Themoonsof Saturnand Jupiter: a spaceoddity?
Darius Joshi
Jupiter and Saturn have a total of 141 confirmed moons. However, the definition of a moon is ambiguous. It is often defined as ‘ an astronomical body that orbits a planet or minor planet ’; 1 yet, if this was the case, this essay would have to cover an indefinite number of moons all of different shapes and sizes – some as large as small planets, others merely a couple metres in diameter. For ease of communication, I will only consider the five largest and most intriguing. Jupiter’s four largest moons (Ganymede, Callisto, Io, and Europa – often described as the Galilean moons) were discovered in 1610 by Galileo Galilei, while Saturn’s largest moon (Titan) was discovered 45 years later by Christiaan Huygens. All were discovered prior to the massive planets of Uranus and Neptune. The characteristic that intrigues me most about these moons is that they all share seemingly habitable environments despite existing outside the Circumstellar Habitable Zone and being more than 790 million kilometres from the sun. I will analyse the extent of their habitability by focussing on four variables: the presence of liquid water, tidal effects, atmosphere, and axial tilt. In astronomy and astrobiology, the Circumstellar Habitable Zone (hereafter CHZ) is the range of orbits around a star within which an astronomical body can support liquid water given sufficient atmospheric pressure. 2 The bounds of the CHZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. The CHZ is also known as the Goldilocks zone; the metaphor describes an environment that is ‘ just right ’ . However, this theory has faced a lot of criticism, mainly owing to its assumption that the only source of radiant energy for an astronomical object is from the Sun – it neglects the effects of tidal forces . The five moons in question orbit the two most massive planets in the solar system. These planets also have the most moons of any in the solar system (Jupiter, 79, and Saturn, 62). Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: tidal energy is dissipated as heat in either the surface ocean or interior of a planet or satellite. 3
Ė Tidal = Tidal Energy
R = Satellite’s Mean Radius (high)
Im(k 2 ) = Efficiency of body dissipation within satellite
n = Mean Orbital Motion
G = Gravitational Forces (low)
e = Eccentricity (high)
1 Wikipedia [online] available at https://en.wikipedia.org/wiki/Natural_satellite. 2 Huang, Su-Shu. (1959 ) ‘Ocurrence of life in the Universe’, American Scientist 47.3: 489-93 3 Wikipedia [online] available at https://en.wikipedia.org/wiki/Tidal_heating.
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