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Exobiology or astrobiology is the study of life elsewhere in the Universe. Exobiology is also speculation about extra terrestrial life. Expbiology assumes that this life is be non-supernatural and existing in the physical context of our Universe, as Earth life does.
The idea of exobiology is about as old as speculations about the existence of Earthlike worlds. Metrodorus of Chios (400 BCE) believed that there were numerous other such worlds, each one approximately the Earth with planets and stars, and he stated that:
- To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow.
His opinions were held by other Atomists, like Leucippus, Democritus, and the Epicureans, but others, like Aristotle, firmly believed in the Earth's uniqueness.
So it remained until Galileo started using his telescope, discovering that other Solar-System objects had Earthlike features. Galileo himself refused to speculate about possible inhabitants of other celestial objects, but that did not stop others. But Johannes Kepler speculated that the Moon's craters were built by Moon inhabitants, on account of their regular shape, and he was followed by many other such speculators. Over the next few centuries, it was a common opinion that the entire Solar System is inhabited, often on the ground that God would not let a world go to waste by leaving it unpopulated.
However, by the nineteenth century, it became apparent that the Moon has very little -- if any -- air and water by Earth standards, and that it was likely lifeless.
Late in that century, however, new "evidence" appeared of life on Mars, in the form of the "canals" observed by Giovanni Schiaparelli in 1877. Their most zealous supporter, Percival Lowell, believed them to be built by Martians, but many astronomers were skeptical about that conclusion -- and even about the existence of those canals. For more, see Animal and Extraterrestrial Intelligent Design.
On a more sober note, a "wave of darkening" was often observed coming from each polar icecap during its hemisphere's spring; that was sometimes interpreted as vegetation growing with the help of that melting icecap's water.
But one of Lowell's contemporaries, Alfred Russel Wallace, sounded a note of skepticism, publishing in 1907 a book called Is Mars Habitable? He concluded that Mars was too cold and its atmosphere too thin to support most familiar life.
The Solar System
Wallace's work was followed by a century of increasing theoretical and observational sophistication, first with ground-based and then with space-based observations. Spacecraft have now been sent to most of the relatively-large Solar-System objects, first flying by, and then sometimes orbiting, landing, releasing rovers, and returning samples. The result has been confirmation of most of extraterrestrial-life skeptics' suspicions.
Mars's "wave of darkening" is a contrast effect produced by windblown dust, and its canals do not exist. There is not a trace of them in the numerous pictures returned by several spacecraft. Wallace was essentially correct about its surface conditions; its average surface temperature is -63 C and its average surface pressure is 6 millibars (compare Earth's 1013 millibars).
Most of the rest of the Solar System is even worse. Venus has a surface temperature of 450 C and a surface pressure of 93 bars. Most of the rest of the Solar System is airless, and the Moon is depleted in volatiles, including water. Jupiter, Saturn, Uranus, and Neptune lack condensed surfaces, meaning that their inhabitants would have to be buoyant in air, something that has never evolved on Earth.
Water on Mars?
Despite its lack of canals and other disappointing features, there is evidence that Mars has had a watery past, and thus possibly an inhabited one. Mars has numerous riverbeds and other flow features, though none of them matches those infamous canals. Though these are now dry, these would have to have been formed by some liquid. Water is the favorite candidate; Mars's early atmosphere could have been much thicker than at present, with the difference coming from evaporation into outer space. Such a thickness would have had to counteract the Sun's lower luminosity early in its history (70% of present about 4 billion years ago).
However, these riverbeds are generally unbranched, suggesting that they had been produced by large flash floods, something like the glacial dam-break floods known on Earth, including the continent-scale Pleistocene ones in the Columbia Basin and Altai Mountains. This suggests that Mars had always been too cold for liquid water -- at least most of the time -- and that liquid water was produced by impacts or volcanism melting permafrost or glaciers.
Alternatively, these features could have been produced by flows of dust entrained in evaporating carbon dioxide.
Life on Mars?
Searches for life on Mars have had inconclusive results -- at best.
Several meteorites are known to have come from Mars on account of their composition, including that of bubbles of trapped atmosphere; these meteorites are Mars rocks that had been kicked up by impacts. The best-known possible container of life is the Martian meteorite ALH84001, found in Antarctica, where opportunities for terrestrial contamination are very limited.
In that meteorite are what look like fossils of bacteria, but at 20-100 nm, they are much smaller than typical Earth bacteria (>= 1 micron). There are also possible chemical fossils, like Polycyclic Aromatic Hydrocarbons (PAH's; possible degradation products) and tiny magnetite crystals (used by some Earth bacteria for navigation). However, it is difficult to rule out nonbiological synthesis of these PAH's and magnetite crystals.
