Life in the solar system
In recent decades, the scientific consensus has been that the most likely places to find life in the Solar system are within the icy moons of the outer planets. Once thought to be inert balls of ice, there is increasing evidence that many of these worlds have subsurface oceans with complex chemistries. The moons now termed by NASA as ‘ocean worlds’ were originally thought to be too far from the sun, too cold, and too small to support liquid water, let alone life. It is now known that there is a significant heat source within many of the outer planet moons resulting from friction caused by tidal effects, thanks to the strong gravitational interactions from the parent planet and the other moons, which compress and stretch their interiors, causing heating. In the case of Io, this causes volcanism, but the majority of bodies in the outer solar system are comprised of ice, not rock. With ice being the primary component material of these objects, the icy crust remains but the interior melts to form a subsurface ocean as a mantle, surrounding a rocky core. The two prime candidates in the search for life amongst the outer planets are Europa (around Jupiter) and Enceladus (around Saturn). There have been plumes of water observed emanating from both, with the plumes from Enceladus having been flown through and chemically analysed by the Cassini spacecraft, while recent reanalysis of data from the Galileo probe confirmed the presence of plumes at Europa. 12 There are also a number of othermoons suspected to have subsurface oceans, including Titan, Triton, Ganymede and Callisto. Little is known about the chemical composition of the subsurface ocean of Europa, other than it is rich in salts. This was determined by magnetic measurements which imply a widespread electrically- conductive layer, such as a saline ocean. The presence of salts improves the chances of life existing on Europa, since it means minerals which are important for biochemistry are present, and also provides a potential chemical energy source. 13 Although data is lacking on the composition of Europa’s oceans, there are direct measurements of the composition of Enceladus’s subsurface water. The Cassini spacecraft flew through the plumes of water being ejected from Enceladus, and collected mass spectrometer readings on component molecules. The ejected water was found to be salty, and accompanied by carbon dioxide, methane, hydrogen, and ammonia gases. These are all biologically relevant, showing there is carbon and nitrogen available to any potential organisms, while the presence of hydrogen and methane shows the possibility of chemical energy sources. Even more interesting was the discovery of organic compounds, such as complex hydrocarbons, including benzene. On Earth, these are primarily biogenic in origin, although they can be formed abiotically. Amines were also found, these are precursors to amino acids, the building blocks of proteins. It now seems clear that the environment is habitable and the prerequisite compounds for life to begin are present on Enceladus. 14 Titan, the largest moon of Saturn, has been of particular interest for habitability for many years. It is a prime candidate for life for several reasons, having a substantial atmosphere, a subsurface water ocean, and most strikingly surface lakes and a weather cycle based not on water, but on ethane and methane. Complex organic compounds, such as polyaromatic cyclic hydrocarbons are formed in the upper 12 https://www.nasa.gov/press-release/old-data-reveal-new-evidence-of-europa-plumes. Consulted 14/08/2020 . 13 Kivelson M.G., Khurana K.K., Russell C.T., Volwerk M., Walker R.J. , Zimmer C. ‘Galileo magnetometer measurements: a stronger case for a subsurface ocean at Europa’, Science 2000;289(5483):1340-1343. doi:10.1126/science.289.5483.1340. 14 https://www.hou.usra.edu/meetings/oceanworlds2019/pdf/6005.pdf. Consulted 14/08/2020.
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