Tuesday, May 18, 2021

How big would methane raindrops be on Titan?

Clouds on Titan create methane rain, causing changes on the surface below. 

Methane raindrops on Titan could grow to be almost a centimeter across, nearly twice the size of large raindrops on Earth (about 6 millimeters). And, thanks to Titan’s thicker atmosphere and lower gravity, they would fall much more slowly, roughly 5.2 feet per second (1.6 m/s), the speed at which snowflakes fall on Earth (compared to rates of terrestrial rainfall at up to 30 ft/s [9 m/s]).

A consequence of falling slowly is that there is more time for raindrops to evaporate before they reach the ground, so the phenomenon of virga, seen over deserts on Earth, is likely much more common on Titan. 

However, we know from observations by the Cassini spacecraft that rain does occasionally reach Titan’s surface. Cassini’s cameras have revealed darkening of the surface in the wake of some of the largest cloud outbursts — like rain on Earth darkens the ground, except that on Titan it’s methane rain wetting a surface covered in solid hydrocarbon material, and it takes weeks to months for Titan’s surface to dry out again. Astronomers only have seen this a few times over more than 10 years of observations by Cassini, suggesting rainfall is rare but intense — another parallel with terrestrial deserts.

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