Looking for life in Europa

After two decades of development and ‘heartbreak’, scientists are on the verge of sending missions to explore the ocean world of Europa. Could this be our best shot at finding life elsewhere in the Solar System? Orbiting the giant planet Jupiter is an icy world, just a little smaller than Earth’s moon. From a distance, Europa appears to be etched with a nexus of dark streaks, like the product of a toddler’s chaotic scribbling. Close up, these are revealed to be long linear cracks in the ice, many of which are filled with an unknown contaminant that scientists have dubbed the ‘brown gunk’.

Jupiter’s immense gravity helps generate tidal forces that repeatedly stretch and relax the moon. But the stresses that created Europa’s smashed up terrain are best explained by the ice shell floating on an ocean of liquid water. “The fact that there’s liquid water underneath the surface which we know from previous missions, in particular from the magnetometer observations made by the Galileo spacecraft as it flew past (in the 1990s), makes it one of the most exciting potential targets to look for life,” says Professor Andrew Coates of University College London’s Mullard Space Science Laboratory in Surrey, UK.

Search for clues underwater
Europa’s briny deep might extend 80 to 170 km into the moon’s interior, meaning it could be holding twice as much liquid water as there is in all of Earth’s oceans. And while water is one vital prerequisite for life, Europa’s ocean might have others - such as a source of chemical energy for microbes. What’s more, the ocean may communicate with the surface through a number of means, including warm blobs of ice from below rising up through the ice shell. So studying the surface could provide clues to what’s going on deep  below.?

Now, National Aeronautics and Space Administration (NASA), USA, is priming two missions to explore this intriguing world. The first is a flyby mission called Europa Clipper that would likely launch in 2022. The second is a lander mission that would follow a few years later.

Dr Robert Pappalardo, from NASA’s Jet Propulsion Laboratory (JPL), is Clipper’s project scientist. “We’re really trying to get at Europa’s potential habitability, the ingredients for life: water, and whether there’s chemical energy for life,” he says.

Clipper carries a payload of nine instruments, including a camera that will image most of the surface; spectrometers to understand its composition; ice-penetrating radar to map the ice shell in three dimensions and find water beneath the ice shell; and a magnetometer to characterise the ocean. However, since the Galileo spacecraft provided evidence for an ocean in the 1990s, we’ve learned that Europa isn’t one of a kind.

At Saturn’s moon Enceladus, for example, ice from a subsurface ocean gushes into space through fissures at the south pole. However, what makes Europa stand out is its neighbourhood. Europa’s orbital path takes it deep into Jupiter’s powerful magnetic field, which traps and speeds up particles. The resulting belts of intense radiation fry spacecraft electronics, limiting the durations of missions to months or even weeks. That said, this radiation also drives reactions on Europa’s surface, yielding chemicals called oxidants.

On Earth, biology exploits the chemical reactions between oxidants and compounds known as reductants to supply the energy needed for life. However, the oxidants made on the surface are only useful to Europan microbes if they can get down into the ocean. Fortunately, the process of convection that pushes warm blobs of ice upwards might also drive surface material down. Once in the ocean, oxidants could react with reductants made by seawater reacting with the rocky ocean floor.

Overcoming challenges
Exploring Europa is costly - though no more so than other NASA ‘flagship’ missions such as Cassini or the Curiosity rover. There are inherent engineering challenges, such as operating within Jupiter’s radiation belts. Spacecraft instruments need to be shielded with materials such as titanium metal but, says Dr Robert, “you can only shield them so much because they have to be able to see Europa.” So to keep Clipper safe, NASA is going to stray from the rulebook somewhat.

“The assumption always was: Galileo flew past Europa, so the next mission has to be an orbiter. That’s just how we do business,” says Clipper’s programme scientist Curt Niebur from NASA. But rather than orbit Europa, Clipper will instead reduce its exposure to mission-shortening radiation by orbiting Jupiter, and make at least 45 close flybys of the icy moon over three-and-a-half years. “We realised we could avoid those technical challenges of orbiting Europa, make the mission much more achievable and still get the science we want if we fly past it a lot,” says Clipper’s programme scientist.

The strength of sunlight near Europa is about a 30th of what it is at Earth. But NASA decided it could power Clipper with solar panels rather than the radioactive generators some other outer planet missions have used. “All those years of study forced us to burn away our pre-conceptions and get us to really focus on reality, not on our wish-list and to focus on the best science,” says Curt.

Some innovative Europa lander concepts have been proposed over the last two decades, reflecting the scientific bounty to be had by touching down. Dr Geraint Jones of the Mullard Space Science Laboratory has worked on one concept called a penetrator. A projectile deployed from a satellite hits the surface “really hard, at about 300m/second, about 700 miles an hour”, exposing pristine ice for analysis by onboard instruments, which could be designed to withstand the impact.

By contrast, NASA’s forthcoming lander would put down softly with the help of Sky Crane technology. During the touchdown, it will use an autonomous landing system to detect and avoid surface hazards in real time. Clipper will provide the reconnaissance for a landing site. The craft would be equipped with a sensitive instrument payload and a counter-rotating saw to help get at fresher samples below the radiation-processed surface ice.

Convincing evidence
Earth’s seas are teeming with life, so it can be hard for us to contemplate the prospect of a sterile, 100km-plus deep ocean on Europa. But the scientific threshold for detecting life is set very high. So will we be able to recognise alien life if it’s there? “The goal of the lander mission is not simply to detect life, but to convince everyone else that we have done so,” Curt explains.

Thus, the lander’s science definition team came up with two ways to address this. First, any detection of life has to be based on multiple, independent lines of evidence from direct measurements. “There’s no silver bullet; you don’t do one measurement and say: ‘aha, eureka we’ve found it’. You look at the sum total,” says Curt. Second, the scientists have come up with a framework to interpret those results, some of which might be positive, while others negative: “It creates a decision tree that marches through all the different variables. Following all these different paths, the end result is: yes, we’ve found life, or no we haven’t,” he says.