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Woyenumcer (c n )

Figure 6.16. Calculated thermal emission spectra of the giant planets for nadir viewing. Note that the scales are different since the integrated flux from the planets decreases as T4, where T is mean emission temperature. The Planck functions at temperatures 50, 100, 150, 200, 250, and 300 K have also been plotted for reference. Radiance units are ^W cm-2 sr-1 (cm-1)-1.

comparing the thermal spectra of the giant planets is to instead plot the log of the radiance (as shown in Figure 6.17), or to plot their brightness temperature spectra (as has been done in Figure 6.18). The brightness temperature is defined as the temperature of a black body that emits the same radiance as that observed at a given wavenumber.

Jupiter

To interpret the spectrum shown in Figures 6.16-6.18, it is useful to consider also the peak levels of the weighting functions for this planet, shown earlier in Figure 6.9. Starting in the far-infrared, the weighting function at cm-1 peaks fairly deeply, but the Planck function for all temperatures tends to zero in this region leading to the

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Woven umber(cirr')

Figure 6.17. Calculated thermal emission spectra of the giant planets on a log scale. Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dash-dotted line. The rapid decrease in brightness, due to the decrease in mean thermal temperature is clearly seen, as is the disappearance of spectral features due to ammonia and water vapor as we go outwards through the solar system. Absorption of methane is clearly visible for all planets as is the emission of hydrocarbons such as ethane and acetylene from the stratospheres. The Planck functions at temperatures 50,100,150,200, 250, and 300 K have also been plotted for reference. The spectra of Jupiter and Saturn have been calculated for cloud-free conditions while deep "H2S" clouds have been assumed for Uranus and Neptune with optical depth of unity at bar.

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Woven umber(cirr')

Figure 6.17. Calculated thermal emission spectra of the giant planets on a log scale. Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dash-dotted line. The rapid decrease in brightness, due to the decrease in mean thermal temperature is clearly seen, as is the disappearance of spectral features due to ammonia and water vapor as we go outwards through the solar system. Absorption of methane is clearly visible for all planets as is the emission of hydrocarbons such as ethane and acetylene from the stratospheres. The Planck functions at temperatures 50,100,150,200, 250, and 300 K have also been plotted for reference. The spectra of Jupiter and Saturn have been calculated for cloud-free conditions while deep "H2S" clouds have been assumed for Uranus and Neptune with optical depth of unity at bar.

small radiance seen. As the wavenumber increases, opacity increases due to H2-H2 and H2-He CIA, rotational absorption lines of mainly NH3, and to a lesser extent PH3 and CH4. Although weighting functions move upwards to cooler levels, the Planck function increases rapidly and thus the spectrum peaks at ^210 cm-1. Between 220 cm-1 and 600 cm-1 the spectrum is smooth and arises due almost entirely to H2-H2, H2-He CIA. As the weighting function continues to drift slowly upwards, the decreasing temperature and the decay of the Planck function causes radiance to decrease. Between 600 cm-1 and 700 cm-1 the weighting function starts to drift downwards again until the 729 cm-1 vibration-rotation band of stratospheric acetylene appears, which in the central Q-branch introduces a second peak to the weighting function at x 10-5 bar (not shown in Figure 6.9). Since the stratosphere is warm at this altitude, the acetylene Q-branch introduces the characteristic "spike" seen in the spectrum at this wavelength. A perpendicular band of ethane next appears

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Wavenumber (cm"1)

Figure 6.18. Calculated brightness temperature spectra of the giant planets. Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dash-dotted line.

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Wavenumber (cm"1)

Figure 6.18. Calculated brightness temperature spectra of the giant planets. Jupiter: solid line; Saturn: dotted line; Uranus: dashed line; Neptune: dash-dotted line.

at ^820 cm-1, although this is more apparent at higher zenith angles. The spectrum between 800 cm-1 and 1,200 cm-1 is mostly dominated by vibration-rotation transitions of upper-tropospheric ammonia, and to a lesser extent phosphine and CH3D. Between 1,200 cm-1 and 1,400 cm-1 there appears one of the main vibrationrotation bands of methane. The high absorption of methane pushes the weighting function high into the stratosphere and the characteristic P, Q, R-branch system is visible. The spectrum between 1,400 cm-1 and 1,800 cm-1 is composed of ammonia, CH3D, and weaker bands of methane. The strength of these absorptions decreases rapidly after 1,800 cm-1, and thus between 1,800 cm-1 and 2,100 cm-1 the weighting functions peak deep in the atmosphere, providing there are no clouds. The absorption features seen in this 5 ^m window arise from deep H20, NH3, CH3D, CO, AsH3, GeH4, and PH3 absorptions and thus allow for abundance determinations of these molecules in the 5 bar to 8 bar pressure region. Above 2,100 cm-1, a strong vibrationrotation band of phosphine appears, which pushes the weighting function back up to approximately the 1 bar level. At higher wavenumbers, the spectrum becomes dominated by reflected sunlight on the day side. There is a small contribution of reflected sunlight in the 5 ^m window, although this is usually negligible provided that the clouds are not too thick, as can be seen in Figure 6.19 where thermal emission from 1,500 cm-1 to 2 ,500 cm-1 has been over-plotted with the calculated reflected solar radiance from a cloud layer at 1 bar with albedo of 0.1.

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Uro nus

Jupiter

Jupiter

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