Up until about 30 years ago, it was believed that all life on Earth was dependent upon energy from the sun. Furthermore, it was thought that you would probably not find life where temperatures were extremely hot, like in geysers or hot springs, or extremely cold, like in the Antarctic desert.
These ideas changed when oceanographers explored hydrothermal vents, openings in the ocean floor where extremely hot, mineral-rich water erupts from the crust. Hydrothermal vents are located several miles below the surface, on the ocean floor, where the surrounding water is at or near freezing, it is absolutely dark and the pressure is high. In organized communities around the bases of these vents, called black smokers, scientists found clams, crabs and exotic, giant tubeworms measuring 6 feet (2 meters) long. The water coming out of these vents is 230 to 662 degrees Fahrenheit (110 to 350 degrees Celsius).
How can these animals survive so far from the sunlight, under these extreme conditions? In the water, scientists found species of bacteria that split hydrogen sulfide from the water to get energy to make organic compounds (chemosynthesis). The tubeworms have bacteria in their tissues that help them derive energy from the water. The clams feed on the bacteria, and the crabs feed on the tubeworms.
The discovery of hydrothermal-vent communities showed that it is possible for life to evolve in places without light from the sun, and in other worlds without sufficient light from the parent star. In view of the discovery of hydrothermal vents, it may be possible that life exists on Europa, an icy moon of Jupiter, which scientists believe has a water ocean beneath its icy crust.
Life has been found in other extreme environments as well. Scientists discovered microcolonies of lichens called cryptoendoliths in rock samples of the Antarctic desert, where temperatures often drop to 100 degrees below zero and there is little or no liquid water. In contrast, thermophilic (heat-loving) bacteria have been found in hot springs where temperatures exceed the boiling point of water.
If life can evolve in extreme environments on Earth, it seems possible that life may exist in the extreme environments of other worlds such as Mars.