospheric turbulence, which arises due to the flow shear between the storm and the surrounding atmosphere. Some of these waves must then propagate vertically upwards, depositing their energy as heat through viscous dissipation. It is unknown, at present, why the two red data points at GRS latitudes (grey shaded region in Fig. 3) differ by 800?K. Perhaps there may be contamination of the H3+ line at 3.454?μm ? by the methane emission line at the same wavelength. Any additional intensity added to this H3+ line results in a lower temperature (for further detail see the Methods section). Thus, the temperature above the southern part
Column-averaged temperature of H3+ (K) 1,800 1,600 1,400 1,200 1,000 800 600 –60 –40 0 –20 20 Planetocentric latitude (°) 40 GRS spectra Non-GRS spectra GRS boundaries

Figure 3 | Jovian H3+ temperatures versus planetocentric latitude. Column-averaged temperatures of H3+ shown here are each derived from model fits to the discrete H3+ emission lines as shown in Fig. 2. Red circle symbols correspond to the co-addition of GRS-related spectra (that is, from the spectral image in Fig. 1b) between 239° and 253° in Jovian system III Central Meridian Longitude (CML). The GRS latitudes are indicated by the grey shading. Blue triangle symbols were derived from exposures taken in the ranges 293°–359° and 0°–82° CML, that is, longitudes well separated from the GRS, representing the ‘ordinary’ background conditions based on solar heating alone. The modelled temperature of the upper atmosphere for these non-auroral regions is 203?K (ref. 1). Uncertainties are standard errors of the mean.
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RESEARCH Letter
of the GRS may be much higher than derived, but only if methane is preferentially brighter in the south. However, as the H3+ and CH4 lines at 3.454?μ? m are not separated spectrally in this work, it is not possible to conclude whether or not contamination is present. An alternative physical explanation may relate to the relative velocities between the zonal wind and the GRS being greatest on the equatorward side of the storm: relative velocities are 75?m?s?1 in the north, 15?m?s?1 in the storm core, and 25?m?s?1 at the poleward edge9. The largest relative velocities would induce the strongest flow shear, leading to the greatest turbulence and therefore the largest contribution to heating above. It is possible that evidence of such energy transfer from the lower to the upper atmosphere would be deposited en route in the intervening troposphere and upper stratosphere (0–150?km altitude), as there is a temperature enhancement of 10?K encircling the GRS at these altitudes23,24. However, this enhancement could also be due to the upwelling of material in the centre of the GRS, followed by increased adiabatic heating when the material downwells around the edges24. The only previous map of Jovian H3+ temperatures that contains the GRS was made using ground-based data obtained in 1993 (ref. 17). The authors of ref. 17 did not mention the GRS, as no obvious signature was present in their temperature map. However, on the basis of their temperature contours and the expected location of the GRS at the time, we estimate that there was a measured temperature enhancement of 50?K above the GRS. Such a minor temperature increase may indicate that the GRS-driven heating of Jupiter’s upper atmosphere is transient, but the spatial resolution of the 1993 observations was 9,800?km per pixel (at the equator), compared with 500?km per pixel in this study. Therefore, the previous data had much cruder resolution in latitude and longitude, and any localized temperature enhancements would have been smoothed out. In this work, the high-temperature region above the GRS is localized in latitude and longitude, indicating a large temperature gradient and perhaps a confinement by currently unknown upper-atmospheric dynamics. If wave heating driven from below is responsible for the temperatures observed in Jupiter’s non-auroral upper atmosphere, then we might expect a relatively smooth temperature profile with latitude, punctuated by temperature enhancements above active storms. The GRS may then simply be the ‘smoking gun’ that dramatically illustrates this atmospheric coupling process, and provides the clue to solving the giant-planet energy crisis.
