Gary Parker looks at images from Saturn's moon Titan and sees the Sangamon, the Illinois, the Wabash and other earthbound rivers.
Oh, there are differences to be sure. That stuff running in Titan's rivers is probably liquid methane, not water, for one thing. And if you plucked a stone from the shore to skip across the surface, you likely would be skipping ice, not rock.
Still, what appear to be rivers on Titan look a lot like rivers on Earth overall, wider maybe, a little deeper, not as steep but hardly as alien in appearance as you might think, said Parker, a University of Illinois civil and environmental engineering and geology professor.
Likewise Mars, where evidence shows that now-dry rivers, probably flowing with water at some point in the planet's history, carved Earthlike features in the surface.
"We know there were rivers on Mars," Parker said. "The rivers that we have seen ... show many Earthlike features."
The whole thing is a matter of physics, Parker said.
"The ultimate goal of science is to determine universal rules," he said. In his case, the rules governing what earthbound rivers do, how the results of that action look and why.
"Those same rules ought to hold for rivers of methane moving stones of ice on Titan," he added. "The rules of physics are the rules of physics."
Parker himself studies an example of that principle right here on Earth.
In addition to looking at rivers on the surface, like the Mississippi, he examines deep sea "turbidity current," in essence a river in the ocean that flows along the bottom cutting channels, even canyons, and otherwise acting like streams on land.
Truly understanding river dynamics at the physical level – whether on the Earth's surface, undersea or on another planet – has a practical side, said Parker, who's collected data on rivers from all over the world.
He's involved in a project aimed at regenerating the Mississippi Delta, for example, which could help mitigate hurricane damage, among other things.
Parker began applying what he knows to Saturn's largest moon after a book on the outer planets and their satellites peaked his interest.
Scientists studying Titan, the second largest moon in the solar system and the only one with an atmosphere, already had indications that methane there, like water on Earth, could change from liquid to solid to gas depending on the weather, so to speak. There were indications of methane oceans, rain and snow as well. Why not rivers?
Most of our knowledge about Titan was indirect when Parker first started looking at the question. Titan's atmosphere is so dense it obscures the moon's surface.
He began applying what he knew about rivers on Earth in earnest with the NASA and European Space Agency Cassini-Huygens mission. The mission put a spacecraft in orbit around Saturn and released a probe that dropped through the atmosphere and onto Titan's surface in January 2005.
Images from the probe's descent show aerial views of what look like river channels. On the surface, the probe captured pictures of ice "rocks" that appear to have been smoothed and rounded like stones in an Earth river, some probably perfect for skipping, assuming one could stand the minus-290-degree temperatures.
The probe stopped working after 90 minutes, longer than expected, but the orbiter is continuing to work the Saturn neighborhood and will make more data-gathering passes by Titan this year.
Parker said Titan's much lower gravity means rivers there should have a lower acceleration rate, leading to channel slopes somewhat wider, deeper and less steep, more like streams in Nebraska's Sand Hills than, say, the Colorado.
He said some differences could have a significant impact, however. Take ice. Water is unusual in that, when it freezes, it floats. Methane ice, on the other hand, might sink, affecting flows on Titan.
"(That) would make rivers do things completely different than on Earth," Parker said.