Discovering extraterrestrial life would be one of the most profound scientific and philosophical revelations that our species has ever made. But such a big discovery won't come easy. Our starting point is to first search other worlds for signs of habitability, that is, the potential for life to exist.
Author
James O'Donoghue
Research Associate Professor in Planetary Astronomy, Meteorology, University of Reading
Nasa is doing just that: launching a spacecraft on October 10 to Europa, a moon of Jupiter holding twice the water of all Earth's oceans combined. Europa's ocean is between 60 and 150 kilometres deep and is hidden beneath an outer shell of ice that's 15 to 25 kilometres thick. The evidence for an ocean began to mount from the late 1990s onwards.
The Europa Clipper mission will carry nine instruments designed to assess whether this ocean world is habitable. A place may be habitable for life as we know it if three ingredients are available: liquid water, energy and carbon-containing compounds called organics.
Earth has been teeming with life for almost four billion years, in spite of no less than five large mass extinction events. Water and organics are abundant on our planet, while sunlight powers photosynthesis in plants, allowing them to produce sugars that pass into the animal kingdom through plant-eating species.
However, Europa's salty ocean is pitch black below a depth of several hundred metres, meaning photosynthesis has no chance there. This is why in 1977, when scientists explored even deeper, near 2,500 metres in a volcanic hotspot on the Pacific ocean's seafloor, they were amazed to find life thriving around hydrothermal vents.
Life at that depth is fuelled not by photosynthesis, but by chemosynthesis, a way for organisms to get energy from chemical reactions. Sunlight was no longer a prerequisite for habitability.
The water in Europa's ocean is kept liquid due to frictional heating. This heating occurs because Europa becomes stretched and then relaxed as it interacts with Jupiter's gravity on its orbital path around the giant planet. For Europa's ocean to be habitable, a steady supply of ingredients is needed to allow some form of chemosynthesis to take place.
If these ingredients exist, they could come from hydrothermal vents on Europa's rocky seafloor, like those on Earth, or from material seeping down through the icy crust, the "sea ceiling" if you like. We do not yet know if these mechanisms are plausible, so we need more data from many different angles.
There is growing evidence that plumes of material are escaping from Europa's surface into space. If this material is from the ocean, measuring its composition would give us insights into the habitability of that ocean.
The long road to Europa
Scientists have advocated for a mission to Europa since at least the 1990s. Nasa's Europa Orbiter was cancelled in 2002, followed by the ambitious Jupiter Icy Moons Orbiter (Jimo) in 2005, which was to orbit moons Europa, Ganymede and Callisto.
In 2008, Nasa and the European Space Agency (Esa) proposed the Europa Jupiter System Mission - Laplace (EJSM-Laplace), which aimed to send orbiters to Europa and Ganymede.
Both were cancelled in 2011, but out of the ashes came Esa's Jupiter Icy Moons Explorer (Juice) and Nasa's Europa Clipper. Juice launched in April 2023 and will arrive in July 2031, while Europa Clipper will launch on October 10 and arrive in April 2030 -- sooner than Juice because it will launch on a more powerful rocket. Both spacecraft will be in the Jupiter system simultaneously for three years, which in the end is not far off the plan for EJSM-Laplace.
Europa Clipper won't orbit Europa, instead it will cleverly orbit Jupiter in such a way that it passes over Europa 44 times, eventually building up a full global scan of the moon. The probe carries nine science instruments ready to give us a comprehensive understanding of Europa's ocean, its geology and current state of activity.
Nasa's main mission statement is: "Europa Clipper's main science goal is to determine whether there are places below the surface of Jupiter's icy moon, Europa, that could support life."
During the flybys, magnetic field instruments will help determine the ocean's depth and saltiness, mass spectrometry can "taste" the plumes to see their composition, ground-penetrating radar can see if water is inside the crust, helping us to understand if materials are exchanged from the ocean to the surface. Infrared instruments will scan the surface to look for signatures of organic materials which could be seeping out, as well as perform thermal imaging.
For decades, planetary scientists have pointed to ocean worlds like Europa as potential habitats for life. Europa Clipper cannot detect life directly, but it marks humanity's first dedicated mission to study an ocean world and search for signs of habitability.
If there is even a hint that the stuff of life exists there, a surface lander could follow to probe deeper, and the surface observations gathered by Clipper will be essential for the planning of that mission. And as ever, this only pertains to life as we know it.
James O'Donoghue receives funding from the UK's Science and Technologies Facilities Council as an Ernest Rutherford Fellow at the University of Reading.
