Imagine a babbling river, making its way over stones and pebbles - the flow occasionally knocking a pebble forward along the riverbed.
Now imagine that scene on Mars.
According to new research, that's no daydream. A novel study has examined the erosion patterns of rocks on Mars and determined they were carried by water for some 30 miles on the Red Planet.
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While recent evidence from Mars has suggested the planet hosts a small amount of liquid water today, this study is based on close analysis of images taken by NASA's Mars Curiosity rover in 2012 and hint at a much wetter world with flowing rivers in the planet's past.
The rover's images included fine details of pebbles on the planet, revealing their unusually smooth and round shapes. For geophysicist Douglas Jerolmack and his colleague, Gábor Domokos, a mathematician at Budapest University of Technology and Economics, the details of the rocks' shapes were enough to understand their history.
"Thousands of years ago, Aristotle pondered the question of pebbles on the beach and how they become rounded. But until recently, descriptions of pebble shape have been qualitative, and we lacked a basic understanding of the rounding process." said Jerolmack, an associate professor in the Department of Earth and Environmental Science in Penn's School of Arts & Sciences.
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How exactly can a pebble's shape alone reveal its past? Enter: math.
While Jerolmack analyzed the geophysics of the Mars pebbles, Domokos did some number crunching based on his past research on the precise link between an object's motion and its shape. Since the balance points of a rock are reduced by natural abrasion, Domokos was able to come up with a geometric model that predicts how a stone's shape and mass is altered as it bangs against another similarly sized object.
The team tested the geometric model with data they gathered from their own terrestrial erosion tests. First they rolled limestone fragments in a drum in a lab and recorded the stones' change in shape and loss in mass over time. Next they examined rocks and pebbles in a mountain river in Puerto Rico.
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"We started at the headwaters, where chunks of angular rock are breaking off from the walls of the stream, and went downstream," Jerolmack said. "Every few hundred meters we would pull thousands of rocks out and take images of their silhouette and record their weight."
Finally they performed similar analysis of sediment from an alluvial fan at the mouth of a canyon in New Mexico. This fan-shaped sediment feature closely resembles features at the location on Mars where the Curiosity rover snapped photos of pebbles.
Data from all three terrestrial experiments agreed nicely with the geometric model Domokos had developed - proving they could accurately use the model to determine how far a pebble had traveled based solely on its shape.
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Applying Domokos' model to the contour data of the Mars pebbles, the team inferred the rocks had lost about 20 percent of their volume. After correcting for reduced Mars gravity, they were able to calculate the pebbles had traveled an estimated 50 kilometers, or about 30 miles from their source.
What might that source be? Based on Jerolmack's estimation of the rock's composition and other clues as to the direction of water flow, they believe the original source was from a crater rim located about 30 kilometers away (as the crow flies).
The team's work is significant not only for what they suggest about a long-gone Mars river, but also for their technique, which could be applied to stones found here at home - and on other far away planets.
"Now we have a new tool we can use to help reconstruct ancient environments on Earth, Mars and other planetary bodies where rivers are found such as Titan," Jerolmack said.