Strange Martian mineral deposit probably from volcanic explosions

The ash falls due to old volcanic explosions are the likely source of a strange mineral deposit near the landing site for NASA's next rover, reveals a new study. The research, published in the journal Geology, could help scientists establish a chronology of volcanic activity and environmental conditions in early March.

"This is one of the most tangible proofs of the idea that explosive volcanism was more common in early March," said Christopher Kremer, a graduate student at Brown University who led the work. "Understanding the importance of explosive volcanism in early March is essential to understand the water balance of Martian magma, the abundance of groundwater and the thickness of the atmosphere. "

Volcanic explosions occur when gases such as water vapor are dissolved in underground magma. When the pressure of this dissolved gas exceeds the maximum capacity of the rock, it explodes by projecting a burning cloud of ash and lava into the air. Scientists believe that this type of eruption should have occurred very early in the history of Mars, while there was more water available to mix with magma. As the planet dried up, the volcanic explosions would have faded and would have given way to more effusive volcanism – a gentler seepage of lava on the surface. There is ample evidence of an expansive phase on the Martian surface, but evidence of the early explosive phase has not been easy to spot with orbital instruments, Kremer explains.

This new study examined a deposit located in an area called Nili Fossae that has long been of interest to scientists. The deposit is rich in mineral olivine, which is common in planetary interiors. This suggests that the deposit originates from subterranean depths, but the way the material has reached the surface has not been clearly defined. Some researchers have suggested that there is another example of effusive lava flow. Others suggested that the material had been dredged by a large impact of asteroids, the impact that had formed the giant basin of Isidis in which the deposit is located.

For this study, Kremer and his colleagues at Brown used NASA's Mars Reconnaissance Orbiter high resolution images to examine the geology of the deposit in detail. Kremer's co-authors on the work are Mike Bramble, another graduate student of Brown, and Jack Mustard, professor in the Department of Earth, Environmental and Planetary Sciences of Brown, and Kremer's advisor .

"This work stands out methodologically from what other people have done in examining the physical form of the land composed of this bedrock," Kremer said. "What is the geometry, the thickness and the orientation of the layers that compose it.We have found that the explanation of explosive volcanism and ash falls meets all expectations what we observe in orbit ".

Work has shown that the deposit extends uniformly over the surface in long continuous layers that uniformly cover hills, valleys, craters and other features. This equal distribution, says Kremer, is much more compatible with falling ashes than lava flow. A lava flow should accumulate in low areas and leave thin or non-existent traces in the highlands.

And stratigraphic relationships in the region exclude an origin associated with the impact of Isidis, say the researchers. They showed that the deposit was based on known characteristics after the Isidis event, suggesting that the deposit itself was also after.

The explanation of the ash falls also allows to take into account the unusual mineral signatures of the deposit, say the researchers. The olivine shows signs of widespread weathering by contact with water – much more weathering than other olivine deposits on Mars. This makes sense if it were ash falls, which are porous and therefore likely to be altered by small amounts of water, explain the researchers.

In total, say the researchers, these orbital data rely heavily on the origin of ash falls. But the team will not have to rely solely on orbital data for a long time. The NASA Mars2020 robot is due to land in the Jezero crater, located in the olivine field. And there are exposures of the deposit in the crater. The olivine-rich unit will almost certainly be one of the mobile's exploration targets, and could have the last say on the nature of this deposit.

"What is exciting is that we will see very soon if I am right or wrong," Kremer said. "So it's a bit scary, but if it's not an ashtray, it's probably going to be a lot more weird – it's just as much fun, if not more."

If it turns out to be a drop of ash, Kremer says, this will validate the methodology used in this study as a way to look at potential ash drop deposits elsewhere on Mars.

But no matter what the rover will find will be important to understanding the evolution of the red planet.

"One of the top 10 discoveries of March 2020 will be determining the nature of this unit containing olivine," said Mustard, Kremer's advisor. "It's something that people will write and talk for a long time."