Are we alone? Where can we look for life? What would it be like if we found it?
These are all equally terrific and daunting questions. With the discovery of organic molecules thought to have been implicated in the beginnings of life on Earth on Titan and Enceladus has come a ramping up of interest in the possibility of non-terran life existing somewhere, tucked away, in our solar system. This review offers a short overview of some thought on the topic. Of particular interest were the articles I have cited on cold-origin theories, these are well worth a look. Thanks to Dr Ian Bennett of ECU for the topic and title.
“It’s life sir, but not as we know it.”
The rise of modern space exploration and astronomical techniques involving ever more sophisticated equipment combined with ongoing in-depth research into the chemical origins of life has lead to some renewed and serious debate about the origins of life and the possibility of life being found on other bodies in our solar system. Among those scientists discussing the possibility of life on other bodies there are a wide range of viewpoints generated from various areas of research including biochemistry, astrobiology, microbiology and geology among others. This essay shall outline current research surrounding the search for life on other planets and moons in our solar system, concentrating on first providing a useful definition of life and continuing on to identify the various theories surrounding the origins of life, where life is most likely to be found in our solar system, what these organisms are most likely to resemble in comparison to terrestrial organisms and how best to detect them.
What is life?
Before considering where we may search for life in our solar system it is first prudent to have an idea of what is meant by the term ‘life’. While a concise definition of what constitutes life is still a topic of frequent debate (McKay, 2008), a factorial approach titled the ‘seven pillars of life’ (Koshland, 2002) may be of use for providing a framework of what does or does not qualify as life in the context of the search for extraterrestrial organisms. These seven pillars are as follows; program, improvisation, compartmentalisation, energy, regeneration, adaptability and seclusion (Koshland, 2002).
In greater detail, program would refer to an organised plan such as DNA or RNA. Improvisation would be the ability of the organism to create new responses to new problems or change its program, for instance mutation. Compartmentalisation refers to the fact that all organisms known to us occupy a limited volume surrounded by a membrane or skin. Energy is needed as any living organism is not in equilibrium and must therefore be a metabolising system. Regeneration is necessary to compensate for damage accumulated through wear and tear and metabolic reactions. Adaptability is similar to improvisation but faster acting, where improvisation refers to mutation or ‘program change’, adaptability refers to the ability to change behaviour such as in pain feedback responses. Lastly, seclusion refers to specificity of enzymes and how this keeps separate chemical processes from interfering with one another (Koshland, 2002). All seven of these factors are seen in Earth life and thus serve as a good general outline when deciding if any potential finding on another planet or moon is indeed life.
Theories of the origins of life
Evidence for the earliest known life forms on Earth comes from cyano-bacteria like microfossils thought to be 3.6 billion years old (Carroll, 2001). The question of how they arose however is one for which there are many theories, none of which have been conclusively proven. There stand two main groups of scientific theory. The first group involves panspermia, the notion that life arose elsewhere and was carried here via meteor impact, this however just moves the question of how life arose rather than answers it (Shapiro & Schulze-Makuch, 2009). The other group of theories involve abiogenesis, or how life arose from non-living organic matter on Earth (or on other bodies).
While it is widely accepted that before the evolution of DNA based life there was an RNA world that had been built from prebiotic chemicals (Orgel, 2004), the conditions that surrounded the development of this RNA world are still much debated. Two main theories are often seen in the literature, the first being that of a ‘hot-origin’ the second being that of a ‘cold-origin’ of life.
Research on archaic life forms living in the deep ocean close to hydrothermal vents has yielded the majority opinion that life had a hot-origin (Moulton et al., 2000), with the RNA world arising in hot, chemical rich waters of the primordial Earth. More recent research however has suggested that the high temperatures associated with a hot-origin would be incompatible with an RNA world due to hyperthermal conditions causing folding of RNA that is incompatible with life (Moulton et al., 2000). This suggests that life may have arisen in icy conditions rather than hyperthermic ones, thus the opposing cold-origin theory. Further evidence for a cold-origin theory is seen in research that demonstrated that hydrogen cyanide (a common molecule) at very low temperatures is capable of producing prebiotic compounds (Miyakawa, James Cleaves, & Miller, 2002).
As will be seen, the bodies in our solar system that are considered to have the greatest likelihood of harbouring life, past or present, either have (or are thought to have) hydrothermal vents of some type or ice. With the hot and cold origin theories in mind it is possible to see how these bodies can be considered likely sites for independent abiogensis.
Where should we look for life in our Solar System?
