A Submarine for Europa

The Search for life in the solar system may one day take us beneath the icy surface of Jupiter’s moon Europa. This moon is undergoing continual gravitational tidal stress due to the gravitational pull of Jupiter and the other Jovian satellites in its vicinity. These tidal stresses are believed to be providing sufficient heat to Europa’s interior. This tidally generated heat may have liquefied the interior enough to have created an extensive ocean under its icy exterior.

There is also ample photographic evidence obtained from the Voyager and Galileo space probes that this ocean may indeed exist. Europa’s surface is essentially smooth. Yet, it is criss-crossed by dark linear features that appear to be cracks in its icy shell. Close examination of these features also shows similarities to ice floes on seen on Earth’s polar seas. The distorted band shapes suggest that there might have been soft ice or liquid water below Europa’s surface. These could be explained if Europa’s surface rotates slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon’s surface from its rocky mantle and to the effects of Jupiter’s gravity tugging on the moon’s outer ice crust.

The prospect that an extensive ocean may possibly exist has fuelled a great deal of speculation that Europa could very well be another abode for life within our solar system. If this alien biosphere does exist it could be an environment whose characteristics resemble those found near Earth’s deep-ocean hydrothermal vents or under the Antarctic’s Lake Vostok. This speculation has also fuelled a great deal of enthusiasm amongst Astrobiologists that Europa should be a top candidate in the next phase of NASA’s exploration of the outer solar system.

There have been many proposals put forth. One mission known as the “Ice Clipper” mission would use an impactor similar to the Deep Impact mission that impacted comet Tempel on July 4th, 2005. Deep Impact was designed to study the composition of the interior of the comet Tempel 1 by allowing a section of the spacecraft to smash into the comet. In a similar way the Ice Clipper mission to Europa would make a controlled crash into the surface of Europa, generating a plume of debris which would then be collected by a small spacecraft flying through the plume to enable direct chemical analysis of the ejecta.

More ambitious proposals call for the direct breach of Europa’s icy surface and the exploration of its conjectured ocean. One proposal calls for a large nuclear powered “Melt Probe” (cryobot) which would melt through the ice until it hit the ocean below and deploy an autonomous underwater robotic submarine known as a hydrobot to explore the ocean underneath. Another possibility, as depicted in this painting by space artist David A. Hardy, is to use a high velocity projectile to breach the surface and create an opening that would allow a robotic submarine to enter the ocean without having to slowly melt its way towards the interior.

Recently Carl T. F. Ross, a professor at the University of Portsmouth in England offers an abstract design of such an underwater vessel in his paper, “Conceptual Design of a Submarine to Explore Europa’s Oceans.”

Ross’s submarine would have to withstand the enormous pressure of an Europan ocean ten times deeper than Earth’s global ocean. Ross proposes a 3 meter long cylindrical sub with an internal diameter of 1 meter. He believes that steel or titanium, while strong enough to withstand the hydrostatic pressure, would be unsuitable as the vessel would have no reserve buoyancy. Therefore, the sub would sink like a rock to the bottom of the ocean. A metal matrix or ceramic composite would offer the best combination of strength and buoyancy. Ross’s conceptual submarine would be powered by fuel cells or some other more efficient power source that may become available as technology advances.

Before such a mission is undertaken a series of precursory missions have been proposed within NASA’s Jet Propulsion Laboratory (JPL) to ascertain if an Europan ocean does indeed exist. However, if it does indeed become a certainty that such an extraterrestrial ocean does exist it will most undoubtedly be a major target of exploration of both robots and future aquanauts.


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