Don't Forget Europa

Credit: NASA / JPL / Ted Stryk

We are living on exciting times , hundreds even thousands of possible planets are being discovered almost every day in other stars opening our mind to the possibility of life somewhere in the vast universe.

But closer to us, much closer, there are places where life can be present , and what a important discovery it would be!


Jupiter's icy moon Europa is slightly smaller than the Earth's Moon. Like the Earth, Europa is thought to have an iron core, a rocky mantle and a surface ocean of salty water. Unlike on Earth, however, this ocean is deep enough to cover the whole surface of Europa, and being far from the sun, the ocean surface is globally frozen over.

Europa orbits Jupiter every 3.5 days and is phase locked - just like Earth's Moon - so that the same side of Europa faces Jupiter at all times. However, because Europa's orbit is eccentric (i.e. an oval not a circle) when it is close to Jupiter the tide is much higher than when it is far from Jupiter. Thus tidal forces raise and lower the sea beneath the ice, causing constant motion and likely causing the cracks we see in images of Europa's surface from visiting robotic probes.

This "tidal heating" causes Europa to be warmer than it would otherwise be at its average distance of about 780,000,000 km (485,000,000 miles) from the sun, more than five times as far as the distance from the Earth to the sun.

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.

Ron Greeley, of Arizona State University, agrees that life is a strong possibility on Europa. "If we think about the basic ingredients for life, we need liquid water, we need an energy source, and we need the right organic compounds. Europa seems to be a place where those three ingredients can be found."

An ice-covered ocean may not seem an ideal place for life; human beings would certainly find it inhospitable. But microbes are far less discriminating.

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