1. Some general considerations
2. Thermal structure and composition
3. Atmospheric Chemistry
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Information about Pluto's atmosphere comes from a variety of sources. Direct information was first obtained during the occultation of a 12th magnitude star by Pluto in 1988. Measurements of the composition and physical state of the surface also bear directly on the atmosphere because we believe that the composition and structure of the atmosphere is determined to a large degree by its interaction with the surface. Less direct information is obtained by comparing Pluto with Triton. Triton is roughly the same size as Pluto, and likely formed in a similar orbit around the sun (McKinnon, 1984; Goldreich et al., 1990). For these reasons, as well as because the atmospheres of both Triton and Pluto are predominantly , Voyager observations of Triton should provide a good guide to the range of phenomena to be expected in Pluto's atmosphere. Finally, we can rely upon physical theories to help frame questions about Pluto's atmosphere, bearing in mind that specific predictions about the physical state of an unstudied atmosphere are difficult and likely to be unsuccessful. What we do know about Pluto suggests an atmosphere which is both varied and extreme in many ways. Because of the expected large variations in the surface temperature (a consequence of the observed albedo patterns and volatile distribution) the atmospheric structure near the surface is likely to exhibit large geographic variations: horizontal temperature variations may be as large as a factor of two (on Earth a 10% variation is considered large). There are suggestions, based on the occultation data, that the vertical temperature gradient in the atmosphere could be as steep as 20-30 K/km. The atmosphere contains at least three condensible species, namely ( , CO, and ). The interplay between these atmospheric species and the associated surface ices should be complex, with an interesting analogy to and on Mars. Pluto has the most weakly bound atmosphere in the Solar System and consequently the atmosphere which is lost most rapidly, relative to the atmospheric bulk.
When thinking about Pluto's atmosphere it is important to remember that the knowledge based directly on observations is limited and that in the history of outer Solar System exploration nature has repeatedly demonstrated an imagination superior to our own. The atmospheres in the outer Solar System have proved to be more varied and interesting than predicted by earthly investigators. It is extremely unlikely, for example, that the geysers on Triton could have been predicted (or that such a prediction would have been taken seriously by the scientific community). The same is likely to be true of Pluto. Planetary exploration remains an observational science, and a mission is needed in order to understand Pluto. The description below follows this point of view.
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Thermal structure and composition
In 1988 Pluto occulted a 12th magnitude star (Hubbard et al., 1988; Elliot et al., 1989; Elliot and Young, 1991; Millis et al., 1993). Our knowledge of Pluto's atmosphere is based largely on observations of this event. The occultation and its implications have recently been reviewed by Yelle and Elliot (1995). A brief summary is given here. The occultation was observed by several ground-based and airborne observatories. Although much can be learned from the simultaneous analysis of the entire occultation data set (Millis et al., 1993), the data obtained with NASA's Kuiper Airborne Observatory (KAO) have the highest signal-to-noise ratio and supplies most of the basic information on the state of the atmosphere. The data along with model fits are shown in figure 2. Both ingress and egress occultations were observed and, to within the accuracy of the data, the light curves appear to be identical. The occultation observations probe the atmosphere in the pressure region from several microbars to several tenths of a microbar and within this region there are clearly some changes in the structure of the atmosphere. An abrupt change in the slope of the light curve occurs at 1215 +11 km, where the pressure is 2.33 + 0.24 bar. The nature of this change is discussed further below. We first describe the atmosphere above 1215 km because the structure in this region appears to be fairly simple.
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In addition to the species supplied by the evaporation of surface ices Pluto's atmosphere should contain molecular, atomic, and ionic species produced by photochemistry. The situation should be similar to that on Triton, with important differences due to the larger abundance, potentially different CO abundance, distinct energetic particle environments, and disparate atmospheric temperatures. The chemistry of Triton's atmosphere has proved to be complex (cf. Summers and Strobel, 1995). The main feature is a very close connection between the neutral photochemistry of the lower atmosphere and the ion-neutral chemistry in the ionosphere. The presence of and in the atmosphere implies the presence of photochemically produced species such as , along with other hydrocarbons and nitriles. Photochemical model calculations for the abundance of minor constituents have been presented in Summers and Strobel (1995). These exploratory calculations are a useful way to study the physical and chemical processes in Pluto's atmosphere. However, there is a lack of observational constraints on the minor constituents and uncertainties in basic atmospheric parameters such as surface pressure, bulk composition, vertical mixing rates, aerosol content, etc., and uncertainties in the values of reaction rates at low temperature. In consequence, a wide range of results are possible and the models do not have much predictive capability. Nevertheless, although the abundance of minor constituents cannot be predicted with confidence, the Summers and Strobel models probably do provide a good guide to the types of minor constituents likely to be in the atmosphere. An illustrative calculation, showing density profiles for some of the photochemically produced species is presented in figure 3.
Figure 3: Calculations of the composition of Pluto's atmosphere from Summers and Strobel (1996). These photochemical calculations assume that the atmosphere is predominantly with small amounts of and CO; the distance of Pluto from the sun is set at 40 AU, yielding a surface pressure less than that derived from the 1988 stellar occultation which occurred near perihelion.
Pluto presents another example of an interesting problem in the evolution of volatile atmospheres in the outer Solar System. is irreversibly lost from the atmosphere because photolysis liberates H and , which rapidly escape. The loss rate for atmospheric due to photolysis is on the order of ten thousand years, much shorter than the age of the Solar System. Either the initial endowment of ice on Pluto's surface is sufficient to resupply the atmosphere over the age of the solar system, or must be supplied from a reservoir in the interior. If the latter possibility is correct, it implies the existence of geological processes which transport from a subsurface reservoir to the surface. Another consequence of photolysis is the production of higher order hydrocarbons (such as those predicted by the chemical models mentioned above) which eventually condense and end up on the surface, yet no other hydrocarbon species have been detected. These same questions appear in slightly different guises on both Triton and Titan. More thorough searches for photochemical products are required and the theoretical mechanisms of photochemistry in the atmosphere need to be observationally tested.
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