Having considered what life is and how it may arise, we are in a position to examine the planets and moons of our solar system in an attempt to identify which of these may have had their own independent abiogenesis. Five bodies that are of interest to astrobiologists not only due to their status as potential homes of extraterrestrial life but due to their viability as reachable research targets are Titan, Mars, Europa, Enceladus and Venus (Shapiro & Schulze-Makuch, 2009). Reasons for and against priority for missions to these bodies can be seen in Fig. 1. Details of why these bodies are of importance in the search for extraterrestrial life will be outlined below.
Fig. 1. Reasons for and against missions searching for extra terrestrial life.
Adapted from Shapiro & Schulze-Makuch, 2009, p. 340.
Titan is the largest moon of Saturn and is of much interest to scientists. Titan possesses a stratified atmosphere that is in some ways similar to that of Earth, possesses a methane cycle that is somewhat similar to our water cycle (methane being a substitute solvent in chemical reactions) and has an abundant organic chemistry with many known prebiotic chemicals such as hydrogen cyanide and cyanoacetylene being present (Raulin, 2008). Finding life on Titan would be of great interest as it is likely to be quite different from Earth life given the radically different chemical environment.
Mars is one of the better understood planets in our solar system having had several landers and obrbiters compile information on its geology and atmosphere in previous years (Shapiro & Schulze-Makuch, 2009). There are many points in favour of Mars when it comes to the search for extra terrestrial life such as the presence of methane in the atmosphere (often a by-product of life), evidence of ice (thus a possible cold origin of life) and evidence of past liquid water (Shapiro & Schulze-Makuch, 2009). Experiments demonstrating that subterranean environs present on Mars can be tolerated by some extremophilic Earth organisms have also been carried out (Reid et al., 2006; Shapiro & Schulze-Makuch, 2009). These experiments lend credence to theories that postulate the possibility of subterranean microbial life on Mars. A source of criticism for searching for life on Mars is the absence of organic matter on the planet’s surface (Shapiro & Schulze-Makuch, 2009). Research however has suggested that any organic matter present may have been destroyed by aggressive oxidants resulting from reactions between UV radiation and chemicals in the soil (Bada et al., 2005). Panspermia is an area of concern when searching for independently developed life on Mars also due to the proximity of this planet to Earth.
Europa is the second of Jupiter’s moons with a diameter of approximately 3160km (Shapiro & Schulze-Makuch, 2009). While Europa has no atmosphere, linear geographic features that can be seen on the surface of the moon are thought to be cracks in the mantle, under which is believed to be a subsurface ocean (McKay, 2008). If this ocean has hydrothermal vents similar to those seen on Earth, it is possible that these vents may be a site of hot origin abiogenisis (Shapiro & Schulze-Makuch, 2009).
Enceladus is a small and icy satellite of Saturn with geological features that seem to indicate recent formation by geologic activity (Shapiro & Schulze-Makuch, 2009). A source of recent great interest in Enceladus was the discovery of a water jet near the southern pole of the moon during the Cassini spacecraft flybys of 2008 (McKay, 2008). Analysis of the water revealed a distinct organic chemistry with carbon dioxide, methane and small quantities of acetylene and other chemicals present (Shapiro & Schulze-Makuch, 2009). The evidence of liquid water lends credibility to the idea that Enceladus has the potential for life either as a result of a hot origin or cold origin.
Finally, Venus is another planet of interest in the search for extraterrestrial life. It is considered a possibility that life may have developed in hot oceans on Venus at about the same time that it originated on Earth, however it is unlikely that any evidence of this life survives (Shapiro & Schulze-Makuch, 2009). It has been suggested that some life may persist in the clouds of Venus which would be of great interest to science if ever proven true (Shapiro & Schulze-Makuch, 2009). Unfortunately, much like Mars, due to the proximity of Venus to Earth it would be very difficult to rule out panspermia in any life detected there (Shapiro & Schulze-Makuch, 2009).
If we find life on other planets or moons, what will it be like?
On Earth, microorganisms are ubiquitous and various, they are adapted to diverse environments from hyperthermophiles that live in high temperatures (around hydrothermal vents or volcanic pools) to psychrophiles that can thrive in frozen lakes and halophiles that tolerate high salt concentrations (Reid et al., 2006). Earth life as can be seen is very pervasive at the microscopic scale. Keeping this in mind, life on other worlds is most likely to be microscopic, unicellular and relatively simple. If we were to consider possible extraterrestrial life forms based on our knowledge of known life on Earth some of the best examples of possible models of extra terrestrial life may come from the extremophiles mentioned already. Research has demonstrated that some psychrophilic Archaea found on Earth such as Methanococcoides burtonii are capable of growth in simulated current-day Mars environments (Reid et al., 2006), therefore it is possible that similar life may survive or have survived on Mars. Similarly, hyperthermophilic lithoautotrophic microbes that feed on abiologically derived hydrogen and carbon dioxide are known to exist here on Earth (Takai et al., 2004), these organisms could present a model for life that may have developed around theorised hydrothermal vents on Enceladus or Europa. Methanotrophs are organisms that feed on methane and are known on Earth (Eshinimaev et al., 2004), if life here can survive on methane, it is possible that a methane rich environment such as Titan may support similar microorganisms.
