Europa, The Frozen Ocean

The Moon Europa as seen by the Galileo probe, via NASA.

Between 1609 and 1610, using his improved telescope design, Galileo Galilei observed four bodies orbiting Jupiter.  These bodies, later named the Galilean Moons, are the largest satellites of the Jovian System (Jupiter and its moons).  The second of these moons and the smallest of the first four observed, Europa has become one of the focal points for the search for life outside of Earth.

Despite most sources giving credit to Galileo for its discovery, Europa and the other three Galilean Moons were also likely discovered at the same time by Simon Marius.  Originally Galileo named the moons the Medician Stars, partly out of his initial uncertainty of their nature and as a way of faltering his future patrons.  Marius, however, named the moons according to the suggestion of fellow astronomer Johannes Kepler, invoking the names of the mistresses of Zeus (who was later renamed Jupiter according to Roman mythology), a tradition that has been used in the naming of all of Jupiter's moons since.

Europa was named after a Cretan moon goddess who was absorbed into Greek mythology as a Phoenician Princess who was abducted and raped by Zeus as he took the form as Bull.  Sought out by Zeus for her beauty, the Jovian moon of Europa follows in the steps of its namesake.

The first human probe to observe Europa was Pioneer 10 in 1973 and was followed the next year by Pioneer 11.  The first images of this sixth moon of Jupiter were grainy and lacked any distinguishing detail.  It wasn't until 1979 when the first Voyager probe passed by that images detailed enough to discern surface features were available.  It was with these first images that scientific curiosity and debate surrounding Europa bloomed.

While studying images obtained from both Voyager 1 and 2, certain anomalous features were noticed.  The most striking were striations that seemed to cover the moon.  Named Lineae, these features reached widths of 20 kms (12 mi) across and hundreds to thousands of kilometers in length. 

Europa with its prominent Lineae, via NASA.

To confound things further, the surface of Europa was surprisingly smooth.  Few craters were observed which suggests that the surface is constantly being remade.  In the cosmic shooting gallery that is the solar system, craters are a common feature.  The older the visible surface, the more craters that will be observed as only changes in surface features will wipe away craters that form.  A quick look at Mercury or our own moon Luna and you will see how a geologically inactive body collects craters.  Europa, on the other hand, has very few craters and those few that do exist are visibly 'young'.  Combined with Europa having a very high reflectivity, or albedo, at 0.64, the surface of the moon can be estimated to be between 20 and 180 million years.  In comparison, regions of the Martian surface have been dated to around 3.3 billion years.

These and other unexpected features seen by future missions such as the the Galileo mission to the Jovian system and the New Horizon probe as it passed by Europa on its way to visit Pluto suggested that the source of the strange features was ice, water ice.  But these surface structures suggests something more then just a cold frozen moon.  The renewing of the surface and the formation of the long Lineae lent credence to the hypothesis that there was liquid water hidden beneath the icy surface.  Not just a little water, but a vast, possibly global ocean.

But with an average surface temperature of 110 K (−160 °C; −260 °F) at the equator, how could there be a subsurface ocean of liquid water?  The answer seems to come from the same process that causes the tides here on Earth.  As Europa and the other Jovian moons orbit Jupiter, they are pulled on gravitationally at different rates at different times.  Similar to the processes that cause the nigh constant volcanism on the innermost of the Galilean Moons of Io, Jupiter's gravity causes tidal forces that heat up the plant.  This could, potentially, cause volcanism akin to the deep ocean hydrothermal vents on Earth.

The 'Ice Rafts' of the Conamara Chaos, via Wikipedia Commons.

With such internal heating, a warm, liquid water ocean is likely to exist.  This isn't a fringe idea either, it has become the leading hypothesis to explain the observable features of Europa.  The liquid ocean has been calculated to have an average depth of 100 km (over 62 miles) with depths up to 170 km (over 105 miles).  This liquid ocean would exist below a cold, hard cap of ice reaching 10 -30 km (6 - 19 miles) slowly becoming a more ductile warm ice before eventually becoming the liquid water below.  This warm ice would be capable of moving up into any cracks that form in the hard surface ice, creating the Linaea.  Still further evidence from the chaoses suggest that there may even be liquid water 'lakes' trapped in the ice.

The amount of water that can be found on Europa is staggering.  If one takes the average depth of 100 km, there is a volume of 3 × 10¹⁸ miles cubed of liquid water.  This is over two times the amount of water that can be found here on Earth.

Europa next to Earth, along with the comparable spheres of water contained by each, via NASA.

In the search for life elsewhere in our solar system, Mars tends to steal the show in the popular media.  With its relatively close proximity, potential for human colonization, and its history of once possessing large quantities of liquid water on the surface, it is understandable.  Liquid water seems to be the key for life as we know it.  Everywhere on Earth that it is found, life can be observed as well.  This being considered, Europa seems to be one of the best sites for the future search for life and the astronomical community knows this.  While there have been many probes proposed to visit Europa, most have fallen through until JUICE.

JUICE is the somewhat tortured acronym for the planned European Space Agency (ESA) probe known as the JUpiter ICy moon Explorer.  Planned for a 2022 launch, JUICE would reach the Jovian system in 2030 where it will serve out at least a three year mission to visit Europa and two other of the Galilean moons, Callisto and Ganymede.  Like Europa, Callisto and Ganymede also appear to have liquid water as well as the observed water ice.  While all have the potential for harboring conditions that could be conducive to life, Europa is seen as the most likely to contain such an environment due to its larger amounts of water, hotter internal environments and a far more dynamic surface.

Artist rendition of the JUICE probe, via ESA.

As a final boost to the potential for the development of life is the oxygen content of Europa.  Oxygen is, as far as we know, required for the development of complex life and was directly responsible for the explosion of multicellular life here on Earth.  Based on new calculations by Richard Greenberg, there could be enough oxygen to support a biomass of around 3 billion kilograms if such multicellular life had developed on Europa.  Even better, the proliferation of oxygen most likely occurred later in its existence which would be required for the formation of life based on how life progressed here on Earth.

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Citations:
Complete Dictionary of Scientific Biography. New York: Charles Scribner's Sons, 2007. ISBN 0-684-31559-9.

Geissler, Paul E.; Greenberg, Richard; et al. (1998). "Evolution of Lineaments on Europa: Clues from Galileo Multispectral Imaging Observations".

McFadden, Lucy-Ann; Weissman, Paul; and Johnson, Torrence (2007). The Encyclopedia of the Solar System. Elsevier. pp. 432. ISBN 0-12-226805-9.

Schmidt BE, Blankenship DD, Patterson GW, & Schenk PM. (2011) Active formation of 'chaos terrain' over shallow subsurface water on Europa. Nature, 479(7374), 502-5. PMID: 22089135

Cosmos Magazine. http://www.cosmosmagazine.com/news/3069/jupiter-moon%E2%80%99s-ocean-rich-oxygen.

Hartmann, William K., and Gerhard Neukum. "Cratering Chronology and the Evolution
of Mars." Space Science Reviews 96 (2001): 165-194.