Data from two instruments aboard NASA’s Mars Reconnaissance Orbiter have led to the discovery of a rare acid-sulfate mineral deposit on Mars, providing the first mineralogical evidence that the planet once had glaciers.

For years, scientists have speculated that glaciers might once have crept through what many people know as ‘the Grand Canyon of Mars’ – the 4,000-km-long and 8-km-deep canyon system called Valles Marineris.

Using satellite images, researchers have identified features that might have been carved by past glaciers as they flowed through the canyon system. However, these observations have remained highly controversial and contested.

Now, planetary scientists from the United States and Germany using data collected by the Mars Reconnaissance Orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High-Resolution Imaging Science Experiment (HiRISE) have discovered what they say is the first mineralogical evidence of ancient Martian glaciers.

They spotted an unusual layer of a mineral called jarosite on the southern wall of Ius Chasma, a large canyon within the Valles Marineris system.

“Acid-sulfate minerals such as jarosite are often used as indicators of environmental conditions on Mars. These minerals form in a diverse set of environments on Earth; however, all known Martian jarosite deposits are most likely evaporative, from evaporation of either groundwater or ponded water,” they explained in a paper published in the journal Geology.

“The morphology and geologic context of the Ius deposit are unique on Mars, and difficult to explain with an evaporative or groundwater mechanism.”

“We propose instead that it was deposited along the margins of a past glacier.”

The scientists speculate that it may have formed during the Noachian (4.1-3.7 billion years ago) or Hesperian (3.7-3.0 billion years ago) period via a mechanism similar to one observed at glaciers in the Svalbard, arctic Norway.

“Atmospheric sulfur becomes trapped in the ice, is warmed by the Sun, and reacts with the water to produce highly acidic sulfate minerals like jarosite along the margins of the glacier.”

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Selby Cull et al. A new type of jarosite deposit on Mars: Evidence for past glaciation in Valles Marineris? Geology, published online September 26, 2014; doi: 10.1130/G36152.1