Online Content Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper. received 14 April; accepted 4 June 2016. Published online 27 July 2016. 1. Strobel, D. F. & Smith, G. R. On the temperature of the Jovian thermosphere. J. Atmos. Sci. 30, 718–725 (1973). 2. Miller, S., Aylward, A. & Millward, G. in The Outer Planets and their Moons Vol. 19 of Space Sciences Series of ISSI 116, 319–343 (2005). 3. Smith, C. G. A., Aylward, A. D., Millward, G. H., Miller, S. & Moore, L. E. An unexpected cooling effect in Saturn’s upper atmosphere. Nature 445, 399–401 (2007). 4. Yates, J. N., Achilleos, N. & Guio, P. Response of the Jovian thermosphere to a transient pulse in solar wind pressure. Planet. Space Sci. 91, 27–44 (2014). 5. Smith, C. G. A. & Aylward, A. D. Coupled rotational dynamics of Jupiter’s thermosphere and magnetosphere. Ann. Geophys. 27, 199–230 (2009). 6. Hickey, M. P., Walterscheid, R. L. & Schubert, G. Gravity wave heating and cooling in Jupiter’s thermosphere. Icarus 148, 266–281 (2000). 7. Matcheva, K. I. & Strobel, D. F. Heating of Jupiter’s thermosphere by dissipation of gravity waves due to molecular viscosity and heat conduction. Icarus 140, 328–340 (1999). 8. Rayner, J. T. et al. SpeX: a medium-resolution 0.8–5.5 micron spectrograph and imager for the NASA infrared telescope facility. Publ. Astron. Soc. Pacif. 115, 362–382 (2003). 9. Parisi, M., Galanti, E., Finocchiaro, S., Iess, L. & Kaspi, Y. Probing the depth of Jupiter’s Great Red Spot with the Juno gravity experiment. Icarus 267, 232–242 (2016). 10. Melin, H., Miller, S., Stallard, T., Smith, C. & Grodent, D. Estimated energy balance in the Jovian upper atmosphere during an auroral heating event. Icarus 181, 256–265 (2006). 11. O’Donoghue, J. et al. Conjugate observations of Saturn’s northern and southern H3+ aurorae. Icarus 229, 214–220 (2014). 12. Uno, T. et al. Vertical emissivity profiles of Jupiter’s northern H3+ and H2 infrared auroras observed by Subaru/IRCS. J. Geophys. Res. 119, 10,219– 10,241 (2014). 13. Miller, S. et al. H3+: the driver of giant planet atmospheres. Phil. Trans. R. Soc. Lond. 364, 3121–3137 (2006). 14. Lystrup, M. B., Miller, S., Dello Russo, N., Vervack, R. J. Jr & Stallard, T. First vertical ion density profile in Jupiter’s auroral atmosphere: direct observations using the Keck II telescope. Astrophys. J. 677, 790–797 (2008). 15. Yelle, R. V. & Miller, S. in Jupiter’s Thermosphere and Ionosphere 185–218 (Cambridge Univ. Press, 2004). 16. Cowley, S. W. H. et al. A simple axisymmetric model of magnetosphereionosphere coupling currents in Jupiter’s polar ionosphere. J. Geophys. Res. 110, 11209 (2005). 17. Lam, H. A. et al. A baseline spectroscopic study of the infrared auroras of Jupiter. Icarus 127, 379–393 (1997). 18. Müller-Wodarg, I. C. F. et al. Magnetosphere–atmosphere coupling at Saturn: 1—Response of thermosphere and ionosphere to steady state polar forcing. Icarus 221, 481–494 (2012). 19. Watkins, C. & Cho, J. Y.-K. The vertical structure of Jupiter’s equatorial zonal wind above the cloud deck, derived using mesoscale gravity waves. Geophys. Res. Lett. 40, 472–476 (2013). 20. Hickey, M. P., Schubert, G. & Walterscheid, R. L. Acoustic wave heating of the thermosphere. J. Geophys. Res. 106, 21543–21548 (2001). 21. Walterscheid, R. L., Schubert, G. & Brinkman, D. G. Acoustic waves in the upper mesosphere and lower thermosphere generated by deep tropical convection. J. Geophys. Res. 108, 1392 (2003). 22. Schubert, G., Hickey, M. P. & Walterscheid, R. L. Heating of Jupiter’s thermosphere by the dissipation of upward propagating acoustic waves. Icarus 163, 398–413 (2003). 23. Flasar, F. M. et al. Thermal structure and dynamics of the Jovian atmosphere. I. The Great Red Spot. J. Geophys. Res. 86, 8759–8767 (1981). 24. Fletcher, L. N. et al. Thermal structure and composition of Jupiter’s Great Red Spot from high-resolution thermal imaging. Icarus 208, 306–328 (2010). Acknowledgements We thank the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration (NASA). We are grateful to the observing staff at the Infrared Telescope Facility and Mauna Kea Observatory. This work was funded by NASA under grant number 9500303356 issued through the Planetary Astronomy Program (to L.M. and J.O’D). The UK Science and Technology Facilities Council (STFC) supported this work through the Studentship Enhancement Programme (STEP) for J.O’D., and consolidated grant support for T.S.S. and H.M. (ST/N000749/1). The Royal Astronomical Society partially funded travel to take the observations. We are grateful for the planetary ephemerides that were provided by the Planetary Data System. Author Contributions J.O’D. collected, analysed and interpreted the data and wrote the paper. L.M. greatly assisted in the data reduction, analysis, interpretation and writing of the paper. T.S.S. helped with the analysis and interpretation of the data. H.M. assisted in the collection and reduction of data, and provided computer code necessary for the analysis of data. All authors provided comments on the manuscript. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper. Correspondence and requests for materials should be addressed to J.O’D. (jameso@bu.edu). Reviewer Information Nature thanks J. Cho, M. Flasar and J. Sinclair for their contribution to the peer review of this work.