On Mars itself, the Viking landers (1976) had four experiments for searching for life by looking for metabolic evidence:
- A gas-chromatograph mass spectrometer (GCMS), for looking for organic molecules directly.
- A gas-exchange experiment (GEX), in which water, and then a nutrient-rich broth were added to some Martian soil; a gas chromatograph was used to detect whatever gases were released, like nitrogen, oxygen, hydrogen, and methane.
- A labeled-release experiment (LR), in which a C14-containing nutrient-rich broth was added to some Martian soil. If any of it was metabolized to gases like CO2 and CH4, those gases could then be detected by way of their C14 content.
- A pyrolytic-release experiment (PR), in which some Martian soil was exposed to light and C14-labeled CO2 and CO. After several days, the soil was baked and C14 searched for in the released gases; this tested for carbon fixation.
However results were mixed:
- The GCMS reported no evidence of organic molecules.
- The GEX reported emission of oxygen, even if the Martian soil was heat-sterilized.
- The LR reported the emission of labeled gases, except when the Martian soil was heat-sterilized.
- The PR reported fixation of carbon, even if the Martian soil was heat-sterilized.
Only the LR results looked like the results of biological processes, and an alternate hypothesis can better explain all the results:
The Martian soil contains peroxides and superoxides produced by solar ultraviolet light acting on adsorbed atmosphere. These would form hydrogen peroxide when exposed to liquid water, which in turn would react with the nutrients to release carbon-containing gases and with the carbon dioxide and monoxide to fix carbon. These substances may also release oxygen by decomposing.
However, there may be subsurface life on Mars, much like some subsurface life that was recently discovered on Earth. Its metabolism would depend on combining relatively reduced gases like hydrogen and hydrogen sulfide with relatively oxidized ones like carbon dioxide. However, for such reduced gases to travel upwards from Mars's interior, Mars must still be geologically (areologically?) active, and there is no clear present-day evidence of such activity. However, Mars's large shield volcanoes have relatively few impact craters on them, implying that they are relatively young features, so Mars may still have some geological activity.
Life elsewhere in the Solar System?
Subsurface life may also exist on Jupiter's satellite Europa. It has very few impact craters on its surface, suggesting that its surface is very young. Its surface also has long lines that look like cracks and chaotic terrain that looks like Earth pack ice. These circumstances suggest that its surface is an ice layer above an ocean of liquid water, a layer which gets burst through and reformed rapidly enough to destroy craters relatively quickly. Such quick destruction is familiar on Earth, which has very few prominent impact craters; most of them have been identified with mineralogical evidence like shock metamorphism.
This liquid water that may harbor life if there is some chemical disequilibrium to derive metabolism, like reduced gases from the interior combining with oxidized gases from nearer the surface, in the fashion of the aforementioned subterranean life. The near-surface gases may easily become oxidized as a result of hydrogen escaping into space relatively easily, leaving oxygen behind.
Europa's ocean must be kept liquid as a result of internal heating, because without it, that ocean would freeze solid. And that satellite does have an internal heat source. Its two neighboring satellites, Io and Ganymede, are in a 1:2:4 resonance, which forces its orbit's eccentricity, which in turn forces changing tides, which in turn heats the satellite. The same effect happens with much greater strength on Io, producing large, currently-active volcanoes; it is much weaker on Ganymede, whose surface is much more cratered, and therefore much older.
This internal heat source may also make Europa's interior release various gases, which may possibly be relatively reduced ones.
The remaining possible habitat of life in the Solar System is Saturn's satellite Titan. It has an atmosphere containing methane and nitrogen and reddish-brown clouds and haze. Prebiotic-chemistry experiments on simulated Titan atmospheres produce a reddish-brown goo ("tholin") with properties remarkably similar to Titan's clouds. However, Titan's surface is too cloud-covered to allow significant photosynthesis, and it likely lacks significant internal heat sources, either radioactive or tidal, that could produce chemical disequilibrium at its surface. Also, Titan's surface temperature is around -178 C, making it too cold for liquid water, meaning that Titan life would have to work with a more limited chemical palette. Thus, in my (LP) opinion, Titan is lifeless.
Elsewhere in the Universe
Turning outside the Solar System, Metrodorus's old argument returns in force. There are something like 100 billion stars per galaxy and 100 billion galaxies in the observable Universe (very rough estimates), yielding 10^22 stars total. But the large majority of these are extremely difficult to reach; travel to even the nearest stars is very difficult. However, if some analogue of Homo sapiens appeared on some other planet, it may be able to communicate across interstellar distances by using radio, as Cocconi and Morrison had first proposed in 1959.
