Friday, March 31, 2023

Mystery of “impossible” Mars mineral solved

Mysterious Minerals: In July 2015, the Mars rover “Curiosity” made a surprising discovery during a borehole – a mineral that shouldn’t actually exist on Mars. This is because the rare tridymite usually forms when silicon-rich lava cools rapidly. But these are not present at the site where the mineral was found in the Martian Gale Crater. Researchers have now clarified how tridymite may have formed.

The Mars rover Curiosity during its exploration of Gale Crater has already provided some surprising insights into the geology and early evolution of Mars. Their measurements have demonstrated the existence of organic molecules in Martian sediments, suggesting that the Martian atmosphere was once oxygen-rich and that a potentially life-friendly freshwater lake once existed in Gale Crater.

Tridymite was among this gray material uncovered by Curiosity’s drilling.© NASA/JPL-Caltech/MSSS

amazing find

However, a discovery made on July 30, 2015 still puzzling: When samples were taken from the sediments of the primordial waters of Mars, the Curiosity rover detected a fine, gray powder whose chemical composition surprised . Since this sample—named “buckskin”—contained about 74 percent silicon dioxide—this was unusual for a predominantly basaltic layer of the Red Planet. What was even more surprising was that these silicates also contained about 16 percent monoclinic tridymite.

“The discovery of tridymite in the sediments of Gale Crater is one of the most surprising discoveries the Curiosity rover has made in its ten years of Mars exploration,” says co-author Kirsten Seeback from Rive University in Texas. Tridymite is formed when silicate is first highly heated and then rapidly cooled. This high temperature modification of quartz is already extremely rare on Earth; It has only been found at two volcanoes in Japan and one site in Greenland.

How did tridymite get into the crater of Mars?

On Mars, and in particular Gale Crater, geologists would not have expected such high concentrations of tridymite. “Typically, tridymite originates from felsic magma, which is rare on Mars,” explained first author Valerie Pere and colleagues from Northern Arizona University. “While there is ample evidence of basaltic eruptions on Mars, this is a different chemistry.” There is also no evidence of volcanoes containing silica-rich magma in Gale Crater.

Also: “We found this mineral at the bottom of an ancient lake,” says Seeback. The source rock is not of magmatic origin, but consists mainly of older sediments. To find out how tridymite might have gotten into this environment, the team went through all conceivable formation scenarios and compared them with data from Gale Crater.

The peaks in this spectrometric analysis indicate the presence of tridymite.© NASA/JPL-Caltech/MSSS

From the Magma Room to the Lake

Result: Tridymite probably originated from a more distant Martian volcano, whose magma stays in the magma orbit longer than usual and partially cools in the process. As a result, the heavier components, some of which had already crystallized, sank to the bottom and the lighter, silicate-rich components of the magma moved upwards. When the volcano erupted, this part of the molten rock, rich in silicate and consisting of tridymite, ejected mainly as lava and ash.

“Based on observations of Gale Crater, there must have been an example of an explosive, siliceous volcanism on Mars in the Hesperian era at least between 3.5 and 3 billion years ago,” Pere and his colleagues wrote. Ash from this eruption was blown by the wind to Gale Crater where it fell on the lake and tributaries and gradually settled to the bottom, enriching it.

‘Because we only found this mineral in this one layer and in high concentrations,’ says Seebach, the volcano must have erupted at about the same time that the lake was present. “Analyses also indicate that the ash was chemically weathered and sorted out by water.” This also speaks to the deposition of volcanic ash on the lake.

“Not a completely basaltic world”

According to the research team, these results suggest that the Red Planet’s volcanism may have been more diverse and complex than previously thought. “Current and future missions should look for further evidence of a geologically evolving volcano and its duration,” the scientist says. “Mars is not a completely basaltic world.” (Earth and Planetary Science Letters, 2022; doi: 10.1016/j.epsl.2022.117694)

That: Rice University

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