Europa, one of Jupiter's 95 known moons, has emerged as one of the most promising places in our solar system to search for signs of extraterrestrial life. This icy moon contains a vast subsurface ocean of liquid water underneath its frozen exterior, possessing some of the key ingredients necessary for life as we know it. As NASA prepares to launch the Europa Clipper mission in October 2024 to study this intriguing Jovian moon, hopes are high that Europa may harbor microbial organisms in its dark subsurface seas.
Of the four large moons of Jupiter discovered by Galileo Galilei in 1610, Europa stands out for its smooth, ice-covered surface crisscrossed by long fractures and ridges. Galileo was the first to observe these cracks in Europa's surface, correctly hypothesizing that they could be caused by an subsurface ocean interacting with the moon's icy shell. Modern observations by the Galileo spacecraft in the 1990s provided further evidence for a global ocean, identifying geologic features like ice rafts and collapsed surfaces consistent with warm ice upwelling from below. Additionally, Hubble Space Telescope observations have detected plumes of water vapor erupting from Europa, confirming that this ocean is in contact with the surface.
But what makes scientists think this ocean could be habitable? For life as we know it, three key ingredients are required: liquid water, chemistry, and an energy source. Europa checks all three boxes. With an ocean more than twice the volume of all Earth's oceans combined, Europa contains plenty of liquid water. This water interacts with Europa's rocky seafloor, producing a rich chemical environment. Tidal forces from Jupiter's gravity provide the energy to keep Europa's ocean liquid, while also driving internal heating mechanisms that could support hydrothermal vents on the ocean floor – environments on Earth teeming with microbial life.
Europa also benefits from the presence of Jupiter itself. The gas giant's powerful magnetic field acts like a protective shield, deflecting harmful radiation from the sun and surrounding space that could otherwise sterilize Europa's surface and ocean. This radiation protection makes Europa's ocean potentially more hospitable than those of fellow ocean worlds Enceladus and Titan.
The Search for Life
When the Europa Clipper arrives in 2030, it will conduct a detailed survey of Europa to gather evidence about the composition and dynamics of its interior ocean. Clipper will fly by Europa 50 times, collecting high-resolution images of the surface and using an ice-penetrating radar to map the ocean depth and seafloor morphology. This data will help identify sites where material from the ocean below has risen up and broken through the icy crust.
In its close passes, Clipper may be able to fly through Europa's sporadic water plumes and directly sample the subsurface ocean. By analyzing the chemical makeup of these samples, scientists could search for biosignatures – chemical signs of microbial life. On Earth, examples of biosignatures include certain ratios of elements and organic molecules that are byproducts of biological processes. Scientists have debated the best approach to look for biosignatures on Europa. Some argue for detecting complex organic compounds like amino acids, the building blocks for proteins that support life on Earth. Others advocate a “follow the energy” mantra, seeking out methanogenesis or other microbial metabolisms that utilize Europa's chemical energy gradients.
While the Europa Clipper will focus on analyzing Europa from orbit, scientists are already thinking about the next phase of Europa exploration – landing a probe directly on the surface. In the future, an advanced lander could attempt to penetrate Europa's icy shell and deploy a robotic submarine into the ocean depths. This underwater vehicle would provide our first direct glimpse of Europa's alien seafloor, snapping photos and collecting samples to analyze for any chemical or geological hints of life.
Challenges and Unknowns
Despite Europa's potential, many mysteries about this moon remain unsolved. The exact thickness, depth, and salinity of its global ocean are still uncertain. Europa's seafloor composition and geology are also entirely unknown – is it a rocky seafloor like Earth or more muddy or icy? Many also wonder if and how material and nutrients cycle between Europa's ocean and surface. Unraveling Europa's complex dynamics and interior structure will be critical for astrobiologists to understand its potential habitability.
The radiation environment around Europa poses challenges both for orbiters and potential landers. Jupiter's intense radiation belts can damage spacecraft electronics over time. Landing and operating surface probes on Europa will also be extremely difficult given the harsh radiation. Engineers will need to find ways to shield sensitive electronics from radiation while designing landers and submersibles durable enough to survive Europa's punishing environment.
Ultimately, the question of life on Europa remains open. But Europa represents our best chance so far to answer the fundamental question – are we alone in the universe? The upcoming Europa Clipper mission and proposed follow-on exploration of Europa seeks to involve all of humanity in this historic search for alien life in our cosmic backyard.