Exobiology: The Hunt for Extraterrestrial Life

Samir Chitnavis (17) discusses the origin of life on Earth and the possibility of other living organisms within our Solar System.

Life Beyond Earth

Astrobiology, the study of extraterrestrial life, is an enigmatic science. Firstly, its uniqueness lies in the fact that it incorporates several scientific disciplines into one. Fields such as biochemistry and astrophysics are used simultaneously to aid the search for life in the universe. Secondly, it is the only field of study yet to prove that its subject matter actually exists. It is clear that astrobiology is a special branch of science.
In 1961, California, United States, astronomer and astrophysicist Frank Drake formulated the \”Drake Equation\”. The so-called \”Drake Equation\” is a probabilistic argument used to estimate the number of intelligent, extra-terrestrial civilisations in our galaxy, the Milky Way.¹ Far from a strictly mathematical equation, the multifactorial expression summarises the most important concepts in determining the likelihood of other communicative life existing within our galaxy. The usefulness of the equation has been greatly criticised since each term is speculative, and as a result a final value is not reliable enough to draw conclusions from. The original estimates in 1961 included a minimum number of intelligent civilisations as 20 and a maximum of 50,000,000.² The reason for such a vast range is due to the fact that each factor itself has a range of values, and so the final value of N is different when multiplying all minimums together than when multiplying all maximums together. For example, the value of n_e can vary between 1 and 5. Current estimates range from a minimum number of 2 and a maximum of 280,000,000. Despite the equation\’s flaws, it does emphasise the need for astrobiologists to contemplate a variety of concepts as they question the likelihood of life beyond Earth.

