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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FULL STORY ========================================================================== 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.

* RELATED_TOPICS
o Space_&_Time
# Mars # Saturn # Space_Missions # Space_Exploration #
NASA # Solar_System # Astronomy # Satellites
* RELATED_TERMS
o Phobos_(moon) o Titan_(moon) o Mars o Mars_Exploration_Rover
o Moon o Spacecraft_propulsion o Science o Exploration_of_Mars

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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.


========================================================================== 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230710180457.htm

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