Titan: Saturn's Moon With Liquid Methane Rain, Lakes, and Seas You Could Almost Sail

Titan is the only moon in the solar system with a thick atmosphere, and the only body other than Earth known to have stable liquid on its surface right now. That liquid is not water. It is methane, the same gas that comes out of a kitchen burner, existing in liquid form because Titan's surface temperature sits around minus 179 degrees Celsius. At that temperature, methane does what water does here: it evaporates, rises, condenses into clouds, and falls back down as rain.
The rain itself is slow and heavy. Titan's gravity is about one-seventh of Earth's, and its atmosphere is four times denser than ours. A methane raindrop falls at roughly one metre per second, about a fifth of the speed a water raindrop hits the ground on Earth. The drops are larger too, and they drift. If you were standing on Titan's surface (in a suit that could survive the cold and the nitrogen-methane air), the rain would feel less like a downpour and more like a thick, deliberate drizzle of something that smells of natural gas.
NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, mapped Titan's surface using radar because the orange haze of its atmosphere blocks ordinary cameras. What the radar returned was extraordinary: hundreds of lakes and seas clustered around the moon's north pole, filled with liquid methane and ethane. The largest of these, Kraken Mare, is estimated to cover roughly 400,000 square kilometres, larger than the Caspian Sea. A second sea, Ligeia Mare, is about the size of Lake Superior.
These are not shallow puddles. Cassini's measurements suggest Ligeia Mare is at least 160 metres deep in places. The liquid is clear, ethane and methane mixed together, with dissolved nitrogen. The shores are made of water ice as hard as rock at those temperatures. There are no waves of any size because the wind at the surface is almost still, so the lakes sit flat and mirror-like under an orange sky.

The southern hemisphere has far fewer lakes, and scientists believe this is a seasonal effect. Titan's year lasts about 29 Earth years (it orbits Saturn, which orbits the Sun slowly). The north is currently in a wetter phase. The south had its lakes perhaps thousands of years ago and may have them again.
The Cassini mission, a joint project of NASA, ESA, and the Italian Space Agency, carried a probe called Huygens that parachuted through Titan's atmosphere in January 2005. Huygens was the first spacecraft to land on a body in the outer solar system. It transmitted for about 72 minutes on the surface before its batteries died. During that descent, it recorded wind speeds, measured the atmosphere's composition layer by layer, and photographed a landscape of rounded pebbles, not rock pebbles, but water-ice pebbles smoothed by flowing liquid methane, the way river stones are smoothed by water.

Cassini itself made 127 close flybys of Titan over 13 years. It detected methane clouds forming and dissipating near the south pole, watched the lakes change slightly between passes, and found evidence of river channels running from highlands to the seas. One channel, Vid Flumina, runs about 400 kilometres into Ligeia Mare and appears to cut deep into the terrain, a methane river with a carved canyon, analogous to a river delta on Earth but made of entirely different chemistry.
Methane in Titan's atmosphere is being destroyed by sunlight constantly. Ultraviolet radiation breaks it apart into heavier hydrocarbons that fall to the surface as a kind of orange haze and soot. The problem is that the methane keeps replenishing. Something is putting it back. The leading hypothesis is that Titan has geological activity, cryovolcanoes, possibly, that vent methane from a subsurface reservoir. No cryovolcano has been directly confirmed yet. The methane cycle on Titan is one of the genuinely open questions in planetary science.

NASA's Dragonfly mission, a rotorcraft lander approved for launch in the late 2020s, is designed to fly across Titan's surface and sample its chemistry at multiple locations. It will not reach Titan until the mid-2030s. When it does, it will look specifically for organic molecules, the kind that, on Earth, are precursors to biology. Titan is not expected to have life as we know it. But it is the best available laboratory for studying what organic chemistry does when it has billions of years and a stable liquid environment to work in.
The methane lakes of Titan and the methane rain that feeds them are not just strange facts about a distant moon. They are the same physical process, evaporation, condensation, precipitation, collection, that makes Earth habitable, running on a completely different molecule, at a temperature where steel becomes brittle. The solar system built the same machine twice, with different parts, and one of them works in the dark, 1.2 billion kilometres from the Sun.