This morning at 1.00 am NASA’s
In typical sensationalist style, the morning BBC newsreader announced, ‘It’s mission? To find life on Mars!’ Soon, some viewers will be disappointed to learn there is no advanced, intelligent life form, human or otherwise on Mars. It is almost certain there isn’t even rudimentary life – like bacteria – there. Not even viruses. [Which have no independent life anyway. Viruses need a living host to thrive].
Actually the lander will search the sub-surface with its robotic arm in the hope of finding any remaining evidence of water, and/or of fossils or of traces of the molecules that we consider essential to the process of life – with experiments similar to those of the earlier Viking lander (see below). We already know that liquid water once did flow on Mar’s surface, although it no longer exists in liquid form anywhere on its surface. The new site is in the far north of the planet and some little water in ice form may exist at the poles.
Let us examine the question, ‘Is it possible that life evolved on Mars?’
It is almost certain that even the most rudimentary life forms (e.g. bacteria) do not and cannot exist on Mars today nor have they likely been on that planet any time over the past several billions of years. Though Mars is a large enough planet with a solid surface and temperatures and pressure just within the limiting range for life forms to survive (it did at one time have an atmosphere capable of sustaining liquid water and has a pressure ~0.1 bar), it has for eons suffered very high ultraviolet radiation bombardment which makes it now sterile (the planet has no excess O and no O – which in the upper atmosphere of our Earth today protects us and other life forms from solar radiation).
Yet the early Mars condensed at the same time as Earth in the Solar System some 4.6 billion years ago and with not radically different conditions (life began on Earth some 4.4 billion years ago). Of course being more than four times more distant from the Sun, it was considerably colder but frequent bombardment and a dense ‘greenhouse’ atmosphere possibly resulted in surface temperatures on both early planets not radically different from that of Earth today. In any case in recent years we have evidence of life forms existing in the most extreme of temperature environments. Though some scientists speculate on an extra-terrestrial origin for Earth life – and a few that our life may have been ‘seeded’ from early Mars – it is at least as likely and vastly more interesting to consider the possibility that life evolved independently on both planets (with possibly an extra-Solar System origin!).
Though our early atmosphere (Earth’s, that is) was composed mostly of carbon dioxide and some nitrogen, we know that here there existed free oxygen (O) from at least two billion years ago – a by-product of two billion years of rudimentary life forms! – and early Earth’s interior was much hotter than that of Mars (our planet is even today volcanic though Mars no longer is) to compensate for the young sun which was ~ 20% less hot. Rudimentary life forms existed here even during the period of extra-terrestrial impacts (up to 3.8 billion years ago). Indeed repeated impacts may have destroyed early life forms so that several independent origins are possible. We know all this from direct rock evidence but we cannot know anything so detailed about early Mars for we have much less evidence.
From the one example of life we possess – that of Earth – we must deduce the conditions necessary for life to evolve (though we cannot yet tell exactly what is sufficient for its evolution). We know that life when it began evolved from the simple (bacteria) to the complex (man) and that despite many major extinctions life has survived and evolved to most complex forms. Many scientists now believe that life will spring up wherever conditions are favourable, which makes early Mars an interesting possible candidate. Let us look to the necessary conditions.
Generally we define life in terms of organic compounds of nucleic acids, proteins and polypeptides and we recognise that these must be contained in a system capable of metabolism and of self-replication. So we need a sufficient supply of suitable chemicals to construct living cells, a suitable energy source, a liquid solvent (water) in which to dissolve molecules and a solid surface to support the development of larger complex organisms. Previous Mars probes found fairly suitable early conditions: for example the Viking (a landing in the 1980s) probe, though inconclusive on the issue of the existence ever of life on Mars, did find
- some gas exchange between a surface sample and the atmosphere in the presence of organic nutrients from Earth
- the conversion of radioactive carbon 14 to radioactive carbon dioxide 14 in the presence of Martian soil
- the uptake of radioactive carbon dioxide 14 and carbon monoxide 14 by Martian soil
- that these processes occurred under a wide variety of conditions and microbes in soil are unlikely to operate under such varied conditions, but
- no evidence was found of indigenous organic molecules and
- no compelling case was therefore made in favour of life on Mars.
It is these and other such technical experiments that will be repeated over the next few weeks in a further search of the surface and the immediate sub-surface.
But we also have the evidence of Martian meteorites found on Earth. ALH 84001 is the oldest of these in our possession and so the most interesting. All are igneous rocks containing, trapped within them, samples of Mars atmosphere, the reason we know their place of origin. The meteorite we name crystallised shortly after Mars accreted but it has been extensively and interestingly altered.
NASA scientists studied it and concluded that the following .. ‘might be considered evidence for primitive life on Mars’:
… the existence of secondary mineral formation and ‘possible’ biogenic activity within fractures and pores
…it contained carbon globules of a younger age that its igneous rock
….some resemblance to terrestrial microfossils or biogenic carbonate structures
….it contained iron sulfides and magnetites which were possibly the result of oxidation and reduction reactions known to be important in terrestrial microbial systems.
….the presence of polycyclic aromatic hydrocarbons (PAHs) associated with surfaces rich in carbonate globules ..
On the other hand other scientists pointed out that …
… its grains were a factor of two times smaller than those characteristic of biogenic processes on Earth
… they don’t appear to contain cavities which in terrestrial microfossils are a sign that the fluids associated with life have been present
… no cell walls have been identified
… PAHs are not a primary biological product: the meteorite gave little indication that the necessary secondary processes had occurred.
Carl Sagan for one concluded on balance than then (some decades ago) the remnants (and remember, it’s remnants we can only hope for) of life on Mars had not been found. But he added ..
‘Who knows what lurks beneath the surface? Could life still exist there today?’
And THAT is what this latest space shot is all about!