Geologists at the Massachusetts Institute of Technology have developed a technique that allows scientists to see how fast rivers flowed on Mars billions of years ago and currently do on Titan, Saturn’s largest moon, researchers said.
“We can use these other worlds to help us understand what keeps planetary climate stable, or in some cases, what allows planetary climate to change really drastically over time like on Mars,” said Taylor Perron, a professor of Earth, Atmospheric and Planetary Sciences at MIT.
Titan is the only world besides Earth that has standing bodies of liquid, including rivers, lakes and seas, on its surface, according to NASA.
In the past, estimating the speed that liquid and sediment moves downstream has required measurements that are taken on the surface or estimates of what those measurements might be, including the size of the sediment being carried, Perron said.
The new technique, which was published in Proceedings of the National Academy of Sciences in July, uses satellite observations to gauge how wide and steep a river is and estimate river flow from that, Perron said.
“ The difference in our technique is that it makes it possible to estimate rates of river flow from things that you can measure remotely,” Perron said.
Understanding geographical features on other worlds and how they change over time can help understand the climate on Earth, said Sam Birch, a co-author of the paper who is now at Brown University.
“Titan and Mars are the only worlds that have had rivers, and when you look at them, you want to understand what the climate was like that can give rise to those features,” Birch said.
On Mars, dried-up rivers from more than a billion years ago have left remnants of old river beds, Perron said. The new technique allowed the MIT team to calculate the depth and speed of some rivers on Mars when they flowed.
They learned the rivers flowed for prolonged periods of time with conditions that could have maintained life, Perron said.
“The Martian rivers we studied were not just fleeting; they appear to have flowed over prolonged time periods during which conditions favorable to life may have been maintained,” Perron said.
Now Mars is a cold desert, and reconstructing how its climate changed over time can help scientists understand why, Perron said.
On Titan, the same measurements were taken for liquid methane that is flowing through the rivers over a bed of mostly ice. Perron said. While the rivers are active, the distance from Earth, along with the planet’s thick atmosphere, makes it difficult for scientists to explore.
Titan’s weather is similar to Earth’s, and scientists are trying to determine whether its climate is stable and, if so, for how long, Perron said.
Birch and Perron both compared rivers to a conveyor belt: a river is carrying sediment from one place to another and will adjust its width, depth, or steepness in order to do that.
Because rivers on other worlds work similarly to rivers on Earth, they offer natural experiments.
“So studying planetary landscapes is a way of finding these natural experiments and learning not only about other worlds but also about Earth,” Perron said.
Each world presents its own challenges. Mars has the same materials as Earth — rocky sediment and water, but the gravity is different.
Titan, not only has different gravity but different materials make up the landscape. Despite the differences, similar landforms still form, although much less is known about the surface of Titan.
Despite the challenges, Birch said he is motivated by the similarities among Earth, Mars and Titan.
“A moon on Saturn has rivers doing the same thing a river in your backyard is doing,” Birch said.
Rivers aren’t the only features on other worlds that provide a record of past climate, and Perron said he is now researching shorelines.
“Titan also has lakes and seas of liquid hydrocarbons,” Perron said. “And we’ve been working with coastal geologists to try to understand what the shorelines that we can see on Titan, to tell us about the past climate and geology.”