Astrobiology

Earth's Microbial Biosphere

Since the late 1980s, advances in genetics and molecular biology have radically altered scientists' view of the biosphere and the contribution of microbial life to planetary biodiversity. The opportunity to compare gene sequences from a wide variety of living organisms and environments has shown that living organisms cluster into one of three biological domains: the Archaea, Bacteria, or Eukarya. Each of these domains is made up of dozens of biological kingdoms, the vast majority of which are microbial. Species inferred to be the most primitive forms so far discovered are all found at high temperatures (greater than 80°C [176°F]) where they use simple forms of chemical energy. However, knowledge of Earth's biodiversity is still very much a work in progress. While biologists have sampled a wide range of environments, it is estimated that only a small fraction, perhaps 1 to 2 percent of the total biodiversity present, has so far been captured. Still, the three-domain structure has remained stable. New organisms are being discovered each year, adding diversity to each domain, but many discoveries still lie ahead.

These advances in biology have led to a growing awareness that Earth is overwhelmingly dominated by microscopic life and that these simple forms have dominated nearly the entire history of the biosphere. Indeed, advances in paleontology have now pushed back the record of microbial life to within half a billion years of the time scientists believe Earth first became inhabitable. This suggests that once the conditions necessary for life's origin were in place, it arose very quickly. Exactly how quickly is not yet known, but in geologic terms, it was a much shorter period than previously thought. This view significantly improves the possibility that life may have originated on other planets such as Mars, where liquid water may have been present at the surface for only a few hundred million years, early in the planet's history.

The Evolution of Complex Life

Studies of the fossil record have revealed that complex, multicellular forms of life (plants and animals) did not appear on Earth until about 600 million years ago, which is recent in geological history. Animals are multicellular consumers that require oxygen for their metabolism. Scientists believe that their late addition to the biosphere was triggered by the buildup of oxygen in the oceans and atmosphere to a threshold of about 10 percent of the present atmospheric level.^* It is clear that the high level of oxygen found in the atmosphere today could have been generated only through photosynthesis, a biological process that captures sunlight and uses the energy to convert carbon dioxide and water to organic matter and oxygen. Clearly, oxygen-evolving photosynthesis has had a profound effect on the biosphere. If oxygen was required for the appearance of complex animal life, then a detailed understanding of photosynthetic processes and their evolution is crucial to create a proper context for evaluating the cosmological potential for life to evolve to the level of sentient beings and advanced technologies elsewhere in the cosmos. This research also provides a context for the SETI program (Search for Extraterrestrial Intelligence), which is currently exploring the heavens for advanced civilizations elsewhere in the galaxy by monitoring radio waves.

Basic Requirements for Life

The most basic requirement of living systems is liquid water, the universal medium that organisms use to carry out the chemical reactions of metabolism. Water is a unique dipolar compound (positively charged on one side and negatively charged on the other) with special solvent properties that allow it to act as a universal medium of transport and exchange in chemical reactions. In addition, the physical properties of water allow it to remain liquid over a very broad range of temperatures, thus enhancing its availability to living systems. In exploring for life elsewhere in the cosmos, the recognition of the importance of liquid water as a requirement for life is reflected in NASA's basic exploration strategy, which seeks to "follow the water."

But to exist, living systems also require sources of nutrients and energy. The common biogenic elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), which comprise the basic building blocks of life, appear to be widely distributed in the universe. These elements are forged in the interiors of stars through nuclear fusion reactions, and through normal processes they produce elements with masses up to that of iron—56. The heavier metallic elements, some of which living systems also require, are formed only in very massive stars during supernova explosions. A key question of astrobiology concerns the distribution of massive stars in galaxies, which in turn may control the distribution of heavy elements essential for life.

By applying new methods of molecular biology and genetics over a broad range of environmental extremes, scientists' knowledge of the environmental limits of life on Earth (and the ways that organisms obtain nutrients and energy) has expanded dramatically. This area of inquiry comprises a relatively new area of biology known as extremophile (extreme-loving) research. This research has revealed that microbial species thrive in environments with broad extremes of temperature, ranging from deep-sea,hydrothermal vents (about 114°C [237°F]) to Siberian permafrost (−15°C [5°F]). (Above about 130°C [266°F], complex organic molecules become unstable and begin to break down. This temperature may comprise an absolute upper limit for life based on the limitations of carbon chemistry.) In addition, microorganisms occupy nearly the entire pH range from about 1.4 (extremely acid) to about 13.5 (extremely alkaline). Microbial life also occupies an equally broad salinity range from freshwater to saturated brines (containing about 300 percent dissolved solids) where salt (NaCl) precipitates. Finally, organisms also survive at very low water availability by creating desiccation-resistant structures that can survive for prolonged inclement periods.

