Life in our Solar System
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. For example, 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.1 It is clear that astrobiology is a special branch of science.
In 1961, in California U.S., 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.2 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 value of intelligent civilisations as 20 and a maximum as 50,000,000. Current estimates range from a minimum number of 2 and a maximum of 280,000,000.3 Despite the equation’s flaws, it emphasises the scale of the task that faces astrobiologists.
Before aimlessly searching, one must consider how life originated on Earth itself. Abiogenesis is the process by which organic matter originates from non-living matter. On Earth, this 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 (ribonucleic acid) “soup” that eventually gave rise to DNA (deoxyribonucleic acid) and cells. The hypothesis is favoured because experiments have shown that complex biotic molecules can in fact be synthesised using early Earth-like conditions. In 1952, Chicago, United States, Stanley Miller and Harold Urey investigated how self-replicating molecules came into existence. The Miller-Urey experiment demonstrated that organic macromolecules 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 and macromolecules were also found. In total 25 amino acids were identified. 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 more complex organic polymers and cells and eventually life developing. Miller later stated, “if God didn’t do it this way, he missed a good bet.” 4
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. If the last common ancestor was formed in an RNA “soup”, it is possible that the compounds within the soup would have concentrated at various locations, including deep-sea hydrothermal vents. From simple prokaryotes, complex eukaryotes 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, favouring aerobic organisms.
Whilst astrobiologists must consider how life began, they must also know where to look for it. It is postulated that extra-terrestrial life forms would be organic and rely on both carbon and 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 in any search for life. On Earth, life requires water as a solvent in which chemical reactions can take place. Water can also act as a reagent in many cellular reactions, such as in photosynthesis or hydrolysis. Quantities of carbon along 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 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 in the nucleotides of genetic material such as DNA. It is also predicted that any extra-terrestrial life found will be in the form of extremophile microorganisms, that is, microorganisms that can survive in extreme conditions, withstanding large temperature or pH ranges for example.
3 Possible Locations for Life in our Solar System
Mars is of great interest to scientists, partly due to its similarity to the early Earth and 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: extremophile microorganisms that thrive in high salt concentrations and cold conditions.6 Mars’s present day conditions differ with its warmer and perhaps wetter past. During the Noachian time period on Mars, high rates of meteorite and asteroid impacts as well as volcanism could have resulted in warmer conditions favouring the presence of abundant surface water as a result of rainfall.7 On December 9th, 2013, NASA reported that, based on evidence from Curiosity studying the Aeolis quadrangle, Gale Crater contained an ancient freshwater lake, confirming that liquid water once flowed on Mars.8 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.9 However, for life to thrive on present-day Mars, it must be able to reproduce and evolve and not just survive.
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 tides generated by the gravitational impact of Jupiter, causing the ocean to remain liquid as well as driving geological activity on the planet, including plate tectonics. The hypothesis 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.10 For life to exist within the moon’s ocean, it could be supported by hydrothermal vents at the ocean floor, fissures in the planet from which geothermally heated water issues. 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 use bacteria in their trophosome to convert carbon dioxide into carbohydrates, a source of food for the worms.11 Alternatively, life could exist clinging to the underside of the moon’s icy crust, similar to ice algae in the Earth’s polar regions.12 If any water discovered was indeed salty, it would favour the existence of halophilic bacteria. Future missions to Europa include a 2022 launch to Jupiter’s Galilean moons: Ganymede, Callisto and Europa. All moons are believed to have possible sub-surface oceans, and the spacecraft will be designed to characterise the moons and evaluate their potential habitability.
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 2014.13 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 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.14 It has since been determined that microscopic particles within the plumes make up Saturn’s second outermost ring, the E Ring.15 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, propane 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 spectrometers to identify amino acids or detect fatty acids within bacterial cell membranes. If life is discovered on Enceladus, it is extremely likely that it is life that had a second origin, an exciting prospect.
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. 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.