by ADAM FRANK
April 09, 2013 8:59 AM
Not too long ago I asked an astrobiologist colleague if he thought the search for life on Mars would be successful. His answer, delivered deadpan, stunned me.
"Oh I think we will find life on Mars," he said. "But the real question will be: Is it our life?" The point my friend was raising is the fundamental question of the solar system's second genesis.
We know life formed once in the Sun's family of planets. What we can't be sure about is where or when that first genesis occurred. In the wake of those uncertainties, it is entirely possible we were Earth's first alien invasion.
The Earth and Mars have been swapping spit (astrobiologically speaking) for eons. Large impacts from comets and asteroids can launch debris deep into interplanetary space. The debris then wanders the vacuum, orbiting the sun for millions of years. The pull of a passing planet may capture these forsaken bits of geology and down the gravity well they go. If they are large enough, these chunks from Mars or Earth or Venus may make it through the atmosphere onto the surface (of Earth or Venus or Mars). That is where the fun and, theoretically, the contamination might begin.
Four billion years ago Mars might have been a blue world (or blue-ish). It was certainly warm and wet enough for liquid water to run across its surface. This epoch of Martian history — called the later Noachian — lasted perhaps just 500 (or so) million years. That is short compared to Earth's four billion years of habitability. But it may have been long enough to allow abiogenesis (life from non-life) to get going.
Then, a chance asteroid impact might have sent a chunk of microbe-laden Martian crust to Earth. Given what we now understand about extremophiles — bacteria on Earth that can live in the most extreme conditions — it is not unreasonable to think some hardy "spores" of Martian descent made it to this warm, wet world, seeding the beginnings our own lineage.
So, life might have started on Mars, not Earth.
Thus what we really are looking for as we explore other planets in our solar system is not just life, but life that is clearly not our own.