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Titan's atmosphere contains few tenths of a percent of hydrogen.

This seems odd to me, as Earth with a 4x greater escape velocity cannot retain hydrogen or even helium in its atmosphere.

Titan is 3x colder then Earth, but this shouldn't be anywhere near enough to retain hydrogen in significant quantities.

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How does Titan have Hydrogen in it's atmopshere?

Hydrogen is a trace gas in Titan's atmosphere (0.099% per the wikipedia article on Titan's atmosphere).

Titan's scant amount of hydrogen in its atmosphere comes from photolysis of methane. See, for example Wilson, Eric H., and S. K. Atreya. "Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere." Journal of Geophysical Research: Planets 109.E6 (2004). Some of this hydrogen reacts with other compounds in Titan's atmosphere, but some does escape. This may eventually lead to Titan becoming Triton-like (i.e., a thin atmosphere and a frozen surface). See, for example, Lorenz, Ralph D., Christopher P. McKay, and Jonathan I. Lunine. "Photochemically driven collapse of Titan's atmosphere." Science 275.5300 (1997): 642-644.

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    $\begingroup$ It seems like the key is that Titan is continuously producing hydrogen. It just has to be produced faster than it is lost into space or reacted into heavier compounds, over a long enough period, for a surplus to be detected by spectroscopy. $\endgroup$
    – alexw
    Dec 3, 2022 at 18:17
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    $\begingroup$ @alexw Of course Titan is continuously producing hydrogen, just as is the Earth. On Titan hydrogen is produced by photolysis of methane. On Earth, it's by photolysis of water. Titan has seas and lakes that contain liquid methane and ethane. Earth has oceans, seas and lakes that contain water. Methane plays a similar role on Titan that water plays on Earth. When methane vapor rises high enough in Titan's atmosphere, it is subject to photolysis. The same happens with water vapor in Earth's atmosphere. $\endgroup$ Dec 3, 2022 at 18:47
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The chemistry of Titan's atmosphere is actually quite complex. Different reactions occur at different altitudes and there are a wide range of minor constituents in the atmosphere. One of those reactions is the photochemical breakup of methane molecules by UV from the Sun generating reactive free radicals which then react with other constituents of the atmosphere. This results in the formation of some hydrogen as well as an orange tinted organic smog of assorted hydrocarbons and nitrogen compounds. The hydrogen generated will slowly escape into space but is continually replenished. The 0.1% level being the equilibrium point where losses balance gains.

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  • $\begingroup$ Are there estimates on when this temporary stability will be exhausted? $\endgroup$
    – user6587
    Nov 30, 2022 at 20:13
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    $\begingroup$ @Slarty Titan's atmosphere is mostly nitrogen, about 95%. Methane on Titan plays a role similar to that of water on Earth. $\endgroup$ Dec 1, 2022 at 0:11
  • $\begingroup$ @David Hammen yes you are right my bad I will correct it - thx. $\endgroup$
    – Slarty
    Dec 1, 2022 at 20:40
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    $\begingroup$ @6587 Not that I'm aware of, but I imagine it would be a very long time as Titan's atmosphere is very extensive and has a significant amount of methane in it. As the temperature and pressure are close to methanes triple point there are also lakes / seas of liquid methane on the surface which would evaporate at lower pressure. $\endgroup$
    – Slarty
    Dec 1, 2022 at 20:42
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Titan's atmosphere contains almost 5% methane. The current understanding based on the Voyager and Cassini readings is that hydrogen is released in atomic and molecular form from the photochemical conversion of methane along with trace amount of more complex, less saturated hydrocarbons.

It is also found that the hydrogen is disappearing at a much rapid rate. Based on the data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer, scientists developed a model of densities of hydrogen in different parts of the atmosphere and the surface. It was predicted that hydrogen will be distributed fairly equally at all parts of atmosphere but according to the new model, they found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escaping out of the upper atmosphere. They are currently not sure where did the hydrogen escaped but a plausible explanation is that the hydrogen is reacting with the water ice on Titan surface and creating a film of an hydrocarbon rich organic compound of at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth making it hard to detect the composition of the film.

Reference

  1. Darrell F. Strobel, Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions, Icarus, Volume 208, Issue 2, 2010, DOI: 10.1016/j.icarus.2010.03.003.
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