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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Gauging the strength of ancient and active rivers beyond Earth    
      
     Date:   
         July 10, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         A new technique allows scientists to see how intensely rivers used   
         to flow on Mars, and how they currently flow on Titan. The method   
         uses satellite observations to estimate the rate at which rivers   
         move fluid and sediment downstream.   
      
      
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   Rivers have flowed on two other worlds in the solar system besides Earth:   
   Mars, where dry tracks and craters are all that's left of ancient rivers   
   and lakes, and Titan, Saturn's largest moon, where rivers of liquid   
   methane still flow today.   
      
   A new technique developed by MIT geologists allows scientists to see   
   how intensely rivers used to flow on Mars, and how they currently flow   
   on Titan.   
      
   The method uses satellite observations to estimate the rate at which   
   rivers move fluid and sediment downstream.   
      
   Applying their new technique, the MIT team calculated how fast and   
   deep rivers were in certain regions on Mars more than 1 billion years   
   ago. They also made similar estimates for currently active rivers on   
   Titan, even though the moon's thick atmosphere and distance from Earth   
   make it harder to explore, with far fewer available images of its surface   
   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 built on the   
   work of co-author Gary Parker, who in the 2000s developed a series of   
   mathematical equations to describe river flow on Earth.   
      
   Parker had studied measurements of rivers taken directly in the field   
   by others. From these data, he found there were certain universal   
   relationships between a river's physical dimensions -- its width, depth,   
   and slope -- and the rate at which it flowed. 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 realized that any estimate of river flow on Mars or Titan would   
   have to be based on the few characteristics that can be measured from   
   remote images and topography -- namely, a river's width and slope. With   
   some algebraic tinkering, he adapted Parker's equations to work only   
   with width and slope inputs. He then assembled data from 491 rivers on   
   Earth, tested the modified equations on these rivers, and found that the   
   predictions based solely on each river's width and slope were accurate.   
      
   Then, he applied the equations to Mars, and specifically, to the ancient   
   rivers leading into Gale and Jezero Craters, both of 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.   
      
   From their predictions of flow rate, the team found that rivers likely   
   flowed for at least 100,000 years at Gale Crater and at least 1 million   
   years at Jezero Crater -- long enough to have possibly supported   
   life. They were also able to compare their predictions of the average   
   size of sediment on each river's bed with actual field measurements of   
   Martian grains near each river, taken by NASA's Curiosity and Perseverance   
   rovers. These few field measurements allowed the team to check that   
   their equations, applied on Mars, were accurate.   
      
   The team then took their approach to Titan. They zeroed in on two   
   locations where river slopes can be measured, including a river that   
   flows into a lake the size of Lake Ontario. This river appears to form   
   a delta as it feeds into the lake. However, the delta is one of only a   
   few thought to exist on the moon -- nearly every viewable river flowing   
   into a lake mysteriously lacks a delta.   
      
   The team also applied their method to one of these other delta-less   
   rivers.   
      
   They calculated both rivers' flow and found that they may be comparable   
   to some of the biggest rivers on Earth, with deltas estimated to have   
   a flow rate as large as the Mississippi. Both rivers should move enough   
   sediment to build up deltas. Yet, most rivers on Titan lack the fan-shaped   
   deposits. Something else must be at work to explain this lack of river   
   deposits.   
      
   In another finding, the team calculated that rivers on Titan should be   
   wider and have a gentler slope than rivers carrying the same flow on   
   Earth or Mars.   
      
   "Titan is the most Earth-like place," Birch says. "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."  This research   
   was supported, in part, by NASA and the Heising-Simons Foundation.   
      
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   Story Source: Materials provided by   
   Massachusetts_Institute_of_Technology. Original written by Jennifer   
   Chu. Note: Content may be edited for style and length.   
      
      
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   Journal Reference:   
      1. Samuel P. D. Birch, Gary Parker, Paul Corlies, Jason M. Soderblom,   
      Julia   
         W. Miller, Rose V. Palermo, Juan M. Lora, Andrew D. Ashton,   
         Alexander G.   
      
         Hayes, J. Taylor Perron. Reconstructing river flows remotely on   
         Earth, Titan, and Mars. Proceedings of the National Academy of   
         Sciences, 2023; 120 (29) DOI: 10.1073/pnas.2206837120   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230710180457.htm   
      
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