Europa possesses vast oceans which may have a chemical makeup close to that of Earth, and could be warmer than astronomers previously believed. This finding increases chances alien life may be found on Jupiter's fourth largest moon. However, another satellite also holds promise of lifeforms living beyond the Earth - Saturn's largest moon, Titan.

Extraterrestrial life in the solar system was once thought to most likely be found on Mars. Science fiction classics, including War of the Worlds, told epic tales of Martians. However, modern research reveals Mars to exhibit extremely dry conditions away from its polar caps, with the exception of ice trapped beneath its surface.

For life to exist as we know it, an alien world would likely need to have vast quantities of a medium, such as water, for organic materials to combine, permitting complex reactions to take place.

A vast ocean is thought to exist on Europa, beneath its frozen surface. Models created mimicking conditions on Europa suggest the salty oceans of the Jovian satellite could harbor enough chemical energy to give rise to lifeforms. These conditions could bring forth life without a need for hydrothermal action, researchers have revealed.

"We're studying an alien ocean using methods developed to understand the movement of energy and nutrients in Earth's own systems.The cycling of oxygen and hydrogen in Europa's ocean will be a major driver for Europa's ocean chemistry and any life there, just as it is on Earth," said Steve Vance from the Jet Propulsion Laboratory.

As the seawater on Europa interacts with its rocky floor, hydrogen may be released through a process known as serpentinization. Such a release, triggered by the formation of minerals as deep as 15 miles beneath the ocean floor of the satellite, could have driven the rise of life there.

Titan, on the other hand, is bathed in massive pools of hydrocarbons. A lack of liquid water leads some astronomers to believe this cloud-shrouded satellite is an unlikely place to discover alien life. Such oceans of methane and other organic materials, however, may provide an ideal birthplace for the creation and evolution of extraterrestrial life, as contended by other researchers.

Methane in the atmosphere of the distant moon was once thought to suggest the frozen body may indeed hold life, even today. The gas is destroyed by exposure to sunlight, but is replaced on Earth by living organisms, suggesting a similar mechanism might be at work on Titan.

"Methane is a by-product of the metabolism of many organisms. On Earth, the simplest biological sources, such as those associated with peat bogs, rice fields and ruminant animals, continuously supply fresh gas to replace that destroyed by oxidation. Could this mean there is life on Titan?" asked the European Space Agency in 2003.

However, astronomers later found a geological explanation was responsible for high concentrations of the atmospheric gas.

So far, the only life we know of lives on our home planet. While a fraction of astronomers search for signals from intelligent beings on distant worlds, others hope to find life, however primitive, on worlds and moons orbiting our companion star. Both Europa and Titan present striking possibilities for finding new lifeforms, as well as daunting challenges.

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