Groundbased Observations Of The Giant Planets

There are a number of extremely large and sensitive telescopes across the world that may be used for planetary observations, each of which is equipped with a range of instruments for recording images or spectra of the giant planets. There are enormous advantages in ground-based observations including: (1) long-term monitoring of slow changes (e.g., such as decadal variation in the disk-averaged albedo of Neptune); (2) the flexibility of being able to record with variable spectral resolutions, in particular very high resolution and thus discriminate between individual gaseous absorption lines; and (3) the ability to observe at short notice should something unusual and unexpected occur on the planet in question. However, there are also disadvantages in that the available observing time for these telescopes is very limited, and any time that might be allocated for investigating a particular question may be foiled by weather conditions. In addition, there are also other significant problems of observing the giant planets from the surface of the Earth, including terrestrial atmospheric absorption, angular resolution, and brightness, which will now be discussed.

7.3.1 Terrestrial atmospheric absorption

From the UV through to microwave wavelengths the absorption of the Earth's atmosphere makes some regions of the giant planet spectra completely unobservable from the ground. This can be seen in Figure 7.3 where the vertical transmission to space from the ground has been calculated from UV to microwave wavelengths, both from sea level in a standard atmosphere and from an altitude of 4,000 m in a dry atmosphere. A number of strong absorptions throughout can be seen. At UV wavelengths below 0.3 ^m, the Earth's atmosphere is effectively opaque due to photolysis of ozone (03) and molecular oxygen (02) is the stratosphere. In the visible/near-infrared most of the absorption below 3 ^m is due to water vapor. At 4.3 ^m a strong absorption band of C02 appears, but then between 6 ^m and 9 ^m (1,600 to

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