What Is Life?
Before life exists, there cannot be any such thing as information. But things get a lot weirder when Davies applies to biology ideas from thermodynamics and the mathematical theory of information. Imagine a box of gas with a partition containing a tiny door. A supernatural intelligence could open the door to let fast-moving particles go one way and slow-moving particles the other. This would eventually result in two different temperatures of gas either side of the partition, reversing entropy and enabling work to get done free. The big idea is that nature itself might take advantage of the superefficiency of a demonic approach to information, and understanding the information flow in organisms might be the missing part of our scientific jigsaw puzzle.
The information flow in genetics, too, is far more complex than once thought, a point piquantly illustrated by some perverted worm-botherers. Cleland and Chyba have suggested that life might be like water, hard to define phenomenologically, but easy to define at the fundamental level.
But life is like fire, not water—it is a process, not a pure substance.
What is the Point of Life?
Such definitions are grist for philosophical discussion, but they neither inform biological research nor provide a basis for the search for life on other worlds. The simplest, but not the only, proof of life is to find something that is alive. There are only two properties that can determine if an object is alive: metabolism and motion.
Metabolism is used here to include an organism's life functions, biomass increase, and reproduction.
All living things require some level of metabolism to remain viable against entropy. Movement either microscopic or macroscopic in response to stimuli or in the presence of food can be a convincing indicator of a living thing. But both metabolism fire and motion wind occur in nature in the absence of biology.follow
What Is LIFE & LEGACY®?
The practical approach to the search for life is to determine what life needs. The simplest list is probably: energy, carbon, liquid water, and a few other elements such as nitrogen, sulfur, and phosphorus McKay Life requires energy to maintain itself against entropy, as does any self-organizing open system. The only other source of primary productivity known is chemical energy, and there are only two ecosystems known, both methanogen-based Stevens and McKinley ; Chapelle et al.
Photosynthetic organisms can use sunlight at levels below the level of sunlight at the orbit of Pluto Ravens et al. Carbon, nitrogen, sulfur, and phosphorus are the elements of life, and they are abundant in the Solar System. Indeed, the Sun and the outer Solar System have more than 10, times the carbon content of the bulk of Earth McKay When we scan the other worlds of our Solar System, the missing ecological ingredient for life is liquid water.
It makes sense, then, that the search for liquid water is currently the first step in the search for life on other worlds. The presence of liquid water is a powerful indication that the ecological prerequisites for life are satisfied. Orbital images, such as the canyon in Figure 1 , show clear evidence of the stable and repeated, if not persistent, flow of a low-viscosity fluid on Mars at certain times in its past history.
What's My Purpose in Life?
The fluid was probably water, but the images could also suggest wind, ice, lava, even carbon dioxide or sulfur dioxide. Recently, results from the Mars Exploration Rover missions have shown that this liquid carried salts and precipitated hematite in concretions. The case for water, we could say, is tight.
The image covers an area 9. This image is the best evidence we have of liquid water anywhere outside the Earth. On Jupiter's moon Europa, the cracks and icebergs on the surface of the ice indicate water beneath the ice, but not necessarily at the present time. Present water on Europa is indicated by the magnetic disturbance Europa makes as it moves through Jupiter's magnetic field, not unlike the way coins in the pocket of a passenger will set off an airport metal detector.
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Europa has a large conductor, and this is most likely a global, salty layer of water. The Viking missions to Mars in the late s were the first and as yet, the only search for life outside Earth. Each Viking conducted three incubation experiments to detect the presence of metabolism in the Martian soil. Each lander also carried a sophisticated Gas Chromatograph Mass Spectrometer for characterizing organic molecules. The results were unexpected Klein , There was a detectable reaction in two of the incubation experiments.
In the Labeled Release experiment, this reaction ceased if the soil was first heated to sterilizing temperatures, but the reaction of the Gas Exchange Experiment persisted. If considered alone, the Labeled Release results would be a plausible indication for life on Mars. However, the Gas Chromatograph Mass Spectrometer did not detect the presence of any organic molecules in the soil at level of one part per billion Biemann It is difficult to imagine life without associated organic material, and this is the main argument against a biological interpretation of the Viking results Klein ; but cf.
Levin and Straat It is also unlikely that the oxygen release in the Gas Exchange experiment had a biological explanation, because the reaction was so rapid and persisted after heating. It is generally thought that the reactivity observed by the Viking biology experiments was caused by one or more inorganic oxidants present in the soil, and was ultimately produced by ultraviolet light in the atmosphere.
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Consistent with the apparently negative results of the Viking biology experiments, the surface of Mars also appears to be too dry for life. Indeed, conditions are such that liquid water is rare and transient, if it occurs at all e. Table 1 shows a categorization of life as we have observed it. Using this diagram, we can speculate about how life might be different on Mars or Europa.
At the bottom of the table, life is composed of matter—a reasonable assumption for now. Carbon and liquid water are the next level; this makes Mars and Europa likely candidates, because they have carbon and have, or have had, liquid water. Other worlds may have a different chemical baseline for life. The usual speculation in this area is that the presence of ammonia and silicon, rather than water and carbon, might be preconditions for life on other planets.
Such speculation has yet to lead to any specific suggestions for experiments, or to new ways to search for such life, but this may just reflect a failure of human imagination rather than a fundamental limitation on the nature of life. Life on Mars is also likely to be the same at the top of the table: at the ecological level. However, that definition would exclude mules, which are born sterile, and would include nonliving things like fire.
Others have suggested that life is something that can metabolize — that is, take in energy to move or grow, and release waste — but many nonliving things, like cars, can do that. Despite the difficulty of pinning down life, some scientists aren't content to give up, saying a working definition of life is necessary if we are to identify living things beyond Earth.
In that case, the adage, "You'll know it when you see it" just doesn't cut it. But maybe we can find extraterrestrial life without knowing what it is. That's where discoveries like the recent arsenic finding come in. The organism was unearthed in California's Lake Mono. Thus the discovery seems to negate the last requirement on McKay's list phosphorus , and opens up the field to a wider array of habitats that could support alien life.
Yet since the finding was announced, other scientists have questioned the researchers' methods , wondering whether the arsenic was really being incorporated into the microbe's DNA, and if there was a chance trace amounts of phosphorus were available to the bacteria. Whether or not this discovery holds weight over time, it's possible that some of scientists' basic assumptions about what life is and what it requires to survive will need to be revised.
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And the quest to understand life is inextricably twined with the search for extraterrestrial life.