Life will find a way
One of the books I have read while in New Zealand is How to Build a Habitable Planet by Charles H. Langmuir and Wally Broecker. It is an updated version of Broecker’s original book of the same name and spends a great deal of time discussing the geological construction of a planet like Earth.
This is particularly relevant in a country like New Zealand where the land seems to be forming under your feet. Fossil beds are exposed on beaches and in the walls of caves. Volcanic activity is generating new land offshore. Geothermal fields result in the generation of silica sinter and mineral deposits. There is a great deal of geological activity across the country.
But at the heart of the book is the question of what constitutes habitable. Neither Langmuir nor Broecker are from New Zealand so might be forgiven their narrow interpretation of what constitutes habitable.
However, walking past a geothermal vent spewing 70 C water and then seeing algae and bacteria growing in mats floating in the resulting pool does give one pause to consider the possibilities for living organisms. As Jeff Goldblum so succinctly put it in Jurassic Park, “life will find a way.”
The range of habitability extends well beyond the range of environments humans can occupy unaided. Our comfortable 15 C average surface temperature with plenty of liquid water is only the most recent environment in which life has thrived. And certainly not representative of all life on this planet. Everywhere scientists have looked, they have found new organisms taking advantage of a particular niche within the world’s ecosystems.
For example, sea sponges, worms, and such have been found clustered around underwater smokers where temperatures can reach 300 C and pressures are over 200 atmospheres. Their metabolism is built around the consumption of hot mineral water which provides the molecules necessary. Other organisms found on the floor of the deep ocean survive by consuming iron and sulfur while excreting sulfuric acid and rust.
Organisms have also been found living in solid rock two kilometres into the Earth’s crust, within a lake half a kilometre under the Antarctic ice at temperatures of -25 C, and floating on the wind at altitudes as high as 10 km above the Earth’s surface. Scientists have even been able to revive 40 million year old bacterial spores.
But perhaps the champion extremophile are the tardigrades. If there was a contest for cutest, they would win hands down. These tiny creatures, measuring no more than a millimetre in length, resemble eightlegged bears or a crumpled up manatee. They are nicknamed water bears and are able to withstand temperatures ranging from -200 C to + 150 C, a complete lack of water and oxygen, immersion in boiling alcohol, the harsh vacuum of outer space and radiation at levels thousands of times higher than we can take. They are life’s survivors.
They survive by entering a state of suspended animation, called cryptobiosis, in which body functions are shut down until conditions improve. They can remain in this state for decades if necessary and possibly longer. Learning their tricks might be the secret to interstellar travel.
Habitable means suitable or good enough to live in and as far as we know, the range of possibilities is quite diverse – perhaps more diverse than we even suspect. It is possible there are forms of life based on elements other than carbon and in media other than water although from our perspective both those possibilities seem far-fetched.
Having water and air certainly seem to be necessary criteria for a habitable planet according to Langmuir and Broecker. Having reduced carbon makes sense as carbon’s ability to concatenate or build up long chains of atoms plays a major role for organisms on Earth.
Having an energy conversion mechanism would seem to be an essential component for any habitable planet. But beyond these few criteria, there is little more required.
A recent report in the Nature Geoscience provides evidence the Earth was entirely covered in water as recently as 3 billion years ago.
This is well after the period in which it is thought life began on our planet and yet life did emerge without land. Their work is based on oxygen isotope ratios which are sensitive to transport across the ocean-atmosphere boundary and undergo isotopic separation in clay minerals.
Their results, though, depends to some extent on assumptions about how land would behave over 3 billion years ago. It is unlikely any soil would exist and this is a key factor in oxygen uptake.
That said, if the Earth was once a waterworld or completely frozen as other research indicates, then our definition of what constitutes habitable would certainly be extended.
We may yet find life in other habitable environments elsewhere in the solar system, on Europa or Titan, and beyond.