Parts of ancient Mars may have been wet for a billion years longer than scientists previously thought, a new study of images of the red planet's surface suggests.
Along with Earth and the other inner planets of our solar system, Mars formed about 4.5 billion years ago. Scientists have long known that flowing water formed many of the features seen on Mars today, but previous studies suggested that water runoff from precipitation had ceased after the first billion years of Mars' history, called the Noachian Epoch.
But one team of scientists thinks these rains and floods persisted into more recent — geologically speaking — periods in Mars' history.
Catherine Weitz, a senior scientist with Planetary Science Institute in Tucson, Ariz., and her colleagues examined close-up images of the plains surrounding the huge Valles Marineris canyon system taken by the HiRISE instrument aboard NASA's Mars Reconnaissance Orbiter (currently still circling the planet). HiRISE can resolve features as small as 3 feet (1 meter) in diameter.
Weitz and her team noticed that light-toned layered deposits in the plains around Valles Marineris had different features from those inside the canyon; these features suggested that water continued to flow on a large scale in these plans after the Noachian, into the Hesperian epoch of Mars, until about 3.7 billion to 3 billion years ago. Phenomena associated with flowing water are called fluvial processes.
"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," Weitz said. "Everyone thought that by then the climate had pretty much dried out."
Another recent study suggested that periods of rain and flooding on Mars were longer in duration, not just short bursts, as had previously been thought.
The deposits that Weitz and her team observed outside the canyon showed "a lot of variations in brightness, color and erosional properties that we don't see for light-toned deposits inside Valles Marineris," Weitz said. "This suggests that the processes that created the deposits outside Valles Marineris were different from those operating inside."
Two locations near the canyon had inverted channels, which, on Earth, form when sediment deposits in a streambed over time. After the stream dries up, the softer terrain surrounding the streambed erodes away, leaving the harder, cemented stream sediments forming a ridge above the surrounding terrain.
Other explanations for the deposits, such as explosive volcanism and wind deposition, can't be ruled out, Weitz said, but the distinctiveness of the features suggests a fluvial origin, she added.
"What we're seeing tells us that this light-toned layering on the plains was associated with fluvial activity that wasn't occurring just in little pockets over very brief episodes, but rather on a much larger scale for sustained time periods," she added. "For some reason, there was precipitation around Valles Marineris that allowed these systems to form out on the plains."
The details of the study are posted online in the journal Geophysical Research Letters.
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