More than two weeks without blogging, that's the longest stretch since I restarted this blog almost one year ago. Sometimes life gets in the way: I had to give a presentation at a meeting in Baltimore (that was last week) and it took me two weeks non-stop work to prepare the talk. Before I left Ames it was still winter, the weird out-of-season Iowan version where it is warm one day and freezing the next. Now that I am back, spring finally arrived in full force. The magnolia in front of my house is peaking, and this post is a perfect excuse to show some of the flowers that survived last week rains.
The trip, and the non-stop maddening work, were worth it though: this was one of the most interesting meetings I have ever attended. The meeting was about habitable worlds across time and space and the participants were equally divided among astronomers, geologists, climate scientists and biologists. Our task was to discuss what we really know about habitable planets outside Earth, and what we need to understand to search for more planets that could host living organisms right now. The fact is, this is not an easy task. Just defining what is alive and what is inanimate matter is not a trivial effort (the best definition I heard is that "it is alive if it can die"). If I look at the magnolia flower on the left, I instinctively know it is alive. But life comes in different forms, and recognizing it may be not so easy. One speaker talked about her expeditions in Antarctica to study a brine subglacial lake that has been isolated from the surface for millions of years. You would expect that whatever was trapped into the lake when the continent froze would have died by now. Not so fast: she found lots of little bugs thriving in the salty frozen water tens of meters below the surface. Well, thriving is maybe too strong a word: to survive in such a resource deprived environment the little buggers had to slow their metabolism to a death-defying point. Each of this bacteria, on average, reproduce only once every 120 years! Slow bloomers, they are.
Earth is completely shaped by life. Even the air we breathe was made by life. Oxygen is too reactive to survive as a free gas in a planetary atmosphere: a planet left to its own devices would rapidly lose all its oxygen as everything will rapidly rust, and Earth would become as red as Mars. The original atmosphere of our planet was rich in CO2, with no O2 whatsoever. Then the bugs arrived, the first wave of them, the cyanobacteria that figured out how to use sunlight to eat CO2 and breath out oxygen. They were tireless; they produced so much oxygen that they completely inverted the ratio between CO2 and O2. This was the Great Oxygenation Event (GOE) without which we would be very small and blue (you need an oxygen metabolism to sustain complex life). But bacteria didn't stop at the GOE, they changed not just the air we breathe, but also the rocks we walk. There are 5,000 minerals on this world, and 2/3 of them would not be here if it wasn't for the presence of bacteria. Some of these minerals are the actual shells of little bugs that died and formed immense carbon-rich layers at the bottom of the oceans. Most other minerals would not have ever formed in absence of an oxygen-rich atmosphere. We literally walk and breath on a planet shaped by life.
So where does this leave us if we want to search for life on other worlds? We can precisely count on the power of life to change its own environment. We can look for the signatures of life that distinguish a world from its dead counterpart, a living breathing oxygen rich atmosphere from a dead CO2 one. A world with rocks born out of living organisms instead of dead volcanic stones. We may not find an alien magnolia in the world next door, but sooner or later we will see the incontrovertible chemical signs that there are other worlds in the stable disequilibrium that would not be possible without the wondrous action of living matter. Other Gaias are out there, waiting to meet us across the aeons of time and space.