The moons of Saturn and Jupiter
vast oceans of water provide a theoretically ample environment for life. However, it is Titan that has an atmosphere most suited to harbouring life. 12 The final variable of habitability is the delicate matter of the moon’s axial tilt; 13 this potentially can solely delineate whether a moon has a good environment for life. The Earth is tilted at an angle of 23.5° relative to our orbital plane causing moderate seasons. Titan's orbital tilt with respect to the sun is very close to Saturn's axial tilt (about 27°). This means that the direction of incoming sunlight is driven almost entirely by Titan's day-night cycle and Saturn's year cycle. The axial tilt causes seasons and weather formations on Titan. Seasonal weather changes include larger hydrocarbon lakes in the northern hemisphere during the winter, decreased haze around the equinoxes due to changing atmospheric circulation, and associated ice clouds in the South Polar regions. 14 We know this for certain because the Cassini spacecraft was able to picture clouds and weather formations in Titan’s thick atmosphere. Titan may lack hydrothermal vents that could potentially kickstart life; however, its thick clouds could do the job just as well with lightning . Although no evidence of lightning activity has yet been observed on Titan, computer models suggest that clouds in the moon's lower troposphere can accumulate enough charge to generate lightning from an altitude of roughly 20 km. 15 Despite the absence of any know hydrothermal vents, the presence of lightning in Titan's atmosphere would favour the production of organic materials on its surface making Titan’s surface an even more viable home of life. On the other hand, the slight axial tilt of Europa and Enceladus (a maximum of 1.5°) 16 excludes the chance of seasons forming on the moons. However, as the habitable environment for life would be far from the surface for both of these moons, the lack of seasons would not be as important. To conclude, it is Titan, Europa and, Enceladus which are the most likely to harbour life, therefore excluding Earth, they are the space oddities of our solar system. Each are unique and have the potential to harbour life of completely different kinds we are used to. In Europa and Enceladus, it is probably in the formof single celled organisms nurtured in the hydrothermal vents in thewarmer pits of the oceans. However, it will be long before we are certain of this. The subsurface oceans are difficult to access; submersible probes have been proposed which would drill through the icy surface. However, plans were scrapped in a bid to prevent contamination of the subsurface oceans 17 (also there is a risk of the detection of Earth organisms from the spacecraft instead of native life). In Titan it is the organicmaterial produced by the energy from the lightning on its surface which is most promising for life. Excitingly, certainty of this could come sooner than later. NASA announced on the 27th June 2019 it will send a spacecraft to Titan to fly across its surface to study how life there could develop. ‘ Dragonfly ’ is scheduled for launch in 2026, arriving at Titan eight years later. The spacecraft will touch down in dune fields in themoon’s equatorial regions. From there ‘ Dragonfly ’ , effectively a flying drone the size of a Mars rover, 12 NASA Science [Online} available at https://www.nasa.gov/image-feature/jpl/seeing-titan-with-infrared-eyes. 13 Axial tilt is the angle between a planet's rotational axis at its north pole and a line perpendicular to the orbital plane of the planet. 14 Lakdawalla, E. (2004) Titan: Arizona in an Icebox? at The Planetary Society https://www.planetary.org/blogs/emily-lakdawalla/2004/20040121-titan-arizona-in-an-icebox.html. 15 Chow, D. (2010). Titan's Thunder Could Point to Alien Lightning at https://www.space.com/8381-titan thunder- point-alien-lightning.html 16 Bills, B. G. (2005) ‘ Free and forced obliquities of the Galilean satellites of Jupiter ’, Icarus 175: 233-47 17 Preventing the Forward Contamination of Europa. (2000) National Academy of Sciences Space Studies Board. Washington (DC): National Academy Press.
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