Last year, for the first time in 14 years, an alignment of stars with Pluto created an opportunity to observe the atmosphere of this most remote of planets. Though tenuous, the atmosphere has, remarkably, expanded.
Pluto is the only planet in the Solar System still unvisited by spacecraft. But an appropriate conjunction of stars with the planet can make some Earth-bound observations possible. In particular, Pluto's thin atmosphere (exerting a surface pressure roughly a million times lower than that at the Earth's surface) comes into view as the planet passes in front of, or 'occults', a bright star. On pages 165 and 168 of this issue, Elliot et al.1 and Sicardy et al.2 report the first new data on Pluto's atmosphere from stellar occultation in more than a decade. The observations are timely: Pluto's orbit over the next few years offers an opportunity to learn more about this planet, at a time when technological developments make it feasible to consider a mission to it. But both time and money are in short supply.
As Pluto is the most distant of the nine planets, its 248-year orbit around the Sun is lengthy compared with human timescales. It is also the most eccentric: Pluto's heliocentric distance varies between 30 and 50 astronomical units (one astronomical unit, or AU, is equivalent to the mean distance between the Earth and the Sun). In accordance with Kepler's second law of planetary motion, Pluto spends most time in each orbit at almost its maximum distance from the Sun and the Earth. But, in 1989, Pluto coincidentally made its closest approach to the Sun (called its perihelion) during a period of heavy spacecraft exploration and in precisely the year that the spacecraft Voyager 2 had its final encounter with Neptune and Neptune's icy satellite Triton. Triton seems to be a very similar body to Pluto and is perhaps a close relative3. The Voyager 2 encounter revealed, through a slight distortion of the spacecraft's radio link to Earth, that Triton too has a tenuous atmosphere, with a surface pressure of perhaps 10 to 20 microbar (atmospheric pressure on Earth is one bar).
At the time of the Triton encounter, Pluto was actually slightly closer to the Sun than Neptune was. But Pluto always maintains a healthy distance from Neptune, because their orbits are synchronized. So a spacecraft visit to Pluto would require a specially planned trajectory and a lengthy trip, rather than any straightforward extension of an existing mission. And as Pluto's diameter is only some 2,400 km, substantially smaller than the Moon's, is it worth the trip?
In 1988, just one year before the Voyager 2 encounter with Triton, distortion of light from an occulted star revealed that Pluto had a tenuous atmosphere, to which Triton's is somewhat similar. In fact, the earliest evidence for Pluto's atmosphere seems to have been gathered in 1985 by the Israeli astronomer Noah Brosch: observing in the Negev desert, he saw a gradual dimming of starlight caused by refraction in Pluto's atmosphere, rather than the knife-edge drop expected when an atmosphere-less planet (such as the Moon) occults a star. Brosch's data were so unexpected that he was widely disbelieved until the case was clinched by observations of the 1988 occultation by several telescopes, including NASA's Kuiper Airborne Observatory4.
What process could produce microbar atmospheres on tiny objects such as Triton and Pluto? Triton and Pluto are thought to belong to the Kuiper belt — a ring of icy bodies beyond the orbit of Neptune — and so bear a generic relation to short-period comets whose orbits pass through the belt. Like comets, they seem to be able to generate fluctuating atmospheres in response to variable solar heating of their icy surface layers. The difference, though, is that Triton and Pluto are just massive enough to retain the gas as a bound atmosphere, whereas comets produce an escaping envelope of gas, known as a coma. And because the solid surface layers of Triton and Pluto never get warmer than about 40 kelvin, any gas that erupts must be much more volatile than the water vapour thought to power the spectacular tails of those comets whose orbits bring them within a few AU of the Sun.
By a process of elimination, a likely candidate for the main gas in Pluto's atmosphere is the volatile molecule nitrogen. As Fig. 1 shows, its vapour pressure could build to a maximum at perihelion well in excess of the pressures measured in Pluto's atmosphere. But because the vapour pressure of nitrogen decreases strongly as the surface temperature falls, the atmosphere ought to collapse as Pluto's surface cools during its retreat from the Sun over the next few decades.
Following the initial detections of Pluto's atmosphere in 1985 and 1988, there was an unproductive 14-year period during which astronomers sought, to no avail, either to place portable telescopes in an occasional narrow path of Pluto's starlight shadow on the Earth, or to predict feasible occultation opportunities at fixed observatories. There were, typically, one or more aborted campaigns every few years or so. The forecast shadow paths invariably turned out to be off Earth or otherwise inaccessible. The shutdown of the Kuiper Airborne Observatory did not help. Planetary scientists have now become strong advocates for a spacecraft mission to Pluto while the planet is still warm enough to exhibit the volatile molecules embedded in its surface layers.
As discussed by Elliot et al.1 and Sicardy et al.2, Pluto's atmosphere has, unexpectedly, expanded rather than contracted over the past 14 years, but temperature variations in planetary surface layers do typically lag somewhat behind solar heating variations. In the long run, cooling and atmospheric contraction are inevitable. To understand what is going on, we need a spacecraft mission to Pluto: an occultation measurement of the atmosphere at radio wavelengths, together with ultraviolet measurements of a solar occultation, could tell us much more — and both of these require an antenna or telescope to aim past Pluto back towards the Earth or the Sun. Meanwhile, possible light extinction caused by particles in Pluto's atmosphere (reported by Elliot et al.) may be bad news for detailed stellar-occultation studies of the atmosphere, as standard analysis methods assume that there is no loss of photons.
The first of NASA's New Frontiers line of space missions will be a Pluto–Kuiper-belt mission called New Horizons5. The mission has a planned launch in 2006 and a Pluto flyby about ten years later (Fig. 1). But New Horizons is in trouble because of unexpected extra costs. Similarly, NASA's Planetary Astronomy programme, which has supported much of the Pluto occultation work in the United States, had to cancel some of the ground stations that could have provided more data. Like mountaineers preparing an assault on Mount Everest, scientists are closely watching the calendar and their budgets, hoping to reach their objective before the snows come.
Elliot, J. L. et al. Nature 424, 165–168 (2003).
Sicardy, B. et al. Nature 424, 168–170 (2003).
McKinnon, W. B., Lunine, J. I. & Banfield, D. in Neptune and Triton (ed. Cruikshank, D. P.) 807–877 (Univ. Arizona Press, Tucson, 1995).
Binzel, R. P. & Hubbard, W. B. in Pluto and Charon (eds Stern, S. A. & Tholen, D. J.) 85–102 (Univ. Arizona Press, Tucson, 1997).
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