The Weather On Titan: Windy With A Chance Of Methane Mist

Posted on Categories Discover Magazine

Back in 2005, the European Space Agency’s Huygens probe made a dramatic landing on the surface of Saturn’s largest moon Titan. During its descent through the dense nitrogen-rich atmosphere, Huygens sent back pictures of a complex landscape shaped by familiar forces.

The images showed hills and valleys, meandering rivers leading to complex deltas that eventually fed into lakes and seas with vast shorelines. Titan, it appeared, had many features in common with Earth.

But there is a crucial difference. Water plays a huge role in shaping the Earth’s surface through erosion, glaciation and so on. That’s because conditions are perfectly balanced to allow it to exist in liquid, solid and gaseous form at the same time, the so-called “triple point”.

But on Titan, where the temperature is -179 degrees Centigrade, water is a rock-hard frozen solid. Instead, it is methane that has shaped Titan’s surface, carving out rivers and deltas and shorelines because conditions there are curiously balanced around methane’s triple point. So it exists as a gas in the atmosphere, as liquid in the rivers and oceans and as a solid in ices.

Rain On The Plane?

All this suggests Titan must experience complex weather patterns with wind, rain, mist and many of the other meteorological phenomena we experience on Earth. Scientists would dearly love to be able to model this weather, perhaps even predict its effects on the surface. But there is a problem.

Meteorologists have long known that seas and lakes on Earth can themselves influence the weather, creating their own microclimates. That’s because the temperature of water changes more slowly than on land. Around the Great Lakes on Earth, for instance, this leads to cooler springs and warmer falls. Evaporation also causes greater rainfall downwind.

Until now, nobody has simulated how Titan’s lakes and seas influence its weather. But that now changes thanks to the work of Audrey Chatain at the Southwest Research Institute in Boulder, Colorado, and colleagues, who have simulated the 3D weather patterns around Titan’s lakes for the first time. “Like water lakes on Earth, these methane lakes on Titan likely profoundly affect the local climate,” they say.

In the past, planetary scientists have attempted to gauge the influence of lakes by looking at wind patterns in two dimensions. This work suggests that large bodies of liquid ought to stimulate “lake breezes, that spread out onto the land.

But these simulations ignored the role of the third dimension, which is an important factor because as warmer air rises, cooler air is sucked in behind it, creating convection cells.

So Chatain and co decided to simulate these effects on Titan using both ideal lakes and models of real lakes on the moon’s surface. This showed that 2D wind models overestimate some properties of lake breeze, such as the extent it travels over land, and underestimates other properties, such as the downward movement of air behind the breeze.

Chatain also modelled specific lakes on Titan such as a 50km-wide lake called Oneida Lacus and other lakes around it to see how the local weather patterns interact. They found that bigger lakes generate stronger winds and that their individual weather patterns can merge to form a local microclimate with its own complex weather patterns.

Moon Mist

These winds have an important influence on evaporation into the atmosphere. Chatain and estimate that the lake evaporates at a rate of 6cm per Earth year (on Earth, the rate is measured in meters per year). “Simulations of several real-shaped lakes located at a latitude of 74°N on Titan at the spring equinox show that some sections of lakes might accumulate enough methane vapor to form a thin fog,” say the team.

Indeed, some images from Huygens, which landed at the edge of a lake, suggest the presence of mist. In one image, there is even a dew drop.

All this has significant similarities to the weather on Earth. Titan’s atmosphere is denser than Earth’s and windspeeds are lower because there is less energy in the atmosphere at that distance from the Sun. But this and other work suggests that the weather must be spectacular, including wind, rain, fog, sleet, waves, maybe even rainbows.

Clearly there is much to learn about this mysterious moon, not least of which is where all the methane comes from. That’s a puzzle because it is broken down by sunlight and so should all have disappeared over the lifetime of the Solar System. Then there is the question of how Titan’s temperature is carefully balanced at methane’s triple point — what kind of feedback mechanisms must be at play? And finally, what kind of complexity is possible in a system like this, at potentially superconducting temperatures.

But all that is for the future. In the meantime, if ESA or NASA or anybody else is listening, Titan is surely worth another visit.

Ref: The impact of lake shape and size on lake breezes and air-lake exchanges on Titan :

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