Data from the Opportunity rover on the rim of Endeavour crater indicates an asteroid impact changed Martian water forever (Source: NASA)
The surface of Mars once flowed with water fresh enough to sustain life, according to data collected by two of NASA's Martian rovers.
The findings, published in the journal Science, suggest freshwater rivers and streams once existed in the Meridiani Plains region near the Martian equator, 3.8 billion years ago.
But all that changed when a huge asteroid impact turned the fresh water into a highly acidic soup.
"All the evidence we have indicates the local Martian water was like spring water that you could drink," according CSIRO's Dr Paulo de Souza, a scientist with NASA's Mars exploration rover program.
"It would have been hospitable to any microbial life that may have existed at the time."
But, according to de Souza, the asteroid impact exposed volcanic rocks that were then subjected to chemical weathering, resulting in salts dissolving into the water.
"As time passed with this and other impacts in the region, a more acidic environment developed, with the water becoming as strong as vinegar," he says.
De Souza and colleagues can determine the chemistry of the water based on the different types of minerals formed in the sediments left behind.
"We not only know that there was water on Mars, but we can see how that water changed over time forming different minerals," says de Souza.
"Looking at those minerals is just like looking at a book of history, with the story of the planet recorded in those minerals."
Eyes in the skyThe researchers sent Opportunity to investigate part of the Endeavour Crater rim where NASA's Mars Reconnaissance Orbitor spacecraft detected iron- and aluminium-rich clay minerals.
The rover sampled a geological feature called the Matijevic formation, a grouping of fine-grained, layered rocks enriched with clay minerals.
Analysis of the spherules, veins and fractures in these mineral rich rocks indicates the Matijevic formation predates the impact event that formed Endeavour crater.
They represent the oldest Martian rocks, as well as the earliest evidence of water activity, so far encountered by Opportunity.
The authors found these older rocks were formed in conditions that would have been favourable to life or prebiotic chemistry.
"The formation on the rim of this crater had evidence of a more suitable environment for life to form and evolve in fresh water," says de Souza.
"We haven't actually found the fresh water, that would have been there four billion years ago, but what we saw were the minerals that could only be formed if we have fresh water present."
Meanwhile, the younger rocks sitting on top of the formation, which are believed to date from after the impact event, contain signatures for super-salty, highly-acidic water, which would have made life tough for even the hardiest extremophile microorganisms.
Ongoing missionNASA's twin six-wheeled Mars exploration rovers, Spirit and Opportunity, landed on opposite sides of the red planet in January 2004, on what was expected to be a 90-day mission to search for geological evidence of past water.
Spirit's mission ended in March 2010, however its twin Opportunity is still exploring the red planet, ten years after landing.
"We've travelled over 38 kilometres across the surface of Mars ... Opportunity has just come out of another Martian winter," says de Souza.
"And a wind storm the other day has cleared the dust off the rover's solar panels."
According to de Souza, that means despite a decade of service, there's lots more science that can still be done.