Researchers looking at data supplied by NASA's Spirit rover have found what could be the best evidence yet there was once life on Mars.
Searching the red planet's Gusev crater nearly a decade ago, Spirit stumbled across a rocky formation bearing a striking resemblance to something found in hot springs and geysers here on Earth.
In a just-released study published in journal Nature, researchers from Arizona State University say the silica structures "satisfy [the] definition of potential biosignatures".
The structures' significance wasn't realised at the time the images were sent back in 2007.
"Here we report remarkably similar features within active hot spring/geyser discharge channels at El Tatio in northern Chile, where halite-encrusted silica yields infrared spectra that are the best match yet to spectra from Spirit," the scientists claim.
"Furthermore, we show that the nodular and digitate silica structures at El Tatio that most closely resemble those on Mars include complex sedimentary structures produced by a combination of biotic and abiotic processes."
In other words, the rocky formations found at El Tatio which best match those found on Mars are produced by a combination of biological and non-biological processes.
Rocks on Mars, left, and at El Tatio in Chile, right (NASA/Nature)
It's strongly suspected life began on Earth in deep-sea geysers. El Tatio has conditions not too dissimilar from Mars - including low precipitation, high evaporation and water than freezes at night and melts in the day.
"Such conditions provide a better environmental analog for Mars than those of Yellowstone National Park and other well-known geothermal sites on Earth."
Spirit landed on Mars in 2004, and its mission was expected to last only 90 days. It ended up rolling around Mars for five years before getting stuck.
One of its goals was to find evidence that water once flowed on the red planet - now six years after contact with the rover was lost, there's hope it might have succeeded in finding much more than that.
Rocks on Mars, left, and at El Tatio in Chile, right (NASA/Nature)
The researchers say they'll probably need to bring some of the Martian structures back to Earth to confirm it though - something not yet possible.
"Because of the challenges in obtaining unambiguous evidence... coordinated microscopic and compositional analyses of samples returned to laboratories on Earth may be required to reach a robust conclusion as to the presence or absence of past Martian life in these rocks."
Last year NASA confirmed not only was there water on Mars, there still is - even in a liquid form.
Spirit's sister rover, Opportunity, is still working - having lasted 50 times longer than intended.
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