The moons of Saturn and Jupiter
temperature. The oxygen within the atmosphere is also essential for life. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being Saturn's moon Titan. 8 It is an extraordinary moon that is more massive than Mercury with an atmosphere made mostly of nitrogen , like Earth’s. Tita n has clouds, rain, rivers, lakes and seas of liquid hydrocarbons likemethane and ethane with towering mountains of solid ice. The Cassini spacecraft (tasked with monitoring Saturn and its moons) took numerous gravity measurements of Titan revealing that the moon is hiding an underground ocean of liquid water (likely mixed with salts and ammonia). Titan’s subsurface water could potentially be a suitable spot to nurture microbial life, while its surface lakes and seas of liquid hydrocarbons could conceivabl y harbour life that uses different chemistry than we’re used to. 9 Titan is like no other moon in our solar system; it was even formed in an entirely different way. Titan’s atmospheric nitrogen ratio (94.2%) 10 suggests the moon’s building blocks formed earl y in the solar system's history, in the same cold disk of gas and dust that formed the Sun (called the protosolar nebula), rather than forming in the warmer disk of material that Saturn later formed from (called the Saturn sub- nebula). This atmosphere makes Titanmore like a planet than a moon (Figure 2). Amazingly, Titan’s air
Figure 2: Infrared images of Titan from Cassini – Normal image of Titan at centre surrounded by infrared images of different faces of Titan expose the previously hidden weather patterns on the moon. It is very much a living moon.
is even dense enough that a human could theoretically walk around on its surface (providing one had an oxygenmask and protection from sub minus 150 ˚ C temperatures). However, an atmosphere is a rare characteristic. It is where most moons do not fulfil the full conditions for habitability. Ganymede and Callisto, the two most massive moons of Jupiter lack any atmosphere that provides the environment to support life. Astronomers using the Hubble Space Telescope found evidence of only a thin oxygen atmosphere on Ganymede in 1995. 11 Similarly, Callisto, the most heavily cratered object in the solar system, has a thin atmosphere that leaves it vulnerable to even the smallest of meteors causing them to scar its surface as meteorites. Without an atmosphere nor a sub-surface ocean there is little chance that these two moons could harbour life. Whilst Europa and Enceladus also have a thin atmosphere, their 8 Wikipedia [online] available at https://en.wikipedia.org/wiki/Habitability_of_natural_satellites#Atmosphere. 9 NASA Science [online] available at https://solarsystem.nasa.gov/moons/saturn-moons/titan/overview/. 10 Catling, David C. & Kasting, James F. (2017). Atmospheric Evolution on Inhabited and Lifeless Worlds . Cambridge 11 Hall, D.T. & Feldman, P.D. et al. (1998). ‘ The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede ’, The Astrophysical Journal 499.1: 475-81
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