The study of the power of ancient and active rivers beyond Earth is a difficult but intriguing field of planetary science and geology. While direct observations are scarce, scientists employ a variety of techniques to assess the power and properties of rivers on other celestial worlds.
Scientists can now visualize how furiously rivers used to flow on Mars and how they currently flow on Titan thanks to a new technology. The approach estimates the rate at which rivers carry fluid and sediment downstream using satellite measurements.
Other than Earth, rivers have flowed on two other worlds in the solar system: Mars, where dried trails 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 devised a novel approach that allows scientists to see how furiously rivers used to flow on Mars and how they currently flow on Titan. The approach estimates the rate at which rivers carry fluid and sediment downstream using satellite measurements.
What makes Titan intriguing is that it is 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 novel technique to compute how fast and deep rivers flowed on Mars more than a billion years ago. They also made similar estimations for currently active rivers on Titan, despite the fact that the moon’s thick atmosphere and distance from Earth make exploration more difficult, with significantly fewer photographs of its surface available than those of Mars.
“What makes Titan intriguing is that it is 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, Cecil and Ida Green Professor of Earth, Atmospheric, and Planetary Sciences (EAPS) at MIT. “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
Perron and Birch’s puzzlement about Titan’s rivers inspired the team’s research. In contrast to many rivers on Earth, photos collected by NASA’s Cassini probe show a remarkable lack of fan-shaped deltas at the mouths of most of the moon’s rivers. Is it possible that Titan’s rivers do not carry enough flow or sediment to support delta formation?
The group expanded on the work of co-author Gary Parker, who devised a series of mathematical equations to characterize river movement on Earth in the 2000s. Parker had reviewed river data made in the field by others. He discovered certain general correlations between a river’s physical measurements (width, depth, and slope) and the rate at which it flowed based on these statistics. He drew up equations to describe these relationships mathematically, accounting for other variables such as the gravitational field acting on the river, and the size and density of the sediment being pushed along a 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 noted that any estimate of river flow on Mars or Titan would have to be based on the few parameters that can be determined using remote pictures and topography, notably the width and slope of the river. He modified Parker’s equations to work just with width and slope inputs with some algebraic tinkering. He then gathered data from 491 rivers on Earth, evaluated the modified equations on these rivers, and discovered that predictions based merely on the width and slope of each river were correct.
The equations were then applied to Mars, notably the ancient rivers heading into the Gale and Jezero Craters, which are estimated 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 projections, rivers likely ran for at least 100,000 years at Gale Crater and at least 1 million years in Jezero Crater – long enough to support life. They were also able to compare their predictions of the average size of silt on the bed of each river with real field measurements of Martian grains acquired by NASA’s Curiosity and Perseverance rovers near each river. These few field observations allowed the scientists to ensure that their equations were correct when applied on Mars.
The crew then turned their attention to Titan. They focused on two sites where river slopes may be measured, one of which runs into a lake the size of Lake Ontario. As it flows into the lake, this river looks to form a delta. However, the delta is regarded to be one of just a few on the moon; practically every visible river running into a lake curiously lacks a delta. The researchers also used its strategy on another of these delta-free rivers.
They assessed the flow of both rivers and discovered that they may be similar to some of the world’s largest rivers, with deltas predicted to have a flow rate as enormous as the Mississippi. Yet, most rivers on Titan lack the fan-shaped deposits. Something else must be at work to explain this lack of river deposits.
