Habitability and Biology - page 2

The Biosphere

Just as the "habitable zone" is the region of our Solar System that can support life-bearing planets, the "biosphere" is the region of Earth that can support life. More specifically, Earth's biosphere is the area of our planet where living organisms survive and interact with both their environment and with other living organisms. This zone includes most of the hydrosphere (liquid water, frozen water, and water vapor), parts of the lower atmosphere, and the upper lithosphere (Earth's crust and upper mantle).

The continued survival of life on Earth, or on any planet, depends upon the one-way flow of energy from a source (in Earth's case, from our Sun); the cycling of critical elements such as carbon and water; and gravity, which prevents atmospheric gases from escaping into space.

The Aleutian Islands, a chain of more than 300 small volcanic islands in the northern Pacific Ocean, are a designated Biosphere Reserve, an international recognition given by the United Nations Educational, Scientific, and Cultural Organization (UNESCO). (Image credit: U.S. Fish and Wildlife Service)

Earth's biosphere is divided into different regions, known as biomes, that can be characterized by their unique plants, animals, and climate. The five major biomes found on Earth are desert, forest, grasslands, tundra, and aquatic areas. Within each biome are groups of plants, animals, and microorganisms that function together as a unit -- known as ecosystems. For example, although the desert of central Australia and the Sonoran Desert in the southwest U.S. and Mexico are both part of Earth's desert biome, they each have very different plants and animals.

International Biosphere Reserves have been set aside in many different regions of the Earth as places to develop and demonstrate various approaches to conservation and sustainable development.

Life in Extreme Places

Recent discoveries have shown that life can exist in conditions that would be considered extreme by human standards, such as volcanic vents on the deep ocean floor and the cold, dry valleys of Antarctica.

A black smoker vents superheated water from a hydrothermal vent near the Atlantic mid-ocean ridge. (Image credit: NOAA)

Many people think of volcanic eruptions as only happening on the Earth's surface. But deep on the ocean floor, volcanic activity releases hot water and molten rock into the ocean water through vents called black smokers. Water emitted from the black smokers can be up to 400 degrees C (750 degrees F). It's difficult to imagine any living organism surviving at these temperatures, but in recent decades scientists have discovered life not only surviving, but thriving in these conditions. Similar organisms live in hot springs, such as the ones in Yellowstone National Park.

Other organisms thrive at the other end of the extreme - in very cold environments. In Antarctica's dry valleys, microbes called lithophiles have been found living in tiny pockets of water that fill pore spaces between grains in rock. These organisms, which get their energy from chemical reactions between the water and surrounding rock, can survive up to several kilometers below the Earth's surface.

Tubeworms, mussels, and scavenging crabs congregate at a hydrothermal vent in the East Pacific Ocean. (Image credit: NOAA, Photographer: C. Van Dover)

It is even possible for bacteria to lie dormant in dry, airless, bitterly cold conditions for millions of years and then become re-activated when conditions become more favorable to their existence. These organisms have special "resting" cells, called endospores, that allow them to go into a dormant state. Such dormant microbial colonies may exist in the Martian arctic, where - due to the periodic wobbling of the planet's axis - liquid water may exist at the surface for brief periods about every 100,000 years. During these temporary intervals of liquid water, the soil environment could transform into a habitable environment for the microbes.

The very word extremophile implies that all of these organisms live in very extreme environments, but the conditions they survive in are only extreme by human standards. In fact, life that exists in these conditions is probably much more common than is life that requires moderate temperatures and oxygen. Most biologists agree that when looking for life elsewhere in the universe, including Mars, we should search for organisms that live in a variety of conditions. Also, because we now know that life can exist in a variety of extreme conditions, the number of places we might find life is much greater than was previously believed.