A world of hydrocarbon seas
On Saturn's largest moon, Titan, a cycle similar to Earth's water cycle unfolds, but with liquid hydrocarbons instead of water. At frigid surface temperatures of around -180 degrees Celsius (-292 F), methane and ethane exist as liquids, forming vast seas and scattered lakes concentrated in the moon's polar regions. These bodies of liquid were confirmed by NASA's Cassini spacecraft, which mapped over 1.6 million square kilometers of liquid on Titan's surface.
The largest of these is Kraken Mare, a sea containing about 80% of Titan's surface liquids and covering an area larger than Earth's Caspian Sea. Data from Cassini's radar altimeter indicates that Kraken Mare is at least 300 meters (about 1,000 feet) deep in its center. The second-largest sea, Ligeia Mare, has a measured depth of over 160 meters (560 feet). The smaller northern lakes are also surprisingly deep, with some plunging more than 100 meters.
The composition of these seas is complex. They are primarily liquid methane and ethane, with dissolved nitrogen gas. Models and measurements suggest that the specific blend varies between lakes. The southern lake, Ontario Lacus, seems to have a roughly equal mix of methane and ethane. Some analyses of Kraken Mare suggest a composition of 70% methane, 16% nitrogen, and 14% ethane. Other models propose that many lakes are predominantly ethane (76-79%) with smaller amounts of propane, methane (5-10%), and other compounds like hydrogen cyanide and acetylene.
The methane cycle's central puzzle
Titan's atmosphere is about 94.2% nitrogen and 5.65% methane. This atmospheric methane is the source of the liquid on the surface, condensing into clouds and falling as rain. However, this cycle is a paradox. In the upper atmosphere, sunlight and chemical reactions continuously break down methane into ethane and other heavier hydrocarbons. This process is irreversible because the resulting hydrogen escapes into space. All of Titan's atmospheric methane should have been destroyed within tens of millions of years—a very short time for the solar system's age.
This means there must be a continuous source replenishing the atmospheric methane, but its source is unknown. One hypothesis is cryovolcanism, where eruptions release methane from Titan's interior. Another theory suggests methane is trapped within a crust of water ice as a clathrate hydrate, a cage-like ice structure. This methane-rich ice could periodically warm and release its gas into the atmosphere. Underground reserves, equivalent to Earth's aquifers but filled with liquid hydrocarbons ("methanofers"), might also contribute.
The stability of the lakes themselves adds to the mystery. The atmospheric conditions on Titan suggest that the methane in the lakes should evaporate, but observations by Cassini showed that the lakes are remarkably stable in size and shape over many years. The details of this equilibrium, balancing precipitation, evaporation, and potential underground seepage, are still not fully understood.