But how likely is such communication? Astronomer Frank Drake has tried to quantify the likelihood of the emergence of communicative civilizations with his famous equations:
N = R(star) * f(p) * n(e) * f(l) * f(i) * f(c) * L
- R(star) is the rate of star formation
- f(p) is the fraction that has planets
- n(e) is the number of habitable planets
- f(l) is the fraction of such planets that life emerges on
- f(i) is the fraction of such life that intelligent life emerges from
- f(c) is the fraction of such intelligent life that invents interstellar-communication capability
- L is the lifetime of those interstellar communicators
Various modifications and variants have been proposed, but it is instructive to examine each of the terms in Drake's original equation:
R(star), the rate of star formation
This is reasonably well-understood, though it may be convenient to restrict this rate to the rate of formation of stars that can have inhabited planets.
This consideration rules out stars much more massive than the Sun, because:
- They have large ultraviolet fluxes, which are dangerous for surface life.
- They last relatively short by geological standards, allowing little time for life to emerge and evolve.
But though such stars are bright, they are also rare, thus making little difference in the overall statistics. Life could evolve in a star system with longer lifespan and develop an advanced technology. An advanced technology could migrate to a bright, massive star. An advanced technology could also shield itself from ultraviolet flares. An advanced technology may or may not seek to colonize more hospitable star systems first.
This consideration may rule out low-mass stars like red dwarfs, because:
- Many red dwarfs are flare stars, having stellar flares that resemble solar ones, but often much brighter. Their emissions are potentially deadly for surface life. Life may overcome the problems of flares. See Wikipedia Article. See How life could evolve in a Red Dwarf star system.
- To be close enough to have liquid water, a planet must be close enough for tidal drag to make it rotate synchronously, possibly limiting liquid water to a narrow region.
However, these low-mass stars are the most abundant ones, reducing the number of habitable-planet stars by sizable factors. Subsurface life could exist, but such life is not likely to evolve interstellar-communication capabilities. Surface life is also possible, see Aurelia.
A possible complication is the presence of tidally-heated Europa-like satellites of Jupiter-like planets. If Europa was a little larger, it could retain enough heat to have a liquid ocean with a liquid surface -- and enough gravity to keep it from evaporating into outer space. And its primary could be a planet at a "safe" distance from its sun.
A further complication is that most stars in our Galaxy had formed early in its history; these "Population II" stars are low in heavy elements or "metals" in astrophysical parlance. These stars reside in the Galactic nucleus, its globular clusters, and its halo; those in the Galactic disk, the "Population I" stars, are typically much younger.
The Sun is a Population I star, though somewhat enriched in metals. Which suggests a further limitation of what stars can have habitable planets -- to Galactic-disk stars or even metal-rich ones.
f(p), the fraction that has planets
Numerous protoplanetary disks have been detected, and some young stars retain observable dust disks. Several extrasolar planets have been (indirectly) detected, but the detection techniques only allow the detection of Jupiter-sized planets. And several of these planets orbit relatively close to their primaries, interfering with the presence of planets in Earthlike orbits. However, these "hot Jupiters" could have Europa-like or even Earthlike satellites.
There are some space missions being planned that will detect evidence of Earthlike planets; these will look at large numbers of stars in the hope of finding eclipses by Earthlike planets.
n(e), the number of habitable planets
If Earthlike planets or Europa-like satellites exist, the next question is how many of them there are. The Solar System has the only examples of such objects detected to date, they are:
- Venus: the right size but too close; its atmosphere has a runaway greenhouse effect.
- Earth: the type specimen.
- Mars: a bit on the far side and a bit small.
- Io: too much tidal heating.
- Europa: a bit small; has an iced-over surface.
- Ganymede: too little tidal heating?
- Titan: too far from the Sun; too little tidal heating.
f(l), the fraction with life
This depends on our understanding on abiogenesis, which continues to be an unsolved problem. Factors suggesting that the origin of life may be rare are:
- All extant Earth life has only one origin, with the most recent common ancestor being at least a RNA-protein organism.
- Some biomolecules, like the simpler amino acids, can easily be produced in prebiotic-chemistry experiments, but some, like ribose, cannot. This is a major difficulty for the otherwise-attractive RNA world scenario.
However, studies of the evolution of the first proteins suggest that the earliest proteins largely used the simpler amino acids, those easily formed prebiotically, and that some of these proteins had been adapted to mineral-surface conditions, a plausible site for the origin of life. So the question is still up in the air.
f(i), the fraction with intelligence
Understanding this requires understanding some large-scale patterns of evolution. Exactly what the evolution of intelligence requires is somewhat controversial, but these factors may be significant:
- Search for scattered food sources, especially by interpreting hints of their presence.
