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April 25, 2022

Determining the history of Mars, how it formed and evolved over time, has been a goal of orbiter and rover missions to the Red Planet for decades.

By analyzing data from several of these Mars missions, a team of researchers led by Steve Ruff from Arizona State University School of Earth and Space Exploration determined that the enigmatic olivine-rich bedrock in Gusev Crater and in and around Jezero Crater may be a type of rock called “ignimbrite”, which is both igneous and sedimentary and forms as a result of explosive eruptions cataclysmic from huge volcanic calderas.

Olivine-rich bedrock outcrop in Gusev Crater observed with Spirit’s Panoramic Camera (Pancam) in 2005. Image by NASA/JPL/Cornell/ASU
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If the team is correct, this may lead to a better understanding of olivine-rich bedrock in other places on Mars, and may also point to a more common style of volcanism in Mars’ ancient history. The results of their study were recently published in Icarus.

“There are many ideas about the origin of the olivine-rich bedrock that covers large parts of an area called Nili Fossae, which includes Jezero Crater,” Ruff said. “This is a debate that has been going on for almost 20 years.”

Outcrops of bedrock rich in olivine and also in carbonate connect the Gusev crater, explored 16 years ago by NASA’s Spirit roverand the Nili Fossae region where March 2020 The Perseverance rover currently exploring in Jezero Crater. Both locations have the highest abundance of olivine ever identified on Mars. The similarities in the composition and morphology of the widely separated olivine-rich rocks had not previously been investigated. Now it looks like they formed the same way.

Olivine is a common silicate mineral that comes from magma generated in the mantle of Mars (this same process also occurs on Earth). Thus, some sort of volcanic process is a reasonable explanation for the origin of olivine-rich rocks on Mars. But scenarios ranging from lava flows to a giant impact dredging olivine from the mantle had been proposed previously.

Ruff and the team aimed to test a primary hypothesis involving gently deposited ash from volcanic plumes. But their observations revealed a far more violent story.

In particular, Ruff examined mosaics of images from the Mars rover Spirit’s Microscopic imager (which looks like a geologist’s magnifying glass) and noticed some rocks with an unusual texture. Ruff searched an online library with images of rocks on Earth and came across volcanic rocks with textures that looked remarkably like mosaics from Mars.

“It was a eureka moment,” Ruff says. “I was seeing the same type of textures in the rocks of Gusev Crater as a very specific type of volcanic rock found here on Earth.”

The left image is a mosaic from Spirit’s Microscopic Imager showing dark-toned diagonally oriented features with flame-like shapes that are possible from the flattened pumice fragments known as fiamme, which include clear crystals ( white rectangle), both resembling those in the Earth ignimbrite example on the right. The dark corner parts are the shadows of the rover’s hardware. The white scale bar represents 1 cm in both images. Images by NASA/JPL/USGS and Scripps Institution of Oceanography

The images were from a type of rock called “ignimbrite”, which is essentially both igneous and sedimentary. Ignimbrites form as a result of flows of pyroclastic ash, pumice and boulders from the largest known volcanic explosions on Earth.

“Imagine a cloud of hot gases and ash and nearly molten pumice streaking across the landscape for tens of miles and piling up in layers up to hundreds of feet thick in just a few days,” Ruff said. .

After emplacement, ignimbrite deposits cool slowly over months to years. This leads to intricate networks of fractures called cooling joints, which form when the thick piles of ash and pumice contract. Ruff recognized particularly similar fracture patterns in olivine-rich rock deposits on Mars, adding to the evidence for an ignimbrite origin.

On Earth, ignimbrites are found in places like Yellowstone National Park in the western United States. The yellow-colored rocks are ignimbrites from a huge volcanic caldera that formed over a period beginning around 2.1 million years ago and is now infilled.

“No one had previously suggested ignimbrites as an explanation for olivine-rich bedrock on Mars,” Ruff said. “And it’s possible that this is the type of rock that the Perseverance rover rolled over and sampled last year.”

Mars has the largest volcano in the solar system and lava flows that cover huge swaths of the planet, so volcanic rocks are a no-brainer. But only a few places had been suggested to contain ignimbrites, and so far only tentatively.

The false color image on the left shows olivine-rich deposits in the Nili Fossae area compared to welded ignimbrite deposits on Earth (true color). The fractures in the Earth example are cooling joints, much like those in the Mars example. Images by HiRISE/Google Earth

With the new discoveries of this team, it is possible that ignimbrites occur in Jezero and Gusev craters. Other locations with olivine-rich bedrock are also candidate ignimbrite deposits, and all appear to have formed early in Mars’ history, around the first billion years or so.

“The olivine-rich composition is unusual for most ignimbrites on Earth, but there is evidence for this composition in older ones. Now, with the strong evidence for ancient olivine-rich ignimbrites on Mars, this may indicate a style of volcanism, cataclysmic explosive eruptions of olivine-rich magma, which occur early in a planet’s geologic evolution,” Ruff said. “The answer in the case of Mars may come from rock samples collected by Perseverance and returned to Earth by future assignments.”

The other authors of this study are Victoria Hamilton of the Southwest Research Institute, Deanne Rogers of Stony Brook University, Christopher Edwards of Northern Arizona University and Briony Horgan of Purdue University.

Abdul J. Gaspar