WHAT KIND OF ALIEN LIFE ARE WE LOOKING FOR?
All life on Earth needs three things: DNA and RNA for storing genetic information, proteins for making structural components of a cell and running biochemical reactions, and fatty lipid molecules that make up an outer membrane of a cell. We therefore know that this chemical system works for biology, but when searching for life on other worlds it’s important to not be too blinkered by the terrestrial example: alien life could be very different from us.
Based on the fundamentals of chemistry, life in the Solar System is thought most likely to be built from organic (carbonbased) molecules, and use water as the solvent (although perhaps life on Titan uses ethane instead). The instruments we design for probes sent to other worlds therefore look for complex organic molecules in general, rather than specific compounds like DNA.
A lot of the work of space missions is about trying to understand what the environment on other planets and moons is like – or was like billions of years ago – and whether these conditions could be habitable for life. Our understanding of the survival limits of terrestrial life is informed by studies of extremophiles – ultra-hardy organisms that tolerate boiling hot or freezing cold temperatures, or high acidity, or punishing levels of radiation, or the desiccating effects of salty surroundings. Some extremophiles live in environments on Earth that are similar to other worlds. For example, Lake Vostok, buried under the Antarctic ice sheet, helps guide us on the chance of life beneath the surface of Europa or Enceladus.