The moons of Saturn and Jupiter
Over time in a two-body system, the initial elliptical orbit decays into a circular orbit (tidal circularization). However, sustained tidal heating occurs when the elliptical orbit is prevented from circularizing owing to additional gravitational forces from other bodies that keep tugging the object back into an elliptical orbit (for example, the other moons of Jupiter). Owing to the many small gravitational forces (G) exerted on the moons by each other, they remain balanced in an elliptical orbit (high eccentricity, e), and thus they have some of the greatest tidal energies in the solar system, as demonstrated by the equation for tidal energy above. This is the reason why Io is the most volcanically active body in the solar system. Volcanic activity can be the perfect ingredients for life, after all, the majority of scientists believe life on earth began in hydrothermal vents underneath the ocean. 4 However, Io is an extreme example, constant volcanic activity and intense radiationmake Io an unlikely destination for life. It is the larger moons with more moderate orbital energies that cultivate a more reasonable surface temperature. Jupiter’s moon Europa and Saturn’s moon Enceladus (discovered a century later than the others in 1789 by WilliamHerschel) have tidal forces that provide the source of a moderate temperature under their icy surfaces. This temperature is warm enough to sustain a hidden
ocean of liquid water under its icy surface. 5 Scientists are certain of this fact on Enceladus where in 2005, NASA’ s Cassini spacecraft discovered that icy water particles and gas gush from themoon’s surface at approximately 800 miles per hour (400 meters per second) (Figure 1). This liquid was even sampled by the passing Cassini spacecraft. 6 It was
Figure 1: NASA’s Cassini spacecraft captured this image of Enceladus on Nov. 30, 2010. It shows the jets of water gushing from the surface.
revealed to contain silica nanograins , which can only be generated where liquid water and rock interact at temperatures above 90 degrees Celsius. In this way, hydrothermal vents are almost certainly under the surface of Enceladus. The vents have the ability to provide a source of heat, liquid water, minerals and, chemicals; all the right ingredients for life. For this reason, Enceladus and Europa have the desired amount of tidal heating that can create a habitable zone under their surface despite being 552 million kilometres outside of the CHZ. 7 An atmosphere is considered by astrobiologists to be vital in developing prebiotic chemistry, sustaining life and for surface water to exist. It blocks almost all of the Sun's dangerous rays from reaching the surface. It traps heat, making the surface a comfortable 4 MIT School of Engineering [Online} available at https://engineering.mit.edu/engage/ask-an-engineer/how- did-life-on-earth-begin/. 5 "Frequently Asked Questions about Europa". NASA. 2012. Archived from the original on 28 April 2016. NASA [online] available at https://www.nasa.gov/. 6 "Deepest-Ever Dive Through Enceladus Plume Completed". Jet Propulsion Laboratory. October 28, 2015. 7 Not a raw photo (enhanced brightness): NASA [online] available at https://www.nasa.gov/.
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