Due to technological limitations and our distance from other celestial bodies, determining the strength of ancient and active rivers beyond Earth is a difficult task. Scientists can now see how intensely rivers used to flow on Mars and how they currently flow on Titan thanks to a new technique. The method estimates the rate at which rivers move fluid and sediment downstream using satellite observations.
Other than Earth, rivers have flowed on two other worlds in the solar system: Mars, where dry tracks and craters are all that remain of ancient rivers and lakes, and Titan, Saturn’s largest moon, where rivers of liquid methane still flow today.
MIT geologists have developed a new technique that allows scientists to see how intensely rivers used to flow on Mars and how they currently flow on Titan. The method estimates the rate at which rivers move fluid and sediment downstream using satellite observations.
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. And on Mars, it gives us a time machine, to take the rivers that are dead now and get a sense of what they were like when they were actively flowing.
Taylor Perron
The MIT team used their new technique to calculate how fast and deep rivers were on Mars more than a billion years ago. They also made similar estimates for currently active rivers on Titan, despite the fact that the moon’s thick atmosphere and distance from Earth make exploration more difficult, with far fewer images of its surface available than those of Mars.
“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,” says Taylor Perron, the Cecil and Ida Green Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “And on Mars, it gives us a time machine, to take the rivers that are dead now and get a sense of what they were like when they were actively flowing.”
Perron and his colleagues have published their results today in the Proceedings of the National Academy of Sciences. Perron’s MIT co-authors are first author Samuel Birch, Paul Corlies, and Jason Soderblom, with Rose Palermo and Andrew Ashton of the Woods Hole Oceanographic Institution (WHOI), Gary Parker of the University of Illinois at Urbana-Champaign, and collaborators from the University of California at Los Angeles, Yale University, and Cornell University.
River math
The team’s study grew out of Perron and Birch’s puzzlement over Titan’s rivers. The images taken by NASA’s Cassini spacecraft have shown a curious lack of fan-shaped deltas at the mouths of most of the moon’s rivers, contrary to many rivers on Earth. Could it be that Titan’s rivers don’t carry enough flow or sediment to build deltas?
The group expanded on the work of co-author Gary Parker, who developed a series of mathematical equations to describe river flow on Earth in the 2000s. Parker had studied river measurements taken in the field by others. He discovered certain universal relationships between a river’s physical dimensions (width, depth, and slope) and the rate at which it flowed based on these data. He devised equations to mathematically describe these relationships, taking into account other variables such as the gravitational field acting on the river and the size and density of the sediment pushed along the river’s bed.
“This means that rivers with different gravity and materials should follow similar relationships,” Perron says. “That opened up a possibility to apply this to other planets too.”
Getting a glimpse
On Earth, geologists can make field measurements of a river’s width, slope, and average sediment size, all of which can be fed into Parker’s equations to accurately predict a river’s flow rate, or how much water and sediment it can move downstream. But for rivers on other planets, measurements are more limited, and largely based on images and elevation measurements collected by remote satellites. For Mars, multiple orbiters have taken high-resolution images of the planet. For Titan, views are few and far between.
Birch realised that any estimate of river flow on Mars or Titan would have to be based on the few characteristics that can be measured using remote images and topography, namely the width and slope of the river. He modified Parker’s equations to work only with width and slope inputs with some algebraic tinkering. He then gathered data from 491 rivers on Earth, tested the modified equations on these rivers, and discovered that predictions based solely on the width and slope of each river were correct.
The equations were then applied to Mars, specifically the ancient rivers leading into the Gale and Jezero Craters, which are thought to have been water-filled lakes billions of years ago. To predict the flow rate of each river, he plugged into the equations Mars’ gravity, and estimates of each river’s width and slope, based on images and elevation measurements taken by orbiting satellites.
According to their flow rate predictions, rivers likely flowed for at least 100,000 years at Gale Crater and at least 1 million years at Jezero Crater – long enough to support life. They were also able to compare their predictions of the average size of sediment on the bed of each river with actual field measurements of Martian grains taken by NASA’s Curiosity and Perseverance rovers near each river. These few field measurements allowed the team to ensure that their equations were correct when applied on Mars.
The team then turned their attention to Titan. They focused on two sites where river slopes can be measured, one of which flows into a lake the size of Lake Ontario. As it flows into the lake, this river appears to form a delta. However, the delta is thought to be one of only a few on the moon; nearly every visible river flowing into a lake mysteriously lacks a delta. The team also used their method on another of these delta-free rivers.
They calculated the flow of both rivers and discovered that they may be comparable to some of the world’s largest rivers, with deltas estimated to have a flow rate as large as the Mississippi. Both rivers should carry enough sediment to form deltas. However, most rivers on Titan do not have fan-shaped deposits. Something else has to be at work to account for the lack of river deposits.
