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Posted: September 18, 2008
According to many studies, flowing water helped carve out giant valleys like Valles Marineris – a huge canyon system that runs nearly a quarter of the way round Mars’ equator – up until around 3.7 billion years ago. But the results of new research, led by Catherine Weitz of the Planetary Science Institute, present strong evidence that sustained rainfall and associated valley-carving events also occurred on the plains surrounding Valles Marineris as recently as three billion years ago.
Valles Marineris is made up of a network of canyons and stretches around nearly one quarter of Mars' equator. It is about 4000 kilometres long and in some places 600 kilometres wide and 10 kilometres deep, seven times deeper than Earth's Grand Canyon. Image: USGS. To arrive at this conclusion the research team used the Mars Reconnaissance Orbiter (MRO) HiRISE camera to study light-toned layered sediments that were likely laid down by flowing water inside Valles Marineris, and compared them to deposits found outside the canyon. The HiRISE camera enabled them to pick out features down to a size of around one metre. The layered deposits that appear inside canyons within the Valles Marineris system were found to share similar characteristics in terms of their composition, brightness and shape, suggesting that they were formed in the same type of environment. On the plains, however, the scientists see a wide variety of different features, such as sediment layers of varying thickness that have eroded at different rates to make a step-like pattern or ‘inverted channels’. Inverted channels form on Earth when sediment is deposited in streambeds over time. When the streams dry up, the softer terrain erodes away, leaving the harder, cemented sediments standing above the surrounding terrain. "What we found was that these light-toned layered deposits on the plains are very different from those within Valles Marineris," says Weitz. "There are a lot of variations in brightness, colour, and erosional properties that we don't see for light-toned deposits inside Valles Marineris. This suggests that the processes that created the deposits outside Valles Marineris were different from those operating inside."
(A) HiRISE image showing inverted channels along the plains west of a canyon called Juventae Chasma within the Valles Marineris system. (B-C) Examples of locations where light-toned beds are exposed within and between the inverted channels (bold arrows). Image courtesy C. Weitz. The scientists speculate that either the properties of the materials changed over time, or there were variations in processes outside of Valles Marineris that affected the deposits in different ways, such as water related activity that occurred on a large scale around the Valles Marineres system for sustained time periods, rather than in local areas for brief episodes as other research groups suggest. “The layering seen in the deposits on the plains shares some morphologic similarities with other sedimentary deposits on Mars that were emplaced by water activity, such as in Holden crater and the Eberswalde delta,” Weitz tells Astronomy Now. "This was a big surprise because no one thought we'd be seeing these extensive fluvial systems in the plains all around Valles Marineris that were formed during the Hesperian Era [3.7-3 billion years ago]. Everyone thought that by then the climate had pretty much dried out." Although it is plausible that the sediment layers could also have been deposited by wind or volcanic processes, the team think this is unlikely since they identified minerals that often form in the presence of liquid water on Earth.
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