Abiogenesis: The Primordial Pond

Before aimlessly searching, one must consider how life originated on Earth itself. Abiogenesis is the process by which living, organic matter originates from non-living matter. On Earth, abiogenesis occurred around 3.8 billion years ago. One of the main explanations for the origin of life on Earth is the \”primordial soup hypothesis\”. The hypothesis states that \”LUCA\”, The Last Universal Common Ancestor, a molecule that stored information as a genetic code and gave rise to life on Earth, was produced in an \”RNA soup\” that eventually gave rise to DNA and cells. In 1952, Chicago, United States, Stanley Miller and Harold Urey investigated how self-replicating molecules came into existence. The Miller-Urey experiment (see Fig. 2) demonstrated that organic monomers could be formed by simulating the environment of the early Earth. The primitive atmosphere of the Earth was created by sealing the chemically reducing gases water, methane, ammonia and hydrogen in a glass flask. The flask was connected to a second flask containing liquid water, which represented the primeval ocean of the Earth. Then, the water was heated to induce evaporation and the water vapour entered the flask containing the reducing gases. After that, an electric spark was continuously struck between two electrodes to simulate a bolt of lighting in the mixture of gases and water vapour. Finally, the reaction mixture was cooled so that the condensed water could settle in a U-shaped trap at the base of the apparatus. After a day, the solution in the trap had turned pink in colour and within a week, the presence of the amino acids glycine, a- and B-alanine had been revealed. Additionally, ribose, a pentose sugar found in RNA, was identified, a result of a reaction between water and formaldehyde. Further organic compounds were also found. In total 25 amino acids were identified, a result of a complex series of reactions involving the reducing gases. The underlying hypothesis of the experiment was that multiple conditions on the primitive earth favoured chemical reactions that synthesised complex organic compounds from non-organic matter. The compounds would have accumulated in a \”soup\” which lead to the formation of complex polymers and cells and eventually life developing. Miller later stated, \”if God didn\’t do it this way, he missed a good bet\”. ³
It is generally accepted that life on Earth descended from self-replicating RNA. Modern cells contain a genetic code in the form of DNA, but RNA is a close relative of DNA and like its neighbour, it is found in all living cells. Importantly, RNA can catalyse chemical reactions by itself and is essential in the translation process in the formation of proteins. All living organisms use proteins to make DNA, but proteins can only be synthesised using DNA as their code- RNA addresses this conundrum. If the last common ancestor was indeed formed in an \”RNA soup”, it is possible that the monomers within the soup would have concentrated at various locations, including deep-sea hydrothermal vents. Hydrothermal vents are fissures in a planet\’s surface from which geothermally heated water issues, containing many dissolved chemicals including methane, carbon dioxide, hydrogen and sulphur, which are necessary for producing the monomers formed in the Miller-Urey experiment. These monomers would then have combined to form polymers. The amino acids from the Miller-Urey experiment would have joined together in condensation reactions to form polypeptides (possibly containing disulphide bonds, from the sulphur at hydrothermal vents) whereas RNA could have formed in a condensation reaction between ribose, a phosphate group and an organic base. The first life would have been a replicating cell with some form of membrane enclosing its chemicals. From simple prokaryotes, complex eukaryotes eventually began to develop through endosymbiosis, stimulating biodiversity and the evolution of early life. It is thought that mitochondria and chloroplasts existed as free-living prokaryotes before being engulfed by other cells to become organelles. Chloroplasts are most likely to have developed from cyanobacteria, which had the ability to photosynthesise, converting the chemically reducing atmosphere of the early earth into an oxidising one, paving the path for aerobic organisms to evolve.
Whilst astrobiologists must consider how life arises on a planetary body, they must also understand the requirements for life to exist. It is postulated that extra-terrestrial life forms would be organic, based on carbon chemistry, and rely on water. Carbon is a unique element. The carbon atom has the ability to make four chemical bonds with other atoms, including their own: these covalent bonds have a direction such that carbon atoms can form the skeletons of complex three-dimensional structures, such as proteins or nucleic acids. It is also true that carbon forms more compounds than all the other elements combined. Therefore, it is unlikely that any other element could replace carbon, even beyond Earth, in composing the backbones of organisms. Along with carbon, water is vital for any organism to survive. \”Follow the water\” is the mantra of astrobiology; water is indeed the theatre of life. On Earth, life requires water as a solvent in which biochemical reactions can take place, for example the series of reactions that first gave life at hydrothermal vents. Water can also act as a reagent in many cellular reactions, such as in photosynthesis or hydrolysis. Quantities of carbon and the other elements with water would potentially allow for the formation of organisms on planetary bodies which share a temperature range similar to Earth. On Earth, the combination of carbon, hydrogen and oxygen in the form of carbohydrates, including glucose, are a source of energy on which life depends. Astrobiologists can also search for an atmosphere that partly reduces carbon and partly oxidises it since life on Earth requires carbon in both reduced and oxidised states. Additionally, nitrogen is required on Earth as a reduced ammonia derivative in proteins and phosphorus is oxidised to form phosphate groups, a component of nucleotides and a vital compound in ATP synthesis. It is predicted that any extra-terrestrial life found will be in the form of extremophile (unicellular) microorganisms, since no planetary body has so far been shown to exhibit such favourable conditions for life to evolve as Earth does.
But what of energy? All life requires energy for processes including the synthesis of organic polymers, cell division, locomotion and bulk transport to name a few. Craig Venter, the pioneer of synthetic biology, has long tried to unravel the essential toolkit for all modern life by designing artificial cells to reveal the minimum number of genes essential for life. These \”minimal genomes\” have revealed many of the genes that LUCA probably possessed. Other teams in the field of synthetic biology have analysed the genomes of bacteria and archaea to discover the most ancient genes present. Their results suggest that although LUCA had genes coding for proteins to be used in cell growth or the repair of replicated genetic material, it lacked the ability to generate a gradient across its cell membrane to form ATP.⁴ So where did LUCA get its energy from? Present-day organisms produce ATP by a process known as chemiosmosis: the energy released during respiration allows for carrier proteins to pump ions across a membrane to generate an electrochemical gradient. Hydrogen ions (protons) flow back across the membrane, releasing energy that ATP synthase uses to complete the oxidative phosphorylation of ATP from ADP. Once ATP is made, it can be hydrolysed to release its energy for the organism to use. The fact that LUCA lacked the ability to form a gradient further promotes the theory that life first evolved at deep-sea hydrothermal vents. Instead of forming its own electrochemical gradient, LUCA could have harnessed the existing natural gradient between vent water and sea water, exploiting the hydrogen ions spewing from the vents, to form ATP. Later organisms could have generated their own electrochemical gradients, allowing life to depart the vents on two occasions: once to give Archaea and the other to give Bacteria, the two first domains of organisms.

Fig. 2: Miller-Urey Experiment

Wikipedia, The Free Encyclopaedia: Wikipedia Contributors, \”Miller-Urey experiment\”.

