Space

Why is the north pole of Saturn's moon Titan a veritable land of lakes, while its south pole is relatively dry? Changes in Saturn's orbit over tens of thousands of years – rather than seasonal variations lasting a few years – may be to blame, giving any potential life in the lakes a longer time to evolve.

The Cassini spacecraft has found numerous hydrocarbon lakes on Titan since it arrived at Saturn in 2004. But as it mapped the moon, it discovered far more lakes on the north pole than the south.

Since the north pole was experiencing winter during Cassini's observations, researchers suggested that the lakes might be a seasonal phenomenon, filling with methane rain in the winter, then evaporating in the summer.

But a new study led by Oded Aharonson of Caltech argues the lakes are much longer lived, moving between the poles over timescales set by gradual changes in Saturn's orbit.

The researchers mapped the distribution of liquid-filled lakes around the polar regions and found that they cover an area 25 times larger in the north than in the south. "The sheer number of filled lakes in the north compared to the south is amazing," says Erika Barth, a planetary scientist at the Southwest Research Institute in San Antonio, Texas, who was not involved in the study.

Too fast

Strangely, the number of dry lake basins is also greater in the north. If the lakes were simply moving back and forth between the poles over the course of a year on Titan, which lasts 29.5 Earth years, the number of dry basins should be the same on each pole.

The depth of the lakes also argues against a purely seasonal explanation. Research suggests that only 1 metre of liquid evaporates from the lakes during Titan's summers – not enough to completely drain or fill the 200-metre-deep basins in a single season.

Instead, the researchers say the lakes wax and wane on cycles lasting about 45,000 years. These are determined by how elongated Saturn's orbit around the sun is – a measure called eccentricity – and the direction of its tilt as it orbits – a measure called precession. Both factors change over time because of the gravitational influence of other bodies in the solar system.

Intense summers

At the moment, Saturn's orbit is not a perfect circle – it comes closer to the sun on one side of its orbit than on the other. The planet is also tilted by about 27 degrees relative to the plane of its orbit.

When the planet is closest to the sun, its south pole – and that of Titan – is pointed towards the star. The researchers calculate that the atmosphere above Titan's southern hemisphere gets about 24 per cent more incident solar radiation during its summers.

They say that might lead to differences in the amount of evaporation and precipitation between the hemispheres, explaining the imbalance in the distribution of lakes. On Earth, similar orbital changes seem to serve as "pacemakers" for shifts in climate.

If the orbital explanation is correct, Titan's north pole hasn't always boasted more lakes than the south. Because Saturn and its moons slowly wobble like a top as they orbit the sun, sometimes Titan's north pole should experience shorter, more intense summers than the south. That should deliver more lakes to the south pole, a situation that Aharonson said last occurred about 30,000 years ago.

Climate probe

If the lakes move between the poles every few tens of thousands of years, why don't we see the same number of dry lake basins in both hemispheres? Aharonson says sludgy material falling out of the atmosphere as a result of sunlight-triggered chemical interactions may have concealed some of the southern basins over the past 30,000 years.

Tetsuya Tokano of the University of Cologne in Germany says the idea that the distribution of lakes on Titan might be used to probe the history of the moon's climate is "amazing".

But he says there is not yet enough evidence to prove that this hypothesis is correct, a fact that Aharonson acknowledges. "At the moment, there is not a definitive mechanism for going from solar variations in sunlight to [rates of evaporation and precipitation]," Aharonson told New Scientist. "We don't know exactly how that works yet."

Incubation time

The idea will soon be tested, however. In August, the northern hemisphere of Saturn – and thus Titan – began to be illuminated by the sun for the first time in 15 years. "We will hopefully see within the next few years of Cassini observations whether the lake distribution remains constant on timescales of years and where and when it rains in the north polar region," Tokano told New Scientist.

If the lakes do in fact last longer than a few years, that would be good news for the potential development of life on the moon.

"Titan's lakes are pretty cold, so the prebiotic chemistry or organic chemistry in general should proceed at a much slower pace than on Earth," says Tokano. "Should the lakes dry up every 10 years or so, this may simply be too short for any relevant chemical evolution under Titan's condition. Tens of thousands of years are better than 10 years... for astrobiology."

Journal reference: Nature Geoscience (DOI: 10.1038/NGEO698)

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