There's something interesting in science-fiction. Go back to the 50s or 60s, and you'll see that Mars was generally populated by an alien race: the little green men of yore. The Martian civilization was perceived as ancient and noble, dessicated and dying.
But life on Mars had appeared much earlier in modern science-fiction. The earliest I know of would be H.G. Well's classic, The War of the Worlds (1898). The Martians launch an invasion of earth, in massive tripod-like machines armed with devastating heat-rays. Earth gets its ass kicked. (Let's be honest; you were rooting for the Martians, weren't you?) Of course, and you probably could guess this even if you haven't read the original, some kind of microbe ultimately kills off the invaders. And what happens to Mars? Alas, the war effort consumed its last resources: Mars is a dead world.
Not long after came the pulpy, misogynistic Barsoom series (1912-1943): Edward Rice Burroughs. He hit a literary gold mine: the ultra-manly (naked, because, why not?) John Carter rescuing the (naked, because, why not?) Dejah Thoris, Princess of Helium, from the hordes of four-armed, green Tharks. If these stories don't make you puke, you'll love 'em.
The War of the Worlds was re-done for a radio show, the infamous broadcast of Orson Welles (1938). The program was a kind of early mockumentary, with breaking “news reports” describing the Martian onslaught. Accounts of what happened are conflicting, but it does appear that a number of people across the US did actually head for the hills, somehow believing that the Martian invasion was happening. (Later the program was billed as a “hoax.” My understanding is that regular announcements were made that a particular show was being broadcast, so it seems rather many viewers just didn't connect the dots.)
I'd love to get my hands on a recording of that show.
Later writers emphasized the pathos of a dying civilization. Witness, for example, the tender Martian Chronicles (1950), a set of short stories by Ray Bradbury. Flower power led to the odd Stranger in a Strange Land (1961; Robert A. Heinlein), in which messianistic foundling returns to Earth with the power and wisdom of the Old Ones of Mars, best described as ghosts.
More recent writers have agreed that there isn't any life on Mars (and perhaps wasn't any in the first place). These stories focus on the colonization of Mars. The decent Red Mars (1992; Kim Stanley Robinson), for example, launches a series in which Mars is “terraformed”-made habitable for human life (I'd link to the Wikipedia article on terraforming, but it's skip-worthy). All that I've read recently is along the same vein; the only life on Mars is the kind that we (or others) would bring to it.
Mars in science
Let's take an overview of the science of Mars over the same time. We must begin with Giovanni Schiaparelli, an Italian astronomer who, in the late 1800s, made observations of Martian “seas” and canali (translated into English as “canals”). Percival Lowell subsequently founded the Lowell Observatory in Flagstaff, Arizona (lovely night skies in Flagstaff) for the purpose of studying the civilization on Mars. Lowell's detailed reports of canals and oases led to the idea that a Martian civilization struggled against an imminent demise.
No one really knows what Percival Lowell and Giovannia Schiaparelli actually saw, but the best guess is that the canals and oases were due to common visual error. Stare at a red dot for a minute, and look away. You'll see green. An amateur can see the same thing happen the first time he or she looks through a telescope at Mars. An experienced observer should have no difficulty accounting for this, but perhaps a little subconscious wishful thinking affected Schiaparelli and Lowell's observations.
(Mars is quite beautiful through a decent telescope. The planet is quite red, with perhaps an ice cap visible if you're looking at the right time, and a variety of darkish markings across the surface of the planet. These marks shift around with the massive dust storms, and in no way should a professional astronomer conclude these are canals.)
Finally in the 1960s, exploration of Mars began. Mariner 4 flew past Mars (1964). The Viking 2 mission included a lander (1976), which was intended to study (among other things) the possibility of whether life might be present on Mars.
One of the experiments involved the introduction of organic materials into a sample of Martian soil. It was found that the organics were converted into carbon dioxide. This is a chemical hallmark of life. The results were, nonetheless, somewhat puzzling (I won't go into detail here), and ultimately when combined with other data, virtually every scientist believes that the Martian tested negative for the presence of life. Just this one particular experiment was tantalizing.
(As an aside, please, please don't believe anything you read on Wikipedia regarding the proven existence of life on Mars. It hasn't been proven. The discussion of “Gillevinia strata”, a putative Martian microorganism, by the Viking lander article is outright lunacy.)
Mars is not a hospitable world. It is dry, dusty, wracked with storms, very cold, essentially without oxygen (what little atmosphere there is, is 95% carbon dioxide, a poison to us but food for plants and many microorganisms), and bathed in strong ultraviolet radiation. Still, we now know that Mars is perhaps not so inhospitable as it seems. Water existed on Mars, in the form of rivers, gullies, river deltas, oceans. Liquid water! This is truly the elixir of life; it is the chemical foundation on which all known life depends. If liquid water existed on Mars, then perhaps, just perhaps, life had arisen on Mars as well. (This is why there is such excitement this month about finding water ice on Mars. This is Big News.)
If life ever did exist on Mars, it could very well have adapted to the conditions now present. Bacteria have been found in a variety of incredibly hostile environments on earth: the extremophiles. Bacteria can thrive in the cold, in extremely dry conditions, and even inside rocks (no, really). It's still tantalizingly possibly that some kind of life might still be present on Mars, perhaps near as-yet-undiscovered underground deposits of frozen water.
Several years ago, researchers reported the presence of fossil life in a Martian meteorite. This was later debunked: the same “fossils” (structures many times smaller than the smallest microorganisms known on Earth) can be produced in the lab through purely non-living reactions. More intriguingly, the same meteorite yielded magnetic minerals that are known on Earth to be formed only by microorganisms. Could this be evidence of (ancient) life on Mars? Hmm...
