IF NASA鈥檚 Martian microfossils really are the remnants of past life on
the Red Planet, then the implications for life and how it got started are
profound. The first thing that would have to be explained is why ancient
microorganisms on Earth and on Mars are apparently so similar.
Some scientists, such as Ian Crawford of University College London, believe
that the resemblance may be only superficial. 鈥淲e have not the slightest idea
whether Martian organisms used anything like DNA or terrestrial cell machinery,鈥�
he says. Even so, the fact remains that the fossils do seem much like some
terrestrial fossils.
One possible explanation is that there is a single route from a primordial
chemical soup to a simple living organism. However, this is a view which Chandra
Wickramasinghe of University of Wales, Cardiff, considers 鈥渂izarre鈥�. 鈥淚t鈥檚
inconceivable that life got going on Earth and Mars and followed the same
谤辞耻迟别.鈥�
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For more than twenty years, Wickramasinghe and his colleague Fred
Hoyle have maintained that planets are 鈥渟eeded鈥� from space by comets carrying
primitive organisms such as bacteria and viruses. 鈥淥ur hypothesis is that life
is a cosmic phenomenon,鈥� says Wickramasinghe. 鈥淭he reason that primitive life on
Earth and Mars were similar was that both planets were seeded by similar
辞谤驳补苍颈蝉尘蝉.鈥�
The seeding idea would also explain the puzzle of how life got off to such a
quick start on Earth and Mars鈥攁lmost immediately conditions were suitable
for life. Crawford accepts the idea as one possibility. The other, he says, is
that given the right chemical brew the emergence of life is almost inevitable.
鈥淢y guess is that simple life is easy to produce,鈥� he says.
Clearly, another big question is: Is there life on Mars today? 鈥淚t certainly
can鈥檛 be ruled out,鈥� says Wickramasinghe. 鈥淚n fact, I feel confident that it
still survives.鈥� On Earth, says Wickramasinghe, life survives under glaciers and
in hydrothermal vents deep in the ocean. 鈥淭he lesson of microbiology over the
past decade is that life can survive extremes that no one guessed before,鈥� he
says. 鈥淚f life got going in an environment that was even remotely like Earth, I
can鈥檛 see anything stopping it.鈥�
Crawford, who is more pessimistic, believes that if there was life on Mars,
it must be extinct. He cites James Lovelock鈥檚 Gaia hypothesis. 鈥淚n Lovelock鈥檚
models, life either flourishes after tailoring the environment to suit its needs
or peters out. If life had survived on Mars, it would have taken over the
planet鈥攊t would not simply be hanging on.鈥�
Crawford admits, however, that conditions for life may have existed on Mars
even in the relatively recent past. 鈥淭he giant volcano Olympus Mons formed only
about 100 million years ago,鈥� he says. 鈥淚t could have outgassed a large amount
of material and water may have run on the surface.鈥�
Wickramasinghe points out that as Mars wobbles, or precesses, on its axis, it
may warm up enough to release much of the carbon dioxide and water in its polar
caps into the atmosphere twice each 50 000-year precessional cycle. 鈥淭here may
be domains of liquid water under the carbon dioxide ice caps even today,鈥� he
says.
Both Wickramasinghe and Crawford agree that if there is life on Mars today it
will probably require a mission to the planet to find it definitively. Before
that, however, there are things that can be done from Earth to bolster the case.
Crawford suggests high-resolution images of Mars might reveal active
hydrothermal vents near volcanoes. He also suggests looking for signs of
sedimentary rocks which might contain fossils of the right age. And stepping up
the search and study of meteorites in the Antarctic ice could help, too.
鈥淓ventually, we鈥檒l have to go there, though,鈥� he says.
Wickramasinghe thinks it may be worth hunting more carefully for the spectral
signature of organic matter from bacteria in the material ejected into the
atmosphere in seasonal eruptions from the Martian ice caps. 鈥淚n the past, people
have only looked half-heartedly,鈥� he says.
There are phenomena elsewhere in the Solar System that are hard to explain in
the absence of biological activity, says Wickramasinghe. For example, there is
the odd coloration of the cracked surface of Jupiter鈥檚 moon Europa, which might
be caused by organic matter, and the small quantities of oxygen on this
moon.
Wickramasinghe is in no doubt that Earth and Mars are not alone in harbouring
life. 鈥淚 believe there is life on every bit of the Solar System that has a
habitable region,鈥� he says. Crawford is not so sure. 鈥淚n their youth, Earth and
Mars were similar but moons such as Europa and Titan were simply too different,鈥�
he says. 鈥淥f course, life may also have got going on Venus but then got wiped
out by the runaway greenhouse effect.鈥�
Crawford says that the prospects for life in the rest of the Galaxy are now
much higher. However, he cautions that we are talking about primitive life not
intelligent life. 鈥淭he fact that it took life on Earth nearly 3 billion years to
go from the single-celled to the multi-celled stage implies that this step is
very hard,鈥� he says. 鈥淧lanets with primitive life may be common but not ones
with advanced civilisations.鈥�
