Searching for Other Life Forms in Extraterrestrial Environments
October 7, 2010 3 Comments
The ancient Greeks were among the first to explain astronomical phenomena in physical terms. It is known, for example, that Aristarco from Samos taught that the Earth was just one planet which, as others, moves around the sun and that stars were at great distances. Epicurus suggested that the universe is filled with other worlds where is possible. Since then, the idea of a universe consisting of many worlds, just like Earth and our solar system, has been raised many times in the course of human history.
To search for life on planets other than the Earth we must be prepared to recognize life as we do not know it. We cannot rule out other planets just because they are not like our world. An infinite number of life forms may have been fashioned in alien environments with characteristics fundamentally different from those found on Earth. In this context, to recognize alien life, we must learn how to escape from our anthropocentric, Earth-centered way of thinking and abandon the pre-Copernican belief that our planet is the center of the biological universe and all life forms are just like us.
Criterion of ‘Being Living ‘
For millennia, philosophers, scientists, and theologians, have attempted define life. And yet, there is no general accepted definition of life.Quoting From Research Paper:
Nowadays scientists are content to define life using the “chemical Darwinian definition” that involves “self-sustaining chemical systems that undergo evolution at the molecular level” (Joyce et al 1994). It is a limited definition considering that life on Earth may have originated on other planets (Joseph 2009a; Rampelotto 2009). There are in fact a number of genetic-studies which purport to demonstrate that the common ancestors for Earthly life forms may have first began to form billions of year before the Earth was fashioned (Jose et al., 2010; Poccia et al., 2010; Sharov 2010). It has been speculated the first steps toward actual life may have begun with self-replicating riboorganisms (Jose et al., 2010) whose descendants fell to Earth and other planets through mechanisms of panspermia (Joseph 2009a) thereby triggering the RNA world and then life as we know it (Jose et al., 2010). However, this model of life is still based on life as we know it. In fact, the concept of a self-sustaining chemical process can be applied with some justification to other catalytic, self-sustaining physicochemical process, such as forest fires.
Life on some planets may be like life on Earth. Life on other worlds may have a completely different chemistry, and may not even possess a genetic code. It would be extremely unfortunate to expend considerable resources in the search for alien life and not recognize it when we find it–or it finds us.
Life that may have been originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus could not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products. Life might be based on molecular structures substantially different from those we know. Therefore, it may be a mistake to try to define life based on a single example – life on Earth. As pointed out by Cleland and Chyba (2002) definitions just tell us about the meanings of words in our language, as opposed to telling us about the nature of the world.
What we really need is a general theory of living systems, analogous to the theory of molecules that permits one to give an unambiguous answer to the question “what is water?”. Prior to molecular theory, the best a scientist could do in characterizing water would be to define it in terms of its sensible properties, such as being wet, transparent, odorless and tasteless. Once we had an understanding of the molecular nature of matter we could identify water in such a way that all ambiguity disappears: water is H2O. Thus, a precise answer to the question “what is water?” was possible only when situated within an appropriate scientific theory.
Again, however, this may trap us into an Earth-centered perspective. Life in the universe may not be like life as we know it. Therefore, the key to formulate a general theory of living systems is to explore alternative possibilities for life. In this context, first of all, we need to understand the fundamental features of life not just based on examples from Earth, but based on how life may form and then evolve on planets completely unlike Earth. By taking a broad view this will greatly improve the possibility of recognizing life if we come upon it elsewhere in the Universe.
The search so far has focused on Earth-like life because that is all we know. Hence, most of the planning missions are focused on locations where liquid water is possible, emphasizing searches for structures that resemble cells of terran organisms, small molecules that might be the products of carbonyl metabolism and amino acids and nucleotides similar to those found in terrestrial proteins and DNA.
However, life that may have been originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus could not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products. We must recognize that our knowledge of the essential requirements for life and therefore our concept on it, is based on our understanding of the biosphere during the later stages of Earth history. Since we only know one example of biomolecular structures for life and considering the difficulty of human mind to create different ideas from what it already knows, it is difficult for us to imagine how life might look in environments very different from what we find on Earth. In the last decades, however, experiments in the laboratory and theoretical works are suggesting that life might be based on molecular structures substantially different from those we know.