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Letter RESEARCH
Additional observing details. In Fig. 1, where we show the acquisition of Jovian spectra, Jupiter’s sub-Earth latitude was +3 ? °. The configuration of the SpeX instrument on the Infrared Telescope Facility was single order with a long slit, at a spectral resolution of R?=??2,500. The slit length and width used were 60?arcsec and 0.3?arcsec, respectively, and one pixel subtended 0.15?arcsec on the sky. In Fig. 2 the model telluric transmission spectrum is obtained from the Atmospheric TRANsmission database (ATRAN; https://atran.sofia.usra.edu) for a spectral resolution of R?=??2,500. The absorption wells near H3+ lines in the centre of the spectrum in Fig. 2 serve to highlight our reasons for avoiding that region in the temperature fitting. The attenuation of the signal in this figure by the sky is constant as a function of latitude because all of the temperature fits are from the same exposure, so any attenuation would affect each temperature as a function of latitude in the same way. Absolute calibration. We flux calibrated the data by using the photometric-standard A0V star HR1019 in the usual manner: that is, by assuming a blackbody curve for the temperature of the star (10,000?K in this case) and comparing it to what we observed. This serves a dual purpose in that by dividing the data by the flux calibration, it converts counts into physical units of flux and also yields a profile of what the sky has absorbed. The mean uncertainty in the absolute calibration as a function of wavelength is 4% of the flux, and the signal-to-noise ratio for the star was 24. Instrumental effects. These are accounted for by flat fielding, dark-current subtraction and hot pixel removal in every frame. The calibrated Jovian spectra (containing uncertainties in absolute calibration above) also include noise from the instrumentation and Earth’s atmospheric attenuation. The uncertainties are thus found by finding the standard deviation of the backgrounds in the final spectrum. All errors are propagated through with the absolute calibration and uncertainty to produce the error bars in intensity displayed in Fig. 2 and the temperature estimates in Fig. 3. H3+ fitting. To find the temperatures from Fig. 1b, we used a spectroscopic H3+ line list25 and the most recent H3+ partition function coefficients26. The spectrum of H3+ can be treated as a sum of Gaussian distribution curves, with each curve a function of temperature. This ‘equation of a spectrum’ is solved in order to derive the temperature27. This technique has been used to derive H3+ temperatures on Jupiter, Saturn and Uranus for decades28, with typical uncertainties of 10%. The fitting routines used are the same as those in previous literature27, and include a list of over three million ro-vibrational transition lines of H3+ (ref. 25). The fitting routine uses the most recent partition function constants to establish a temperature; these constants are applicable for temperatures between 100?K and 10,000?K (whereupon the ion dissociates)26.

Methods

Handling of non-H3+ intensity. We now address the possibility of attenuation of H3+ by other sources at Jupiter. Possibility 1 is that there is enhanced reflection of sunlight from haze at the location of the GRS, but this is not seen adjacent in wavelength to any lines in Fig. 1 and can consequently be ruled out. Possibility 2 pertains to emission from neutral gases. Only the two intensity peaks overlaid with solid red lines are included in the final fit, though the left peak contained the H3 lines at 3.38285?μ? m and 3.38391?μ? m, whereas the right peak line included 3.45502?μm ? , 3.45483? μm ? and 3.45468?μm ? . Methane (CH4), the dominant hydrocarbon in Jupiter’s atmosphere, is known to emit at a number of wavelengths in this region, namely 3.380?μ?m, 3.392? μ?m, 3.404? μ?m, 3.415? μ?m, 3.440? μ? m and 3.454?μ?m. Some of these are visible in Fig. 1 (for example, 3.404?μ? m) and some are not (for example, 3.380?μ? m), but we are mainly interested in any that could affect the fitted H3+, which means ignoring, for now, the central portion of Fig. 2. The CH4 emission line at 3.454?μ? m is the only line that could possibly fall on a fitted H3+ line, and the effect of it doing so would mean that the line ratio between the H3+ lines denoted by the solid red fit would be larger. For this particular set of lines, if the ratio is increased, then the temperature estimate decreases: this can be seen by comparing the ratios of lines in Fig. 2, with the lower-ratio GRS spectrum corresponding to 1,644?K?±??161?K, while the higher-ratio non-GRS spectrum is fitted as 900?±??42?K (standard errors of the mean). In other words, if methane was contributing emission to this line, then accounting for it in some way by removing an arbitrary amount would result in the GRS temperature fitted being even higher than the 1,600?K derived here. Code availability. The H3+ spectroscopic line list used in the model is available online at http://www.exomol.com/data/molecules. In addition, an online H3+ intensity calculator is available at http://h3plus.uiuc.edu. The model-fitting routines and reduction code used in this work are available on request from J.O’D. (jameso@bu.edu). Our data reduction pipeline makes substantial use of the NASA Astronomy IDL library, available online at http://idlastro.gsfc.nasa.gov.
25. Neale, L., Miller, S. & Tennyson, J. Spectroscopic properties of the H3+ molecule: a new calculated line list. Astrophys. J. 464, 516–520 (1996). 26. Miller, S., Stallard, T., Melin, H. & Tennyson, J. H3+ cooling in planetary atmospheres. Faraday Discuss. 147, 283–291 (2010). 27. Melin, H. et al. On the anticorrelation between H3+ temperature and density in giant planet ionospheres. Mon. Not. R. Astron. Soc. 438, 1611–1617 (2014). 28. Stallard, T. S. et al. Temperature changes and energy inputs in giant planet atmospheres: what we are learning from H3+. Phil. Trans. R. Soc. 370, 5213–5224 (2012).

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