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Space Topics: JupiterEuropa
Diameter: 3,121.6 kilometers -- 0.2447 Earth diameters -- 15th largest solar
system body What could Europa be hiding beneath its smooth, bright ice? The Galileo spacecraft has confirmed that there is a liquid, briny ocean at some depth below Europa’s surface. (The exact depth to the liquid layer is one of many hot topics of debate about Europa.) The ocean would be a thrilling place to explore with a future submersible probe. It would also be an environment in which primitive unicellular organisms, called extremophiles on Earth, might be able to survive. As a result, any mission planned to land on Europa will have to pass extremely strict “planetary protection” qualifications in order to prevent the contamination of Europa by Earth organisms. That way, if life is ever detected on Europa, we can be sure that it didn’t get there from Earth. Galileo also returned images of a Europan surface that displayed a stunning variety of features, from grooves and cracks to pits, spots, domes, and jumbled “chaos” regions. At the same time, there are very few craters, suggesting a surface age for Europa of fewer than 30 million years. The cracks and grooves likely resulted from the flexion of Europa’s crust due to tidal interactions with Jupiter and Io, Ganymede, and Callisto. But while some of the cracks and grooves are oriented in the proper direction with respect to all of the tidal forces acting on Europa, other, older features are not properly aligned. This leads scientists to believe that Europa’s outer icy shell may be “decoupled” from its rocky interior because of the global subsurface ocean, so that Europa’s icy shell can rotate independently of the interior. For years after Galileo’s arrival at Jupiter there was a hot debate about whether or not the Europan chaos regions represent areas where liquid water actually erupted through to a cracked surface and then refroze. That’s certainly what the chaos regions look like -- icebergs in a frozen sea. But appearances can be deceiving. Europa’s surface temperature only reaches 110 Kelvin (-160 degrees Celsius, -260 degrees Fahrenheit) at the equatorand only 50 Kelvin (-220 degrees Celsius, -370 degrees Fahrenheit) at the poles. At such low temperatures, water ice behaves like rock does on Earth; liquid water reaching the surface would produce features that look like volcanic vents or flows. Scientists now believe that many of Europa’s strange structures result from convective motions of relatively warm, but still solid, regions of ice in Europa’s outer shell. Europa has a very tenuous atmosphere consisting mostly of oxygen. Unlike oxygen on Earth -- which is created as a byproduct of photosynthesis in plants -- oxygen on Europa likely originates when charged particles from the Sun hit water molecules on Europa’s surface. The water molecules are broken into hydrogen and oxygen atoms, and sometimes recombine to form hydrogen and oxygen gas. Hydrogen, being less dense than oxygen, escapes more easily from Europa’s surface. Over time, the preferential escape of hydrogen has left behind a ghostly atmosphere of oxygen. Global views of Europa
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