What are the best ways to detect life on other planets or moons?
While detecting and finding an extant life form on another world would be of very great interest to science, it is important to remember that the chances of doing so may be very small. This is due not only to the fact that it is highly likely that any extraterrestrial life is microscopic but that much life that may have existed may now be extinct (McKay, 2008). While there is some argument about the universal nature of Earth’s biochemistry, it is agreed upon that life in general will follow a pattern of use and production of certain organic molecules in certain proportions particular to that variety of life (McKay, 2008). Following this logic, the best way to search for life may be to search for biological molecules that will be present with extant life and also be indicative of past life. Extending from this, we then need a way to differentiate molecules of nonbiological origin from those of biological origin (for example organic chemicals detected on Titan or in the water jet of Enceladus may or may not be of biological origin). Here is where the study of the patterns of organic chemicals present by relative number may be of great use, as can be seen in Fig. 2. Where organic chemicals of nonbiological origin will present a smooth distribution curve when graphed, those of biological origin are more likely to be present in a series of marked spikes due to their specific production by life forms (McKay, 2008). Upon finding such a pattern, more detailed searches for fossil evidence or extant organisms would be warranted.
Fig. 2. Comparison of patterns of organic material.
Adapted from McKay, 2008, p. 53.
While the possibility of finding life on other bodies in our solar system, particularly life with an independent origin to our own, is of great interest to scientists much more research needs to be conducted before we know with any great certainty what to look for or where. The data available at this point seems to point us in the direction of microorganisms, probably what we would term extremeophiles, that may exist or have existed on Titan, Mars, Europa, Enceladus or Venus. At this point, the search for evidence of extant or extinct life through the examination of distribution patterns of biological molecules seems to be the most promising technique for finding these postulated organisms. With further advances in space technology, robotics and our understanding of life chemistry in the future we may look forward to a time when we know for sure if we are alone in our solar system.
Bada, J. L., Sephton, M. A., Ehrenfreund, P., Mathies, R. A., Skelley, A. M., Grunthaner, F. J., et al. (2005). New strategies to detect life on Mars. Astronomy & Geophysics, 46(6), 6.26-26.27.
Carroll, S. (2001). Chance and necessity: The evolution of morphological complexity and diversity. Nature, 409(6823), 1102.
Eshinimaev, B. T., Medvedkova, K. A., Khmelenina, V. N., Suzina, N. E., Osipov, G. A., Lysenko, A. M., et al. (2004). New Thermophilic Methanotrophs of the Genus <i>Methylocaldum</i>. Microbiology, 73(4), 448-456.
Koshland, D. (2002). The seven pillars of life. Science, 295(5563), 2215.
McKay, C. (2008). An Approach to Searching for Life on Mars, Europa, and Enceladus. Space Science Reviews, 135(1-4), 49.
Miyakawa, S., James Cleaves, H., & Miller, S. (2002). The Cold Origin of Life: A. Implications Based On The Hydrolytic Stabilities Of Hydrogen Cyanide And Formamide. Origins of Life and Evolution of Biospheres, 32(3), 195-208.
Moulton, V., Gardner, P. P., Pointon, R. F., Creamer, L. K., Jameson, G. B., & Penny, D. (2000). RNA Folding Argues Against a Hot-Start Origin of Life. Journal of Molecular Evolution, 51(4), 416-421.
Orgel, L. (2004). Prebiotic Chemistry and the Origin of the RNA World. Critical Reviews in Biochemistry and Molecular Biology, 39(2), 99-123.
Raulin, F. (2008). Astrobiology and habitability of Titan. Space Science Reviews, 135(1-4), 37.
Reid, I. N., Sparks, W. B., Lubow, S., McGrath, M., Livio, M., Valenti, J., et al. (2006). Terrestrial models for extraterrestrial life: methanogens and halophiles at Martian temperatures. International Journal of Astrobiology, 5(02), 89-97.
Shapiro, R., & Schulze-Makuch, D. (2009). The Search for Alien Life in Our Solar System: Strategies and Priorities. Astrobiology, 9(4), 8.
Takai, K., Gamo, T., Tsunogai, U., Nakayama, N., Hirayama, H., Nealson, K. H., et al. (2004). Geochemical and microbiological evidence for a hydrogen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME) beneath an active deep-sea hydrothermal field. Extremophiles, 8(4), 269-282.