Pluto and Charon

In 1930, Tombaugh discovered Pluto, the outermost known planet (Reaves, 1997 Marcialis, 1997). Several authors have derived the radius of Pluto with very small uncertainties unfortunately, the derived values do not overlap. Consequently, only a broad range can be quoted (1145 to 1200 km) within which the true radius of Pluto may fall (Tholen Buie, 1997). Pluto is by far the smallest planet of our Solar System it is even smaller than many planetary satellites. Pluto s orbit is highly eccentric and inclined by more than 17° to the ecliptic plane (Malhotra Williams, 1997). At perihelion (29.7 AU), Pluto is closer to the Sun than Neptune (30.1 AU), and at aphelion it reaches a heliocentric distance of almost 50 AU. Pluto s orbital period, 248.35 sidereal years, is locked in a 3 2 ratio with that of Neptune (Cohen Hubbard, 1965). The axis of rotation is nearly in the orbital plane therefore, this small planet undergoes rather complex seasonal changes (Spencer et al., 1997). Malhotra (1993, 1999) provides interesting discussions of the possible evolution of Pluto s orbit and that of other planets (see also Stem etai, 1997). 

In 1978, Christy Harrington (1978) discovered Pluto s rather large, but close-by satellite Charon. The radius of Charon is between 600 and 650 km, which is more than half of that of Pluto (Tholen Buie, 1997). In comparison, the lunar radius is 0.27 that of Earth. Charon orbits Pluto at a distance of 16.5 Pluto radii with an orbital period of 6.4 Earth days. It can safely be assumed that the bodies are tidally locked to each other, which means the rotation periods of Pluto and of Charon equal the orbital period of Charon (Dobrovolskis etal, 1997). Charon must be an impressive sight observed from Pluto hovering over the same equatorial area, it would appear nearly 7.5 times the diameter of the Moon as seen from Earth. Even more dramatic would be Pluto observed from the surface of Charon its apparent diameter would be nearly 14 times the lunar diameter. In contrast to this, the diameter of the Sun subtends only 38 and 48 arcsec as seen from the aphelion and perihelion positions of Pluto, respectively. The maximum angular diameter of Jupiter seen from Earth is about 46 arcsec. 

It is difficult to resolve Pluto and Charon using Earth-based telescopes, because their angular separahon never exceeds 0.9 arcsec. An early picture showing both objects clearly resolved and separated was taken in 1990 with the 3.6 m Canada-France-Hawaii telescope on Mona Kea, Hawaii (see the note by Cmikshank et al.. Science, 27 August, 1999, page 1355). The complicated dynamics of the Pluto-Charon binary system is discussed by Dobrovolskis et al. (1997). 

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). 

Spectroscopic evidence collected from Pluto before the discovery of Charon is in general applicable to both bodies. For example, the first near infrared multispectral radiometry of Pluto (and Charon) showed the signature of methane. At this time, Pluto was close to perihelion (Cmikshank et al., 1976). After the discovery of Charon, it became pmdent to take advantage of the transit and occultation of Charon mentioned above to separate the spectra of both objects. Near infrared spectra were taken first with both bodies in the field of view and then at a time when Pluto occulted Charon completely. In the latter case, only Pluto contributed to the signal, while the difference spectmm (both objects minus Pluto only), was then due 

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In August 2006, the International Astronomical Union redefined the term planet and decided that the former ninth planet in the solar system should be referred to as a dwarf planet with the number 134340. The dwarf planet Pluto and its moon, Charon, are the brightest heavenly bodies in the Kuiper belt (Young, 2000). The ratio of the mass of the planet to that of its moon is 11 1, so the two can almost be considered as a double planet system. They are, however, quite disparate in their composition while Pluto consists of about 75% rocky material and 25% ice, Charon probably contains only water ice with a small amount of rocky material. The ice on Pluto is probably made up mainly of N2 ice with some CH4 ice and traces of NH3 ice. The fact that Pluto and Charon are quite similar in some respects may indicate that they have a common origin. Brown and Calvin (2000), as well as others, were able to obtain separate spectra of the dwarf planet and its moon, although the distance between the two is only about 19,000 kilometres. Crystalline water and ammonia ice were identified on Charon it seems likely that ammonia hydrates are present. 

Krasnopolsky, "V.A. Middle ultraviolet spectroscopy of Pluto and Charon. Icarus 2001, 153, 211-2 4. 

The Pluto-Charon system raises additional questions because it lies within the Kuiper Belt, which extends about 20 AU (astronomical units about 3 x 109 km) beyond the orbit of Neptune. NASA s New Horizons space mission, launched in January 2006, should help provide a better understanding of the nature of Pluto and Charon, especially in relation to other Kuiper Belt Objects. 

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. 

The history of the formation processes of the two now very close but distinctly different objects (Pluto and Charon) is discussed by Dobrovolskis etal. (1997) and by Stem et al. (1997). A collision scenario, similar to that invoked in the formation of the Earth-Moon system, is presently the preferred theory. Information on Pluto and Charon can also be found in articles by Lunine et al. (1989) and by Buie (1992). 

Binzel, R. P. Hubbard, W. B. (1997). Mumal events and stellar occultations. In Pluto and Charon, 85-102, ed. S. A. Stem D. J. Tholen. Tucson University of Arizona Press. 

Pluto has a large satellite, Charon, which was discovered in 1978 (Fig. 3.20). A comparison between these two objects shows that Charon might be the result of a large impact on Pluto (like the formation of the Moon). Pluto and Charon are gravitationally locked. They always keep the same hemisphere to each other. The average distance between Pluto and Charon is only 19 570 km. The average density of Charon is 1.65 gcm and its mass is 0.11 that of Pluto. 

Owen et al., 1993 [256] studied surface ices and composition of Pluto by observations in the 1.4 to 2.4 micrometer range. They claimed that frozen nitrogen is more abundant than other ices. Analyzing spectra of the two objects from Pluto and Charon in the range from 1.0 to 2.5 micrometers showed clearly that ... 

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