Thursday, April 21, 2011

Worlds Like Titan

A reddish colour dominated everything, although swathes of darker, older material streaked the landscape. Towards the horizon, beyond the slushy plain below, there were rolling hills with peaks stained red and yellow, with slashes of ochre on their flanks. But they were mountains of ice, not rock. An ethane lake had eroded the base of the hills, and there were visible scars in the hills' profiles.
- Stephen Baxter, Titan

In human terms, Titan is a cold and frigid world with an average surface temperature of minus 180 degrees Centigrade and a surface atmospheric pressure that is 1.45 times the atmospheric pressure at sea-level on Earth. These conditions allow for the existence of liquid methane on Titan’s surface in the form of lakes and seas. A large number of these lakes and seas can be found in Titan’s north polar region and the largest of them is named Kraken Mare - a large sea of liquid methane and ethane that is estimated to be similar in size to the Caspian Sea on Earth. Titan is also characterised by a thick atmosphere which extends hundreds of kilometres above its surface and a global atmospheric haze layer that is transparent to infrared wavelengths but opaque to ultraviolet and visible wavelengths. In this article, I will be considering how Titan will be like if it were to orbit a red dwarf star instead of the Sun and also if it were a rogue planet wandering in the dark depths of interstellar space.

The global atmospheric haze layer of Titan blocks incoming ultraviolet and visible light but allows infrared radiation from the surface to freely escape into space, thereby creating an anti-greenhouse effect. In comparison, a greenhouse effect allows visible light in but blocks outgoing infrared radiation. The clouds in the atmosphere of Titan rain liquid methane and ethane, completing a ‘methanological cycle’ that is akin to the hydrological cycle on Earth. Benner et al. (2004) were the first to suggest that liquid methane on Titan could potentially be the basis for life there, playing the same role as water does for life on Earth. Methane-based life on Titan could consume organic molecules similar to Earthly life, but they would probably inhale hydrogen instead of oxygen and exhale methane instead of carbon dioxide. The discovery of any methane-based life on Titan will have incredibly interesting implications. In this article, it will be assumed that methane-based life on cryogenic Titan-like worlds is a possibility. Hence, the term liquid methane habitable zone (LMHZ) will correspond to Titan-like worlds while the term liquid water habitable zone (LWHZ) will correspond to habitable Earth-like worlds.

Suddenly I was aware of something new. The air in front of me had lost its crystal clearness… I was aware of a faint taste of oil upon my lips, and there was a greasy scum upon the woodwork of the machine. There was no life there. It was inchoate and diffuse; extending for many square acres and then fringing off into void. No, it was not life. But might it not be the remains of life? Above all, might it not be the food of life, a monstrous life, even as the humble grease of the ocean is the food for the mighty whale?
- Arthur Conan Doyle, The Horror of the Heights

Red dwarf stars have much lower masses than our Sun and they comprise the vast majority of stars. Being much more numerous that Sun-like stars, red dwarf stars are particularly interesting in the search for potentially habitable worlds; both in the LMHZ for Titan-like worlds and in the LWHZ for Earth-like worlds. The much lower luminosities of red dwarf stars mean that a planet orbiting a red dwarf star will have to be located much closer in just to receive the same amount of radiation as if it were located around the Sun. For a habitable Earth-like planet around a red dwarf star, the LWHZ will be situated very close to the star, causing the planet to be in a tidally locked state whereby one hemisphere of the planet perpetually faces its host star. However, the LMHZ for a Titan-like planet around a red dwarf star is located much further out from the star and this gives the planet a much better chance of not being in a tidally locked state, thereby creating a less stringent condition for life to exist.

