Aerosol influence on energy balance of the middle atmosphere of Jupiter

Nature Communications, Dec 2015

Aerosols are ubiquitous in planetary atmospheres in the Solar System. However, radiative forcing on Jupiter has traditionally been attributed to solar heating and infrared cooling of gaseous constituents only, while the significance of aerosol radiative effects has been a long-standing controversy. Here we show, based on observations from the NASA spacecraft Voyager and Cassini, that gases alone cannot maintain the global energy balance in the middle atmosphere of Jupiter. Instead, a thick aerosol layer consisting of fluffy, fractal aggregate particles produced by photochemistry and auroral chemistry dominates the stratospheric radiative heating at middle and high latitudes, exceeding the local gas heating rate by a factor of 5–10. On a global average, aerosol heating is comparable to the gas contribution and aerosol cooling is more important than previously thought. We argue that fractal aggregate particles may also have a significant role in controlling the atmospheric radiative energy balance on other planets, as on Jupiter.

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Aerosol influence on energy balance of the middle atmosphere of Jupiter

Abstract Aerosols are ubiquitous in planetary atmospheres in the Solar System. However, radiative forcing on Jupiter has traditionally been attributed to solar heating and infrared cooling of gaseous constituents only, while the significance of aerosol radiative effects has been a long-standing controversy. Here we show, based on observations from the NASA spacecraft Voyager and Cassini, that gases alone cannot maintain the global energy balance in the middle atmosphere of Jupiter. Instead, a thick aerosol layer consisting of fluffy, fractal aggregate particles produced by photochemistry and auroral chemistry dominates the stratospheric radiative heating at middle and high latitudes, exceeding the local gas heating rate by a factor of 5–10. On a global average, aerosol heating is comparable to the gas contribution and aerosol cooling is more important than previously thought. We argue that fractal aggregate particles may also have a significant role in controlling the atmospheric radiative energy balance on other planets, as on Jupiter. Introduction As on Earth, Jupiter’s atmospheric temperature profile exhibits a strong inversion above the tropopause1, implying that its middle atmosphere, or the ‘stratosphere’, is convectively inhibited. Therefore, the energy budget should be dominated by radiation and the stratified middle atmosphere is in global radiative equilibrium. A first-order question is: which constituents in the atmosphere control this energy balance? About half of the incoming solar radiation on Jupiter penetrates deep into the troposphere and one third is reflected back to space (Fig. 1)2. The bulk constituents, hydrogen and helium, are not radiatively active except via H2–H2 and H2–He collisional-induced absorption (CIA) at pressures >10 hPa (refs 3, 4). The next most abundant gas, methane (CH4), diffuses upward from the deep atmosphere and heats the stratosphere by absorbing the near-infrared solar flux3,4,5,6,7,8. The methane photochemical products acetylene (C2H2) and ethane (C2H6), together with H2–H2 and H2–He CIA, absorb the upward mid-infrared radiation from the troposphere and re-radiate it to space, resulting in an efficient net cooling of the middle atmosphere3,4,5,6,7,8,9 to compensate the solar heating. Figure 1: Globally averaged heating and cooling fluxes on Jupiter. The heating (yellow branch) and cooling (cyan branch) fluxes are in units of W m−2. The stratosphere is shaded. The heating flux is associated with the incoming solar radiation and the cooling flux is related to the outgoing thermal radiation. Of the 13.5 W m−2 of solar radiation incident to Jupiter’s atmosphere, 0.1 W m−2 is reflected back to space and 11.8 W m−2 is transmitted to the troposphere. Tropospheric hazes and clouds absorbed 7.1 W m−2 and 4.7 W m−2 is reflected back to space2. The remainder of the solar energy is absorbed in the middle atmosphere by fractal haze particles (0.7 W m−2) and CH4 gas molecules (0.9 W m−2). The total outgoing thermal radiation from our radiative calculation is ∼13–14 W m−2, consistent with that from Cassini and Voyager observations9. The thermal cooling flux is mainly emitted from the troposphere (12–13 W m−2). In the middle atmosphere, the net cooling flux is 1.4 W m−2 emitted by gas molecules H2, CH4, C2H2, and C2H6 (black and white molecule diagrams). The upper limit of the outgoing thermal flux from the fractal aggregates (blue diagrams) is ∼0.2 W m−2 as determined in this study. Full size image The global maps of temperature and C2 hydrocarbons were recently retrieved from the Jupiter flyby data from Cassini and Voyager-1 spacecraft in 2000 (refs 4, 10, 11, 12) and 1979 (refs 4, 12), respectively. On the basis of a state-of-the-art radiative transfer model (see Methods section), we investigate the global energy balance of Jupiter4. Surprisingly, the global average cooling flux by gaseous constituents in the middle atmosphere is estimated to be ∼1.4 W m−2, about 1.5 times larger than solar flux absorbed by the stratospheric CH4 (∼0.9 W m−2; Fig. 1). Vertically, the gas solar heating rate is substantially smaller than the gas thermal cooling at pressures >10 hPa (ref. 4). The energy imbalance consistently revealed by the Voyager and Cassini data is not a seasonal effect because Jupiter has nearly zero obliquity. The Jupiter–Sun distance was different for the two flybys, varying from northern fall equinox (Voyager) to the northern summer solstice (Cassini), but the global average heating is not altered significantly. Long-term ground-based observations from 1980 to 2000 also show that the global average temperature at 20 hPa does not substantially vary with time10, and thus neither does the thermal radiative cooling. The violation of the radiative energy equilibrium thereby suggests the presence of an additional strong heat source other than CH4 in the middle atmosphere of Jupiter, which absorbs the missing ∼0.5 W m−2. Here we show that the missing heat source (...truncated)


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Xi Zhang, Robert A. West, Patrick G. J. Irwin, Conor A. Nixon, Yuk L. Yung. Aerosol influence on energy balance of the middle atmosphere of Jupiter, Nature Communications, 2015, Issue: 6, DOI: 10.1038/ncomms10231