The Physical Limits of Life: The Habitable Zone
- The universal nature of biochemistry
Thought on where in the solar system life might occur was limited historically by the belief that life relies ultimately on light and warmth from the sun and, therefore, is restricted to the surfaces of planets. The inner boundary of the "habitable zone" in our solar system was considered as approximately between Earth and Venus, not so close to the sun as to be too hot for life. The outer boundary was considered to lie between Mars and Jupiter, not so far from the sun that the surface of a body would necessarily be frozen or receive too little light for efficient photosynthesis. Light probably is not directly required for life to arise, however, except as it may be involved in the formation of organic compounds during the accretion of a planetary system. On the other hand, the biological use of light energy, photosynthesis, may be a prerequisite for persistence of planetary life over billions of years. The reason for this conjecture is that light provides a continuous and relatively inexhaustible source of energy. Life that depends only on chemical energy inevitably will fail as resources diminish and cannot be renewed.
Nonetheless, we know that life occurs in Earth's crust, away from the direct influence of light, and that many organisms have metabolisms that function independently of light. Thus, the outer boundary of the potentially habitable zone extends into the far reaches of the solar system, to any rocky body with internal heating, regardless of its distance from the sun. [I specify "rocky" body to accommodate chemicals expected to be required for metabolism (below).] Life can persist in the absence of light by using inorganic energy sources, as do lithotrophic organisms, or organic sources deposited in planetary interiors during their accretion, as do heterotrophs (4). Therefore, rather than proximity to the sun, it seems more useful to define the habitable zone for life in terms of the chemical and physical conditions that are expected to be required for life. Our view of life's possible extremes currently is limited to the extremes of terrestrial life. Considering the intrinsic fragility of complex organic systems, coupled with the powerful force of natural selection, I venture that the physical limits of life are likely to be about the same anywhere in the universe. The window of chemical and physical settings that permit life are broad, however. Some important considerations are the following.
Chemical Setting. Although the general energy requirement of life is a state of chemical disequilibrium, in which some oxidation-reduction reaction can occur, the specific thermodynamic requirements of biological energy-gathering strategies constrain the sites where life can occur. For example, a setting for lithotrophic organisms requires the occurrence of an appropriate mix of oxidized and reduced chemicals. Photosynthetic organisms require sufficient light of appropriate frequency. The light must be sufficiently energetic to support biosynthesis, but not so energetic as to be chemically destructive. These considerations constrain photosynthesis-based life to the spectral zone of about 300-1,500 nm in wavelength. (Terrestrial photosynthesis is limited to about 400-1,200 nm.) Beyond the requirements for energy metabolism and CO2 as a carbon source, terrestrial life requires only a few elements: H, N, P, O, S, and the suite of metals.
Physical Setting. Physical constraints on life include temperature, pressure, and volume. The extreme diversity of terrestrial life probably provides an analog for life's diversity anywhere.Temperature is a critical factor for life. Temperatures must be sufficiently high that reactions can occur, but not so high that that complex and relatively fragile biomolecules are destroyed. Moreover, because life probably depends universally on water, the temperature must be in a range for water to have the properties necessary for solute transfer. Water can be stabilized against boiling by pressure, but at too-low temperatures, water becomes crystalline and inconsistent with transport. Currently, the upper temperature record for culturable microbes is 112-113°C, held by hyperthermophilic archaeons of the genera Pyrolobus and Pyrodictium (5). Even the spectacularly durable bacterial endospore does not survive extended heating beyond ca. 120°C. The lower-temperature boundary for life is not established, but microbes are recoverable from ice, and growth of organisms has been detected in ice to 20°C (6). The physical properties of ice can allow solute diffusion at temperatures much lower than the freezing point (7). Thus, if based on aqueous organic chemistry, the temperature span for life anywhere in the universe is likely to be less than 200°C, within roughly 50° to 150°C.
Pressure and Volume. The pressure required of a setting for life is probably limited at the lower end only by the vapor pressure required to maintain water or ice. An upper limit for pressure tolerance is unknown. Organisms on the terrestrial seafloor experience pressure over 1,000 atmospheres, and microbes recovered from deep oil wells are exposed to far higher pressures. The upper pressure limit for life probably is determined mainly by the effect of the pressure in reducing volume for occupancy. Life can be remarkably small, however. It is estimated that cells only a few hundred nanometers in diameter can contain all of the components considered necessary for life (8).The expected commonality of chemistry in life's processes assists in life detection because it predicts that terrestrial types of biochemicals are useful targets for analysis even in an extraterrestrial setting. On the other hand, the expected similarity of terrestrial and potentially alien life complicates the interpretation of positive chemical tests for biochemicals. Thus, analyses of simple terrestrial-like biochemical compounds might not distinguish between a signal of life on one hand and an abiotically derived organic chemical, or between an alien life form and a terrestrial contaminant. Distinction between organisms with different evolutionary origins may require analysis of macromolecules and genes. Particularly, the nature and detail of the genetic information would be telling.
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