For decades, astronomers have come at that question by confining their search to organisms broadly similar to the ones here. In 1976, NASA’s Viking landers examined soil samples on Mars, and tried to animate them using the kind of organic nutrients that Earth microbes like, with inconclusive results. Later this year, the European Space Agency’s ExoMars Trace Gas Orbiter will begin scoping out methane in the Martian atmosphere, which could be produced by Earth-like bacterial life. NASA’s Mars 2020 rover will likewise scan for carbon-based compounds from possible past or present Mars organisms. [...]
To open our minds, we need to go back to basics and consider the fundamental conditions that are necessary for life. First, it needs some form of energy, such as from volcanic hot springs or hydrothermal vents. That would seem to rule out any planets or moons lacking a strong source of internal heat. Life also needs protection from space radiation, such as an atmospheric ozone layer. Many newly discovered Earth-size worlds, including ones around TRAPPIST-1 and Proxima Centauri, orbit red dwarf stars whose powerful flares could strip away a planet’s atmosphere. Studies by the James Webb Space Telescope (JWST), set to launch next year, will reveal whether we should rule out these worlds, too. [...]
The search for exotic life therefore must begin close to home. The moons of Saturn and Jupiter offer a test case of whether biology could exist without an atmosphere. Jupiter’s Europa and Saturn’s Enceladus both have inner oceans and internal heat sources. Enceladus spews huge geysers of water vapour from its south pole; Europa appears to puff off occasional plumes as well. Future space missions could fly through the plumes and study them for possible biochemicals. NASA’s proposed Europa lander, which could launch in about a decade, could seek out possible microbe-laced ocean water that seeped up or snowed back down onto the surface.
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