M.I.T. geologists have devised a new technique that lets them see how intensely rivers used to flow on Mars, and how they currently flow on Titan.

Rivers have coursed through celestial bodies other than Earth within our solar system. One such location is Mars, where remnants of ancient rivers and lakes exist solely as dry tracks and craters. Meanwhile, Titan, Saturn’s largest moon, boasts an intriguing feature—ongoing rivers of liquid methane.

In a significant development, geologists from MIT have devised a novel technique enabling scientists to gauge the past and present intensity of river flow on Mars and Titan. This method leverages satellite observations to estimate the rate at which fluid and sediment move downstream within rivers.

By employing their innovative approach, the MIT team managed to compute the historical velocity and depth of rivers in specific Martian regions over a billion years ago. Likewise, they made similar estimations for presently active rivers on Titan, despite the challenges posed by the moon’s thick atmosphere and its considerable distance from Earth. Obtaining images of Titan’s surface proves more arduous compared to Mars, with limited resources available.

Taylor Perron, the Cecil and Ida Green Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), expressed enthusiasm about Titan’s dynamic nature.

Perron remarked, “What’s exciting about Titan is that it’s active. With this technique, we have a method to make real predictions for a place where we won’t get more data for a long time.”

Furthermore, Perron emphasized the significance of their findings for Mars, noting that the technique functions as a time machine, enabling researchers to envision the characteristics of ancient rivers when they flowed actively.

The outcomes of this research have been published in the Proceedings of the National Academy of Sciences. Perron’s co-authors from MIT include Samuel Birch, who is the lead author, along with Paul Corlies and Jason Soderblom. Additional contributors to the study comprise Rose Palermo and Andrew Ashton from the Woods Hole Oceanographic Institution (WHOI), Gary Parker from the University of Illinois at Urbana-Champaign, and collaborators from the University of California at Los Angeles, Yale University, and Cornell University.

River Science

The inspiration for the team’s research stemmed from the intriguing nature of Titan’s rivers, which left both Perron and Birch puzzled. Examination of the images captured by NASA’s Cassini spacecraft revealed a peculiar absence of fan-shaped deltas at the mouths of most rivers on the moon. This contrasted with the formation of deltas observed in numerous earthly rivers. The researchers questioned whether this discrepancy was due to the insufficient flow or sediment-carrying capacity of Titan’s rivers.

To unravel this mystery, the team drew upon the groundwork laid by co-author Gary Parker in the early 2000s. Parker had devised a series of mathematical equations to elucidate the mechanics of river flow on Earth. His investigations involved analyzing field measurements of rivers conducted by other scientists. From this data, Parker discerned consistent patterns connecting a river’s physical characteristics—such as width, depth, and slope—to its flow rate. He mathematically formulated equations to describe these relationships, accounting for additional factors like gravitational forces acting on the river and the size and density of sediment transported along the riverbed.

Perron explained, “This means that rivers with different gravity and materials should follow similar relationships. That opened up a possibility to apply this to other planets too.”

Gaining Insights

On Earth, geologists can directly measure a river’s width, slope, and average sediment size in the field. These measurements can then be plugged into Parker’s equations to accurately predict the river’s flow rate—the amount of water and sediment it can carry downstream. However, for rivers on other planets, such as Mars and Titan, measurements are more restricted and primarily reliant on remote satellite images and elevation data. Mars has been extensively imaged by multiple orbiters, whereas views of Titan are scarce.

To overcome this limitation, Birch realized that estimating river flow on Mars and Titan would necessitate utilizing the few characteristics that can be derived from remote images and topographic data—specifically, the width and slope of the rivers. By making some adjustments to Parker’s equations, he adapted them to work solely with these inputs. To validate their modified equations, he gathered data from 491 rivers on Earth and found that predictions based solely on width and slope were accurate.

Next, the team applied these equations to Mars, specifically examining the ancient rivers leading into Gale and Jezero Craters, which were once believed to have contained water-filled lakes billions of years ago. By incorporating Mars’ gravity, as well as estimates of each river’s width and slope derived from satellite imagery and elevation data, Birch predicted the flow rate of each river. The team’s calculations indicated that rivers likely flowed for a minimum of 100,000 years at Gale Crater and at least 1 million years at Jezero Crater—sufficient time to potentially support life. They also compared their predictions regarding the average sediment size on each riverbed with field measurements taken by NASA’s Curiosity and Perseverance rovers. These limited field measurements validated the accuracy of their equations when applied to Mars.

The team then extended their approach to Titan, focusing on two locations where river slopes could be measured. One of these rivers feeds into a lake roughly the size of Lake Ontario and exhibits the formation of a delta. However, deltas are notably absent from most observable rivers flowing into lakes on Titan. The researchers also employed their method to analyze one of these delta-less rivers.

Upon calculating the flow rates of both rivers, the team discovered that they could rival some of the largest rivers on Earth, with deltas estimated to have flow rates comparable to that of the Mississippi River. These rivers should carry enough sediment to form deltas. However, the majority of rivers on Titan lack these characteristic fan-shaped deposits. This suggests that additional factors are at play in explaining the absence of river deposits.

Furthermore, the team determined that rivers on Titan should possess wider dimensions and gentler slopes compared to rivers on Earth or Mars, carrying the same flow rate.

Birch remarked, “Titan is the most Earth-like place.

”We’ve only gotten a glimpse of it. There’s so much more that we know is down there, and this remote technique is pushing us a little closer.”

Support for this research was provided by NASA and the Heising-Simons Foundation.

Source: 10.1073/pnas.2206837120

Image Credit: NASA/JPL/USGS