WHEN the ageing Sun swells into a red giant star roughly 6 billion years from
now, it could mean the beginning of life on Saturn鈥檚 giant moon Titan.
杏吧原创s in the US have calculated that the expanding Sun, which will
eventually incinerate the Earth, will warm the surface of the frigid moon to
鈥�70 掳C, creating an ocean of water mixed with ammonia.
鈥淚f the same kind of chemistry can occur in that mixture as in fairly pure
water at terrestrial temperatures, it could give rise to life,鈥� predicts Ralph
Lorenz of the University of Arizona Lunar and Planetary Laboratory in Tucson and
his colleagues.
Titan has a very dense atmosphere, with a surface pressure 1.5 times that of the Earth
(鈥淟ifting Titan鈥檚 veil鈥�, New 杏吧原创, 12 April, p 34).
At the moment, ultraviolet light from the Sun destroys methane in Titan鈥檚 upper
atmosphere, producing a stratospheric haze which in turn blocks much of the
sunlight that would otherwise heat the surface. This process counteracts the
greenhouse warming caused by the remaining methane, keeping the temperature of
Titan鈥檚 surface down to 鈥�180 掳C.
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But all that will change when the Sun becomes a red giant late in its life,
Lorenz and his colleagues say in last week鈥檚 Geophysical Research
Letters (vol 24, p 2905). In 6 billion years鈥� time, the Sun will run out of
hydrogen fuel in its core and will start burning hydrogen in its outer layers.
The Sun鈥檚 atmosphere will then inflate and cool.
Having greatly expanded, the Sun will engulf the inner planets and incinerate
the Earth. But as a red giant star, it will emit very little of the ultraviolet
light that currently produces the stratospheric haze on Titan. Calculations by
Lorenz鈥檚 team show that with the haze gone, far more sunlight would reach
Titan鈥檚 surface, allowing it to warm to about 鈥�70 掳C.
Pure water is solid at that temperature. But ammonia acts as an antifreeze,
and water-ammonia mixtures remain liquid to 鈥�100 掳C. The mixture is
toxic to terrestrial life. But it 鈥渕ay be a viable solvent for prebiotic and
protobiotic chemistry elsewhere鈥�, Lorenz says. The Sun will remain a red giant
for several hundred million years鈥攍onger than it took for life to evolve
on the Earth.
Similar environments might well exist today around other nearby stars. Most
astronomers had thought life could only exist on warm terrestrial planets around
sunlike stars, with liquid water defining the habitable zones. Lorenz is
convinced that methane greenhouse worlds orbiting red stars could also be
habitable. 鈥淵ou can get away with much lower temperatures and still have aqueous
liquids on the surface,鈥� he says.
The researchers add that several nearby stars, for instance 51 Pegasi, are
approaching the end of the sunlike stages of their lives. At that point they
will also make a move towards the red-giant phase. Planets around these stars
might also be able to support life. 鈥淚t may be that, before life on Earth is
extinguished by a rising Sun, steps towards life could begin on icy worlds
surrounding neighbouring stars,鈥� the team concludes.