Wovenurnte's [cm ]
Figure 6.19. Overlap spectral regions between thermal emission (solid) and reflected sunlight (dotted) for the giant planets. Calculated reflection is from a Lambertian layer with albedo 0.1 placed at 1 bar for Jupiter and Saturn, and 3 bar for Uranus and Neptune. Radiance units are again ^Wcm-2 sr-1 (cm-1)-1.
The thermal emission spectrum of Saturn is in many respects rather similar to that of Jupiter, but of significantly lower overall brightness. Spectral features are formed from similar gas absorptions, with the main differences in the observed spectra coming from the lower mean atmospheric temperatures and the greater scale height of Saturn's atmosphere. Between ocm—1 and 6oocm—1, the overall shape of the spectrum is similar to that of Jupiter's with the exception that the low abundance of ammonia above 1 bar leads to greatly reduced rotational absorption lines at wavenumbers less than 2oocm—1. Stratospheric emission from acetylene is again seen at 729 cm—1, together with the clear appearance of the ethane perpendicular band at 82ocm—1. The absorption bands of ammonia between 75ocm—1 and 1,1oocm—1 are much less strong, and instead the absorption features of phosphine are more prominent in this region. The strong vibration-rotation band of methane is again seen between 1,200 cm-1 and 1,400 cm-1 and the radiation in this region thus again comes mostly from the stratosphere. Between 1,400 cm-1 and 1,800 cm-1, the spectrum looks similar to Jupiter's (but colder) until we arrive at the 5 ^m window. The weighting functions here peak at slightly lower pressures than for Jupiter, both because of the extra opacity introduced by significantly supersolar abundances of gases such as phosphine and ammonia, and also because of Saturn's greater scale height. Combined with Saturn's lower-tropospheric temperatures, thermal emission in the 5 ^m window is substantially smaller than that of Jupiter. In fact, thermal emission is so low that reflected sunlight is found to be a substantial component in the 5 ^m spectrum at longer wavenumbers (Figure 6.19) and thus must be carefully modeled for day-side observations. At wavenumbers greater than 2,100 cm-1, the strong absorption of PH3 pushes the weighting function back up into the upper troposphere and the thermal emission drops to levels insignificant compared with reflected sunlight.
The thermal emission spectra of Uranus and Neptune were poorly known before recent observations with Spitzer, but are substantially similar. The spectra are formed in the same way as for Jupiter and Saturn, but since the atmospheres of these planets are so cold, the power of the spectra is extremely low, which makes them experimentally very difficult to measure. Between 200 cm-1 and 400 cm-1, the spectrum was fairly well measured by the IRIS instrument on Voyager 2, and has the appearance shown in Figures 6.16 to 6.18. Ground-based microwave observations indicate very low abundances of ammonia in the observable troposphere and thus the rotational absorption lines of this gas are predicted to be completely lacking at all wavenumbers as shown. The acetylene spike at 729 cm-1 has been detected on both planets, but the signature of ethane is very weak in Uranus' atmosphere, both due to its low estimated stratospheric abundance of 1 x 10-8 and to Uranus' lower-stratospheric temperature. A similar effect can be seen in the center of the methane vibration-rotation band between 1,200cm-1 and 1,400cm-1, where Neptune shows enhanced stratospheric emission compared with the neighboring tropospheric emissions, while Uranus shows little contrast. While there are expected to be no ammonia absorption features between 750 cm-1 and 1,200 cm-1 as shown, there may conceivably be phosphine absorption features observable. We have assumed a solar abundance in these calculations, since to date the abundance of phosphine in both planets' atmospheres has not been measured, although an upper limit of 2x the solar value has been derived for Uranus from VLT observations (Encrenaz et al., 2004). Additional absorption features in this region are due to CH3D and H2-H2, H2-He CIA. In the range between 1,400 cm-1 and 1,800 cm-1, absorption features of CH3D and CH4 dominate, but then the absorption becomes due effectively to H2-H2, H2-He CIA alone since the abundance of the strong 5 ^m absorbing gases—water vapor and ammonia—is estimated to be zero and the abundance of other 5 ^m absorbing gases is unknown (although we have assumed solar abundances of GeH4 and AsH3, in addition to PH3). Hence, the calculated 5 ^m spectra appear rather smooth.
