Phantom lakes are seen appearing and disappearing on the surface of Titan in data obtained by the Cassini orbiter. However, this is not the only mystery of Titan recently uncovered by investigators analyzing results from our last close encounter with the second-largest moon in the Solar System.
Titan, the largest moon of Saturn, exhibits a hydration cycle, much like the water cycle on Earth. However, on this massive satellite, methane and ethane (instead of water) evaporate into the atmosphere and are returned back to the surface in the form of hydrocarbon rain.
A trio of lakes once seen at the north pole of Titan by the Cassini orbiter disappeared in observations made seven years later, during the spacecraft’s final pass by the massive moon. When the lakes were seen, it was winter in the northern hemisphere of Titan, while summer reigned at the time of the final observations. The most likely explanation is that seasonal changes, together with absorption of liquid by the ground, was responsible for this disappearing act.
“One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface,” Shannon MacKenzie, planetary scientist at the Johns Hopkins Applied Physics Laboratory said.
Titan is the only body in the solar system, other than the Earth, known to hold bodies of stable liquid at its surface. Lakes are concentrated at the northern and southern poles of Titan, although they are larger and more numerous in the north. Composed of methane and ethane, the lakes at the north pole of Titan were found to be odder than anyone expected.
Cassini found lakes near the north pole of Titan were far different on one side of that world than the other. The low-lying large seas on the eastern side of Titan are surrounded by canyons and dotted with islands. The opposite side of that world is filled with smaller bodies of methane and ethane, measuring just tens of miles in diameter, but running up to 100 meters (330 feet) in depth. Mesas, or buttes, rising far over the surface, are seen with deep lakes at their tops.
“It is as if you looked down on the Earth’s North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does,” Jonathan Lunine of Cornell University said.
The mesas on Titan provide geologists with clues to the question of how these bodies formed. Similar to karstic lakes on Earth, these liquid bodies likely formed as bedrocks of ice and organic material dissolved and collapsed, leaving behind mesas. On our home world, this process is driven by the collapse of limestone.
Ethane and methane, the two major components of lakes and ponds on Titan, are found as a gas on Earth, but the much colder temperatures on Titan hold the materials in liquid form on that world. Of the two compounds, methane evaporates far more easily, providing further evidence seasonal changes are responsible for the disappearance of lakes as seasonal changes take place on Titan.
“If this were the case, the phantom lakes could be interpreted as shallow ponds, with either a pure methane composition or a regolith porous enough to remove the less volatile ethane,” researchers wrote in an article published in Nature Astronomy.
Cassini arrived at Saturn in 2004, carrying out observations of the second-largest planet in our solar system and its mighty ring system for 13 years. Hitching a ride with Cassini was the Huygens probe, which touched down on the surface of Titan on January 15, 2005, becoming the first spacecraft to ever land on the surface of a world in the outer solar system.
The Cassini spacecraft made its final pass of Titan on April 22,2017, before mission engineers purposely directed the spacecraft onto a collision course into the atmosphere of the mighty planet, preventing a possible crash landing on one of the many moons in the system.
During its time exploring Saturn’s system, Cassini mapped over 1.6 million square kilometers (620,000 square miles) of lakes and ponds on the surface of Titan, nearly seven times larger than all the Great Lakes combined. It completed 294 orbits of the planet, traveling 7.9 billion kilometers (4.9 billion miles) from launch to its final plunge into Saturn. The spacecraft created over 453,000 images, discovered six moons, and the data it collected resulted in the publication of roughly 4,000 scientific papers.