One of the fundamental features of life is its chemical complexity, which is based on polymeric molecules joined by covalent bonds. Carbon appears to be the only element capable of forming polymers that readily undergo chemical alterations under the physical conditions prevailing on Earth.
When we discuss about the search for extraterrestrial life, one of the most enticing questions that emerge in our mind is “how such exotic forms of life might look?” or “how similar or different from us will they be?”
Life is likely a result of physical and chemical contingencies presented in the world where it arises. Most of the geochemical and environmental processes of any world remain unclear. Even the conditions that were present in early Earth are not clearly understood, which makes the origin of terrestrial life a mystery far to be resolved. Furthermore, the history of life on Earth shows us that the evolutionary trajectory of a living system cannot be predicted. The diverse and unimaginable forms of life which arose during the Cambrian period are a good example of the variety of forms life may take. Therefore, the details of form and function that a different history of life elsewhere would take, cannot be known until we find it. However, despite the possibility of so much diversity, at the molecular level underlying mechanisms guide the development of any unimaginable living system. Thus, based on biochemical principles, it is possible to make predictions about the nature of exotic forms of life which may be found in our solar system.
Thus, instead of searching for specific biosignatures that appeared later in the Earth’s history, future missions should focus to search for the general characteristics of life, which means search for life’s material-independent signatures. Most of our universe appears to be a hostile place for life to exist with no planetary bodies except Earth harboring life as we know it. However, similar notions were previously thought of Earth’s extreme environments such as acidic hot springs, deepsea vents or solar salterns, which were believed to be too “extreme” to nurture life. Yet numerous studies over the last decades have shown that these extreme environments actually harbor an incredible diversity of bacteria and archea.
Imagine with me and let’s go to planet venus which may harbour silicon based life forms. In some cases it may also harbour Boron based life forms. Boron have some interesting chemical and physical properties which make it a possible candidate to constitute exotic life forms under some condition. Though, this time it is not going to make complex compound by going through the formation of covalent bonding but it may be hydrogen bonding. It is also capable of forming long chain compounds with hydrogen at normal pressure and temperature conditions. The nido boranes are extremely stable(boranes are compound of hydrogen and boron). So I can’t discard its role to develope sentient exotic alien beings.
Ammonia Based Life Forms
An alternative biochemistry could be conceived in which water was replaced as a solvent by liquid ammonia.1 Part of his reasoning was based on the observation that water has a number of ammonia analogues. For example, the ammonia analogue of methanol, CH3OH, is methylamine, CH3NH2. Haldane theorized that it might be possible to build up the ammonia-based counterparts of complex substances, such as proteins and nucleic acids, and then make use of the fact that an entire class of organic compounds, the peptides, could exist without change in the ammonia system. The amide molecules, which substitute for the normal amino acids, could then undergo condensation to form polypeptides which would be almost identical in form to those found in terrestrial life-forms. This hypothesis, which was developed further by the British astronomer V. Axel Firsoff, is of particular interest when considering the possibility of biological evolution on ammonia-rich worlds such as gas giants and their moons.
On the plus side, liquid ammonia does have some striking chemical similarities with water. There is a whole system of organic and inorganic chemistry that takes place in ammono, instead of aqueous, solution.4, 5 Ammonia has the further advantage of dissolving most organics as well as or better than water,6 and it has the unprecedented ability to dissolve many elemental metals, including sodium, magnesium, and aluminum, directly into solution; moreover, several other elements, such as iodine, sulfur, selenium, and phosphorus are also somewhat soluble in ammonia with minimal reaction. Each of these elements is important to life chemistry and the pathways of prebiotic synthesis. The objection is often raised that the liquidity range of liquid ammonia – 44°C at 1 atm pressure – is rather low for biology. But, as with water, raising the planetary surface pressure broadens the liquidity range. At 60 atm, for example, which is below the pressures available on Jupiter or Venus, ammonia boils at 98°C instead of -33°C, giving a liquidity range of 175°C. Ammonia-based life need not necessarily be low-temperature life!
