Basic Properties of Europa

Semimajor axis        = 671079 km
Orbital period        = 3.551810 days
Heliocentric Distance = 5.203 AU
Rotational Period     Synchronous
Orbital inclination   = 0.464 degrees
Eccentricity          = 0.0101
Radius                = 1565 km
Mass                  = 4.797E22 kg
Mean density          = 2.99 g/cm3
Surface Gravity       = 0.135 of Earth's
Escape Velocity       = 2.02 km/s
Geometric Albedo      = 0.6
Surface Temperature   = 128 K (-145 C)
Surface Composition   = Water Ice
Tenuous O2 Atmosphere = Surface Pressure about 10-11 Earth's           

Recent images taken by the Galileo SSI camera show regions on Europa resembling ice flows in Earth's polar regions, along with possible geyser-like eruptions and long dark bands centered with white stripes that stretch across Europa's surface. In some areas, the ice is fragmented into large pieces (as large as 30 km across) that have shifted away from one another. This indicates that the ice crust (as thick as 100 km) of Europa has been or still is lubricated from beneath by warm ice or maybe liquid water. Heat generated by tidal forces due to Jupiter's strong gravitational forces may be sufficient to liquefy some portion of Europa's icy crust. If Europa does indeed have a liquid ocean below its crust, could this ocean's environment support some type of primative life forms?

The high resolution image below shows the ice-rich crust of Europa, one of the moons of Jupiter. Seen here are crustal plates ranging up to 13 km across, which have been broken apart and 'rafted' into new positions, superficially resembling the disruption of pack-ice on polar seas during spring thaws on Earth. The size and geometry of these features suggest that motion was enabled by ice-crusted water or soft ice close to the surface at the time of disruption.

The images below show what appears to be the most convincing evidence yet for a global ocean under Europa's icy crust. The images show the so called "cycloidal" features, or "flexi" near Europa's south pole. These cycloidal cracks form in Europa's solid-ice surface with the daily rise and fall of tides in the subsurface ocean (Gregory V. Hoppa, B. Randall Tufts, Richard Greenberg and Paul Geissler of the Lunar and Planetary Laboratory, University of Arizona). Due to perturbations from Jupiter's moons, Io and Ganymede, Europa is in a slightly eccentric orbit. As Europa approaches Jupiter, the tides are higher (stronger tidal forces) and as Europa moves away from Jupiter, the tides fall (weaker tidal forces). This causes Europa's ice shell to flex. A crack forms when tidal stresses reach the tensile strength of ice. The crack then propagates across the ever-changing stress field, following a curving path until stress drops below the tensile strength of the ice, when it halts. A few hours later, the tidal stress again exceeds the tensile strength of ice, and the crack begins a new curve in another direction. One of the most interesting findings is that each arc segment forms in 3.5 days, roughly the time it takes Europa to make one orbit around Jupiter.

Recent images taken by the Galileo spacecraft appear to enhance Europa's prospects as one of the places in the Solar System that could have hosted the development of life. They also demonstrate that there was enough heat to drive the flows on the surface. Chaotic features seen in many images of Europa's icy surface are probably created by Europa's tides, and are believed to be evidence of melt-through needed for exposing the oceans. The mixing of substances needed to support primative life may be driven by the tides on Europa, with maximum heights of 500 meters (much larger than Earth tides). Circulation of liquid water through cracks produced by tidal forces could bring salts and organic compounds dissolved in the water up to Europa's surface. This circulation also brings biologically useful chemicals, such as formaldehyde (as well as organic compounds dumped on Europa's surface by cometary impacts) down to the subsurface ocean. Other chemicals, formed by radiation near the surface, such as sulfur, hydrogen peroxide, and free oxygen, would also provide primative life with sources of energy and nutrients. Hydrothermal vents would produce organic compounds (seen as dark material coloring cracks?) and provide a heat source. Undersea volcanism could also lead to large melt-throughs, and tidal heat, created by internal friction could also melt the ice. The melted-through ice provides light and surface chemicals to the oceans. Any creatures inhabiting these oceans could use photosynthesis for energy. One possibility is that organisms trapped in the ice could be thawed out during the next tide that flows through, effectively releasing them.

The following image shows reddish spots on Europa's icy surface. These spots may indicate the location of warmer ice that is rising from below. The upwelling could provide a mechanism for material in the ocean below to rise to the surface. This convective process continuosly carries surface material downward to the subsurface ocean , and, if they exist, transports organisms up toward the surface.

Future cryobots could probe the European oceans for evidence of life currently thriving in a warm, nutrient rich environment. It could also reveal evidence of primative life that may have existed in an ancient European ocean, when the moon was warmer. Such lifeforms could have evolved to spend millions of years in a frozen state within the ice, coming to full life and reproducing only when they find themselves within a liquid-brine pocket. Europa thus has a high potential to meet the criteria for exobiology.