(ORDO NEWS) — Determining the history of Mars, how it formed and evolved over time, has been the goal of orbital and surface missions on the Red Planet for decades.
After analyzing data from several such missions, a team of researchers led by Steve Ruff of the School of Earth and Space Science at Arizona State University determined that the mysterious olivine rocks in Gusev Crater, as well as in and around Jezero Crater, could be a rock called “ignimbrite,” which is simultaneously igneous and sedimentary and is formed as a result of cataclysmic explosive eruptions from huge volcanic calderas.
If the team is right, it could lead to a better understanding of olivine-rich rocks elsewhere on Mars, and could also point to a style of volcanism more prevalent in Mars’ early history. The results of their study were recently published in the journal Icarus.
“There are many ideas about the origin of the olivine-rich rocks that cover much of the region called Nili Fossae, which includes Lake Jezero Crater,” Ruff said. “This is a debate that has been going on for almost 20 years.”
Explosions of rocks rich in olivine as well as carbonate link Gusev crater, explored 16 years ago by NASA‘s Spirit rover, and the Nili Fossae region, where the Mars 2020 Perseverance rover is currently exploring Jezero crater.
Both sites contain the largest amount of olivine ever found on Mars. The similarity of composition and morphology of widely separated olivine-bearing rocks has not been previously studied. Now it turned out that they were formed in a similar way.
Olivine is a common silicate mineral that forms from magma formed in the mantle of Mars (the same process occurs on Earth).
Thus, the volcanic process is a reasonable explanation for the origin of olivine-rich rocks on Mars. However, various scenarios have already been proposed before – from lava flows to a giant impact that scooped up olivine from the mantle.
Ruff and his team set out to test the lead hypothesis, which is soft deposition of ash from volcanic plumes. But their observations revealed a much more turbulent history.
In particular, Ruff studied a mosaic of images taken with the Spirit rover’s microscopic camera (which looks like a geologist’s handheld lens) and noticed rocks with an unusual texture. Ruff turned to an online rock image library on Earth and came across volcanic rocks with textures remarkably similar to those seen from Mars.
“It was a eureka moment,” Ruff says. “I saw the same textures in the Gusev crater rocks as in a very specific kind of volcanic rock found here on Earth.”
The images were taken from a rock called “ignimbrite”, which is essentially both igneous and sedimentary. Ignimbrites are formed as a result of flows of pyroclastic ash, pumice and blocks from the largest volcanic explosions,
“Imagine a cloud of hot gases, almost molten ash and pumice, moving through the landscape for tens of miles and accumulating in layers up to hundreds of feet thick in just a few days,” Ruff says.
Once formed, ignimbrite deposits cool slowly over several months or years. This results in complex networks of cracks known as “cooling seams” that form as thick piles of ash and pumice are compressed. Ruff found similar fault patterns in olivine-rich deposits on Mars, further supporting the origin of ignimbrites.
On Earth, ignimbrites are found in places like Yellowstone National Park in the western United States. The yellow-ocher rocks are ignimbrites from a huge volcanic caldera that formed during a period that began about 2.1 million years ago and is now filled.
“No one has previously proposed ignimbrites as an explanation for the olivine-rich rock on Mars,” Ruff said. “And it’s quite possible that this is the rock that the Perseverance rover has traveled and sampled over the past year.”
Mars has the largest volcano in the solar system and lava flows covering vast swaths of the planet, so volcanic rocks are a given. But only in a few places the presence of ignimbrites was assumed, and so far only speculatively.
Taking into account the new finds of this group, it is possible that ignimbrites occur both in the Jezero crater and in the Gusev crater. Other olivine-rich rock sites are also candidates for ignimbrite deposits, and all appear to have been formed early in Mars’ history, within the first billion years or so.
“An olivine-rich composition is unusual for most ignimbrites on Earth, but there is evidence of such a composition in the oldest of them.
Now that strong evidence has been obtained for the existence of ancient olivine-rich ignimbrites on Mars, this may indicate a style of volcanism, cataclysmic explosive eruptions of olivine-rich magma that occur early in the planet’s geological evolution,” Ruff said. The answer, in the case of Mars, may come from rock samples collected by the Perseverance rover and returned to Earth by future missions.”
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