Ecology: the biology of interaction. 2.16. (supplement) The search for life in the Solar System
Most contemporary efforts to search for life are based on the notion that extraterrestrial life will resemble terrestrial life. There is even a concept—“water‑carbon chauvinism”—the idea that alien life must have the same basis as Earth life. And if chem...
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2.15. (addendum) Venus, Earth, Mars
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Section 2. Biospherology
2.17. (addendum) Anthropogenic Paradox
2.16. (supplement) The search for life in the Solar System Is there a multitude of other worlds, or only one world? This is one of the greatest questions that drives the study of Nature. Albert the Great. 13th century. The main candidate in the search for extraterrestrial life is, of course, Mars. In the past it resembled Earth closely, and even now it probably retains conditions necessary for the existence of simple life forms. But the search for life in the Solar System is not limited to this planet. Titan, the largest moon of Saturn, also draws attention (its size is larger than that of Pluto and Mercury). As the results of the American Cassini mission and the Huygens probe have shown, seas and rivers are located on Titan’s surface. In its dense atmosphere (pressure — 1.5 times Earth’s) clouds are present, from which rain falls. Unfortunately, Titan is too cold for water to exist there as a liquid: the rivers on this celestial body are formed of methane and other substances that are gases under Earth conditions. However, the rocky material through which Titan’s streams and rivers cut their way is water ice! For optimists who dream of life based on something other than water‑carbon, Titan provides rich material for imagination. Europa, a moon of Jupiter, is very interesting from the perspective of the search for life. Its surface is covered with ice, beneath which, apparently, lies an ocean of liquid water! This celestial body is slightly smaller than the Moon and is much farther from the Sun, but heating of its interior may be aided by the nearby giant Jupiter. Water may also exist on Enceladus, one of Saturn’s moons. However, it is likely that this body does not receive enough heat to keep water in a liquid state. Io, another moon of Jupiter, is of interest for its geology. This body, only a little larger than the Moon, has a metallic core and a silicate mantle. Moreover, traces of geological processes, including volcanic eruptions, have been recorded on its surface! Unfortunately, water reserves on Io have not been found. Finally, Jupiter itself presents a certain interest for the search for life—a gas giant, the largest body in the Solar System aside from the Sun. Its atmosphere consists of hydrogen and helium with traces of methane, water, and ammonia. In its centre, there is probably a relatively small solid core, above which lies a massive layer of atmosphere. It is likely that one of the layers of the Jovian atmosphere contains water‑ice clouds. Could life exist in them? Most modern efforts to search for life are based on the assumption that extraterrestrial life will resemble terrestrial life. For example, from the standpoint of the chemical basis of life we can easily imagine its existence based on aqueous solutions and organic compounds, and we cannot readily conceive it on any other basis. There is even a term—“water‑carbon chauvinism”—the notion that alien life must have the same chemical basis as Earth life. Are other possibilities possible? There is no answer yet. And if the chemical basis of life matches that of Earth, will its development proceed as on our planet, or differently? Modern science lacks established ideas about how regularly, during evolution, the particular living organisms we observe on Earth should appear. If the hypothesis that many key stages of Earth‑life evolution were traversed by several evolutionary branches simultaneously is correct, it indicates a regular character of evolution. We will not prove this claim in detail here, but we note that it is quite probable that vertebrates colonized land several times; that winged, flying organisms—birds—arose several times; that embryonic envelopes or the flower of angiosperms could have arisen repeatedly… The commonality of evolutionary pathways of terrestrial organisms is a consequence of their similar “design” and the identical adaptive challenges they must meet during evolution. How will life develop on other planets with conditions incomparable to Earth’s? This can only be conjectured. Here we may consider a possible variant of alien life imagined by Carl Sagan (a well‑known scientist who studied extraterrestrial life at NASA, the American space agency). “On a giant planet like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water vapour and ammonia, a solid surface is unattainable, yet there are fairly dense cloud layers into which organic molecules can fall from the sky like heavenly manna, as has been observed in our laboratory experiments. Such a planet also has a characteristic impediment to life: the atmosphere is turbulent and its lower layers are heated to very high temperatures. Organisms must beware of being blown downwards and roasted. To demonstrate that life is not excluded on such planets radically different from Earth, my colleague at Cornell, E. E. Solpiter, and I performed some calculations. Of course we cannot know exactly what life would look like in such a place, but we wanted to examine, within the known laws of physics and chemistry, whether a world of this type could in principle be habitable. One way to preserve life under the described conditions is to reproduce before being roasted, hoping that convection will carry some offspring to higher, cooler atmospheric layers. Such organisms could be very small. We called them sinkers (from English sinker — a weight). However, they could also become floaters (from English float — to float) — huge hydrogen balloons that vent helium and other heavier gases, retaining only the lightest gas—hydrogen; another variant is a hot‑air balloon that maintains buoyancy by keeping a high internal temperature, powered by energy obtained from food. As with familiar terrestrial balloons, the deeper a floater descends, the greater the lift that returns it to the upper, cooler, safer regions of the atmosphere. Floaters may feed on organic molecules formed in the atmosphere or synthesize them themselves using sunlight and air, much as plants do on Earth. It should be noted that the larger a floater, the more viable it becomes. Solpiter and I imagined floaters with a cross‑section of several kilometres—size of an entire city, far larger than the biggest whales ever existing. Floaters could move through the atmosphere by emitting jets of air, like a jet aircraft or rocket. We imagined them gathered in huge lazy herds stretching as far as the eye can see, with characteristic protective coloration indicating that they also face problems. Because in the considered medium there is at least one other ecological niche—hunting. Hunters (from English hunter) are fast, mobile creatures. They hunt floaters not only for their organics but also for the pure hydrogen they store. Hollow sinkers could have evolved into the first floaters, and self‑propelled floaters into the first hunters. There cannot be too many hunters, otherwise they would consume all floaters and die out themselves. Physics and chemistry allow such forms of life. Art endows them with a certain charm. Nature, of course, is not obliged to follow our conjectures. But if there are billions of habitable worlds in the Galaxy, perhaps some of them will be populated by sinkers, floaters and hunters that we have invented, staying within the laws of physics and chemistry.” (C. Sagan, 2005).
Additional materials: Column: Aliens among us!
2.15. (addendum) Venus, Earth, Mars
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Section 2. Biospherology
2.17. (addendum) Anthropogenic Paradox