The light from a red dwarf star contains a higher proportion of infrared radiation as compared to the light from the Sun. If Titan were orbiting around a red dwarf star instead of the Sun, a greater proportion of the light from the red dwarf star will reach the surface of Titan as the atmospheric haze of Titan is transparent to infrared wavelengths. If Titan is placed at an appropriate distance from the red dwarf star such that it receives the same amount of radiation as it currently receives from the Sun, the increased infrared fraction of the incoming radiation that makes it to Titan’s surface will warm the surface by an additional 10 degrees Centigrade or so. This warming effect is based on the assumption that a Titan-like world orbiting around a red dwarf star has a haze layer that is as thick as Titan’s. However, because red dwarf stars produce a lower proportion of ultraviolet light as compared to the Sun and because red dwarf stars can also produce a greater deal of high energy radiation that is associated with flares as compared to the Sun, the haze production rate for a Titan-like world in orbit around a red dwarf star can range from being much lower to much higher than that for Titan.

A habitable Titan-like world orbiting within the LMHZ of an M4-type red dwarf star will now be investigated. The M4-type red dwarf star is assumed to have a surface temperature of 3130 degrees Kelvin and a luminosity that is 2500 times less than the Sun’s. For a Titan-like world with a haze layer thickness that is reduced by a factor of 100 in comparison to Titan’s haze layer, it will have to orbit its parent M4-type red dwarf star at a distance of 0.23 AU in order to maintain a surface temperature of minus 180 degrees Centigrade. However, if the haze layer thickness of the Titan-like world is increased by a factor of 100 in comparison to Titan’s haze layer, the planet will need to orbit its parent M4-type red dwarf star at a much closer distance of 0.084 AU in order to maintain the same surface temperature. The temperature of minus 180 degrees Centigrade is the current surface temperature of Titan and it allows for the existence of liquid methane. Therefore, within a range factor of 10000 for the haze layer’s thickness, the liquid methane habitable zone (LMHZ) for a Titan-like world around an M4-type red dwarf star varies from 0.084 AU to 0.23 AU.

Instead of orbiting around the planet Saturn in the solar system, now imagine Titan as a lone planet drifting in interstellar space, with no parent star to provide any form of light and warmth. This is the case of Titan as a rouge planet and how it might still support a surface temperature of minus 180 degrees Centigrade as it drifts in the much colder depths of interstellar space. In order to maintain such a surface temperature, Titan with its current haze layer thickness will require an average geothermal heat flux of 1.4 watts for each square meter of its surface. Nevertheless, this value is around 20 times more than the average geothermal heat flux for the Earth and although this value might be consistent with a planet that is somewhat larger than the Earth, it is not realistic for a world the size of Titan. However, if Titan’s atmosphere is 20 times thicker than its current thickness, a more plausible average geothermal heat flux of 0.1 watts for each square meter of its surface will be sufficient to maintain a surface temperature of minus 180 degrees Centigrade.

Thus, for billions of years, Titan waited… An object looking a little like a comet streaked across the sky of Titan, battering atmospheric gases to a plasma twice as hot as the surface of the Sun itself. Cooling, it fell towards the surface slush. A parachute blossomed above it.
- Stephen Baxter, Titan

If any methane-based life is discovered on Titan, it should be widespread on Titan’s surface because liquid methane is also widespread on the surface. Direct evidence from the Huygens Probe has shown that the surface of Titan at the probe’s landing site is soaked with methane and radar imagery from Cassini has revealed numerous lakes on both the northern and southern polar regions of Titan. Life on a cryogenic world which runs on a methanological cycle will be extremely interesting. This is because the discovery of methane-based life on Titan or on any other Titan-like worlds will greatly improve our understanding of the range of worlds and chemical models that might support liquid-based life.

1. Ashley E. Gilliam and Christopher P. McKay “Titan under a Red Dwarf Star and as a Rogue Planet: Requirements for Liquid Methane” (2011), Planetary and Space Science, doi:10.1016/j.pss.2011.03.012.
2. Steven A Benner et al. “Is there a common chemical model for life in the universe?” (2004), Chemical Biology, doi:10.1016/j.cbpa.2004.10.003.