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Pluto is the only planet apart from Earth with an atmosphere consisting
mostly of nitrogen, say astronomers from the US and Europe. Their discovery
offers clues to the origin of the Solar System, because Pluto has changed
little since its birth.

Pluto, the smallest planet, is at present 4.5 billion kilometres from
the Sun, farther than any planet except Neptune. Its atmosphere was confirmed
in 1988 when the planet passed in front of a star and dimmed the star’s
light gradually rather than abruptly (New Å®ÉúСÊÓƵ, Science, 30 June 1988).
However, this event could not reveal the identity of the atmospheric gases.

Now a team of astronomers, led by Tobias Owen of the University of Hawaii,
has discovered nitrogen ice on Pluto’s surface (International Astronomical
Union Circular, No 5532). In the weak sunlight illuminating Pluto, some
of the nitrogen ice must vaporise to produce a nitrogen atmosphere around
the planet.

Owen and his colleagues observed Pluto on the nights of 27 and 28 May
using the UK Infrared Telescope on Mauna Kea in Hawaii. They detected a
weak spectral line at 2.15 micrometres, a wavelength absorbed by nitrogen
ice.

Methane ice was discovered on Pluto in 1976, so methane gas must also
make up part of its atmosphere. Owen’s team found many spectral lines due
to methane, which is easier to detect than nitrogen.

But Owen and his colleagues say the dominant gas in Pluto’s atmosphere
must be nitrogen. Nitrogen has a much greater vapour pressure than methane,
and the vapour pressure indicates the ease with which a solid can become
gaseous at a given temperature. If Pluto’s surface harbours both nitrogen
ice and methane ice, Pluto’s atmosphere must contain more nitrogen gas
than methane gas, say the astronomers.

Owen’s team also discovered carbon monoxide ice on Pluto. Carbon monoxide
has a vapour pressure intermediate between that of nitrogen and methane,
so Owen and his colleagues believe that carbon monoxide is the second most
abundant gas in Pluto’s atmosphere.

‘I think the discovery is very important,’ says Jonathan Lunine of the
University of Arizona. He notes that interstellar clouds contain nitrogen,
carbon monoxide, and methane – just as Pluto and Neptune’s biggest moon
Triton do. ‘What we’re seeing is that these very cold, outermost objects
have probably retained a lot of the original molecular mix of the interstellar
cloud from which the Solar System formed,’ he says. ‘So one can use these
objects as chemical probes of the processes by which the Solar System originated.’

Two worlds, other than the Earth and Pluto, have nitrogen atmospheres,
but they are moons rather than planets: Titan, Saturn’s biggest moon, and
Triton. Like Pluto, Triton also has carbon monoxide ice.

‘Pluto and Triton seem to have the same substances on their surfaces
and in their atmospheres,’ says Lunine, ‘and that strongly suggests that
Pluto and Triton are of the same ilk.’ Since 1989, astronomers have known
that Pluto and Triton also have nearly the same size, mass, and density.

Even with the new discovery, Pluto remains mysterious planet, because
no spacecraft has yet flown past it. But American scientists hope to launch
a spacecraft in 2001 that would reach Pluto in 2015 (Astronomy, May 1992,
p 34).