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What do all lifeforms on Earth have in common with our Last Universal Common Ancestor (LUCA)? a. RNA as the genetic material and carbon as molecular backbone. b. Water as the solvent for biochemical reactions and a cell nucleus. d. the ATP molecule to store energy and DNA for the genetic material. How do astronomers define the habitable zone?
- Overview
- The search for extraterrestrial life
Astrobiology, a term coined for the study of all life anywhere in the universe (including Earth), has replaced exobiology, the study of extraterrestrial life exclusively and therefore criticizable as “a science that lacks a subject matter.” Unlike exobiology, astrobiology respects the scientific possibility that life beyond Earth may never be found. Indeed, no evidence for life beyond Earth has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life.
There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, physiology, and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance.
The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be illuminating. Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases (atmosphere and hydrosphere) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments.
Chemical, mechanical, or spectral disequilibria may also be sought. Earth’s atmosphere contains large amounts of molecular oxygen and about 1.7–2 parts per million (106) of methane, but in thermodynamic equilibrium the abundance of methane should be less than one part in 1035. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic hydrocarbons could have been produced nonbiologically early in Earth’s history, and slow degradation may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to bacteria. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life.
By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 metres (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale accoutrements of Earth’s technical civilization would be extremely difficult to discern. As resolution progressively improves, it becomes increasingly easy to distinguish the regular geometric patterns of cultivated fields, highways, and airports. However, these are products of recent civilization; thus, only 100,000 years ago no clear sign of life would have been visible with remote-sensing techniques. Today lights of the largest cities are detectable from Mars, as are seasonal changes in the colour of plants.
Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of energy put out by Earth into space at certain radio frequencies. According to an estimate made by the Russian astrophysicist Iosif S. Shklovskii, if this radiation were to be interpreted as ordinary thermal emission, the implied temperature of Earth would be hundreds of millions of degrees. This radio “brightness temperature” of Earth would have steadily increased over the last several decades. The frequency and the time variation of these signals are not purely random noise.
Astrobiology, a term coined for the study of all life anywhere in the universe (including Earth), has replaced exobiology, the study of extraterrestrial life exclusively and therefore criticizable as “a science that lacks a subject matter.” Unlike exobiology, astrobiology respects the scientific possibility that life beyond Earth may never be found. Indeed, no evidence for life beyond Earth has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life.
There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, physiology, and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance.
The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be illuminating. Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases (atmosphere and hydrosphere) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments.
Chemical, mechanical, or spectral disequilibria may also be sought. Earth’s atmosphere contains large amounts of molecular oxygen and about 1.7–2 parts per million (106) of methane, but in thermodynamic equilibrium the abundance of methane should be less than one part in 1035. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic hydrocarbons could have been produced nonbiologically early in Earth’s history, and slow degradation may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to bacteria. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life.
By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 metres (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale accoutrements of Earth’s technical civilization would be extremely difficult to discern. As resolution progressively improves, it becomes increasingly easy to distinguish the regular geometric patterns of cultivated fields, highways, and airports. However, these are products of recent civilization; thus, only 100,000 years ago no clear sign of life would have been visible with remote-sensing techniques. Today lights of the largest cities are detectable from Mars, as are seasonal changes in the colour of plants.
Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of energy put out by Earth into space at certain radio frequencies. According to an estimate made by the Russian astrophysicist Iosif S. Shklovskii, if this radiation were to be interpreted as ordinary thermal emission, the implied temperature of Earth would be hundreds of millions of degrees. This radio “brightness temperature” of Earth would have steadily increased over the last several decades. The frequency and the time variation of these signals are not purely random noise.
Some astronomers are searching for extraterrestrial intelligent life (SETI). Because other planetary systems are so far away, traveling to the stars is either very slow or extremely expensive (in …
People also ask
What is extraterrestrial intelligence?
How can we find extraterrestrial life?
What is the difference between interstellar and extraterrestrial intelligence?
Can extraterrestrial intelligence be detected?
Extraterrestrial intelligence, hypothetical extraterrestrial life that is capable of thinking, purposeful activity. Searches for radio signals or optical flashes from other star systems that would indicate the presence of extraterrestrial intelligence have so far proved fruitless.
