Update, 29 March 2004.
Traces of Methane
have recently been detected in the Martian atmosphere; which suggests the possibility of bacterial activity on Mars. While this is evidence against the conclusion that Mars is entirely barren, it vindicates the method described below. That is, you don't need to look under rocks on Mars to find the life there. The Martian methane can and has been detected by observatories located on the surface of Earth. Stay tuned.
Dr James Lovelock was first to articulate the reasons why Mars is barren. Put simply, let us look at the only example that we have of a world where life exists: Earth. How could we devise a subtle test to determine the existence of life on earth's surface? We don't need to; it sticks out a million miles away. Green continents. Odd atmospheric composition.
Life radiates to all available niches, it diversifies, it takes over, it envelops and transforms. Life doesn't just keep a foothold on a planet. If it is present at all, expect it to be almost everywhere on or near the surface. Expect entire geological phenomena such as coal and chalk to be caused by living things. Expect the planetary atmosphere to have puzzling components, like 21% highly reactive oxygen and traces of methane.
Sure, earthly life would have a tough time just keeping a foothold on Mars. But with Martian life, we would even expect like to not arise at all unless it did so in a form suitable to the prevalent conditions, and be further honed by hundreds of millions of years of adaptation.
It is likely that the concept of a planet having traces of life is not a valid one: there will be diversity and many filled niches, or nothing. Even if there are or were a few bacteria on mars, then it's not what we think of as life: there is no ecology, no biosphere, no diversity.
Looking for life on Mars is like the old story of the drunk looking for his keys under the lamppost because there's more light there. We look for life on Mars because it is nearby, not because it is a good place to look.
Of course, these thoughts led Lovelock on to viewing earth's biosphere as in integrated homeostatic system, the the Gaea hypothesis.
I have pilfered Lovelock's own words from Resurgence Issue 187
With Mars the question was, is life there? In the early 1960s not much was known about Mars. Its surface was poorly visible through telescopes and it was easy to imagine that the seasonal wave of darkening that moved across the planet was due to the growth of vegetation.
My colleagues at the Jet Propulsion Laboratory and in American universities were all busy designing instruments to test for life or lifelike chemicals on the Martian surface. And they were trying to put into practice in an automated form the very procedures that they were familiar with in their own laboratories here on Earth. Some of these experiments involved applying Martian soil to culture media to see if organisms would grow; others looked for metabolism to see if oxygen was produced in sunlight, or CO2 in the dark. I found this detailed reductionist approach to life detection for Mars unconvincing. It could fail to detect the presence of life for many reasons. It might not be bacterial, the experiment might land at a barren site, or Martian biochemistry might be different. I suggested a more general experiment, such as a top-down view of the whole planet instead of a local search at the site of landing. The experiment I proposed was simply to analyse the chemical composition of the Martian atmosphere.
If the planet were lifeless, then it would be expected to have an atmosphere determined by physics and chemistry alone and be close to the chemical equilibrium state. But if the planet bore life, organisms at the surface would be obliged to use the atmosphere as a source of raw materials and as a depository for wastes. Such a use of the atmosphere would change its chemical composition. It would depart from equilibrium in a way that would show the presence of life.
Dian Hitchcock joined me then and together we examined atmospheric evidence from the infrared astronomy of Mars. We compared this evidence with that available about the sources and sinks of the gases in the atmosphere of the one planet we knew bore life, Earth. We found an astonishing difference between the two atmospheres. Mars was close to chemical equilibrium and dominated by carbon dioxide, but the Earth was in a state of deep chemical disequilibrium. In our atmosphere carbon dioxide is a mere trace gas. The coexistence of abundant oxygen with methane and other reactive gases, is a condition that would be impossible on a lifeless planet. Even the abundant nitrogen and water are difficult to explain by geochemistry. No such anomalies are present in the atmospheres of Mars or Venus; their existence in the Earth's atmosphere signals the presence of living organisms at the surface. Sadly, we concluded, Mars was probably lifeless.
And the same theme from Gaia, Our Living Earth
There was a senior person at the Jet Propulsion Labs who was a nice but authoritarian man, whom people were a bit frightened of. One day, he called me into his office and asked, "What do you think of these life detection experiments?" I said I did not think much of them. Even if there were life on Mars, I thought there was a poor chance of finding it with the kind of equipment they were proposing to send. For the most part, scientists were just automating their own labs and sending them to Mars. That is not a good way of finding life somewhere else. I said so, and then he asked me what I would do. I replied that I would look for an entropy reduction on the whole planet. He laughed, saying that was a cop-out, because if the discovery of entropy reduction would indicate the presence of life, he needed to know how to do it. So I said, "Give me three days, and I will come back with an experiment for discovering entropy reduction."
I returned with the idea of analyzing the chemical composition of Mars's atmosphere. It was based on the premise that if there was life on Mars, it would be obliged to use the atmosphere as a source of raw materials and as a depository for wastes, thereby altering its atmosphere and disturbing its chemical equilibrium. In 1965 I had a little paper in Nature called "The Physical Basis of Life Detection" that proposed physical tests for the presence of planetary life. One was a top-down view of the whole planet, instead of a local search at the site of landing. Using it, an observer on a spacecraft beyond our solar system could recognize that Earth is the only planet in our system that has life.
My idea, when applied to what was known about Mars from infrared astronomy, suggested Mars was lifeless. Therefore it was not a popular experiment among scientists looking for life on Mars. NASA liked my approach, but the biologists did not. In the end, my experiment was never flown. The team thought they could get enough compositional analysis of Mars's atmosphere from a mass spectrometer attached to the Viking spacecraft. In fact, the instrument was too dedicated to analyzing the soil to look for life-characteristic substances.
Since this has been aired on slashdot, I have accumulated a few arguments against this position. Lets go through them:
Mars may be barren now, but it was once warmer, wetter and capable of supporting earth-like life. Mars may have had an ecosphere.
Wouldn't that have left rather large relics behind? Strata of coal or chalk? How long does it take a planet's atmosphere and surface to return to an entropic equilibrium? It seems to be at equilibrium now, so there is no recent large-scale life. I have no idea on how long recent means in this context.
The wastes of Antarctica support life, albeit microscopic and slow-growing life. Why should the wastes of Mars be different?
The Antarctic bacteria did not evolve there, they moved in from more hospitable climes. They are originally evolved for more temperate climes, and certain irreversible design decisions (e.g. liquid water as a solvent) were made.
No wonder they don't thrive at Antarctica even after adaptation. Ok, so life doesn't really always 'take over'.
But maybe Mars once had life suited to the old conditions, that just got as far as simple, single-celled life, before the conditions turned hostile. It could still be there, eking out a toehold, just like the Antarctic bacteria do. If life either adapts or dies out, why isn't Antarctica either green or completely barren
This may be a valid point, though it ignores some of Dr Lovelock's other work that life actively works to maintain a homeostatis of the plantary ecosphere.
Still you'd expect some subtle atmospheric changes if there was still some limited life on Mars. So far, none have been found. To investigate this further, start by calculating what effect the Antarctic bacteria have on Earth's atmosphere.