We’re living in a golden age of exploration. As humanity’s unmanned spacecraft traverse the unknown corners of the Solar System, they continue to beam data back to Earth that is unravelling the secrets of the mysterious worlds that orbit our Sun.


From recent encounters like the Cassini mission around Saturn to New Horizons’ close-ups of Pluto or the Juno spacecraft currently orbiting Jupiter, these odysseys have revealed the Solar System to be an active, dynamic place with stormy gas giants, volcanic moons and icy, rocky bodies.

Perhaps the grandest ambition of this exploration is the search for life. Could it be that our Solar System is entirely barren, or is there the slightest chance that some form of life is thriving beyond Earth?

Subterranean Ocean

One of NASA’s current projects is Europa Clipper. Due for launch in 2022, the spacecraft will fly into orbit around Europa, the smallest of Jupiter’s four ‘Galilean’ moons. This icy world is of particular interest to scientists because there is strong evidence of a salty ocean hidden beneath the planet’s frozen surface. And water is a key ingredient for life as we know it.

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Europa Clipper is one of many missions that will search for regions in our Solar System where even the smallest microbial life might have a chance of grasping hold. The study of so-called ‘extremophiles’ on Earth has revealed organisms that can survive in the harshest environments, from boiling hot or freezing cold temperatures to extreme pressures, high acidity and saturated salty solutions (see page 41). Perhaps organisms like these could inhabit a moon like Europa. It isn’t a completely frozen world, after all. Its surface may be ice, but tidal flexing of the moon caused by its orbit around Jupiter could be enough to generate hydrothermal activity and maintain a liquid subsurface ocean.

The Clipper will perform 45 flybys of Europa, diving from 2,700km to just 25km above the surface, and is equipped with nine science instruments to collect data along the way. The spacecraft will search for any plumes bursting through Europa’s frozen crust, as these could contain water vapour and might point to a subsurface ocean. It will then analyse the material, searching for potential water particles in its atmosphere and collecting data. Other data should also give scientists a detailed picture of Europa’s magnetic field and gravitational pull, the thickness of its crust and the depth of its ocean.

If life does exist beyond our home planet, Europa is currently one of our best chances of finding it. On Earth we tend to see an abundance of life wherever water exists, so the prospect of a salty ocean hidden on an icy moon is too tempting to pass up. Subsequent missions may even be able to put a lander on Europa and explore the salty liquid below its surface. And, if scientists can find what they’re looking for, it may be enough to at least partly answer the question as to whether we are alone in the Universe.

The hunt for H20

Recent discoveries show that water may be found in many regions of the Solar System, hinting at potentially suitable conditions for life, past and present.

March 2015: Astronomers announce that observations with the Hubble Space Telescope show evidence of a salty, subsurface ocean on Jupiter’s largest moon Ganymede.
October 2015: The Cassini spacecraft dives through erupting plumes on Saturn’s moon Enceladus, revealing salty liquid water and organic molecules below its icy crust.
January 2016: New Horizons data reveals more water ice on dwarf planet Pluto than previously thought.
October 2017: Data from the Dawn mission shows evidence that there was once a global ocean on dwarf planet Ceres, the largest body in the Asteroid Belt.
July 2018: The European Mars Express orbiter finds evidence that a lake of liquid water could exist below the surface of the Red Planet today.
August 2018: NASA detects chemical signatures of water above Jupiter’s deepest clouds.

Studies over the past decade have also suggested the presence of subsurface oceans in ‘Trans-Neptunian Objects’ on the edge of the Solar System, billions of kilometres from the Sun.

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