A year ago on September 15, NASA’s Cassini spacecraft was deliberately plunged into Saturn’s atmosphere after orbiting the planet for 13 years. During its final distant encounter with Saturn’s giant moon Titan 4 days prior, Cassini captured this last photo of the moon’s north polar landscape of lakes and seas, which are filled with liquid methane and ethane. With surface temperatures of 292° below zero Fahrenheit, water on Titan is as hard as quartz while the hydrocarbons ethane, methane and propane, which have much lower freezing points, can pool into lakes and seas especially in the colder polar regions.
The photo, a mosaic of several images stitched together, was taken from a distance of 87,000 miles (140,000 km) using a filter sensitive to near-infrared light, which can penetrate the moon’s perpetually hazy, hydrocarbon-laden atmosphere. Punga Mare (pronounced MAH-ray) at 240 miles (390 km) across, Ligeia Mare (300 miles / 500 km) and the vast Kraken Mare, the largest sea on Titan, at 730 miles (1,200 km) wide or twice the length of Lake Superior, dominate the left half of the image above. At top and right, numerous smaller lakes can be seen. Besides Earth, Titan is the only known solar system body with surface lakes. How did they get there and can you swim there?
Titan’s atmosphere has a significant amount of methane, which is broken down by sunlight into smaller bits that reassemble into more complex organic compounds that fall to the surface as rain droplets in a seasonal cycle equal to the time it takes Saturn to orbit around the sun, 29.5 years. That and underground sources keep the lakes filled. As to how the depressions formed that contain those fluids — the lake basins — no one knows for sure. Scientists suspect that a process similar to how caves and sinkholes form on Earth also operates on Titan. Caves on Earth form when mildly acidic rainwater percolates through limestone, dissolving it. Hydrocarbon liquids on Titan may over time carve out lake beds.
As you may have guessed already, Titan has a methane cycle much like Earth’s water cycle of evaporation, cloud formation, rainfall, surface runoff into rivers, and collection in lakes and seas.
And now, the all-important question — can you swim in a Titan lake? Assuming you could survive the super-subzero temperatures, liquid ethane-methane is roughly half as dense as water, so you might be able to swim if you kicked and scooped hard. If not, you’d sink like a stone.
But maybe there’s a wild card here — the pull of gravity. On Titan, gravity is only 14% what it is on Earth, so you’d weigh a lot less there. A 200-pound person would tip the scale at just 28 pounds, so maybe you could get in a swim after all. (Note: this is corrected from an earlier version of this text where I inadvertently used methane/ethane gas densities!)
For a beautifully-written tribute to the Cassini mission in the form a letter to the spacecraft I encourage you to read Shannon Stirone’s Dear Cassini: Why the Saturn Spacecraft Brings Me to Tears.