When the Perseverance rover takes off from Cape Canaveral in Florida as scheduled on Thursday it will be Nasa’s first mission to the surface of the red planet with the primary goal of finding signs of past life.
For astrobiologists, this search for life beyond Earth can be compared to a game of darts. Hitting the bullseye would mean finding “life as we know it”. In other words, life based on a biology analogous to that found on Earth. But as any darts player knows, the bullseye is a small target, hard to hit and not even the highest scoring area on the board.
That’s why new research supported by Nasa’s astrobiology programme has developed a novel and broader definition of life – a definition that encapsulates life on Earth but also the possibility of “life not as we know it” elsewhere on the board. They call it lyfe.
There is no universally agreed definition of life. But Nasa has a good working description: “a self sustaining chemical system capable of Darwinian evolution”.
All living organisms on Earth take in energy, use and change it, and then release it in a less useful form as waste. All use the same 20 amino acids to make proteins. All use RNA and DNA molecules to store genetic information.
Weighing just over a tonne, the Perseverance rover will land on the Jezero crater, an area once thought to have been flooded with water and therefore a promising candidate for signs of past such life.
Accompanied by a drone helicopter to scout locations, Perseverance will identify, collect and store samples of rock and soil. These samples will then be picked up and returned to Earth by a future mission. Scientists will be looking for certain biosignatures that could indicate evidence of past life. If life as we know it ever existed on Mars, there is a good chance it will eventually be discovered by Perseverance.
But Earthly life represents only one example of biology. Despite the diversity of life on our planet, it appears that every living organism can be traced back to a common ancestor. Beyond Earth, life may have emerged differently and as such our standard definition of life may be too restrictive.
This led Stuart Bartlett, a complexity scientist at Caltech, and Michael L Wong, an astrobiologist at the University of Washington, to develop a new hypothetical concept: lyfe.
In their recent paper, they define a “lyving” organism as satisfying four criteria: dissipation (the ability to harness and convert free energy sources); autocatalysis (the ability to grow or expand exponentially); homeostasis (the ability to limit change internally when things change externally); and learning (the ability to record, process and carry out actions based on information).
With this definition, life is just one specific instance of lyfe. And in Bartlett’s opinion, there is a higher probability of finding lyfe on Mars than life. A chance event in Martian history may have sculpted lyfe differently from that on Earth.
Perhaps Martian organisms use a different molecular structure. Perhaps they use a smaller set of amino acids for proteins. Perhaps they only need a doublet genetic code, instead of a triplet like life on Earth
Evidence for such lyfe may, however, be harder to find, existing deep beneath the surface and beyond the reach of Perseverance.
Beyond Mars, this distinction between life and lyfe offers a useful perspective in the exploration of our solar system. The moons Enceladus and Europa, for example, are prime candidates for life. But Titan is more likely to contain lyfe.
Hitting the bullseye, finding life as we know it, may be hard shot. But lyfe may be a far more common, but no less significant, occurrence in the evolution of our universe.