Saturn’s moon Titan has lakes and rivers of liquid hydrocarbons, temperatures that hover around -300 degrees Fahrenheit, and a thick haze that surrounds it and has cloaked it in mystery. An unusual place for sure, but perhaps what’s most unusual is that Titan more closely resembles Earth of all the planets and moons in our solar system.
This is because like only Earth it has that flowing liquid on its surface, it has a climate featuring wind and rain that form dunes, rivers, lakes, deltas and seas (probably of filled with liquid methane and ethane), it has a thick atmosphere and it has weather patterns that change with the seasons. The moon’s methane cycle is quite similar to our water cycle.
And now astronomers have used data from NASA’s Cassini-Huygens mission to map the entire surface of Titan for the first time. Their work has found a global terrain of mountains, plains, valleys, craters and lakes . Again, this makes Titan unlike anywhere else in the solar system other than Earth.
“Titan has an atmosphere like Earth. It has wind, it has rain, it has mountains,” said Rosaly Lopes, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena. She and her colleagues wove together images and radar measurements taken by the spacecraft to produce the first global map of the moon.
“Titan has an active methane-based hydrologic cycle that has shaped a complex geologic landscape, making its surface one of most geologically diverse in the solar system,” she said. “It’s a really very interesting world, and one of the best places in the solar system to look for life,”
Cassini orbited Saturn from 2004 to 2017 and collected vast amounts of information about the ringed gas giant and its moons. The mission included more than 100 fly-bys of Titan, which allowed researchers to study the moon’s surface through its thick atmosphere and survey its terrain in unprecedented detail.
Their work, which now adds the surface of Titan to the kind of geological mapping done of the surfaces of Mars, Mercury and our moon, was published in Nature Astronomy.
Tracy Gregg, a planetary geologist at the University at Buffalo, described the mapping as “an incredibly powerful tool” that will allow scientists to answer key questions about Titan. One of those questions, she said, is “what kinds of changes has Titan gone through that are seasonal, and what sort of changes might be driven by the internal [heat] of the moon?”
Lopes expanded on that line of thought: “Despite the different materials, temperatures and gravity fields between Earth and Titan, many surface features are similar between the two worlds and can be interpreted as being products of the same geologic processes.”
Nearly two-thirds of Titan’s surface consists of flat plains, the map reveals, and 17% is covered in sandy dunes shaped by the wind, mostly around the equator. Around 14% of the surface is classified as ‘hummocky’ — hilly or mountainous — and 1.5% is ‘labyrinth’ terrain, with valleys carved by rain and erosion. There are surprisingly few impact craters, suggesting that the moon’s surface is fairly young.
Most of the moon’s lakes are near its north pole. Scientists think that this is because Saturn’s elliptical orbit around the sun causes Titan’s northern hemisphere to experience a longer summer than does its southern hemisphere, and Titan’s summer is when rain falls and liquid accumulates on the ground.
Just 1.5% of the moon’s surface is covered with liquid hydrocarbons, while about 70 percent of the surface of the Earth is covered with water.
Frequently described as a planet-like, Titan is 50% larger than Earth’s moon and 80% more massive. It is the second-largest moon in the solar system after Jupiter’s moon Ganymede, and is larger than the planet Mercury, but only 40% as massive.
Planetary geologist David Williams of the School of Earth and Space Exploration at Arizona State University (ASU) worked with the JPL team to identify the geologic units on Titan. First they used radar images and then extrapolated those units to regions not covered by radar.
Williams, who is also the director of the Ronald Greeley Center for Planetary Studies at ASU, has considerable experience working on the geologic mapping of small and unusual planetary objects, including Jupiter’s volcanic moon Io, the asteroid Vesta, the dwarf planet Ceres and now Titan.
“We held meetings at ASU earlier this decade to figure out how to map Titan using the higher-resolution radar and lower-resolution visible images,” Williams. explained. “Titan mappers from NASA’s Jet Propulsion Laboratory and Cornell University came to ASU to figure out how to do geologic mapping of this strange new world.”
The experience of mapping small and unusual planetary objects will help Williams on an important task he will undertake in the next decade, which is to make the first global geologic map of metal asteroid Psyche, the target of an ASU-led NASA mission scheduled to launch in 2022.
The Cassini-Huygens mission launched in 1997, reached Saturn in 2004, explored the planet and surrounding moons for 13 years and then the capsule plunged (intentionally) into Saturn in 2017.
It was a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency. The Huygens module traveled with Cassini until its separation from the probe in 2004; it landed by parachute on Titan the next year. It returned data to Earth for around 90 minutes, using the orbiter as a relay. This was the first landing ever accomplished in the outer solar system and the first landing on a moon other than Earth’s moon.
NASA plans to send a drone to Titan on the Dragonfly mission by 2034, which will fly across the surface and study it in a variety of locations. Some scientists are of the view that Titan, with all its organic molecules, could be home to forms of extreme life. (A Many Worlds story on the mission is here.)
There are no current plans to send orbiters to Saturn or its moons, so this map is likely to remain our best global view of Titan for the foreseeable future.