Since
the subject of life on Mars has been well-documented, I won't linger
long on the topic, but present a brief overview, as no guide of
extraterrestrial life would be complete without a mention of Mars.
A brief history of life on Mars
We
have long been fascinated by the idea of there being life on the Red
Planet. The popularity of the concept of Martians underwent a mass
explosion during the 19th
century. It was theorised by scientists of the time that Mars was a
dying world which was slowly becoming cooler and drier, and was the
home of a civilization which was attempting to communicate with
Earth. It was even suggested that a set of giant mirrors should be
constructed in Texas so that we might signal to our Martian friends.
The
idea of Mars being a dying planet was reinforced in novels such as H.
G. Wells' The War of the Worlds,
in which he depicts the Martians invading Earth.
It
wasn't until more modern times that we finally recognised Mars not as
a dying world inhabited by strange beings, but as a barren landscape
on which life was impossible.
. . . Or was it?
Is it possible?
The
argument for the habitability of the Red Planet was that liquid water
once existed on the surface of Mars, and water is, according to our
current scientific understanding, essential to life as we know it.
Canyons and dried up riverbeds carved by flowing water can be seen on
Mars, clear evidence that rivers once flowed across the planet.
Indeed, the planet is in the habitable zone – or Goldilocks zone –
along with Earth and Venus. Scientists are now fairly certain that Mars was once a
watery, habitable world not unlike our own planet. There's a catch,though. New studies on
Martian rocks suggest that water was most likely too
acidic and salty for most life to exist there, and that only a few
Earthly microbes would even have a chance of surviving these
conditions. I have to say, although this piece of evidence towards
the lack of life is interesting, I'm not convinced. After all, we're not
talking about life on Earth,
are we? We're talking about Martian life. It is feasible that a
native Martian microbe could have evolved to be an extremophile –
in other words, an organism adapted to survive extreme conditions.
These Martian organisms could have evolved a resistance to briny and
acidic conditions, making it a halophile (an organism that can surive
and thrive in very salty conditions. Earthly examples of these
include Halobacterium, Nitzschia, Wallemia icthyophaga,
and others).
But
anyway, does it really matter? Watery Mars existed more than 3.8
billion years ago, and we don't really care about microbes that could
have hypothetically existed then. Well, it does matter, for 2
reasons. If we found fossil microscopic organisms on Mars, then this
would still be an immense scientific breakthrough – it would prove,
once and for all, that life can and does exist on other planets. But
besides that, if life could have once lived on Mars, then is it
possible that life could have survived the drying-up of Mars and
lives on today?
Life underground
The
surface of Mars is not a nice place for an organism. It's cold, and
it's being showered with radiation – and no rover has yet found any solid evidence of an
organism on the surface. Therefore, it seems more likely that if life
does exist on Mars, then it will be present under the surface. Past
volcanic activity could have feasibly created underground caverns and
openings, creating aquifers, which in turn form pools of salty water,
filled with essential minerals and possibly heated geothermally. If
this turns out to be true, then we may well find halophilic organisms
in these underground pools.
If
we are to speculate a little more, then we might ask ourselves, what
are the organisms down there feeding on? There's no sunlight, so
there's no chance of photosynthesizing algae-like species. A clue
to a possible solution lies with an odd looking organism called
Acidithiobacillus thiooxidus,
a form of bacteria which forms biofilms which drip from cave ceilings
and resemble mucous, earning them the nickname 'snottite'. It gains
energy from sulphur compounds, and its waste product is sulphuric
acid. Other organisms use iron and manganese, oxidizing it, and they
gain small amounts of energy from this. These microbes are using the
elements in the stone around them to get energy and perform bodily
functions.
It's
easy to imagine Martian microbes using a similar process to use the
stone around them and what's in it to gain energy.
Who
knows? Maybe there is microscopic life living under the surface of
Mars, in salty, acidic caves, oxidizing elements in the stone around
them to survive.