Hydrothermal systems are locations where water (hydro) and high temperatures (thermal) come together. These conditions are believed to be a possible environment for the earliest life forms on Earth. These systems are associated with both volcanic activity, where water comes in contact with magma, and with areas where deeply circulating water rises up from deep and hot layers in the Earth's crust.
You may have experienced a hydrothermal system if you have ever visited a hot spring, or seen Old Faithful at Yellowstone National Park. While not uncommon on land, the majority of the hydrothermal systems on Earth are actually deep in the ocean; a result of volcanic heating at mid-ocean ridges. Hot magma at these spreading centers, sometimes called hydrothermal vents, heats the water to temperatures of 300-400 °C. However, the temperature of the water drops off dramatically away from the heat source, falling to 2 °C within just a few inches of the vent. The water circulates through the basaltic seafloor and mixes with volcanic gases, resulting in superheated water that is rich in sulfur, iron, and other metals. When the hot water mixes with colder water, minerals precipitate out and sometimes turn the water black (“black smokers”) or form “chimneys” around the vent.
Absolutely no sunlight reaches the bottom of the ocean where we find hydrothermal vents. Nonetheless, these vents teem with aquatic life. These life forms utilize chemical reactions instead of sunlight as a primary energy source. In this warm, chemical rich environment, life is plentiful. There is a wide variety of microbial life, as well as larger organisms such as tube worms, albino crabs, and giant clams. Submarine trips to hydrothermal vents have even revealed the presence of “snow”, white colonies of sulfide-dependent bacteria that are spewed from the vents and have the appearance of snow. All of these life forms are thriving at crushing pressures, high temperatures, and in the complete absence of sunlight. This environment may sound hostile to us humans, but life is not only abundant near hydrothermal systems, some of Earth's earliest life forms may even have started there! A species of thermophilic, or heat-loving, organisms is thought to be the last common ancestor of all living organisms on Earth. During the final stages of heavy bombardment on Earth, deep-sea hydrothermal sites may have provided a protected haven for single-celled life forms.
While most hydrothermal systems on Earth are driven by volcanic energy sources, other sources of energy may also be important. Some minerals in ancient impact craters were formed in hydrothermal systems that were initiated by the high energies of the impacts. On Mars, there is evidence that impacts could melt enough ice beneath the surface to create lakes in some craters. Europa, a large satellite of Jupiter, may have liquid water oceans that are heated from below by tidal flexing. These hydrothermal systems are possible sites for life on other planets.