Unfortunately the real, observable 5 ^m spectra of these planets are not at all well known due to the very low power of emitted radiance in this spectral region. However, the spectra from regions on the sunlit side of these planets are expected to be dominated by reflected sunlight, as can be seen in Figure 6.19 where the reflection from a Lambertian layer placed at 3 bar and with an albedo of 0.1 has also been plotted. An early attempt to measure Uranus' 5 ^m spectrum, Orton and Kaminski (1989) found the 5 ^m spectrum to be very different in character from that of Jupiter or Saturn, but the data did not have sufficient accuracy to allow unambiguous conclusions concerning composition and cloud structure. More recently, VLT observations of Uranus' 5 ^m spectrum have allowed the detection of CO, and revised the upper limit on the abundance of phosphine (Encrenaz et al., 2004).
Although the Planck function tends to zero at longer wavelengths, the thermal emission signal is measurable with ground-based microwave telescopes and has proved extremely useful since the opacity of any aerosols, and most of the atmospheric gases, becomes small, allowing the weighting function to probe down to almost 100 bar in some cases. For Jupiter the spectrum is complicated by synchrotron emission radiated by relativistic particles trapped in Jupiter's strong magnetic field, which dominates at decametric wavelengths. The wavelength where thermal emission from the atmosphere and synchrotron emission from the radiation belts are equal is approximately 7 cm (Berge and Gulkis, 1976). Fortunately the thermal and synchrotron components of Jupiter's microwave spectrum have different polarizations and other properties which make them separable. Synchrotron emission from the radiation belts of the other giant planets is negligible.
The microwave spectra of all four giant planets are well reviewed by de Pater and Lissauer (2001), de Pater and Massie (1985), and de Pater and Mitchell (1993), and the mean spectral observations are shown in Figure 6.20. The main gaseous absorber in this region is ammonia, which has an inversion band at 1.3 cm, and the absorption of ammonia is clearly visible in ground-based microwave spectra of Jupiter and Saturn. With the excellent spatial resolution achievable with current microwave observatories such as the Very Large Array (VLA), described in Section 7.6.2, the giant planets can be mapped from the Earth at microwave wavelengths. For Jupiter it has been possible to map the abundance of ammonia across the planet and it is found that belts are regions of depleted ammonia and zones regions of enhanced ammonia (de Pater, 1986; de Pater and Dickel, 1986) (Figure 6.21) as is expected from the generally accepted view that, above the water cloud, zones are regions of upwelling, moist air, and belts are regions of downwelling, desiccated air. Similar belt/zone contrasts are seen at these wavelengths on Saturn. Although the microwave spectra of Uranus and Neptune apparently lack a clear ammonia absorption feature, indicating depletion of this species at great depth, we saw in Section 5.7.1 that variations in the abundance of this species may account for the latitudinal variation of brightness temperature seen on Uranus at microwave wavelengths; imaging of Neptune's disk at
wavelengLh (cm) wavelength (cm)
Figure 6.20. Microwave and radio emission spectra of the giant planets (from de Pater and Lissauer, 2001; and de Pater and Mitchell, 1993). Courtesy of Cambridge University Press.
these wavelengths has also been achieved. In addition, carbon monoxide and HCN have been detected at these wavelengths on Neptune.
The Planck function defines the radiance emitted by a surface of unit emissivity as a function of wavelength. There are two forms that are commonly used in IR spectroscopy depending on the unit of wavelength.
6.14 cm Total emission
6.14 cm Synchrotron Lad. subtracted
6.14 cm Total emission
6.14 cm Synchrotron Lad. subtracted
Appearance of Jupiter at 2.0, 3.56, and 6.14cm as observed by the VLA (courtesy of Imke de Pater). The increasing contribution of synchrotron emission from the radiation belts with wavelength is clear.
In terms of wavenumbers (cm !), the Planck function is defined as
where the units of B(v, T) are Wcm"2 sr _1 (cm"1)"1. In terms of wavelength (^m), the Planck function is defined as
v where the units are Wcm "2 sr"1 ^m"1. 6.9 BIBLIOGRAPHY
Andrews, D.G. (2000) An Introduction to Atmospheric Physics. Cambridge University Press, Cambridge, U.K.
Goody, R.M. and Y.L. Yung (1989) Atmospheric Radiation: Theoretical Basis (Second Edition). Oxford University Press, Oxford, U.K.
Hanel, R.A., B.J. Conrath, D.E. Jennings, and R.E. Samuelson (2003) Exploration of the Solar System by Infrared Remote Sensing (Second Edition). Cambridge University Press, Cambridge, U.K.
Herzberg, G. (1945) Molecular Spectra and Molecular Structure, II: Infrared and Raman Spectra of Polyatomic Molecules. Van Nostrand Reinhold, New York.
Hollas, J.M. (1992) Modern Spectroscopy (Second Edition). John Wiley & Sons, New York.
Houghton, J.T. (1986) The Physics of Atmospheres (Second Edition). Cambridge University Press, Cambridge, U.K.
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