127 WHAT DOES LIFE DO? When it comes to working out what life is, that’s the one question not to ask, says Michael Russell of NASA’s Jet Propulsion Laboratory. “Never ask what anything is. Ask what it does,” he says. “What does life do?” We may be tempted to answer that life passes on heritable information by the process of reproduction. But that’s a limiting statement, according to Russell, because it looks at things from the point of view of our biology, rather than the underlying physical processes. “Life hydrogenates carbon dioxide,” he says. It takes hydrogen from water and joins it together with carbon dioxide, which originally emanated from volcanoes in abundance. This rights what would otherwise be an unresolvable chemical disequilibrium, and produces a supply of organic molecules in the process. Doing this requires a source of free energy to drive the chemical reaction and a small engine to make it happen. In this picture, life is made of small electrical engines, driven by the movement of free electrons in our environment. Replication – or reproduction – is just something that evolved to keep that going. As the Hungarian Nobel prizewinner Albert Szent-Györgyi put it: “Life is nothing but an electron looking for a place to rest”. “Put electrons in the system and they will try to escape. That very flow of electrons is what drives these little engines to generate the organic molecules,” says Russell.
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fly-bys of Titan by the
Cassini mission
need a source of free energy. That takes us back once again to 1977, this time to that deep-sea dive off the Galapagos Islands. In February of that year, oceanographers Jack Corliss of Oregon State University, and Tjeerd van Andel of Stanford University travelled to the sea floor in Alvin, the submersible best known for exploring the wreck of the Titanic. They were looking for hydrothermal vents, which jet warm water out from beneath the seabed into the cold ocean. They not only found these “black smokers” – so-called for the colour of the minerals that precipitate out of the hot water – but also an extraordinary abundance of life around them.
Extraterrestrial tests Corliss and others concluded that such places could have been the backdrop for the origin of life, pitting them against those advocating the idea of organic chemistry in the atmosphere. Russell, then a deep sea vent geologist working on underwater mineral deposits, soon joined the discussion. He thought the black smokers were too energetic: fragile organic molecules would be as easily destroyed as created. Instead, he proposed that life began at the calmer, warm water vents, in the mineral deposits he was studying. The trouble was how to test the hypothesis. To simulate the sea floor would require water at pressures 10 times that at sea level. It would have to be acidic to represent the higher concentration of carbon dioxide in Earth’s early atmosphere, and the whole lab would have to be more sterile than an operating theatre to ensure that no ordinary Earth bugs were inside – and nothing that sprang to life inside could get out. The Galileo probe, which reached Jupiter late in 1995, showed the experiment might already be going on elsewhere. In particular, its images of Europa showed huge cracks in the moon’s icy surface and areas where the ice blocks had moved, as if transported by currents in a subsurface ocean. Readings from Galileo’s magnetometer revealed that a churning saltwater system encircled the whole
moon. This ocean probably contains more water than Earth, and must be powered by a heat source at Europa’s centre. That gives rise to the hope of hydrothermal vents – and a perfect place to put Russell’s theory through its paces. “I’m lucky enough to be at the beginning of a big extraterrestrial test of these ideas,” says Russell. NASA and ESA are both planning return missions to Jupiter’s moons. NASA’s is called Europa Clipper and will launch in the 2020s. Because of the intense radiation around Europa, in the form of high-energy electrons trapped in Jupiter’s powerful magnetic field, the spacecraft will not orbit the moon, but circle Jupiter and perform 45 fly-bys at altitudes varying from 2700 kilometres to just 25 kilometres. Cameras and spectrometers will determine the surface ice composition, radar will determine its thickness, and a magnetometer the depth and salinity of the underlying ocean. Together, those readings should confirm whether the conditions are suitable for life to have arisen. But Europa is not the only object of interest. The ESA mission, called JUICE for Jupiter Icy Moons Explorer, is planned for a 2022 launch. It will dive close to Europa to take similar