Friday, April 29, 2016

The History of Europa

Europa, one of Jupiter’s moons, has a diameter of 3,100 kilometers, which is about 20 percent smaller than that of the Earth’s moon. Almost all evidence indicates that Europa has an iron core, a rocky mantle, and a salty ocean lying under an icy shell. It orbits Jupiter every three and a half Earth days, and it is very nearly tidally locked with its planet, meaning that its orbital period is the same as its rotational period; one hemisphere of Europa constantly faces Jupiter. As with most planetary bodies, this moon probably formed with a rotation that was not originally synchronous. Over time, tidal torque would have slowed the spin so that it would be nearly synchronous with its orbit. It is generally thought that Europa has a synchronous orbit; however, Richard Greenberg, author of Unmasking Europa, claims otherwise. A non-synchronous orbit would help support the theory that there is a subsurface ocean. The asymmetry of Europa’s mass distribution would offset the tidal torque, and the surface would be separated from the interior by a layer of liquid. The strength of the tidal torque is uncertain, and theory alone does not reveal if Europa is truly tidally locked with Jupiter. Jupiter’s gravity is slightly stronger on the near side of the moon than the far side, so the magnitude of the gravity changes throughout the orbit. This push and pull of gravity stretches and relaxes the surface, and it is believed that this tidal action might cause volcanic or hydrothermal activity on the seafloor, supplying chemicals that could have generated life.

Between its discovery by Galileo and when the Pioneer and Voyager spacecraft passed by it, Europa was an unresolved point of light. However, by passing Europa’s light through a spectrograph, 20th-century scientists were able to infer the material that made up its surface. There were no instruments available to determine what would lie beneath the surface, and even now, it would be challenging to confidently determine the substances that lie within Europa unless scientists were to drill through the surface layer to see what lies underneath.

The presence of water was predicted long before Voyager or Galileo by Guy Consolmagno, who based this prediction on the theoretical models of how Jupiter’s moons form. These models suggest that Jupiter’s moons formed from a swirling cloud of dust and gas that surrounded the planet, just as the planets themselves grew in a similar cloud around the sun. As this Jovian “nebula” cooled, solid particles condensed out, and they gradually came together to form the satellite. The sequence of condensation determined the composition while internal heating separated the materials by density within each moon. Dense metal would sink to towards the center to form a core whereas lighter material would form the surface.

It is worth the cost and time to explore Europa as much as possible because there are still many features of it that remain a mystery. The combination of strong evidence of a subsurface ocean and the possibility of life should make exploring Europa a priority for future missions, and hopefully the mysteries of Europa will one day be revealed.


Unmasking Europa: the Search for Life on Jupiter’s Ocean Moon by Richard Greenberg