When the Curiosity rover started returning pictures from the Gale Crater on Mars, scientists were somewhat surprised to find that there appeared to be pebbles on the ground, likely transported and molded by an ancient river. A new study has now modeled the extent of that river in remarkable detail, thanks to the shape of the pebbles.
The research, published in the journal Nature Communications and partially supported by the National Science Foundation, created a mathematical model based on pebbles on Earth, using geometry to work out how their eroded shape arose as they were dragged along the river bed. Applying that model to Gale Crater, the scientists found it could explain the river that flowed there based on the geometry of Martian pebbles.
The study found that pebbles in Gale Crater, which is 140 kilometers (90 miles) wide, were transported 50 kilometers (30 miles) by a river 3 billion years ago from a source on the crater wall. This river would have flowed at about 1 meter (3 feet) per second and it would have been tens of centimeters deep. This is one of the first and most detailed analyses ever on what a Martian river may have looked like.
“Just by looking at the shape, you can deduce the history of the pebbles,” co-author on the study Gabor Domokos from the Budapest University of Technology and Economics told IFLScience. “This paper is about Martian pebbles, but the same could be done in principle on many other planets.”
Some of the rounded pebbles analysed in the study can be seen here. NASA/JPL-Caltech/MSSS.
The research relied not just on images of rounded pebbles taken by Curiosity, but also on data provided by some of the instruments on board. This allowed scientists to deduce that some of the pebbles were made of basaltic rock, and by comparing those to similar rocks on Earth, they could model what sort of river would have eroded the pebbles into their shape.
“This kind of channel would be something you could stand in, it would have been a brisk current, not one that would knock you over, it would just be pushing the little pebbles that were rolling along,” co-author Douglas Jerolmack from the University of Pennsylvania also told IFLScience.
The researchers said the same shape and geometry of the pebbles, and the way they had been deposited, was not likely to be the result of other processes such as wind. They added that it was unlikely that there were bigger rivers in Gale Crater than this, based on what they had seen so far, as these were the largest pebbles spotted – larger rivers would be able to transport bigger rocks.
Interestingly, this method of studying the shape of the pebbles to infer how they were moved by a river could be applied to other worlds, not just Mars. “I think the best bet is probably [Saturn's moon] Titan,” said Jerolmack. “In terms of promise for where we might find another potential rival channel, it would be Titan, where we have one – and only one – image of very rounded pebbles there [from the Huygens lander in 2005]. That’s the mouth of what looks like a river.”
Image in text: Our sole picture from the surface of Titan, taken by the Huygens lander on 14 January 2005. NASA/ESA.