NASA's Mars orbiter: Water flowed on Mars much longer than previously assumed

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 findings, which were published in the journal AGU Advances, focus on the chloride salt deposits left behind as freezing meltwater evaporated across the terrain.

While the structure of some 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 new questions about how long, if at all, microbial life could 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.

One of the keys to dating the salts was the number of craters: the less craters a terrain has, the younger it is. Scientists can estimate the age of an area 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) colour camera to zoom in on details 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.

The researchers also discovered twisting, dry channels nearby, which were once streams that supplied surface runoff (from ice or permafrost melting) into these ponds. They were able to date the deposits using crater counting and indications of salts on top of volcanic landscape.

"What is amazing is that after more than a decade of providing high-resolution image, stereo, and infrared data, MRO has driven new discoveries about the nature and timing of these river-connected ancient salt ponds," said Ehlmann, CRISM`s deputy principal investigator.

The researchers also discovered twisting, dry channels nearby, which were once streams that supplied surface runoff (from ice or permafrost melting) into these ponds. They were able to date the deposits using crater counting and indications of salts on top of volcanic landscape.

NASA's Mars Odyssey orbiter, which launched in 2001, detected the salt crystals for the first time 14 years ago.

MRO, which has higher-resolution equipment than Odyssey, was launched in 2005 and has been researching the salts, among other things, on Mars since then.