Article: Astrobiology

Astrobiology

Nobody has yet seen an extraterrestrial, which may sound like a problem in
establishing a science of astrobiology. But in the past 20 years or so,
scientists have found clues that life may be quite common in the universe,
and many are hopeful that they will soon find hard evidence of life beyond
Earth.

Some hints come from terrestrial life. Biologists have discovered many
species of extremophile - micro-organisms that thrive in extreme
environments, such as alkali lakes and rock fissures deep underground. Life
may have originated on the ocean floor around thermal vents or black
smokers, which may be common features of other planets and moons.

And chemical traces of metabolism appear in Earth's rocks shortly after the
planet's ferocious Late Heavy Bombardment by meteorites, implying that life
might be able to get started quickly and easily.

Meteorites from Mars occasionally hit Earth. Bacteria or their spores can
probably survive the journey though space, despite the cold and intense
radiation, which means that primitive life might once have been carried
between the planets of the solar system, an idea called panspermia.

Martian microbes
In 1996, a team claimed that one Martian meteorite, ALH84001, contains
fossilised Martian nanobacteria. Although much of their evidence has been
discredited, sceptics cannot yet fully explain the crystals of magnetite
found on ALH84001, which closely resemble crystals made by terrestrial
bacteria.

Whereas Mars was once thought to be dry and barren, the latest evidence from
ESA's Mars Express, and the NASA rovers, Spirit and Opportunity, shows signs
of liquid water on or near the surface. It seems that billions of years ago,
Mars was briefly warm and wet. And even now, water may occasionally flow on
the surface.

Probably a better haven for microbes is beneath the Martian surface, where
liquid water may be permanent and where the Sun's dangerous ultraviolet
radiation does not penetrate. And newly discovered methane in the atmosphere
of Mars may be the by-product of such deep-dwelling bacteria. A range of
missions are planned to look for clearer signs of life, including a new
version of the lost Beagle 2, and eventually an ambitious US sample-return
mission.

Deep dark oceans
The other inner planets seem less promising for life. Although it has been
suggested that Venus may once have been habitable, and that microbes could
still survive in its clouds - 50 kilometres above the surface where the
temperature falls below 70°C.

And Thomas Gold of Cornell University, US, has suggested that "deep hot
biospheres" exist on Mercury and our Moon, with micro-organisms digesting
the rocks as much as 10 kilometres into their respective crusts. He suggests
this could also be true of most of the rocky planets and large moons in the
solar system.

Orbiting the outer planets of our solar system, several large moons are
thought to have a deep, dark ocean beneath their icy crusts. Life may feed
off of heat and chemicals spilling into these oceans from volcanic vents on
Jupiter's moons Europa, Ganymede and Callisto, as well as Saturn's Titan and
Enceladus. The same may be true of Neptune's moon, Triton. And these oceans
may be kept liquid with a dash of ammonia antifreeze. Scientists hope to
send probes to explore Europa's ocean.

Meanwhile, Rosetta and other spacecraft are on a mission to find out whether
comets carry a payload of complex organic chemicals, as scientists suspect.
Comets might deliver this chemical "feedstock" to young worlds, giving life
a head start. Many organic molecules are created by red giant stars, and
detected by astronomers in interstellar clouds. Amino acids, the building
blocks of proteins, were found on the Murchison meteorite.

Stephen Battersby, 18 April 2005

Full Text at NewScientist
http://www.newscientist.com/channel/space/astrobiology

Posted by
Robert Karl Stonjek


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