The vital solvent of a living organism should be capable of dissociating into anions (negative ions) and cations (positive ions), which permits acid-base reactions to occur. In the ammonia solvent system, acids and bases are different than in the water system (acidity and basicity are defined relative to the medium in which they are dissolved). In the ammonia system, water, which reacts with liquid ammonia to yield the NH+ ion, would appear to be a strong acid – quite hostile to life. Ammono-life astronomers, eyeing our planet, would doubtless view Earth’s oceans as little more than vats of hot acid. Water and ammonia are not chemically identical: they are simply analogous. There will necessarily be many differences in the biochemical particulars. Molton suggested, for example, that ammonia-based life forms may use cesium and rubidium chlorides to regulate the electrical potential of cell membranes. These salts are more soluble in liquid ammonia than the potassium or sodium salts used by terrestrial life.
Silicon Based Life Forms
The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations. Complex long-chain silicone molecules are still less stable than their carbon counterparts, though.
For example, polysilanes with molecular weights of above 106 have been synthesized. Although polysilanes are not stable at the temperature and pressure conditions of Earth’s surface they are adequately stable at low temperatures, especially at higher pressures. These studies altogether suggest that whether silicon-based life exist, it may be restricted to an environment with minor amounts of oxygen, scarcity of water, a compatible solvent such as methane and low temperatures (at least below 0°C). Titan provides the best target in our solar system for investigating this possibility. It meets all the described criteria (Fulchignoni et al 2005; Naganuma and Sekine 2010). Although has been considered that the abundance of carbon compounds on Titan may compete with silicon as the building block of life, silicon may have advantage in such extreme cold environment due to its higher reactivity.
Sulphur Based Life Forms
Sulphuric acid has the reputation to be a strong corrosive agent. However, what is not realized is that the process, called hydrolysis, actually requires water. It is the water molecules that split proteins into small pieces; acid merely catalyses the process. Thus, due to its capacity to support chemical reactivity, sulphuric acid may be a reasonable solvent capable to sustain metabolism in non aqueous environments.The Venusians atmosphere is the most proper ambient in the solar system where this exotic form of life may flourish. The clouds of Venus are composed mostly of aerosols of sulfuric acid and water is scarce.
Life could have possibly originated in an early ocean on Venus when the planet’s surface was younger and cooler; then retreated into the clouds when the planet heated. To protect them from the high amount of UV radiation received, such hypothetical living systems may use the compound cyclic-octa-sulfur (S8), which does not react with sulfuric acid. An analogous process is observed on Earth, where some purple sulfur bacteria, green sulfur bacteria and some cyanobacterial species deposit elemental sulfur granules outside of the cell (Tortora et al 2001). Such Venusians life forms may be phototrophic, using hydrogen sulfide, which is oxidized to produce granules of elemental sulfur (Schulze-Makuch et al 2004). Terrestrial purple sulfur bacteria use such anoxygenic process as source of energy.
The discovery of exo-planets around stars other than the Sun continues to stimulate public and media interest. Undoubtedly, this attention has been driven by the prospects of finding evidence of alien life. At the moment, life on Earth is the only known life in the Universe, but there are compelling arguments to suggest we are not alone. As Carl Sagan said, the absence of evidence is not evidence of absence. This thought is well known in other fields of research. Astrophysicists, for example, spent decades studying and searching for black holes before accumulating today’s compelling evidence that they exist. The same can be said for the search for room-temperature superconductors, proton decay, violations of special relativity, or for that matter the Higgs boson. Indeed, much of the most important and exciting research in astronomy and physics is concerned exactly with the study of objects or phenomena whose existence has not been demonstrated.
[Ref: The Search for Life on Other Planets: Sulfur-Based, Silicon-Based, Ammonia-Based Life by Pabulo Henrique Rampelotto ,WIKIPEDIA, DavidDarling]