Alternative Energy Sources

Within the basic constraint of liquid water, barriers to life appear to be few. However, it is important to understand that the level of productivity possible for living systems is strictly constrained by the quality of the energy sources they are able to exploit. On Earth, more than 99 percent of the energy powering the biosphere is derived from photosynthesis. This is not surprising given that, per unit area of Earth's surface, energy from the Sun is several hundred times more abundant than the thermal and chemical energy sources derived from within Earth. Clearly, there is a great advantage (energetically speaking) in exploiting solar energy. But the potential importance of chemical sources was also made clear in 1977 when American oceanographers Jack Corliss and Robert Ballard piloted the deep submersible,Alvin, to hydrothermal springs on the seafloor located more than 2.4 kilometers (1.5 miles) deep. At this depth, no sunlight exists for photosynthesis, and yet complex ecosystems were found there in which the organisms (including large, multicelled animals) derived their energy entirely from chemical sources provided by the hot fluids. This discovery shocked biologists, as they realized that even though photosynthesis provides much more energy, simple forms of chemical energy are still capable of supporting complex ecosystems. Since 1977, many other examples of deep-sea vent ecosystems have been found in virtually every ocean basin on Earth.

A Deep Subsurface Biosphere

As methods of exploration and observation have improved, life's environmental limits have continued to expand. In 1993 American biochemist Thomas Gold suggested that single-celled forms of life survive and grow in the deep subsurface of Earth, residing within tiny pore spaces and fractures in indurated rocks. In fact, volumetrically, such subsurface life forms could comprise more than half of Earth's biomass. Microscopic life is also thought to exist in a deep subglacial lake called Vostoc, which lies more than 3 kilometers (1.9 miles) beneath the ice cap of Antarctica. While many subsurface microbes appear to depend on photosynthetically derived organic matter that washes down from the surface, some species can make their own organic molecules from inorganic sources. Called lithoautotrophs (which literally means "self-feeding on rocks"), these organisms use the byproducts of simple weathering processes in which carbon dioxide dissolved in groundwater reacts with rocks to yield hydrogen. Hydrogen in turn is exploited for available energy. These organisms hold special importance for astrobiology because their existence allows the possibility that subsurface life can exist completely independently of surface (photosynthetic) production. Such lifestyles hold important implications for Mars and Europa (one of Jupiter's largest moons), where deep subsurface habitats are postulated to exist.

Studies of extremophiles have revealed that terrestrial life occupies virtually every imaginable habitat where liquid water, chemical nutrients, and simple forms of energy coexist. This observation has dramatically expanded the range of habitats available to life as well as the potential for life elsewhere in the solar system or beyond.

Exploring for a Martian Biosphere

Liquid water is unstable in surface environments on Mars today, thus imposing a formidable barrier to the development and survival of Martian life. Nevertheless, models suggest that a global groundwater system could exist on Mars today at a depth of several kilometers below the surface. Indeed, the Viking orbiters revealed many ancient channel features on Mars that formed when groundwater escaped and flooded onto the surface. But could groundwater still exist there today? In 2001 planetary scientists Michael Malin and Kenneth Edgett, using a high resolution camera onboard the Mars Global Surveyor mission, detected more than 140 sites on Mars where water appears to have seeped out of the subsurface, carving small channels in the surface. Under current conditions, average crustal temperatures on Mars are well below the freezing point of freshwater almost everywhere on the surface. Such surface springs of liquid water, however, could be sustained by warm, saline brines (salt lowers the freezing point of water) derived from deep hydrothermal sources. If this hypothesis is proven, the presence of liquid water—even hot, salty water—will substantially enhance the biological potential of Mars.

On Earth, scientists have found fossil biosignatures in sedimentary rocks going as far back as there are sedimentary sequences to sample. By studying the processes that govern the preservation of fossil biosignatures in similar environments on Earth, scientists are continuing to refine their understanding of the factors that govern fossil preservation. This provides a basis for the strategic selection of sites on Mars to explore with future landed missions and for sample returns. Due to the lack of plate tectonic recycling and extensive aqueous weathering on Mars, rocks preserved in the heavily cratered, ancient highlands appear to extend back to the earliest history of the planet. The rocks of these old crustal regions could be much better preserved on Mars than they are on Earth. In fact, a meteorite of Martian origin (ALH 84001), which has been dated at about 4.56 billion years, shows very little evidence of aqueous weathering.

