Water on Mars is thought to have evaporated roughly 3 billion years ago. However, two scientists reviewing data from NASA's Mars Reconnaissance Orbiter discovered traces of liquid water on Mars as recently as 2 billion to 2.5 billion years ago, implying that water flowed there for a billion years longer than previously thought.
The results, which were published in the journal AGU Advances, focus on the chloride salt deposits left behind when freezing meltwater drained across the terrain.
While the structure of certain valley networks suggested that water may have flowed on Mars lately, the salt deposits are the first mineral evidence that liquid water existed. The discovery raises fresh concerns about how long, if at all, microbial life may have thrived on Mars. Wherever there is water, there is life, at least on Earth.
Ellen Leask, a post-doctoral researcher at Johns Hopkins University's Applied Physics Laboratory, and Bethany Ehlmann, a professor at California Institute of Technology (Caltech), used data from the MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument to map the chloride salts across the clay-rich highlands of Mars' southern hemisphere, which are pocked by impact craters.
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One of the keys to date the salts was the number of craters: the less craters a landscape has, the younger it is. Scientists can determine the age of a region of the surface by counting the number of craters on it.
MRO has two cameras that would be ideal for this task. With its black-and-white wide-angle lens, the Context Camera aids scientists in mapping the area of the chlorides.
Scientists use the High-Resolution Imaging Science Experiment (HiRISE) color camera to zoom in on features as small as a Mars rover from orbit, allowing them to observe details as small as a rover from space.
Leask and Ehlmann discovered several of the salts in depressions on gently sloping volcanic plains, which were originally home to little ponds, using both cameras to build digital elevation maps.
"What's astonishing is that MRO has led to fresh findings regarding the nature and timing of these river-connected ancient salt ponds after more than a decade of supplying high-resolution image, stereo, and infrared data," said Ehlmann, CRISM's deputy main scientist.
NASA's Mars Odyssey spacecraft, which launched in 2001, detected the salt crystals for the first time 14 years ago.
(Keywords: Water, Mars, Leask and Ehlmann, High-Resolution Imaging Science Experiment (HiRISE), California Institute of Technology (Caltech), Compact Reconnaissance Imaging Spectrometer for Mars (CRISM))