- Dependence on senses that require massive cerebral processing for interpretation, like vision or echolocation as opposed to chemical senses.
- Living in groups, especially when the members must cooperate to acquire food.
All of these require animal-like organisms, and metabolically-efficient ones. Primary producers like photosynthesizers are excluded, because they only have to bask in the Sun or in a hydrothermal vent, and because motility is impractical for macroscopic photosynthesizers.
And oxidizing food molecules with oxygen is practically a necessity; other oxidizers are either too weak, like other parts of those molecules (fermentation), or are necessarily dilute or spottily concentrated, like nitric or sulfuric acid. Oxygen is abundant in the Earth's atmosphere because it is produced from water by oxygen-releasing photosynthesizers -- and the Earth has big oceans of water.
So a prerequisite would seem to be the evolution of oxygen-releasing photosynthesis; such photosynthesis has not only produced an abundance of oxygen, but also an abundance of biomass to eat, which sometimes supports several levels of food chain. So we consider the evolvability of photosynthesis.
Earth organisms have two types of photosynthesis, identified here by their major photosynthetic pigments:
- Rhodopsin-based, used by organisms like the archaebacterium Halobacterium. Bacteriorhodopsin absorbs light energy and transports hydrogen ions out of the cell. The ions then return through ATPase complexes, their energy being tapped to assemble ATP, as is typical of many organisms. Rhodopsin-based photosynthesis does not release oxygen, and it may be difficult for it to do so.
- Chlorophyll-based, used by many eubacteria. Chlorophyll is typically present in large complexes (photosystems) that contain proteins and that may contain carotenoids, an additional photosynthetic pigment. It works by transferring electrons, which can pump hydrogen ions out of the cell, to be returned through ATPase, or else can be used to fix carbon and other essentials. Many bacteria have only one type of photosystem, but cyanobacteria, which release oxygen, have two, allowing them to utilize water.
Cyanobacteria have apparently evolved relatively late in the Earth's history, about 2.3 billion years ago, when the Earth's atmosphere started becoming oxygenated. So it may be difficult for oxygen-releasing photosynthesis to evolve.
Examining further steps in evolution, the multicellular animals (Metazoa) have evolved only once; and from them, complex bilaterally-symmetric ones (bilaterians) evolved only once. However, bilaterians have some deep branches, suggesting that they may have preempted their competition by rapid early diversification.
And what features might be necessary for even an approximation to human-scale intelligence? The species that approach our species are various other primates, notably the chimpanzee, and cetaceans, notably the bottlenose dolphin (Tursiops truncatus). Pigs, dogs, and elephants are also known to be relatively intelligent, even if not quite at a human scale. Some Mesozoic theropods like Troodon seem to have been relatively endowed cerebrally, though details of their behavior are difficult to learn. Of present-day theropods (birds), crows (Corvidae) and some parrots (Psittaciformes) are relatively intelligent. All these species have in common that they are vertebrates, and usually relatively large ones at that.
This may indicate a human-sized or similar-sized body is necessary to house a brain with human-scale intelligence. On Earth, such size is rare among invertebrates; are there any examples other than the giant squid (Architeuthis)? Vertebrates are assisted in attaining their size by their internal skeleton; however, that has evolved only once, while shells and external skeletons have evolved several times. And these other solutions do not scale as well as vertebrate skeletons; arthropods' skeletons are their outer skins, which they periodically molt as they grow.
This suggests that it may be unlikely for internal skeletons to evolve. Combined with the single evolution of oxygen-releasing photosynthesis, multicellular animals, and sophisticated bilaterian ones, this suggests reason for pessimism.
But on a more optimistic note, some features necessary for high intelligence, or at least convenient for it, have evolved several times:
- Lens-camera eyes, the highest-resolution kind, have evolved twice, in vertebrates and in cephalopods.
- Sophisticated echolocation has evolved twice, in bats and in cetaceans.
- Warm-bloodedness and four-chambered hearts, with consequent high levels of physical activity, have evolved separately in the ancestors of mammals and birds.
- Lungs and lunglike structures, for living on land, have evolved in many lineages, such as arachnids, coconut crabs, the gastropod order Pulmonata and, of course, tetrapods.
f(c), the fraction that can do interstellar communication
A very likely prerequisite is the ability to manipulate one's environment and construct tools with ever-increasing sophistication. Doing so with any efficiently requires organs adapted for gripping; mouths are generally insufficient.
