December 19th

Up to now we have no evidence for life on any planet other than the Earth.

CAPTION: Geysers of water from a deep interior ocean shooting out of cracks in the icy crust of Saturn’s moon Enceladus. Photograph taken by NASA’s Cassini spacecraft in 2005. https://solarsystem.nasa.gov/resources/13779/jet-blue/

Here liquid water seems essential for life, so those of us interested in searching for extra-terrestrial life are also keen to know whether other planets have water. Perhaps even oceans.

Water itself – the molecule H₂O – is virtually ubiquitous in space. Radio and infrared observations reveal water (in vapour form or as solid ice) in all sorts of places, from the winds of giant stars to the huge interstellar clouds of gas and dust in which stars and planets form.

It is also present in the rocky surfaces and atmospheres of all the terrestrial planets of our solar system. This water is a mix of interstellar ices present in the nebula from which the planets formed, combined with later additions carried from the outer solar system by asteroids and their fragments crashing into the newly formed planets.

Even the Moon appears to harbour water ice in the permanently shadowed crater floors at its poles. But although Mars may show evidence for the existence of past oceans, the only ones thought to exist right now in the solar system are deep below the surface of the moons of the giant planets, most famously Jupiter’s Europa and Saturn’s Enceladus. 

Sadly, I doubt these oceans are much like the subterranean sea imagined by Jules Verne in 1864 in his Journey to the Centre of the Earth. In this low-energy environment far from the Sun, we don’t expect to find Verne’s ichthyosaurs but it’s possible that microbial life may exist. And rather than sailing on its surface, like Lidenbrock and his companions, or patrolling the depths in a robotic submarine, our fear of polluting a potential site for extra-terrestrial life means flyby sampling of the geysers is the best option, at least initially.

Two missions are in the pipeline for the 2020s. ESA’s Jupiter Icy Moons Explorer will study Ganymede, Callisto and Europa, whilst NASA’s Europa Clipper will focus on Europa’s habitability and may include a biosignature explorer and be followed by a lander.

Of course, our solar system is only one of billions in just our own galaxy. We recently detected water in the atmosphere of K2-18b, a planet in its parent star’s potentially habitable zone, where water can exist in the liquid phase. We don’t yet know whether this particular planet has an ocean, but with so many planets to choose from, huge numbers of alien oceans must certainly exist. Sadly, even the closest star is 4.2 light years away so their exploration by robot or human will have to wait a while yet.

See what lies behind all the windows of the Sea Shambles Advent here.

Sea Shambles is a one night only live extravaganza celebrating the oceans. Hosted by Robin Ince and Helen Czerski with Steve Backshall, British Sea Power, Josie Long, Lemn Sissay and many others it’s a night of science, comedy, music, lasers and more, in which we’ll be turning the Royal Albert Hall into an underwater playground, the likes of which you’ve never seen! May 17 2020. Tickets start at just £10! Book here.

Tim O’Brien is a Professor of Astrophysics in the Department of Physics and Astronomy at The University of Manchester. He is also an Associate Director of Jodrell Bank Centre for Astrophysics and one of the scientific curators of the Blue Dot Festival. His research primarily focuses on the study of exploding stars using telescopes around the world and in space.

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