Published: 10 March 2022
The classical definition of the habitable zone around a host star is the zone of orbits that allows liquid water on a planet’s surface, which recognizes the importance of water for life as we know it. Add rock with its nutrients, the CHNOPS elements (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) to the recipe and you have all you need to make life flourish. Or have you? How about the supply of energy and the thermodynamic disequilibrium that is needed? What does this mean for the environments in which life emerged on Earth? Hypotheses range from deep sea vents, to beaches, to rivers and lakes or subaerial hot springs on land, and even deep faults! If life emerged on land and not in the sea, this precludes the appearance of life on the icy satellites in our Solar System or on ocean worlds.
Whatever the environment (and hydrothermal systems, whether subaqueous or subaerial, are the most popular), life emerged on Earth probably during the Hadean era (4.5-4.0 Ga) and appeared to have evolved very rapidly because, by 3.5 Ga, the more primitive chemotrophs (microbes obtaining their energy from oxidation of inorganic or organic substances) were joined by the more efficient phototrophs (that obtain energy from sunlight). These organisms were anaerobic, i.e. required environments without oxygen.
However, further evolution required substantial changes, not only the evolution of the oxygenic phototrophs that excreted oxygen, thereby oxidizing the surface of the Earth as well as the atmosphere, but also geological and tectonic changes to the Earth. Plate tectonics contributed to burying carbon, exporting it from the sediments and atmosphere and allowing oxygen levels to rise. Plate tectonics also recycles essential nutrients. A planet without plate tectonics would not remain habitable after all nutrients at the surface had been utilized.
All of these factors need to be taken into consideration when looking for life elsewhere, in our Solar System or on exoplanets.
Frances Westall is director of research at the Centre de Biophysique Moleculaire Equipe exobiology of the French National Research Laboratory CNRS in Orléans. The CNRS laboratory is associated with the Université d’Orléans. Frances was born in Johannesburg, South Africa but grew up in the UK and studied geology at the University of Edimburgh. Her research focuses on the earliest life on Earth and its geological context. She does field studies of the earliest supracrustal terrains – including the Kapvaal Craton in South Africa and the Pilbara in Australia – and of fossil bacteria from the early Archaean. She is a leading member of the ESA ExoMars mission science team to search for life on planet Mars.
Webinar was recorded on March 10, 2022