Human hands are excellent organs for such manipulation, having both a "power grip" (fingers-palm) and a "precision grip" (thumb-forefinger). Other primates, like chimps, mostly have a power grip; despite also having opposable thumbs, their precision grip is not quite as good.
Gripping organs have evolved several times:
- Primate limb extremities (hands/feet)
- Tails of some New World monkeys
- Elephant trunks
- Feet of perching birds
- Gripping claws (pincers) of scorpions and several crustaceans
- Cephalopod tentacles
Some Mesozoic theropods may have had some gripping ability, since their front limbs were made available for gripping by their bipedal walking.
Also a likely prerequisite is living on land; it is difficult to run electrical devices underwater, since electricity easily leaks through water.
Both considerations rule out cetaceans, since even the most intelligent dolphin (1) lacks manipulative organs, and (2) is confined to water.
A further prerequisite is likely to be complex language, because describing how and why to build the necessary technology will require such language. Across the animal kingdom, many utterances and other signals typically reduce to something like
- Follow me!
- Come to me!
Though they can be elaborated in various ways, like honeybees' dances and vervet monkeys having separate calls that mean "Snake!", "Eagle!", and "Leopard!", they nearly always have no syntax (multi-word arrangements). Even chimpanzees have trouble learning syntax, though they can learn a couple hundred symbols. Dolphins may have language, but if they do, it has yet to be decoded, so they are still an unknown.
Human languages, however, have complex syntax, and often word morphology, a sort of pseudo-syntax. And human spoken languages are acquired with very little formal instruction, starting in early childhood. And not only is there are no known human societies that lack full-scale language, the human brain contains some parts that are specialized for language interpretation and generation.
It would seem that our species passes with flying colors. But there are problems with using spoken language in constructing advanced technologies, notably the limitations of human memory. It is difficult to learn large quantities of uncoordinated information; such seemingly uncoordinated information is necessary for developing advanced technologies. Consider that one of the earlier scientific triumphs was describing the orbits of the planets. In order to properly test hypotheses of their motions, one must work with a large amount of position data.
Such difficulties are why large memorized epics, like the Iliad and the Odyssey, are in verse -- the verse form helps jog the epic-teller's memory. Furthermore, such epics often contain stylized phrases and motifs, like Homeric "bright-eyed Athena", that further jog the epic-teller's memory.
Written language lacks that difficulty, and it has the nice feature of being persistent, meaning that it can be distributed to others with no memorization effort needed, and that it can easily survive its writers.
But written language is a latecomer in the history of Homo sapiens, with the first decodable writing being around 5000 years old, a tenth or less of the age of our species. Writing systems have often been invented as a result of "stimulus diffusion", being provoked by awareness of the existence of writing. But without that awareness, writing has been invented only a few times in humanity's history. But once invented, it has been widely spread.
Furthermore, writing has to be explicitly learned; written language is more difficult to master than spoken language, and competence in written language varies widely. This is understandable in evolutionary terms -- we are not directly adapted to using written language.
Writing is just one example of a now-important technology that was invented only late in humanity's history. It was likely invented to improve bookkeeping in societies made large-scale by the development of agriculture and the resulting increased population densities. And bookkeeping is difficult to memorize for the reasons discussed above.
Even with the development of agriculture and writing, the development of interstellar communication was not guaranteed. Abstract mathematics and hints of the scientific method only got started 2500 years after the invention of writing, and full-scale scientific method took around 2000 years more to develop. And even then, it took some centuries to develop the necessary technology for interstellar communication.
So it's an open question whether human-scale intelligence necessarily leads to interstellar communication.
L, their lifetime
This is very difficult to estimate; there are numerous calamities that can befall a technological civilization:
- Very destructive wars
- Super plagues
- Ecological disasters
- Exhaustion of convenient but limited resources
- Loss of interest in interstellar communication
- Astrobiology at UCLA
- Astrobiology Instant Expert on New Scientist
- Australian Centre for Astrobiology
- The Astrobiology Web
- Astrobiology Magazine
- Astrobiology Selected Links
- NASA Astrobiology Institute
- The Astrobiology Society of Britain
- Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth
- Scientists Find Clues That Life Began in Deep Space - NASA Astrobiology Institute
- Stars and Habitable Planets
- Life Around a Red Dwarf Reading Exercise
- Mark Elowitz's Web site on Exobiology and SETI
- Let's Build an Extraterrestrial
- Influenza 1918, A Venus Connection?
- NASA-Macquarie University Pilbara Education Project
- Conditions for Life Everywhere
- Snaiad, a world-building project with creatures designed with evolutionary biology in mind.