This is a quotation from the book “Greetings, Carbon-based Bipedals!” by the famous science writer, the late Arthur Clarke — Editor: “Is there life on Mars? Report in 1000 words”. Reporter: “Nobody knows”, repeated 500 times.
NASA has just landed the rover “Curiosity” on Mars, and ISRO is sending a (yet-to-be-named) spacecraft in 2013 which will orbit Mars and transmit data from Mars to us. Among the various issues that these ships want to interrogate on/in Mars is the question of whether the red planet harbours life forms in it.
People have long fantasized on extraterrestrial life and even Martian equivalent of humans. Sci-fi books and movies play on this theme. Even Clarke’s own book title above paraphrases such aliens greeting us humans on earth.
Note he refers to carbon-based life, which is what all organisms living on earth are. Why this bias on carbon, when there are a hundred other elements? It is the extraordinary ability of the carbon atom, with its penchant to link bonds with four other atoms — be it other elements such as N, O, S, or C itself, at temperatures, pressure, moisture and other conditions that Earth provides which makes life possible.
For life to occur, flourish and sustain, the information content — both hardware and software — needs to be encrypted, stored, read and passed on down generations. The best material to do this is through molecules made of chemical bonds.
And of the 100-odd elements in the universe, carbon fits the bill under the conditions ambient on earth.
Carbon makes long chains or tapes using itself as the backbone of this chain. And carbon makes stable chemical links with other abundant elements such as N, O, S, and P. The chemistry of life on earth is written using this handful set of elements as the basic alphabet. DNA (and occasionally RNA), the genetic tape of all life forms on earth is such a carbon-based tape.
And it is the shuffling of the information in this genetic tape that has led to the diversity of life forms on earth, all evolved from an ancient ancestral life form, our own pro-biont.
Why carbon? Why not its neighbours nitrogen or boron, or its follower silicon? Nothing really, except under the ambient conditions on and surrounding the earth, the carbon chains stay stable, easily formed and changed, so that diversity in biological information encoded in the chemistry is facile under the environmental conditions obtained on earth.
There could well be conditions elsewhere in the universe where, say, silicon chains can play the biological information storehouse, but we are yet to find them. Herein lies the excitement of extraterrestrial life-seekers.
What have been the special conditions on Earth that made life form, flourish, diversify and sustain? What makes Earth life-friendly?
First, the surface and the top crust of the earth have provided the laboratory for making the molecules of life. Go beneath, it is too hot, too metallic and too high pressure.
Second are climatic conditions. Terrestrial temperature ranges between -40 to +50 degrees C (average around 20) on the surface. Higher than this, chemical reactions speed up (every 10 degree rise doubles the speed) and can change the chemical composition; below this, reactions run too slow.
Third is water, which is a liquid under these climatic conditions.
It is an excellent solvent, keeping in solution many life molecules, allowing them to mix and react. Water has the right size, melting and boiling points, vapour pressure, viscosity, polarity.
Chemistry in the liquid solution phase is optimal. In solids, reactants are hampered in motion while in the gas phase they tend to fly off.
Fourth, our moon is of the right size, able to control the spinning of the earth and its axis of rotation, thus stabilizing our climate and seasons.
Fifth is the ozone layer, which shields molecules on earth from being zapped away by cosmic radiation.
Sixth, the gravitational strength of Earth keeps useful gases such as N, O, CO and O from escaping off into space.
Seventh, we have a geomagnetic field which protects us from solar flares and storms.
Now look at Mars. It is deficient in several of this seven-fold path that sustains life on Earth. We are yet to determine whether there are regions that have soil on its crust, helping any life chemistry. Curiosity will hopefully let us know.
Martian climate is a disaster for carbon-based life. Its surface temperature is minus 55 degrees, too cold for covalent chemical reactions to run at desirable speeds.
Its two tiny moons are more adornment than adjusters of climate or seasons. Water — perhaps there was some water in earlier times, but many believe that the dry channel like strips seen on the Martian surface come from such runoffs of water that once might have been.
Discovery of water bodies on or in Mars, if any, would be important. Also, given the atmosphere (or paucity of it) and the resultant pressure it exerts on the Martian surface, even if water were there, it would stay as liquid only in the 0-10 degree range, above which it would boil off. In addition, Martian gravity is just about one fourth of Earth’s; so, it would not be able to keep gases like N, O, CO and O (if at all) as well as we do. And its magnetic field is too feeble to shield off solar flares. (For more details, please look up www.biocab.org/LifeOn
Mars.html, from where I got these data).
So, is Mars devoid of life? Not necessarily. Recall even on Earth, we have life forms that live at 120 degrees, in frigid ice-cold climes, and we find microbes that handle radioactive rays with elan.
Hence, let us not dismiss such possibilities in yet-to-be-discovered enclaves in Mars. NASA and ISRO have their jobs cut out. But Carbon-based Bipedal Martians are pure sci-fi.
What then about non-carbon life, or non-water life? Say, silicon-based, or ammonia-based life chemistry? As the reporter wrote 500 times: “nobody knows”, but let us not dismiss it yet.
This article has been corrected for a factual error.