Searching for Life in the Outer Solar System

The discovery that life can survive in deep subsurface environments on Earth, where no sunlight exists, has dramatically reshaped the ways scientists think about the potential for subsurface life on other planets. In the outer reaches of the solar system, energy from sunlight is inadequate to maintain the temperatures required for liquid water at the surface, much less for photosynthesis. However, where internal heat sources exist, liquid water could in principle be present in the subsurface.

Three of the larger satellites of Jupiter (Io, Europa, and Ganymede) appear to possess actively heated interiors that are maintained by gravitational tidal forces. These forces continually distort the shapes of these moons, creating internal friction that is capable of melting rock. In one of Jupiter's satellites, Io, the internal heating is manifested as widespread, active volcanic activity at the surface. On Europa, however, interior heating is manifested in a complexly fractured and largely uncratered (constantly renewed) outer shell of water ice. In many places, blocks of crust have drifted apart and liquid water or warm ice has welled up from below and frozen out in between, forming long, narrow ridges in the spaces between. Over time, some ridge segments have shifted laterally, offsetting older ridge segments along faults. Other more localized areas appear to have melted over broad regions and blocks of ice have foundered, tilted, and become refrozen. At an even finer scale, there are smaller, mounded features that are thought to have formed as ice "volcanoes" erupted water or warm ice erupted water from the subsurface.

While the concept of a Europan ocean is still controversial, measurements of the magnetic field of the moon obtained during the Galileo mission have strengthened the case. In order to account for the induced magnetism measured by Galileo, it is likely that a salty ocean exists beneath the water ice crust. (Similar arguments have also been made for two other large satellites of Jupiter, Ganymede and Callisto.) The idea of an ocean of brine beneath the icy crust is consistent with infrared spectral data from orbit, which suggest that magnesium and/or sodium sulfate salts are present in surface ices.

In assessing the potential for life on Europa, the presence of liquid water is regarded as crucial, both as a medium for biochemical processes and as a source for the chemical energy necessary to sustain life. There does not appear to be enough solar energy at the surface of Europa to support life. However, in 2001 planetary scientist Chris Chyba proposed a model that predicts that chemical energy sources for supporting life may exist from radiation processing of Europa's surface ice, in combination with the decay of radioactive potassium. Together, these processes could decompose water to hydrogen and oxygen (with the hydrogen escaping to space) and the chemical disequilibrium created potentially exploited for energy by organisms.

Habitable Environments Beyond the Solar System

The discovery of planets orbiting other Sun-like stars in the galaxy is a key scientific discovery that has played a central role in the astrobiological revolution. The original discoveries, made in the mid-1990s, have continued. By the early twenty-first century,extrasolar planets have been found orbiting almost seventy solar-mass stars in the nearby region of the galaxy. Six of these discoveries are of planetary systems with two or more planets. Present discovery methods are based on the detection of a slight shift or "wobble" in the position of the star that results from the gravitational pull of an orbiting planet(s). With existing technologies, this method allows for the detection of planets that are Jupiter-sized or larger. Some of the extrasolar planets detected occupy orbits within the habitable zone where liquid water could exist. Gas giants (such as Jupiter and Saturn) are planets that lack a solid surface, but they could contain interior zones of liquid water, or might have large (undetectable) satellites with solid surfaces and liquid water. These discoveries have revealed planets around other stars to be commonplace in the Milky Way, thus widening the possibilities for life elsewhere in the cosmos.

SEE ALSO EXTRASOLAR PLANETS (VOLUME 2); JUPITER (VOLUME 2); MARS (VOLUME 2); MARS MISSIONS (VOLUME 4); PLANETARY PROTECTION (VOLUME 4); SCIENTIFIC RESEARCH (VOLUME 4); SETI (VOLUME 2); TERRAFORMING (VOLUME 4).

Jack D. Farmer

Bibliography

Chang, Sherwood. "The Planetary Setting of Prebiotic Evolution." In Early Life on Earth, ed. S. Bengston. New York: Columbia University Press, 1994.

Chyba, C., and K. Hand. "Life without Photosynthesis."Science 292 (2001):2,026-2,027.

Fredrickson, J. K., and T. C. Onstott. "Microbes Deep Inside the Earth."Scientific American 275, no. 4 (1996):42-47.

Klein, H. P. "The Search for Life on Mars: What We Learned from Viking."Journal of Geophysical Research 103 (1998):28,463-28,466.

Lemonick, Michael D. Other Worlds: The Search for Life in the Universe. New York:Simon & Schuster, 1998.

Pappalardo, R. T., J. W. Head, and R. Greeley. "The Hidden Ocean of Europa."Scientific American (October 1999):34-43.