In recent past, we have been searching the universe far and wide for signs that we are not alone. What if life is much closer than we thought? What if life is right on our doorstep? Our perceptions of what is habitable have long been limited to the ‘habitable zone’, a fabled area orbiting a star whereby water and thus life might exist. It seems, however, that this might limit our search for life beyond Earth. The icy Galilean moon Europa doesn’t play by our rules.

As far as we know it, life requires three things; liquid water, the correct biological elements and a free source of energy to harness. The habitable zone is the area where it is hot enough for liquid water to exist, almost always defined by proximity to a planet’s star. Europa is well out of the habitable zone for the Solar System. So is it possible for the three requisites of life to be present on this frozen world?

A conceptual image from the Europan surface, with Jupiter and Io looming over the horizon. Illustrating the potential water vapour plumes.
A conceptual image from the Europan surface, with Jupiter and Io looming over the horizon. Illustrating the potential water vapour plumes.  © Flickr/ NASA Goddard Space Flight Center

How can liquid water exist on an icy moon?

When we look up at our Moon, we always see the same hemisphere. This is because the moon is ‘tidally locked’ to the Earth; so the Moon’s rotation is in perfect resonance with its orbital period. This process is illustrated in the image below.

Image illustrating how the rotation of the moon is synchronous with its orbital period. Spinning in such a way that the same hemisphere always faces the Earth. Adapted from (SOURCE).
Image illustrating how the rotation of the moon is synchronous with its orbital period. Spinning in such a way that the same hemisphere always faces the Earth. Adapted from (SOURCE).

Unlike the moon, the icy world of Europa isn’t quite tidally locked to Jupiter; due to its relationship with the other large Galilean satellites, Io and Ganymede. This has a significant effect on the orbit of Europa; rather than having a perfectly circular orbit, its orbit is ever so slightly elliptical, e=0.01. For comparison, the planet with the most elliptical orbit in the Solar System is Mercury where e=0.2. This likely causes the rotation of Europa to be slightly faster than synchronous.

The effect of this is that at different times, Europa’s distance from Jupiter varies. At its periapsis (the point of its orbit that is closest to Jupiter) it would experience a greater gravitational force than it would at its apoapsis (the point of its orbit where it is furthest from Jupiter). This difference in gravitational force leads to ‘tidal flexing’, which generates heat within the satellite. It is thought that this tidal flexing would lead to sufficient heat generation for there to be a thick oceanic layer beneath the icy surface.

There is an abundance of evidence to suggest that this is true. The strongest evidence comes through the distortion of Jupiter’s magnetic field. The strength and direction of the field change in response to the position of Europa. For this to be possible there would need to be a global conducting layer near the surface; the most likely culprit for this conducting layer would be a global layer of salty water with a similar salinity to that of our own oceans.

 Biogenic Chemicals & Free Energy

It is commonly thought that Europa has a carbonaceous chondrite meteorite composition. If this is correct then by definition, biogenic elements are sufficiently abundant for life. If this is not the case, then it is still likely that Europa has still received sufficient biogenic elements via cometary impacts. At least 1012 kg of Carbon should have been accumulated this way.

On Earth, life is primarily powered by solar energy, converted into chemical energy via photosynthesis. The thick ice sheet above the ocean precludes the possibility of photosynthesis in the oceans of Europa. Life on Europa would need to utilise oxidation – reduction reactions in order to fuel metabolism and create energy. This requires there to be a balance of both oxidants and reductants available in the system.

“The oxidants from the ice are like the positive terminal of a battery, and the chemicals from the seafloor, called reductants, are like the negative terminal. Whether or not life and biological processes complete the circuit is part of what motivates our exploration of Europa” – Kevin Hand, Planetary Scientist at JPL.

Oxidation occurs via the radiolysis of water. Solar wind ions from the Sun are deflected by Jupiter’s magnetic field. Creating radiation belts of concentrated radiation as illustrated by the image below. This leads to an average surface irradiance of 125mWm−2 on Europa. This is relatively high; compared to its larger counterpart Ganymede, for example, which receives 6mWm-2. The radiation splits water molecules in the Europan surface ice up to a depth of 10cm, oxidising the surface, causing the accumulation oxygen-rich materials (eg. O2, SO4, CO­2).

Image displaying inner radiation belts of Jupiter. The giant planet's magnetic field captures particles from solar winds, concentrating radiation. © NASA-JPL
Image displaying inner radiation belts of Jupiter. The giant planet’s magnetic field captures particles from solar winds, concentrating radiation. © NASA-JPL

However, for there to be life in the oceans these chemicals have to find their way into them. The surface is thought to recycle completely over a period of 10 million years, as evidenced by the crackingchaos terrain of the surface. Furthermore, based on the amount of cometary activity Europa is expected to have received, more craters would be expected to be present on the surface.

Reduction occurs via a process called ‘Serpentinisation’. This is where water percolates into the spaces between mantle rock and reacts with the rock to form new minerals, releasing hydrogen in the process. New cracks are likely being created on the seafloor of Europa as the rocky interior cools down. This constantly exposes new rock to seawater, so more hydrogen producing reactions can take place. Serpentinisation also occurs on Earth, where seawater is thought to penetrate up to around 5-6km. Seawater is thought to penetrate up to 25km into the rocky interior on Europa, making a greater amount of rock available for serpentinisation.

These two processes must be in balance, otherwise, it is possible that the oxidants from the surface would overwhelm the reductive sinks, making the oceans toxic. Thus, they would be unable to support any sort of life as we know it. It has previously been speculated that the process of serpentinisation alone would be insufficient keep the balance, a recent paper argued that because of this greater penetration of seawater into the interior, sufficient reduction would occur to keep the chemical balance stable.

What’s next for Europa?

A photograph of the icy moon Europa, taken by the Galileo spacecraft in 1996. Note the chaotic terrain on the surface. ©NASA/JPL/DLR
A photograph of the icy moon Europa, taken by the Galileo spacecraft in 1996. Note the chaotic terrain on the surface. ©NASA/JPL/DLR

It is likely that the three prerequisites for life are available on Europa. As Kevin Hand said, it’s up to us to explore the oceans of Europa and see if life completes the circuit; all the other components seem to be there. NASA recently announced the possibility that Europa shoots plumes of water vapour to high altitudes as illustrated in conceptual image at the top of the page. This would allow us to sample the oceans of Europa without having to orchestrate a tricky landing or drill through miles of ice.

President-elect of the United States Donald Trump announced in the lead up to the election that he planned on focusing NASA on space exploration and in particular exploration of Europa. If he follows through with that promise, then we might find out if we are alone in the universe sooner than we thought. There could be life lurking in the dark oceans of Europa.

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