Europa, one of Jupiter’s largest moons, is considered to be one of the most likely places within the solar system that might harbor life. Europa possesses a great number of characteristics that might lend themselves to the independent evolution of life, similar to what occurred on Earth. In this essay, I will outline some of those key characteristics and highlight where and why they might suggest Europa is a potential breeding ground for, at the very least, microbial organisms undergoing Darwinian selection.
One of the primary characteristics of Europa that suggest it might harbor life is the presence of a water-ice crust (that is, instead of a rocky crust like on Earth, Europa has a crust made up of frozen water-ice). The presence of frozen water-ice in of itself, however, is not a major astrobiological finding. More importantly, beneath the water-ice crust of Europa, it is hypothesized that a liquid ocean of water exists, warmed from a likely volcanically active iron-nickel core. This liquid ocean is most likely trapped between a rocky nickel-iron mantle and frozen water-ice crust, forming a bubble where temperatures are warm enough to allow liquid water to exist, with the help of high levels of salts. Evidence for a liquid ocean beneath the frozen crust has been identified by the Hubble Space Telescope in the form of liquid vapor jets (cryogeysers) erupting from the surface of Europa. This suggests that the ocean is under pressure, most likely created by the thermal heat generated by Europa’s core, and rocky ice layers, causing increased pressure on the liquid ocean trapped between two rocky layers.
Heat is most likely generated in the core and rocky layers due to tidal flexing, that is, the gravitational pull of Jupiter causes deformation in the metallic core and rocky ice sheets. This deformation is the result of bending, crystalline structures – the act of which generates heat. This heat is most likely enough to allow the liquid ocean layer to persist beneath the rocky crust.
The composition of Europa’s surface is hypothesized to contain a high level of dissolved “sea salt” (sodium chloride), which would contribute to maintaining its liquid form at low temperatures and present an oceanic environment similar to that on Earth’s. However, because the concentration of sea salt is so theorized to be so high on Europa’s ocean, only extreme halophilic bacteria-like organisms could survive such conditions. With a subsurface temperature of -171 degrees Celsius, and a salt concentration significantly higher than Earth’s ocean, this seems like a plausible conclusion. However, this leaves open the possibility that pockets of warmer water, or haloclines (areas of lesser or greater salt concentrations), that may provide environments for more complex life forms to exist.
The search for evidence of life on Europa continues with NASA’s Europa Multiple-Flyby Mission (http://solarsystem.nasa.gov/missions/europaflyby) which will conduct multiple, low-angle flybys of Jupiter’s moon Europa. Interestingly, the EMFM probe will posses an ice-penetrating radar, which should allow for scientists to take a closer look beneath the surface. Unfortunately we would not see the results from this mission until, at the earliest, 2026. Until then we will have to rely on near-earth telescope data and the image data that other probe missions have produced.”
Ceres, unlike Europa, is not a moon – it is a dwarf planet. Interestingly, it is the only dwarf planet that makes its home within the inner solar system. Specifically, Ceres orbits around the sun among the other asteroids and comets within the Kuiper Belt. Similar to Europa, however, Ceres sports a multi-layered crust that houses a large body of frozen water-ice. Unfortunately, it is not currently known whether or not any of the water on Ceres is still liquid. However, Ceres poses an interesting conundrum for astrobiology. Since it is a member of the inner solar system, it stands as one of the possible originating points for life in the solar system. How could life have evolved on this cold, icy, rock that is similar in size and shape to Pluto? Well, most likely, due to Ceres’ small size, it would have cooled and formed a proto-planetary disc much earlier than the Earth (4.5 billion years ago). If Ceres cooled enough to have a stable atmosphere (albeit a small one due to its small gravity), then the organic chemical reactions needed to produce complex nucleic acids, proteins, and lipid structures may have begun much earlier than they would have had on Earth.
The next step would have been for some asteroid or comet to impact with Ceres and drag along any proto-bacteria type life forms with it, all the way to Earth. According to this hypothesis, Ceres would have been the “founder” of life in the solar system, giving rise to the earliest forms of bacteria that populated an early Earth. Of course, conditions on Ceres would not have remained favorable to life for very long (in geological time), so any life forms that did evolve on Ceres would not have likely evolved much further than a simple bacteria. In that sense, Ceres might be a good place look for early signs of bacterial life, but we shouldn’t expect to find much more than that.