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and also the orbits of their satellites around them. We shall see in Chapter 2 that even this very simple data led to profound implications for the internal structure of the giant planets.

Information on the temperature, composition, and cloud structure of the giant planets may be determined from ground-based observations at UV/visible through to infrared and microwave wavelengths in a number of spectral windows where the absorption of the Earth's atmosphere is low (Chapter 7). The absorption features of many molecules are observed in these spectra, which may be used to constrain composition. At certain wavelengths dominated by absorption features of well-mixed gases such as hydrogen, helium, and methane, thermal-infrared and microwave observations of the "brightness temperature" may be used to infer atmospheric temperatures over a wide range of pressure levels. Another technique that is sometimes used is stellar occultation. Occasionally the planets move in front of a star as seen from certain points on the Earth, and observations of the star's light curve during one of these occultations provides unique information on the upper atmospheres of these planets.

Ground-based observations have been improved by the advent of adaptive optics, together with data-processing techniques such as speckling and deconvolution (discussed in Chapter 7), which have greatly increased their spatial resolution. In addition, Earth-orbiting telescopes such as the Hubble Space Telescope (HST) and the Infrared Space Observatory (ISO) are capable of not only greater spatial resolution (owing to the lack of an intervening, turbulent terrestrial atmosphere), but are also unencumbered by terrestrial absorptions and so may measure the entire visible and infrared spectrum and also the ultraviolet spectrum, which contains additional information on composition and clouds.

Together with continually improving telescopic observations from the ground or Earth orbit, observations of the giant planets entered the space age on December 3, 1973 when Pioneer 10 became the first spacecraft to fly by a giant planet, Jupiter. Spacecraft remote observations of planetary atmospheres from ultraviolet to far-infrared wavelengths offer excellent spatial resolution and unrivaled phase angle coverage (the angle between the direction of the Sun and the direction of observation) of these atmospheres. Remote observations by subsequent missions—Pioneer 11, Voyagers 1 and 2, Galileo and Cassini/Huygens—have greatly increased our knowledge of the atmospheres of the giant planets. In addition to remote observations, it is possible to record the strength of the radio signal broadcast by these spacecraft as they go behind or come out from behind the planets in their orbital trajectory and such radio occultations provide highly precise measurements of the vertical density profile, from which the thermal structure can be determined. Furthermore, the Galileo mission included an entry probe, which parachuted through the atmosphere of Jupiter on December 7, 1995 providing the first ever in situ measurements of the atmosphere of a giant planet.

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