Water on Other Planets: Oceans in Our Solar System

If life as we understand it is to exist on a planet or moon other our home planet, it has been long understood that there must be a presence of liquid water to support it. While there was a time in which scientists speculated whether water might only be present in appreciable quantities here on earth, discoveries in recent years have led many to conclude that water is not nearly as rare a commodity as once thought. In our solar system, surface oceans of water appear to be unique to Earth, but some of our neighboring planets and moons appear to be awash in a variety of bodies ranging from oceans of ice to methane seas to subsurface liquid oceans spanning an entire moon or planet.

Earth

Here on Earth, some 72% of the surface of our world consists of salt-water oceans, and this is the only known planet with liquid water on the exterior. Surface level liquid water is possible on Earth due to combination of the planet's mass, temperature, and atmosphere. The oceans of earth are a single continuous body of water that are divided into smaller bodies of water largely only by man's customs. This World Ocean consists of some 1.3 cubic kilometers of salt water with a maximum depth of nearly 11,000 meters. Of all of the Earth's water, only 3% is available as fresh water.

Mars

The presence of water on the surface of Mars has been suggested by some party or another since the earliest days of western astronomy. Striations along the surface of the planet suggested the presence of irrigation or river routes, and the various darkened areas of the planet were thought to have possibly been seas. These speculations were largely dashed in 1965 as a result of a fly-by of Mariner 4, which revealed a decidedly barren Martian surface. If this planet was home to surface oceans, they ebbed and flowed only in the dim past of Mars. The geographical evidence gathered in recent years by orbiting satellites, landers and rovers suggests that Mars did once have a considerable amount of surface water under a different set of atmospheric conditions, and the large polar caps of Mars have been confirmed to consist of water ice. Beneath the hard surface of Mars, it is also believed that there may be a considerable amount of liquid water, but it remains to be seen whether this water exists in the form of subterranean seas, rivers, or any appreciable quantity at all.

Europa

Europa has long been touted as one of the more likely locales for extraterrestrial life, with what many believe to be a vast under-ice ocean of liquid water warmed by tidal heat. If the current mainstream thinking on Europa holds true, the moon includes a layer of subterranean liquid ocean that spans the entire glob with a surface comprised entirely of a few kilometers of ice. Because of Europa's small size, the core of the moon is cold metal. The liquid layer of water under the ice is created through the movements of the tides generated by Jupiter. Some speculate that the ice crust is thin enough for the liquid water layer to occasionally break loose of the crust, as evidenced by certain aspects of the surface terrain, but most scientists believe that this would occur very rarely, if at all.

Titan

Although there are seas that lap at the shores of Titan, they're quite alien to what we are familiar with on earth. The first definitive evidence of the presence of surface liquid located beyond Earth was discovered on Titan, a large Saturn moon. Although the presence of liquid methane was suggested by the Voyager program, concrete evidence was not obtained until 2008 by the Cassini mission, which eventually found bodies of liquid methane and ethane that were smaller than the oceans that were expected. These hydrocarbon lakes and seas are possible due to a surface temperature of -179 °C (-290 °F) and consist of liquid methane and ethane that represent hundreds of times more surface organics than the Earth's supply of oil. It is also likely that Titan has a layer of subterranean liquid water and ammonia between varying layers of ice similar to the case of Europa, but this water-ammonia ocean would have the benefit of a warm core that could provide the basis of chemosynthetic life.

Enceladus

Enceladus, the sixth-largest moon of Saturn, is perhaps the most recent addition to the list of locations in our solar systems that may have large quantities of water. In fact, results from the Voyager program implied that the moon may be made of primarily water in the form of ice, although more recent fly-by's have suggested that Enceladus has a mass that would require other materials present in the water ice. In 2008, a plume of water was observed gushing out from the underground of Enceladus at the powerful rate of 2,189 miles per hour. The presence of such a geyser suggests that there is a considerable amount of subterranean liquid water on this moon being heated by a hot interior mantle. Evidence of such dynamics present a tantalizing case for the possibility of life, as we know that microscopic life thriving on the cusps of such submarine geysers through chemosynthesis provides the support for complex aquatic ecosystems in the oceans of earth that survive entirely deprived of the sun's energy.

Sources:

http://www.solarviews.com/eng/enceladus.htm

http://www.scientificamerican.com/article.cfm?id=signs-of-hidden-ocean-under-titans-crust

http://people.msoe.edu/tritt/sf/europa.life.html

http://www.space.com/scienceastronomy/080620-phoenix-ice-update.html