Written by Shanna Freeman
This striking Jovian moon seems like a wasteland of ice, but may be capable of supporting life beneath its frozen exterior.
Jupiter has more confirmed moons than any other planet in the Solar System, with 67 natural satellites. However, just four moons make up more than 99 per cent of the total mass. These are the Galilean moons, so-named after their discoverer, Galileo Galilei. The smallest of these four, Europa, is Jupiter’s sixth-closest moon. It’s just a bit smaller than our Earth’s Moon, with a diameter of 3,122 kilometres (1,940 miles). Europa’s volume is 0.015 that of Earth’s, and its mass is 0.008 of Earth’s. It may not be large, but it looms large in astronomical circles for more than a few reasons. Visually, the moon captures attention for its smooth, marbled appearance – mostly bluish white, with reddish orange streaks and splotches – due to tidal flexing, a phenomenon caused by gravitational forces from the bodies around it. Tidal flexing is also the potential cause for a liquid water ocean beneath the young and active surface.
The moon has an orbital radius of 670,900 kilometres (417,000 miles) and takes about 3.5 Earth days to make its circuit around Jupiter as well as rotate on its axis. Its orbit is mainly circular, with an eccentricity of 0.0094 (compared to our Moon’s 0.0549). Europa is tidally locked to Jupiter, with the same side facing the gas giant at all times. There is a sub-Jupiter point on the surface of the moon, so that if you were standing on it, it would appear that the planet is hanging directly above you. However, some researchers believe that the relationship between Jupiter and Europa isn’t a full tidal lock. There’s evidence that Europa rotates faster than its orbit, or at least it used to – the icy crust of the moon may rotate faster than its interior. It also has an iron core, a rocky mantle and a liquid ocean under the crust. There’s evidence of a weak magnetic field that can vary wildly as the moon passes through Jupiter’s strong magnetic field.
This ocean has intrigued researchers due to its potential to harbour extraterrestrial life, and yet we didn’t really know much about the moon at all until the Galileo spacecraft arrived in the mid-Nineties. There’s also some thought that humans could colonise the moon, although at first it doesn’t seem likely. Europa doesn’t seem to be very hospitable – it’s very far from the Sun, so temperatures don’t reach higher than -160 degrees Celsius (-260 degrees Fahrenheit) at the equator and -220 degrees Celsius (-370 degrees Fahrenheit) at the poles. It also has a thin atmosphere of mostly oxygen, and radiation levels high enough to kill a person in a day. But perhaps we could use the ocean for drinking water and extract its oxygen for breathing – both issues that serve as roadblocks to colonising other planets and moons. There’s even been speculation about building a base underneath the crust to use the ice as a radiation shield. Before seriously entertaining the idea of extraterrestrial life or living on the moon, we have to return to it – but that’s not scheduled to happen for another decade or so.