Well, maybe there was life on Mars billions of years ago, when Mars was warmer and wetter. Of course no one knows whether life forms easily or extremely rarely. If life forms very easily and rapidly, well, then likely Mars did have life... and if it did, then maybe it still has life now. If life is unlikely to form, well, the odds are against it having been on Mars in the first place.
(I have absolutely no space to discuss the possible transition between a planetary chemical bath and living, eating, breathing, reproducing life, but please follow the link in this sentence. On Earth, life seems to have arisen after several hundred million years and then it took another three billion years of evolution to produce multi-cellular organisms. Let's be humble, however: microorganisms are the dominant form of life on this planet. Multicellular living creatures, while familiar to us, are squatters in a world owned by bacteria.)
Of course, if life evolved according to a chemical basis that we are not familiar with... well, then, perhaps life is there all along, but we haven't managed to recognize it. This is unlikely: the periodic table imposes pretty rigid constraints on chemistry. It's very unlikely that life could exist without liquid water. (Chemists would have figured out that possibility.) Still, scientists wouldn't be scientists if they didn't want to be surprised, and who knows? Maybe there is a kind of life on Mars based on chemical cycles essentially unfamiliar to us here on Earth. We are, perhaps, liquid-water-loving, DNA-and-RNA-bigots.
Gaia and Martian chemistry
We can approach this discussion even more simply. We ask the question, “On which of these planets can we recognize life?”
(“Blue Marble” image courtesy Visible Planet and NASA Goddard Space Flight Center; Mars image courtesy NASA/JPL-Caltech.)
Let's use the Gaia Hypothesis (Lovelock and Margulis) to give us a little perspective. The idea is contentious, and not necessarily correct, but still gives us a simple point of view: consider the planet as a single organism. (This is similar to the fact that you are composed of cells, yet somehow all these little bits of life make up one organism.) The planetary “organism” (Gaia) regulates itself, in terms of temperature and chemistry, and that regulation leads to conditions conducive to life.
In other words, life has already changed our planet to make conditions better for itself. (We don't need to explore this in detail, but anyone who lives on or near limestone should appreciate that all of this rock, the rocks on which your houses are built, were produced by microorganisms. I hope that makes you blink for a second; the ground you walk on, the air you breathe, and the water you drink are all intimately connected with life.)
If we examine Earth from a distance, we find that it is not in chemical equilibrium. Most dramatically, the atmosphere contains a hefty amount of oxygen. Oxygen?! That's just not possible; oxygen is highly reactive and couldn't possibly last as an atmospheric component. Something must be driving the production of oxygen, and the only reasonable chemical explanation is life.
Now when we examine Mars from a distance, we do find that it is in chemical equilibrium. The atmosphere is largely carbon dioxide. If there were life on Mars, then Mars wouldn't look like Mars: Mars would look like Earth.
That is the inevitable conclusion you reach, if you accept the Gaia Hypothesis. Mars would have been regulated: it would be warmer, wetter, and more Earth-like. Life would have produced more carbon dioxide, which would have warmed the planet (greenhouse effect), which would have released frozen carbon dioxide at the poles, which would have warmed the planet, which would have helped more life grow, which... Mars can only look the way it does, because it is not alive. (This very simple argument was made, far more eloquently, by Dr. Lynn Margulis in a talk I once attended.)
Of course this is just a very simple analysis, too simple to be conclusive. As I said, this is just for a little perspective.
Is there no hope for sci-fi?
But let's say we want to allow for life on Mars. What is a science-fiction writer to do?
Well, I think there is one big unknown regarding the chemistry of Mars. Over millions and billions of years, meteorites would have “seeded” Mars with organic carbon (yes, there's plenty of organic material in Outer Space). But there's no organic carbon on Mars.
Where'd it go?
Something must be reacting with the organic carbon. On Earth, that would be oxygen, which would form carbon dioxide. But Mars doesn't have oxygen.
So what is oxidizing the organic carbon? And remember that Viking experiment? The one that showed that something in the Martian soil was reacting with organic molecules?
The most likely explanation is that the fairly intense ultraviolet light on Mars directly produces the superoxide radical (O2-, a negatively-charged oxygen molecule; the extra electron makes oxygen even more reactive) in the Martian soil. The superoxide radical is extremely reactive, and would immediately oxidize organic carbon into carbon dioxide. Thus, any incoming organic carbon won't last on Mars.
To make this sci-fi, let's assume that the superoxide radical is not produced inorganically from Martian soil. We'll just pretend that it is a result of Martian life-microorganisms that are ubiquitous in the Martian soil. And we reject the Gaia Hypothesis.
Instead of a rich and plentiful source of energy as on Earth (light, carbon dioxide, oxygen), life on Mars is limited by a scarce resource: meteoritic organic carbon (with plenty of UV light to drive the conversion of carbon to carbon dioxide). Life is rare-and slow. Growth and reproduction would always be limited by an essentially unavailable resource (imagine a planet-wide algal bloom constantly held in check by an unending lack of key nutrients).
This scenario is relatively plausible, largely fits with the known science of Mars, and I think, interesting. Life on Mars is not little green men; it's little starving microorganisms spread throughout the Martian soil.
This is just the merest hint of a sketch of an idea. Please, add to it in your own imagination. (I do not claim to be the first or only person to have come up with this idea--certainly I am not--but I am not aware of any author who has developed something similar. Please comment if you are.)
Do you wonder, though, just what would happen when humans arrive?
(Edited for grammar and clarity. Gotta remember to proofread, sheesh.--Mister Troll)
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