Habitable environments in the solar system
Alex Crabb
The question of ‘Are we alone ? ’ has long been of interest to the scientific community, writers, philosophers, and the general public alike. Stories of life on other worlds date back to at least the second-century of the common era in Europe and tenth-century Japan. 1 However, it was not until the early 1900s that scientific understanding and observational tools had progressed to the point that it was possible to move attempts to address the question past speculative fiction towards theoretical study based on known facts about biochemistry and our solar system. 2 In the 1960s, with the advent of interplanetary probes, hopes for finding life elsewhere in the solar system faded. However, increasingly detailed knowledge of the environments and chemistries of solar system bodies, along with discoveries on Earth of organisms adapted for extreme environments, has reopened the possibility of finding life in the solar system. To assess the possibility of life on other planets, it is important to know the basic requirements and constraints for life to exist. Accurately determining what these are is not simple, since we only have life on Earth as a sample. One requirement which is certain is that all life requires an energy source. On Earth, almost all life depends on the Sun, directly or indirectly, for energy. Plants, algae and some types of bacteria directly harness sunlight to convert water and carbon dioxide into energy-rich sugars via photosynthesis, and other organisms (e.g. animals, fungi) consume these producers, thereby also deriving their energy from the sun. Until 1977, all life was thought to use sunlight as an energy source. However, the discovery of hydrothermal vents teeming with life on the ocean floor led to the realization that energy could alternately be derived from chemosynthesis, breaking down, in this case, hydrogen sulphide to release energy. 3 Since then, many different types of organism have been discovered using a wide variety of different chemical processes to obtain energy, for example the increasing microbes that reduce metals for energy. 4 There have even been discoveries of fungi which use gamma radiation for energy, found in the ruins of Chernobyl reactor 4. 5 The existence of life which does not rely on sunlight shows that life may be possible both underground on other planets and also in the outer solar systemwhere little solar energy is available.
Radiation can be a major challenge to life in the Solar system. There are three classes of radiation that are of concern; ionizing electromagnetic radiation (Ultraviolet, X-Rays, Gamma), solar particle radiation (consisting primarily of protons and electrons from the solar wind, but also occasional intense
1 A True Story , Lucian of Samosata; The Tale of the Bamboo Cutter , trad. 2 Salisbury FB. ‘ Martian Biology: Accumulating evidence favors the theory of life on Mars, but we can expect surprises ’, Science 1962: 36 (3510):17-26. 3 https://www.whoi.edu/feature/history-hydrothermal-vents/discovery/1977.html Consulted 11/08/2020. 4 Lloyd J. R. ‘Microbial reduction of metals and radionuclides’, FEMS Microbiology Reviews, Volume 27, Issue 2-3, June 2003, Pages 411 – 425, https://doi.org/10.1016/S0168-6445(03)00044-5. 5 Mironenko N.V., Alekhina I.A., Zhdanova N.N., Bulat S. A. ‘Intraspecific variation in gamma -radiation resistance and genomic structure in the filamentous fungus Alternaria alternata: a case study of strains inhabiting Chernobyl reactor no. 4’, Ecotoxicol Environ Saf. 2000;45(2):177-187. doi:10.1006/eesa.1999.1848.
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