It’s late summer not just here for us northern hemisphere dwellers, but nearly one billion miles away at Titan’s south pole too.
But what does summer mean on a world that never gets much warmer that -300 degree Fahrenheit?
NASA’s Cassini orbiter has photographed fog in the south polar regions. This conjures up visions of misty valleys under an orange sky. But it means a lot more for astronomers. The fog may show that Titan really does have liquid methane presently flowing across the surface.
Titan is the only moon in our solar system with a substantial atmosphere, as well as open bodies of some kind of liquid near the north and south poles. It has been assumed that methane rain would explain the streams and erosion seen on Titan. But are the black lakes really standing pools of methane on Titan now?
Astronomers don’t positively know what the mirror-smooth dark material in the polar lakes really is. At least some of it could be ethane. Over geological time ethane could accumulate to form lakes on Titan. Sunlight breaks down atmospheric methane to form ethane much in the same way sunlight breaks down car exhaust fumes to form smog in big cities. The ethane would drizzle out of the skies, pool, and eventually sink into the crust.
However, dwarf planet sleuth Mike Brown of Caltech says that the fog seen on Titan is compelling evidence for methane lakes instead. They would show that Titan’s methane hydrological cycle is truly analogous to the water cycle on Earth in that it deposits liquid methane on the moon’s surface.
Fog forms whenever air is fully saturated at 100 percent humidity. On Earth, morning fog can be caused by ground-level air cooling overnight. We also get fog when warm wet air passes over the cold ground. As the air cools, the water condenses.
But on Titan, as on Earth, the only way to add humidity is to evaporate liquid. Only methane – not ethane – could evaporate off Titan’s surface. This would support the notion of a rain cycle where methane droplets make it all the way down to the ground.
Brown says that the low level fog seen by Cassini can best be explained by having the air cooled just a little by contacting something colder -- ideally a pool of evaporating liquid methane.
Apparently, the southern summer polar rainy season that Cassini has photographed over the past few years has fed lakes of liquid methane at polar latitudes. Brown says the methane must be slowly evaporating back into the atmosphere where it will eventually drift to the northern pole where he predicts another stormy summer season will arise. But we’ll have to be patient. Northern summer solstice is not until 2016.
Cassini scientists have requested that NASA provide funds to extend the mission to 2017. Titan is the only other body in the solar system where we can watch how a hydrological cycle evolves seasonally.
The irony is that any exotic cyrolife on Titan would have such a slow metabolism that the waxing and waning of the seasons might transpire so comparatively rapidly that it is never noticed by surface dwellers.
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