Europa is one of Jupiter’s ‘big’ Galilean moons and the one that has received the most scrutiny when it comes down to finding life in another part of the solar system. The moon is covered with a layer of ice around five kilometres thick and beneath this an ocean up to 160km deep. The ocean survives in the bleak, frozen wastes of that part of the solar system due to gravitational interactions between Jupiter and Europa over the course of its orbit, warming it a little inside.
Heat and water, check, possible hydrocarbons from cometary debris check. But for Earth like organisms to flourish, a little something else is required – oxygen. We know that ice molecules on the surface of Europa are broken into oxygen and hydrogen atoms and molecules through both particle and photonic radiation from the giant planet. We also know impacts can push the material down, but only down to a certain depth well short of the subsurface ocean – Europa’s air is trapped in its throat unable to get to its lungs. However, a study of resurfacing on the moon suggests the oxygen can get a little deeper.
Europa is criss-crossed with ridges, rather like those on the seafloor on Earth, and like the seabed ridges here, they are places where material is upwelling from the warm interior of the body to form a new surface. In the case of the Earth, new seafloor comes from magma rising to fill the ridge and push it apart. On Europa, water is rising and forming new ice. Where there is formation, there must be destruction in order for Europa to have maintained such a thin outer crust. Somewhere, subduction must be happening, where old ice is forced underneath another layer of old ice and into the ocean, where it melts – similar to the edges of tectonic plates on Earth. As the old ice is mixed into the water below, it takes with it the oxygen on the surface.
In addition, erosion of the bottom of these ‘plates’ during the warmer periods of the orbit (when gravitational stresses are high, generating internal heat) and refreezing of the same blocks of ice could also lead to migration of oxygen rich material into the ocean. Calculations performed based on what we know of the icy tectonics of Europa suggest that sufficient oxygen to support the equivalent of millions of fish could be finding its way into the alien world. No proof that they are, and indeed one would suspect with upwelling of material, there would be an indication on the surface close to the ridge, but maybe that’s a game changing observation still to be made.