Friday, March 30, 2012

MARS: Dusty, Acidic Glaciers Could Explain Layered Deposits

A number of recent publications have suggested that the sulfate-bearing ILDs formed by groundwater upwelling, where subsurface water breaches the surface during occasional upwelling events. 

This process has been invoked to explain most of the sulphate-bearing deposits on Mars.

Researchers from the Planetary Science Institute (PSI) and NASA Johnson Space Center (JSC) have proposed a new hypothesis to explain a class of enigmatic geologic features on Mars that have puzzled scientists for decades.

The new results, published recently in the journal Geology, suggest that large sedimentary deposits in the Valles Marineris termed Interior Layered Deposits (ILDs) may have formed in a cold, dry ancient Martian climate as the remnants of massive dust-rich glaciers that may have once filled this canyon system.

"Icy weathering might be a major part of the geologic story on Mars," said PSI Research Scientist Joseph Michalski, "The planet has been in a cold, frozen state for a long time. In the distant past, it was also cold, but volcanoes were much more active, periodically pumping huge amounts of sulfur into the atmosphere, which could have ultimately ended up trapped within ice alongside plentiful dust."

Valles Marineris is a 3,000 kilometer-long tectonic trough system on Mars, which reaches depths of approximately eight kilometers (five miles) below the surrounding terrain.

Inside the canyon are vast mounds of layered sediments of enigmatic origins. Since their discovery by the Mariner 9 spacecraft about 40 years ago, the ILD deposits found within the Valles Marineris have escaped explanation.

Their setting within the trough and canyon system has prompted some previous researchers to suggest that the ILDs formed from volcanic processes because the faulting and rifting that formed the canyon could easily lead to thinning of the crust, high heat flow, and ascent of magma.

However, in the late 1990s, the Thermal Emission Spectrometer instrument showed that the deposits contain gray hematite, similar to deposits explored at Meridiani Planum by the Mars Exploration Rover Opportunity and shortly after, a French team of researchers produced intriguing new results that further complicated the interpretations of ILDs on Mars.

They showed that the deposits contain sulfate minerals - which are typically found in desert playa or shallow sea environments on Earth and are not dominant phases in volcanic terrain.

One big part of the problem is the size of the altered, layered sediments that rise several kilometers from the canyon floor in places.

Proposed ideas have included the suggestion that the canyon once housed a vast system of deep lakes but, the canyon is not topographically constrained on all sides, so it is difficult to imagine how a lake could have existed there without spilling into the low topography to the north.

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