It was believed that at the dawn of space age all life is ultimately dependent on the energy from the Sun. It seemed impossible that any life could exist on the frozen ice balls of outer planets. The discovery of vibrant ecosystems in the depths of the Earth’s oceans, which rely on hydrothermal vents as both energy and fuel, has changed everything. We now know that life can flourish in environments completely cut off from the sun.
Europa’s liquid, inner ocean is believed to have the ability to support simple, microbial living beneath its icy surface. It has all three of the essential requirements for life: biochemically useful molecules; energy sources and liquid solvents (water), in which dissolved materials can react chemically with each other.
Europa’s energy is derived from its slightly elliptical planetary orbit around Jupiter, and the gravitational interactions with two other satellites. The combination of these forces causes Europa to flex with each orbit and release heat. This prevents the water from freezing.
Europa’s biochemically valuable molecules could come from comet impacts or deep within the rocky core of the moon.
Ice penetrating radar
Europa Clipper and JUICE both carry radar instruments that will probe beneath Europa’s ice surface. Radar has been used to detect sub-glacial lakes in Antarctica since the 1970s and more recently on Mars.
Europa is a better environment to test this because radar can see through ice that has gotten colder. Europa’s surface temperature is typically -170degC because it is so far away from the sun. Europa’s goal is to determine the depth of the ocean that will form when the ice sheet melts. Models predict that it is between 15-25km.
Europa Clipper and Jupiter in the background. NASA/JPL-Caltech
Liquid water could also be located closer to the surface. This would make it easier to reach. Hubble Space Telescope images show plumes erupting in the southern hemisphere. These plumes could be produced like a volcanic eruption, with liquid water rising from the ocean.
Under enough pressure, water will push its way through the fractures and voids in the ice to reach the surface, where it will erupt into geysers. The liquid water that doesn’t make it to the surface can still fill cracks and voids in the ice. This is similar to sub-glacial lake formations on Mars and Antarctica.
The missions should find these features if they are present. All of this helps to achieve one of the mission’s ultimate goals, which is to find the best place for a future lander that could drill through the ice to reach the mysterious ocean beneath.
Gravity maps
Europa’s interior. NASA
Spacecraft traveling near the surface can detect subtle changes in the gravitational fields of an object by using slight variations in rocket speed. These “gravitational abnormalities” are caused when the density of the material beneath the surface of the planet changes as the spacecraft passes overhead.
As an example, a spacecraft can experience an extra gravitational pull if it encounters a dense rock in a mountain range. For many years, gravitational anomalies have been detected on Earth to see underground structures like oil fields, metal deposits, and the famous Chixculub impact crater in Mexico.
Europa Clipper and JUICE will be able to detect gravitational anomalies, which could allow scientists to discover interesting features on the ocean floor. A smooth ocean floor with minor gravitational anomalies could be a boon for the chances of life on the moon.
How to get through the ice
To find out if there is life on Europa, we will have to put a lander, possibly carrying a submersible, on the surface one day. This won’t be easy even if Europa Clipper or JUICE can identify the thinnest ice.
Europa is very close to Jupiter. This means that the spacecraft will need a lot of fuel to be able to escape Jupiter’s gravity and enter an orbit around the moon. JUICE will be the first spacecraft ever to manoeuvre Ganymede, another of Jupiter’s moons. It will also use 3,000 kilograms of fuel.
The radiation from Jupiter can also damage spacecraft over time. Europa Clipper is, therefore, designed to stay in a long loop orbit around Jupiter and be repeatedly removed from the radiation field. Instead of studying Europa, it will fly by the moon.
Europa’s lack of atmosphere is another issue. We can’t use parachutes and heat shields to slow down a lander. Rockets are required to do everything, which means more fuel. While the lander is at the surface, the lack of atmosphere offers very little protection against radiation.
Even if the spacecraft is able to survive a landing, the ice will still be there. It is not likely that a mechanical drilling device can bore through miles of granite-hard ice. It is being explored more exotic ways to get through, like using Lasers and heat from a Nuclear Reactor.
Europa is currently a clean environment. This means that these complex tasks cannot be performed without accidentally contaminating oceans with pollutants or microbes from the spacecraft.
We will make it. After the spacecraft has reached the ocean and is able to move freely, the final challenge will be to ensure that it doesn’t get eaten up by anything swimming in the deep.
