Rain on Saturn’s Moon Titan Could Make this World the Life of the Party

Evidence of rainfall at the north pole of Titan is welcome news to astronomers, who have been searching for the phenomenon since 2004. This could increase chances that life, however primitive, may be found on Saturn’s largest satellite.

A slick shimmer, called the sidewalk effect, was recorded by the Cassini orbiter on June 7, 2016, an effect not visible in other images of the region. This confirms predictions made by researchers years ago. The shimmer seen in this Cassini image — known as the wet sidewalk effect — covers 120,000 square kilometers (46,332 square miles), roughly half the size of the Great Lakes.

“The whole Titan community has been looking forward to seeing clouds and rains on Titan’s north pole, indicating the start of the northern summer, but despite what the climate models had predicted, we weren’t even seeing any clouds,” said Rajani Dhingra, a doctoral student in physics at the University of Idaho in Moscow.

Titan seen in natural colors against the backdrop of Saturn, in a composite image taken by the Cassini spacecraft in 2012. Image: NASA/JPL-Caltech/Space Science Institute.

The best explanation for the appearance and disappearance of the reflection is the deposit and evaporation of methane rain.

The Cassini probe arrived at Saturn in 2004, beginning its study of the ring planet, along with that planet’s retinue of moons. At that time, it was summer in Titan’s southern hemisphere, and Cassini observed clouds and rainfall there. Naturally, astronomers expected to see the same processes happening in the northern hemisphere when summer arrived there, peaking in 2017. However, rain clouds were not seen in the northern hemisphere, puzzling researchers, who started referring to the mystery as “the case of the missing clouds.”

An image showing evidence of rainfall in the northern hemisphere of Titan, highlighted by pink arrows in the bottom close-up photo. The red arrow highlights reflections from the largest lake near the north pole, Xolotlan Lacus, while the largest sea in the region, Kraken Mare, is seen between the light blue arrows. Image: NASA/JPL/University of Arizona/University of Idaho.

If Titan possesses a methane cycle, similar to the water cycle on Earth, with rain and evaporation, the process would feed seas full of organic material, potentially providing a home for life on that distant world.

Like our own Moon, Titan is tidally locked to Saturn, so one face of the clouded world always faces its parent planet. Each season on that massive moon lasts seven Earth years, and one year on Titan lasts 30 times longer than it does on our home world.

“[I]t is extremely difficult to detect rainfall events on Titan due to its thick atmospheric haze and very limited opportunities to view the surface (and its changes),” researchers reported in a paper published in the journal Geophysical Research Letters.

Looking in infrared wavelengths, we are able to peer through the thick haze of Titan, viewing the surface of the massive moon. This image of the Saturn-facing side of Titan clearly shows the dark dunes near the equator, named Fensal (to the north) and Aztlan (to the south). Image: NASA/JPL/University of Arizona/University of Idaho.

One additional theory for why it was so hard to view the reflections holds that the methane rain fell on a pebble-like surface, draining between crevices on the ground. Also, most of the lakes and seas on Titan are centered near the north pole, potentially altering expected seasonal changes.

Titan is the largest moon orbiting Saturn, and the second-largest in the solar system. It is the only moon known to have a thick atmosphere, mostly composed of nitrogen. This thick haze could protect any life on Titan from the brutal effects of radiation.

Recently, NASA announced a possible mission to send a probe called Dragonfly to Titan, where the vehicle could soar in the thick atmosphere of the moon, before landing on the surface to explore the world up close. That mission would launch in 2025, arriving at Titan in the year 2034.