3 Possible Locations for Life in our Solar System

Mars

Mars is of great interest to scientists, partly due to its similarity to the early Earth and the fact that it holds a promising record of conditions required for abiogenesis. However, evidence of biosignatures of current life are yet to be found. Due to the high levels of radiation on the Martian surface and the fact that it is completely frozen mean that life is unlikely to survive overland. The Curiosity Rover recently described the radiation levels as “so high that any biological organisms would not survive without protection”. ⁵ Consequently, more likely locations for discovering life may be sub-surface environments. Water on Mars exists almost entirely in the form of ice, mainly located in polar ice caps. Despite a lack of pure liquid water on the surface, spectrometer readings have given conclusive evidence of hydrated brine flows on recurring slope lineae (seasonal flows of salty water) during the warmest months. These lineae are a habitat that could be suited to halophile psychrophiles.⁶ Mars’s present day conditions differ from its warmer and perhaps wetter past. During the Noachian time period on Mars, high rates of meteorite impacts and volcanism could have resulted in warmer conditions favouring the presence of abundant surface water as a result of rainfall. On December 9th, 2013, NASA reported that, based on evidence from Curiosity studying the Aeolis quadrangle, Mars\’s Gale Crater contained an ancient freshwater lake, confirming that liquid water once flowed on Mars.⁷ Future Martian studies will search for evidence of ancient life and environments that may have been habitable. Astrobiologists are currently analysing habitats on Earth that are similar to those on Mars. For example, the Antarctic Dry Valleys, with their low humidity and lack of snow and ice, are considered to be the closest of any terrestrial environment to Mars. In April 2012, scientists reported that extremophile lichen survived and showed impressive photosynthetic adaptability under Martian conditions in the Mars Simulation Laboratory in Germany.⁸ However, for life to thrive on present-day Mars, it must be able to reproduce and evolve and not just survive.

Europa

Unlike Mars, the present conditions on Jupiter’s moon Europa favour the existence of life. The moon’s crust is primarily composed of ice and the smoothness of the surface has led to the hypothesis that a water ocean exists beneath, a possible site for life. It is believed that this ocean is heated by a phenomenon known as \’tidal flexing\’: the gravitational impact of Jupiter causes the ocean to remain liquid whilst driving geological activity on the moon, including plate tectonics. The hypothesis for a liquid ocean gained support in May 2015, when NASA reported that sea salt from a sub-surface ocean may be coating some of the moon’s unique geological features, including the abundant dark linear fractures on Europa’s surface, implying that the ocean is interacting with the rocky sea floor.⁹ For life to exist within the moon’s ocean, it could be supported by hydrothermal vents at the ocean floor. On Earth, hydrothermal vent organisms, in the absence of sunlight energy, feed off the nutrients provided by chemosynthetic bacteria. These bacteria use hydrogen sulfide as an alternative source of energy to sunlight to produce organic material from mineral-rich hydrothermal water. For example, giant tube worms are involved in a symbiotic relationship with bacteria in their trophosome, which convert carbon dioxide into glucose, a source of food for the worms. Similar prokaryotic organisms to the chemosynthetic bacteria would be expected if hydrothermal vents were discovered on future sub-surface missions. Alternatively, life could exist clinging to the underside of the moon’s icy crust, similar to ice algae in the Earth’s polar regions.¹⁰ Future missions to Europa include a 2022 launch to Jupiter’s Galilean moons: Ganymede, Callisto and Europa. All moons are believed to hold sub-surface oceans, and the spacecraft will be designed to characterise the moons and evaluate their potential habitability.

Enceladus

Saturn’s sixth largest moon is an intriguing possible home for extra-terrestrial life. Like Europa, it is believed to harbour oceans hidden beneath an exterior of ice, as reported by NASA in 2014.¹¹ Using evidence obtained by the Cassini-Huygens probe, they believe that the moon has a 10 kilometre thick sub-surface ocean of liquid water in its south pole. Flybys of Enceladus in 2005 revealed an astrobiologist\’s fantasy: plumes of water vapour erupting from cryovolcanoes at the moon’s south pole, with 250 kilograms of water vapour being released into space every second.¹² It has since been determined that microscopic particles within the plumes make up Saturn’s second outermost ring, the E Ring.¹³ Over 100 geyser-like jets have since been identified, supporting the case for a liquid water ocean. Mass spectrometers on the Cassini spacecraft have detected the life-supporting molecules carbon dioxide, methane and nitrogen within the plumes. However, the probe was not designed to capture or detect life and so future missions must take advantage of the free samples at their disposal, possibly using more advanced apparatus to identify amino acids or detect fatty acids within bacterial cell membranes. More importantly, future probes will try to identify the source of detected hydrogen molecules: whether they have come from deep-sea hydrothermal vents or simply the spitting of water. If life is discovered on Enceladus, it is extremely likely that it is life that had a second origin, an exciting prospect.

Conclusions

Astrobiology has long been based around the theory that where there is water, there could be life. If this is indeed the case, there are a great number of possible locations at the disposal of space agencies such as NASA. By understanding how life can arise, survive and thrive on Earth, astrobiologists hope to apply their knowledge to other planets and moons within our Solar System. The new emerging field of astrobiology will continue to excite the scientific community, as it attempts to make one of the greatest discoveries in the history of science.

Key Terms

References

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