Tenuous atmospheres

The planet Pluto as well as many satellites in the outer Solar System, such as Charon, Triton, and others belong to the group of relatively small objects with significant amounts of ices on the surface (see also Schmitt et al, 1998). The atmospheric surface pressure is then controlled by the surface temperature, the vapor pressure of the frozen volatiles, and the escape rate of the gases. Even Mars, with its CO2 polar caps, may be included in this group. Pluto and Charon are discussed together with comets and asteroids in Chapter 7. 

Measured radiation from planetary objects up to Neptune 

Though lacking the strong wavenumber variations that atmospheric spectra show as a result of gaseous vibration and rotation bands, spectra of solid and powdered surfaces can display their own characteristic signatures. More subdued than those of atmospheres (where thermal contrast is large), the spectra of solid bodies nonetheless provide significant information about the surface properties of these bodies. 

The simplest form a surface spectrum can take in the thermal infrared is that of a single Planck function (blackbody). In such a case the temperature of the observed surface can be inferred directly. More often, however, a weighted sum of blackbody spectra is required to obtain an acceptable fit, especially if the observed spectrum covers an extended wavenumber range. This implies an inhomogeneous thermal stmcture across the field of view, which can arise from different causes. 

Two common causes are lateral albedo variations, and variations in topography (surface roughness) across the field of view. The surface temperature is always 

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The cometary coma The coma and the nucleus form the head of the comet the streams of dust and gas released by the comet form a very large, extremely tenuous atmosphere called the coma, which can have a spread up to around 104—105 km. The coma is not developed when the comet is a long way from the sun, but when it comes closer (at around 5 AU), the ice mixture begins to sublime and is ejected as a gas stream. Dust particles are entrained at a velocity of around one kilometre per second. 

One other mechanism has been suggested71 which might involve recycling of carbon already in the samples or in meteorites. Carbon species vapourised and contributed to the tenuous atmosphere as a result of impact, are expected to become ionized by interaction with solar wind ions or ultraviolet radiation, (then accelerated by the electric fields present), and subsequently implanted like solar wind ions, but at lower energy. At present, however, there is no proof that this mechanism does operate. 

The active volcanoes of the Jovian moon lo release large quantities of sulfur and other materials (Spencer and Schneider, 1996) that recover the surface at a rapid rate and maintain a tenuous atmosphere. This sulfur is largely as sulfur dioxide, which is also found as condensate that covers some three-quarters of the surface. However, sulfur is also found as elemental sulfur, with perhaps traces of hydrogen sulfide (Russell and Kivelson, 2001 Zolotov and Fegley, 1998). Low pressures in the atmosphere of lo mean that sulfur can remain in seemingly exotic forms such as sulfur monoxide (SO), which has been calculated to have an SO/SO2 ratio of 3-10% (Zolotov and Fegley, 1998). Others suggest that OSOSO, and its cation, are likely present in the lo s atmosphere (Cacace et al., 2001). 

This chapter provides an overview of available noble gas data for solar system bodies apart from the Earth, Mars, and asteroids. Besides the Sun, the Moon, and the giant planets, we will also discuss data for the tenuous atmospheres of Mercury and the Moon, comets, interplanetary dust particles and elementary particles in the interplanetary medium and beyond. In addition, we summarize the scarce data base for the Venusian atmosphere. The extensive meteorite data from Mars and asteroidal sources are discussed in chapters in this volume by Ott (2002), Swindle (2002a,b) and Wieler (2002). Data from the Venusian and Martian atmospheres are discussed in more detail in chapters by Pepin and Porcelli (2002) and Swindle (2002b). Where appropriate, we will also present some data for other highly volatile elements such as H or N. 

The Moon and Mercury are usually called atmosphereless bodies. However, they are actually surrounded by tenuous atmospheres with near-surface densities not unlike those of cometary comae (cf. Stem 1999b). Atoms or molecules in both atmospheres interact predominantly with the planet s surface rather than with each other. The atmospheres of the Moon and Mercury are therefore usually called exospheres, although the behavior of an exosphere lying above a denser atmosphere is much simpler and better understood than that of an exosphere interacting with a solid planetary surface (Hunten and Sprague... 

Uranus and Neptune, are large, massive bodies, distant from the Sun, with atmospheres composed mostly of hydrogen and helium. The remaining planets Mercury and Pluto are small bodies. Mercury orbits close to the sun and has a tenuous atmosphere In which atomic sodium and potassium have been seen. Pluto lies at a great distance from the Sun and Its surface Is frozen. It may have an atmosphere containing methane, similar perhaps to those of the satellites Titan of Saturn and Triton of Neptune, which have atmospheres of nitrogen with an admixture of methane. 

While some planets have deep atmospheres, others, such as Mercury and Mars, have relatively tenuous atmospheres. For these planets (and the Moon and satellites) the atmospheres are nearly transparent at radio wavelengths, except possibly in narrow wavelength ranges, where resonant absorption lines can produce strong absorption. Thermal emission from the surfaces of these planets is easily observed at radio wavelengths it is possible to interpret the measurements in terms of the physical properties of the near-surface materials. 

A 1987 stellar occultation by Charon did not reveal an atmosphere, but it did provide a fairly good estimate of the diameter of this satellite. Stellar occultations of small objects are rare. Fortunately, in 1988 Pluto occulted a star and the light curve was observed from a number of ground-based telescopes and from the air-bome Kuiper observatory (Millis et al., 1993). Clear evidence of a tenuous atmosphere as well as fairly good estimates of the diameter were obtained. Another fortunate event was the alignment of Charon s orbital plane as seen from Earth. In 1988, that plane could be observed edge on and, consequently, Charon passed directly in front of Pluto and disappeared completely behind it. This orientation helped in the determination of the radii and in the separation of the spectra of both objects (Binzel Hubbard, 1997). 

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