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I just read the popular press version of a new paper, and now I have a question about Titan.

Here are some quotes from the pop article:

A rocky, ocean-bearing planet with more than 0.1 percent methane in its atmosphere should be considered a potentially inhabited planet, the researchers say. And if the atmospheric methane reaches levels of 1 percent or more? In that case, “potentially” doesn’t cut it—such a world would “likely” be home to alien life.

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Thus, methane and carbon dioxide together, unaccompanied by carbon monoxide, on a rocky, ocean-bearing world would best be interpreted as an airtight sign of anoxic life.

Now, some information about Titan from the Wikipedia page about Titan:

Titan's atmosphere
    Stratosphere:
        98.4% nitrogen
        1.4% methane
        0.2% hydrogen
    Lower troposphere:
        95.0% nitrogen
        4.9% methane
        -OR, depending on who you ask-
        97% nitrogen
        2.7% methane

The density of Titan is consistent with a body that is about 60% rock and 40% water.

There are trace amounts of other hydrocarbons, such as [...] carbon monoxide [...].

If I understand correctly, 'trace amounts' of CO is in line with the requirements stated in the Catling paper.

So, is Titan covered in microbes, or what? Since the article didn't mention Titan, I presume I missed something. What was it?

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    $\begingroup$ I believe the paper was referring to "oceans" of water; Titan is far too cold for that. The temperature of Titan disqualifies it from the argument presented in the paper (not that it is impossible that there is life on Titan, either in sub-surface ocean or in some exotic cold-tolerant form on the surface). $\endgroup$
    – antlersoft
    Commented Jan 25, 2018 at 16:33
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    $\begingroup$ The underlying article refers to a planet whose atmosphere contains free oxygen, which alone might be a sign of life. (But there are non-biological sources of free oxygen. For example, a planet without life that is undergoing a runaway greenhouse or moist greenhouse will exhibit a strong oxygen signature, at least until its oceans evaporate/boil to nothingness.) Titan has close to zero free oxygen, so this paradigm doesn't apply. $\endgroup$
    – David Hammen
    Commented Jan 25, 2018 at 16:57

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The underlying assumption in the paper is that the planet has water in liquid form.

The paper suggests that finding both CO2 and CH4 in the atmosphere of a rocky planet in the "habitable zone" (in which water can exist in liquid form) would be hard to explain, without life. Thus one doesn't need to detect atmospheric oxygen. If you can detect both CO2 and CH4, this should be taken as evidence of anoxic life (probably in the form of microbes)

Titan has CH4, but it doens't have much CO2 and there is no liquid water. Titan is not in the habitable zone. On Titan, H2O is a forms part of the "rocks", and the oceans are made of hydrocarbons. Titan is not a good location to look for life as it doesn't have free liquid water. We have a photo from Titan's surface and it isn't covered in Microbes

enter image description here

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    $\begingroup$ I see no microbes, confirmed. $\endgroup$
    – Scott Weaver
    Commented Jan 25, 2018 at 20:44

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