In 1995, Kevin Costner starred in a sci-fi movie called Waterworld which became notorious for being extremely expensive to produce and not doing well at the box office. In the context of the astronomy, being a “water world”, an extraterrestrial body containing liquid water, is a rare and desirable commodity. Liquid water is essential to life because it is a medium for chemical reactions. Satellites that potentially have water have attracted great deals of attention and potential space exploration prospects. If Titan were not to work out for the first extraterrestrial settlers, another body with liquid water may be a good target.
Previous posts have discussed the methane atmosphere and potential liquid methane oceans of Titan. Titan also potentially is home to liquid water. It is known that the moon is largely made up of a mixture of ice and rock. Analysis of data from the Cassini probe in 2006 indicated that Titan distorts slightly during its orbit around Saturn - it is “squishy”. This strongly suggests that there is a layer of water below its surface. Titan may be warmer below the surface, and liquid water may be possible, especially since the presence of ammonia may allow water to remain liquid even at temperatures as cold as -142 degrees Farenheit.
The idea that there may be liquid water oceans in the far solar system is an exciting and counterintuitive one! After all, these satellites are very far from the “habitable zone” where the sun’s energy provides enough heat to create liquid water. Many of these moons are heated by “tidal heat” which means due to the gravitational influence of other bodies (such as their host planet and other moons), the planet is stretched and squeezed. This creates friction and heat, warming the planet – it can even drive volcanic activity!
Below I have outlined a few examples of worlds were scientists suspect the presence of liquid water.Enceladus
In May 2011, NASA scientists commented that Enceladus may be the most habitable spot outside the solar system. Enceladus has a shiny surface, which suggests that it is made from ice that was recently generated (and still actively being generated). The most exciting feature of Enceladus is the phenomenon of cyrovolcanism, or the eruption of water which appears to be happening out of its south pole. The plume is known to be mostly water vapor because Cassini analyzed it during flyby. A false color image of Enceladus’ south pole region is below.
These features of Enceladus suggest the existence of liquid water beneath the surface. It seems unlikely that the higher temperature on the inside of Enceladus is due to nuclear decay, so the most plausible explanation is “tidal” forces where the push and pull of Saturn’s gravity creates heating that keeps the inside of Enceladus warm enough to support liquid water. A geological feature of Enceladus which continues to provoke further study are the “tiger stripes”, four depressions near the south pole, pictured below:Europa
Europa is the smallest of the four Jovian (Jovian means "related to Jupiter") moons discovered by Galileo, but it is one of the largest moons in the solar system. In 1989, the Galileo mission flew close to Galileo and provided a great deal of Its atmosphere is composed primarily of oxygen. The surface appears to be smooth, without craters, which indicates a young surface. The most likely explanation of the shiny, young, smooth surface is that there are warmer layers of ice beneath the surface and tidal movements cause the warmer ice to spread to the surface. Some have hypothesized that there is a liquid ocean beneath the surface, with the squeeze of Jupiter and two other moons (Io and Ganymede) providing the heat. A futuristic imagined probe of Europa's ocean is featured in this YouTube video: http://www.youtube.com/watch?v=sPCpNlVJBr8
While the existence of ice and potentially a liquid ocean is exciting, colonization of any Jovian moon will face a significant obstacle in that there is intense radiation near Jupiter, enough radiation to likely cause major health effects such as cataracts, sterility, and death from radiation sickness in just one day. Callisto
Callisto is approximately the same size as Mercury, and its atmosphere is very thin and primarily composes of carbon dioxide and possibly oxygen. Callisto is made up of approximately 50% water ice. Callisto possibly also has a liquid ocean of salty water far beneath its surface. Callisto is not tidally heated, but its hypothesized ocean may be able to stay liquid because of a combination of high pressure and high salt content. Ganymede
Ganymede orbits around Jupiter and is about half rock and half water ice. Observations by Galileo suggested the presence of certain salts, which may originate from an underground ocean. The liquid ocean would come from tidal heating.