On the surface

With its thick ice and frosty brine-spouting cryovolcanoes, the smooth surface of Europa is a strange place to observe.

Europa has an unusual-looking surface. It’s incredibly smooth for the most part – one of the smoothest objects in existence in the Solar System. That doesn’t mean that the moon is a featureless ball of ice, however. The icy surface is also cracked in places, and criss-crossed with numerous reddish-orange lines and splotches. There are also ridges, domes and possibly even cryovolcanoes. The exact mechanism for the formation of these features is unknown, and there are a number of contradictory theories. The prevailing theory is that they’re likely due to intense tectonic activity within, caused by tidal heating. Opposing gravitational influences from Jupiter and other Jovian moons work to keep the moon’s interior on the move. This generates heat, warming ice below the surface and causing the colder crust on top to crack and shift. This tidal flexing may also spawn cryovolcanoes – the icy equivalent of Earth’s volcanoes that spew ice and gases into Europa’s atmosphere.

The dark lines, or lineae, arcing across Europa were likely produced by a series of eruptions of warm ice and are coloured dark because of contaminants such as magnesium sulphate in the ice. The spots, or lenticulae, may be the result of melted water that pushed up through the surface, then froze. There are also jumbled chunks of ice, known as chaos regions, scattered around. Some researchers believe that these are areas where the subsurface ocean has melted through the crust, but a newer hypothesis has emerged. There may be pockets of liquid water – separate from the ocean – encased just under the icy crust. These could be the source of the chaos regions, not the ocean. Regardless, all of this shifting seems to have got rid of all but the largest impact craters.

Europa has a very thin, tenuous atmosphere, mostly comprising oxygen, that exists at a much lower pressure than Earth’s atmosphere. This atmosphere doesn’t come from biological processes on the moon itself; instead, it’s a result of ultraviolet radiation from the Sun and charged particles from Jupiter’s magnetosphere hitting Europa’s surface. The radiation splits water into separate oxygen and hydrogen molecules, which are drawn to the surface of the moon. The hydrogen molecules are lighter and quickly escape Europa’s atmosphere, joining with other gases to form a neutral cloud around the moon. The denser oxygen stays in the atmosphere and may even reach the subsurface ocean.

At the equator, temperatures on Europa average approximately -160 degrees Celsius (-260 degrees Fahrenheit) and -220 degrees Celsius (-370 degrees Fahrenheit) at the poles. That hasn’t kept us from speculating about the habitability of the moon, or the possibility of life existing there right now. The subsurface ocean has been compared to the deep ocean on Earth, where microbial life exists near hydrothermal vents. There is no evidence yet, but a NASA researcher wrote in March 2013 that there is likely an abundance of hydrogen peroxide on the surface. When hydrogen peroxide is mixed with liquid water, it decays into oxygen. This would make the oxygen concentration high enough to theoretically support life.

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