[LINK] Two Titan links

[rfmcdonald]

First, via james_nicoll, is the abstract for a paper describing the conditions that would support worlds with Titan-like environments of a cryogenic nitrogen-methane atmosphere and liquid methane.

Titan has a surface temperature of 94 K and a surface pressure of 1.4 atmospheres. These conditions make it possible for liquid methane solutions to be present on the surface. Here, we consider how Titan could have liquid methane while orbiting around an M4 red dwarf star, and a special case of Titan orbiting the red dwarf star Gliese 581. Because light from a red dwarf star has a higher fraction of infrared than the Sun, more of the starlight will reach the surface of Titan because its atmospheric haze is more transparent to infrared wavelengths. If Titan was placed at a distance from a red dwarf star such that it received the same average flux as it receives from the Sun, we calculate the increased infrared fraction, which will warm surface temperatures by an additional not, vert, similar10 K. Compared to the Sun, red dwarf stars have less blackbody ultraviolet light but can have more Lyman α and particle radiation associated with flares. Thus depending on the details, the haze production may be much higher or much lower than for the current Titan. With the haze reduced by a factor of 100, Titan would have a surface temperature of 94 K at a distance of 0.23 AU from an M4 star and at a distance of 1.66 AU, for Gliese 581. If the haze is increased by a factor of 100 the distances become 0.08 and 0.6 AU for the M4-star and Gliese 581, respectively. As a rogue planet, with no incident stellar flux, Titan would need 1.6 W/m2 of geothermal heat to maintain its current surface temperature, or an atmospheric opacity of 20× its present amount with 0.1 W/m2 of geothermal heat. Thus Titan-like worlds beyond our solar system may provide environment supporting surface liquid methane.

Second is a post by the Planetary Society blog's Emily Lakdawalla, "Are there more Titans than Earths in the Milky Way?", which expands on the above paper to wonder if Titans are more common than Earths.

Where might Titan-like worlds exist? One tantalizing possibility is that there may be lots of them orbiting red dwarf stars. Red dwarfs are much more common than Sun-like stars. Less luminous than our Sun, they're poor candidates for systems that might support water-based life, because in order for a planet to be close enough to a red dwarf to be warm enough for liquid water, it would be close enough to be tidally locked, resulting in overheating of the star-facing hemisphere and overcooling of the star-shunning hemisphere. But they're great candidates to have planets that are as cool as Titan.

Red dwarfs put out more of their light as infrared than the Sun does. Since Titan's haze is transparent to infrared, that means more of a red dwarf's radiation would reach our theoretical Titan-ish planet's surface than it would if it orbited the Sun; Titans can be farther from red dwarfs than they would be from Suns and have the same surface temperature.

[. . .]

With a range of possible red dwarf star activities, they found that Titan-sized worlds could have Titan-like conditions at distances of 0.24 to 0.52 astronomical units from their stars. What if the world is big enough to have geothermal heat -- what if you have an Earth-sized world around these stars? It doesn't make much of a difference, it turns out; production of internal heat increases the distance that the planet could be from the star and still have Titan-like surface conditions by about 5%. With a large enough internal heat flux, there could even be Titan-like "rogue planets" wandering around the galaxy.

With a range of different amounts of haze production, they wound up with a pretty large "liquid methane habitable zone" around red dwarf stars -- 0.084 to 0.23 AU for less active stars, and 0.63 to 1.66 AU for a more active one like Gliese 581 -- a star that we already know supports several giant planets. There could easily be more Titan-like moons orbiting those planets.

Or these planets themselves could support Titan-like conditions.

This all has implications for possible life: if the solvent liquid methane can support life of one kind or another, liquid methane might be a more common solvent for living things than water.