From the May 2015 issue

How big would methane raindrops be on Titan?

Douglas Kaupa, Colorado Springs, Colorado
By | Published: May 25, 2015
Clouds on Titan create methane rain, causing changes on the surface below.
Clouds on Titan create methane rain, causing changes on the surface below. The left image shows an area near the moon’s equator May 13, 2007, while the other two were taken 15 hours apart January 15, 2011. The bright points in the latter two photos appear to be low clouds above where rain fell recently.
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]). The slower speed and larger drops would make it easier to see that raindrops (on Titan and Earth) tend to be distorted and flattened by the atmosphere as they fall.

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.

It is currently late northern spring on Titan, and based on atmospheric models similar to those used to understand weather on Earth, titanian forecasts have called for an increasing likelihood of clouds as the Sun rises higher over Titan’s north polar seas. However, storms have not materialized as early as anticipated. Plans are for Cassini to continue its mission in the saturnian system until just after the northern summer solstice. So we will be watching Titan closely over the next few years to see if and when summer storms arrive. And if the timing of a storm is just right during one of Cassini’s close Titan flybys, its radar instrument could even detect rain as it falls.

Elizabeth Turtle
Johns Hopkins Applied Physics Lab, Laurel, Maryland