Scientists at MIT have developed a groundbreaking technique that enables them to study the flow of rivers on other celestial bodies in our solar system. By utilizing satellite observations, the geologists can estimate the rate at which fluid and sediment move downstream in rivers on Mars and Titan, Saturn’s largest moon.
The team from MIT used this innovative method to determine the historical intensity of river flow on certain regions of Mars over a billion years ago. Additionally, they made similar assessments for the currently active rivers on Titan, despite the moon’s challenging exploration conditions due to its thick atmosphere and distance from Earth. The scarcity of available surface images makes Titan particularly difficult to study compared to Mars.
One of the significant advantages of this technique is its ability to make predictions about Titan, where data collection is expected to be limited for the foreseeable future. This enables scientists to gain insights into an active environment that may otherwise be inaccessible.
On Mars, this method acts as a sort of “time machine,” allowing researchers to understand what the ancient rivers were like when they were actively flowing, even though they are now dry tracks and craters. By applying this technique, the MIT team aims to unravel the mysteries of rivers and their dynamics on these celestial bodies.
The research findings, which have been published in the Proceedings of the National Academy of Sciences, involved collaboration between MIT and various institutions such as the Woods Hole Oceanographic Institution, the University of Illinois at Urbana-Champaign, the University of California at Los Angeles, Yale University, and Cornell University. The study opens up new possibilities for investigating the geological history and processes of other worlds within our solar system.
River math
The team’s research was motivated by Taylor Perron and Samuel Birch’s curiosity regarding the peculiar nature of rivers on Titan. Images captured by NASA’s Cassini spacecraft revealed a lack of fan-shaped deltas at the mouths of most of the moon’s rivers, a phenomenon that differs from Earth’s rivers. This raised the question of whether Titan’s rivers carry insufficient flow or sediment to construct deltas.
To tackle this question, the scientists expanded upon the earlier work of Gary Parker, a co-author of the study. Parker had developed a set of mathematical equations in the 2000s to characterize river flow on Earth, based on direct measurements obtained by field researchers.
By analyzing these field data, Parker identified certain fundamental relationships between a river’s physical attributes—such as its width, depth, and slope—and the speed at which it flowed. He formulated mathematical equations that described these relationships, accounting for variables such as the gravitational field acting on the river and the size and density of sediment transported along the riverbed.
Perron explains that these findings imply that rivers on different planets, with varying gravitational forces and material properties, should exhibit similar relationships. This realization opened up the possibility of applying Parker’s equations to investigate rivers on other celestial bodies.
Getting a glimpse
On Earth, geologists have the advantage of making direct field measurements of a river’s characteristics such as width, slope, and sediment size. These measurements can be plugged into Gary Parker’s equations, accurately predicting the flow rate of the river and its ability to transport water and sediment downstream. However, when it comes to studying rivers on other planets, measurements are more limited and primarily rely on images and elevation data collected by remote satellites.
Recognizing this challenge, Samuel Birch devised a solution. He realized that estimates of river flow on Mars or Titan would need to be based on the few measurable characteristics obtainable from remote images and topography—specifically, the width and slope of the river. Through some algebraic adjustments, he adapted Parker’s equations to work exclusively with these inputs.
Birch compiled data from 491 rivers on Earth and tested the modified equations using their width and slope. The results demonstrated that the predictions based solely on these two parameters were accurate.
The team then applied these adapted equations to Mars, specifically examining the ancient rivers leading to Gale and Jezero Craters, which were once believed to be filled with water billions of years ago. By considering Mars’ gravity and estimates of each river’s width and slope derived from satellite images and elevation measurements, they predicted the flow rate of each river.
Based on their flow rate predictions, the team determined that rivers likely flowed for at least 100,000 years at Gale Crater and at least 1 million years at Jezero Crater—potentially long enough to support life. They also compared their predictions of sediment size on each riverbed with actual field measurements of Martian grains near the rivers, obtained by NASA’s Curiosity and Perseverance rovers. These limited field measurements confirmed the accuracy of their equations when applied to Mars.
Next, the team turned their attention to Titan. They focused on two locations where river slopes could be measured, including a river that flows into a lake comparable in size to Lake Ontario. While this particular river formed a delta as it entered the lake, the majority of observable rivers on Titan mysteriously lack these fan-shaped deposits. The team applied their method to one of these delta-less rivers as well.
Through their calculations, they determined that both rivers on Titan may rival some of Earth’s largest rivers in terms of flow rate, with deltas estimated to have a similar magnitude of flow to the Mississippi River. These rivers should transport enough sediment to create deltas. However, the absence of fan-shaped deposits suggests the presence of other factors that prevent their formation.
In another intriguing finding, the team discovered that rivers on Titan should exhibit wider widths and gentler slopes compared to rivers on Earth or Mars that carry the same flow rate. Samuel Birch notes that Titan is the most Earth-like place discovered thus far, and this remote technique brings scientists closer to understanding its secrets.