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Evolution - Jovian subnebula due to the greater...

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Evolution Since Callisto and Ganymede are structurally similar to Titan, it is unclear why their atmospheres are insignificant relative to Titan's. Nevertheless, the origin of Titan's N 2 via geologically ancient photolysis of accreted and degassed NH 3 , as opposed to degassing of N 2 from accretionary clathrates , may be the key to a correct inference. Had N 2 been released from clathrates, 36 Ar and 38 Ar that are inert primordial isotopes of the solar system should also be present in the atmosphere, but neither has been detected in significant quantities. [31] The insignificant concentration of 36 Ar and 38 Ar also indicates that the ~40 K temperature required to trap them and N 2 in clathrates did not exist in the Saturnian subnebula . Instead, the temperature may have been higher than 75 K, limiting even the accumulation of NH 3 as hydrates . [32] Temperatures would have been even higher in the
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Unformatted text preview: Jovian subnebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH 3 inventory accreted by Callisto and Ganymede. The resulting N 2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood. [32] An alternative explanation is that cometary impacts release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter . That could erode the atmospheres of Callisto and Ganymede, while the cometary material would actually build Titan's atmosphere. However, the 2 H/ 1 H (i.e., D/H) ratio of Titan's atmosphere is 2.3±0.5×10 −4 , [31] nearly 1.5 times lower than that of comets . [30] The difference suggests that cometary material is unlikely to be the major contributor to Titan's atmosphere....
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