What’s making this moon crack up?
Europa is one of the most stunning moons of Jupiter. About a hundredth of the mass of Earth, this lacerated icy satellite may hide a secret biosphere beneath its surface. When the Galileo probe flew by in the 1990s, it found that Europa was producing a magnetic field, thanks to its interaction with Jupiter.
How could it make such a magnetic field? If it had a salty liquid ocean, then maybe it could. The gravitational influence of Jupiter kneads the satellite like a piece of dough as it orbits, heating the water and keeping it liquid. We can’t see the liquid ocean because Europa’s atmosphere is very thin (providing a pressure less than a trillionth of the pressure of Earth’s atmosphere). This means the surface temperature is a chilly -160 degrees Celsius freezing any surface water immediately into an icy shell.
Its surface might be one of the smoothest in the Solar System, but it isn’t completely smooth: you can see there are huge cracks in the ice, (called lineae). Where do these cracks come from? The first thought is that Jupiter is again the culprit; that it might be kneading the ice sheet along with the rest of the moon. If that was true, then the cracks would have a distinctive pattern. Europa is tidally locked to Jupiter – this means that it shows the same face to Jupiter throughout its orbit (just as our Moon does to the Earth). This means that we expect the cracks to occur at the same stress points at all times. Interestingly, that’s not true – only the youngest cracks conform to this pattern: the older cracks don’t follow this pattern at all.
Does this mean that Jupiter isn’t to blame? Maybe. Maybe Europa is more geologically active than we realised, and that plate tectonics are the cause. Or maybe something even more subtle is happening. What if the ice sheet rotates at a different speed to the rest of the moon? This could happen if there is a subsurface ocean – the ocean could act as a lubricant to grease the ice’s motion. This is a source of hot debate, with implications for more than just Europa.
All this builds upon the possibility of extraterrestrial life on Europa. Life exists on Earth where the Sun cannot reach, in hydrothermal vents deep in the ocean. The vents supply geothermal energy and a chemical gradient, which are critical to the formation of life. It might even be possible to oxygenate this ocean: cosmic rays impacting on the surface could oxidise the ice, which may then be passed into the ocean through the formation of lineae. How do we find out? All our knowledge of Europa’s composition has come from the flybys by Galileo, Voyager and Pioneer probes. They’ve been incredibly successful – but we’ll soon need to go a step further. We’ll have to land on the surface: start taking samples of the ice, maybe even drill through it to reveal the ocean below.
A mission of this complexity will prove an incredible leap forward in technology, from the remote drilling technology (which may have to drill through thirty kilometres of ice without human assistance), to the extreme sterilisation of the equipment to avoid terrestrial contamination. NASA and ESA (and the Japanese space agency JAXA) have their thinking caps on as to how to do this (a nuclear powered ice melter, anyone?), and are planning a joint mission to the Jovian system with a launch date in 2020.