Is this threat of cross-contamination of microbial life a legitimate one? Are these scientists right to be concerned about it? And is it possible that microbes brought back from Mars could imbalance or even threaten existing life on planet Earth?
First of all, there are plenty of microbes right here on planet Earth that can threaten existing life on planet Earth. We have Ebola, SARS, the bird flu virus and other infectious diseases that have the perfect environment right here on Earth in which to mutate, spread and multiply. In fact, human beings are quite susceptible to being killed by viruses. Ebola has a 90% fatality rate, for example.
At the same time, microbes from Mars pose an entirely new threat because they are unknown. They may present something never before encountered by molecular biologists. And that could potentially pose a serious threat to human life on planet Earth (although I think the real risk of that is very small). Yet there is a lot at stake -- the health of all of humanity -- and so it makes sense to take precautions.
The greater concern in all of this is that we would contaminate Mars with microbes from Earth. In the ongoing search for life on Mars, we are seeing increasing evidence of past life on Mars. We know, for example, that Mars once had oceans -- vast masses of liquid water -- and that Mars is close enough to the Sun to put it within the reach of sufficient solar radiation to support not just simple life forms, but ultimately complex life forms. So it has water, it has sunlight and it has an atmosphere even today (although that atmosphere is certainly not of the density of Earth's atmosphere).
There's little doubt in my mind that we will find evidence of past life on Mars. In fact, I'm even optimistic about the possibility of finding living microbes on Mars today. We've seen extremophiles on planet Earth: microbes that can survive and thrive in extreme environments such as boiling water and the frozen tundra. So we know that microbes are capable of thriving under environmental conditions that we once thought could never support life. Mars, it seems, is far more hospitable than some of these extreme environments on Earth, and if we're finding microbes in such environments on planet Earth, there is a very real possibility that they may be found on Mars.
In terms of life in the universe, I'm an optimist, because I think there is life on other planets in our solar system, not to mention the other solar systems in other galaxies in our universe. I think life is very successful, and that if there's any possible way to exist in a climate, you will find life there.
I do support caution when it comes to possible cross-contamination between Earth and Mars, but let's realize it is this cross-contamination that is most likely responsible for seeding life on Earth, Mars, and other planets in other solar systems. Comets carry the seeds of microbial life. And when comets slam into planets, those seeds are not entirely destroyed. In fact, there was a recent study showing that microbes are capable of surviving these enormous impacts that scientists once thought would kill every living thing.
As a side note, it's difficult to gain a true appreciation of the amount of energy that's released during the impact of a comet with a celestial body such as a planet. Even a small comet (5 meters wide, for example) striking a planet's surface at sufficient speed is far more powerful than a typical nuclear bomb. It can even be a global killer -- so devastating that it wipes out entire species by blocking sunlight from reaching the surface of the planet for a period of several years, thereby destroying the food supply and causing mass extinctions. This mechanism is thought to be responsible for the extinction of dinosaurs on planet Earth which, interestingly, opened the way for the human beings.
We now know that, even at extremely high velocities, one in ten million microbes can survive such collisions. And if they happen to land in an environment that has moisture and nutrients and solar radiation, they can thrive and begin to replicate. Life on planet Earth may have actually begun with seeds brought to the planet riding on bits of rocks and chunks of other planets or comets from somewhere else in the galaxy. And it is perhaps through such mechanisms that life spreads throughout the galaxy: a series of collisions spreading seeds of life that then, through a process of evolution and natural selection over hundreds of millions of years, end up creating more complex organisms such as insects, mammals and even human beings.
I point all of this out because worrying about Earth contaminating Mars seems rather silly on the large scale of things, since Earth and Mars have probably cross-contaminated or, more accurately, cross-pollinated each other many times over the last five billion years. In fact, it may have been microbes from Mars that catalyzed life on Earth in the first place.
And that would make us all Martians, right?