A funny thing happened recently on the way to Mars.
A few days after the successful launch of NASA's behemoth Curiosity rover with its Mars Science Laboratory instruments on November 26th 2011, a somewhat muted piece of news came out admitting that the strict biological planetary protection rules had not been adhered to quite as everyone expected. What this meant in practical terms was that the rover's drill bits were not sealed up for launch with quite the same protocols for sterility as everyone had expected. Thus there is an added possibility that alien invaders from Earth are heading for Mars.
The reasons for trying to keep Curiosity and all of its bits and pieces effectively free of any Earthly biological (microbial) contamination are twofold. First, you don't want to get to Mars, start sniffing for interesting organic chemistry and end up detecting someones nasal microbiome, or some other bits of our rich and soupy broth of organisms. That really messes up one's ability to find martian life, whether extinct or extant. The second reason is that we don't want to forward contaminate Mars, unleashing our alien fauna on what might be a pristine or ecologically fragile world. Even the landing site of Curiosity has been chosen to avoid any obvious water-ice deposits within 3 feet of the surface, for fear of contaminating the Martian hydrological system.
The minor breach in protocol for Curiosity's drills is unlikely to spell impending biological apocalypse for Mars, but it does raise some fascinating questions, including whether humans have already contaminated Mars, and whether Nature has beaten us to it by hundreds of millions of years anyway.
Consider for example the case of the Viking landers. In 1976 these two large, stationary, laboratories touched down with parachutes and retro-rockets on opposite sides of the northern hemisphere of Mars. It was well understood that terrestrial biological contamination was a major issue - not least because of the sensitive biological experiments to be undertaken - and the landers were put through sterilization procedures before launch. The problem was that in the 1970's our understanding of the microbial world was different than it is today.
The protocol for sterilizing the Viking landers before launch included baking them inside their aeroshells under dry pressure at about 230 F (110 C) for almost 2 days. But nearly forty years later we know that extremophilic organisms exist which, if present on the Viking hardware, could have potentially survived such conditions with nary a shrug. Indeed, a hardy organism like the single-celled archaea known as Strain 121, not only survives at temperatures of 121 Celsius (250 F, a typical medical autoclave setting) but reproduces in these conditions. It's foodstuff? Well, Strain 121 metabolizes iron oxide for a living, producing magnetite as a byproduct. While we might not expect such organisms to be necessarily lurking in NASA's clean rooms, the problem is also that what was thought to be clean in 1976 is not so today. More than 99% of microbial organisms are not readily culturable (think Petri dish), and it's only with our recently innovative biomarker detection techniques, and metagenomics that we stand a chance of spotting the presence of these elusive, but pervasive, lifeforms.
There's a pretty good chance therefore that the Viking probes carried some number of intact, viable microorganisms - especially of the extremophilic variety - from Earth to the surface of Mars. This is not particularly controversial, one need only read the National Academies of Science 2006 report on "Preventing the Forward Contamination of Mars" to see it clearly stated that tools such as heat sterilization of spacecraft had, at that time, been untested for the case of extremophilic life. So, how big a deal would it be if Viking unwittingly carried organisms like Strain 121, or "Conan the Bacterium" (the infamous Deinococcus Radiodurans) or cold-loving and caustic-chemistry-loving critters? We don't know. The martian surface is terribly unforgiving, even for battle-hardened Earth microbes, so any release could very well remain highly confined and short-lived.
But before we go patting ourselves on the back, there is another route for biological alien invasion, one that has nothing to do with us, and which has been active for approximately 4 billion years. This is known as "impact transfer", the ejection of material from a planetary surface during collisions with asteroids or comets, and its subsequent travel through space until (sometimes) falling into the gravity well of another planet or moon. The chain of events may go like this: a large (kilometer scale) asteroid hits the Earth's continental surface at an oblique angle. During the moments of impact a "spall layer" of Earth's crust can be accelerated to escape velocity, thrusting a mess of rocky particles and chunks up out of the atmosphere and into space. Although this material experiences severe g-forces and temperature fluctuations, experimental studies indicate that conditions would certainly allow for hardy microbial or microscopic hitchhikers to be carried aloft.
What happens next is a fascinating result of celestial mechanics. The range of velocities and trajectories of impact ejected material result in a multitude of pathways. Some of the spall will fall back to Earth promptly, some will fall back over weeks and months, some will enter orbital routes that carry it away for tens of thousands of years before it too returns to Earth (a pathway that could put terrestrial chemical and biological material into "cold storage" during an episode of apocalyptic destruction on the homeworld). Other chunks go much further, entering what can be thought of as a very slow and very inefficient orbital conveyor belt. Some of this material can traverse interplanetary space only to be swept into the gravity well of another world or moon, including that of Mars, where it can rain down onto the planetary surface.
Simulations of these impact ejecta "transfers" indicate that over billions of years collisions will have resulted in bits of Earth (as well as other solid bodies) being spread out across the solar system, even arriving at places as distant as Europa or Titan. In these cases the 'hit rate' is low, perhaps one in ten million bits of Earth ejecta might ever make it to Europa or Titan over a million years, a smaller number would make the journey significantly faster. For Earth-to-Mars transfers the rate can be much higher, one in every thousand chunks of material ejected in a single impact event will make it to Mars in every million year interval after that initial dispersal into space. So over time the flux of transferred material (adding up all impact events) can be very significant.
While the jury is definitely out on the survival rate of organisms carried along within ejected pieces of Earth's upper layers (radiation damage, temperatures, and nutrient availability are all factors), there seems little doubt that the opportunity exists for viable critters to undergo planetary transfer, and at very least for biochemical components to make the trip largely intact.
So, it's quite possible that we ought to revise our preconceptions about planetary contamination. Nature may have already done a good job at mixing things up in our solar system (whose orbital architecture seems well suited to the transfer of impact ejecta), and the issue is perhaps less of whether Terran bio-filth has been dumped on a place like Mars, but more of when it last happened.
In the bigger picture we're left with the prospect of discovering whether our biology has invaded other worlds or whether we are the results of biology from (for example) a wet and warm Mars 4 billion years ago, or whether both things have happened. The odds seem good; we may be aliens, we may also be interplanetary mongrels.