A team of researchers at the Arizona State University’s School of Earth and Space Exploration has determined that the olivine-rich bedrock in the Gusev crater and in and around the Jezero crater on Mars may be a special type of rock called “ignimbrite”. This rock is both igneous and sedimentary and forms due to explosive eruptions from volcanoes. The results of their study have been published in a research article in the journal Icarus.
If the team’s hypothesis is current, it will lead to a better understanding of the olivine-rich bedrock in other places on Mars. Bedrock rich in olivine and carbonate links the Gusev crater, which was explored 16 years ago by NASA’s Spirit rover, and the Nili Fossae region, where the Mars 2020 Perseverance rover is currently exploring. Both locations have the highest abundance of olivine yet observed on Mars.
Olivine is a magnesium iron silicate mineral that is the primary component of Earth’s upper mantle and is also common in Mars’s mantle. It is named olivine for its typical olive colour that may also appear reddish to the oxidation of iron.
The similarities in composition and morphology between the olivine-rich rocks in the two widely-separated regions of Mars have not yet been investigated before. But this study seems to indicate that they formed in a similar way. Even though this mineral is common in the mantle of Mars, there is yet to be a conclusive explanation for the beds on the surface of the planet.
Scientists have proposed scenarios ranging from lava flows to a giant meteor impact dredging up the mineral in the past. The research team tried testing a hypothesis which involved volcanic ash being gently deposited by plumes of smoke but their observations pointed towards a much more violent history.
The team examined mosaics of images from the Spirit rover’s Microscopic Imager and noticed rocks with an unusual texture. The team then consulted an online library with images of rocks on Earth and came across some volcanic rocks with textures that looked remarkably similar to those in the mosaics from Mars.
The images of rocks from earth featured ignimbrites, which form as the result of flows of pyroclastic ash (fast-moving mixture of rock fragments, gas, and ash), pumice and blocks from the largest volcanic explosions on Earth.
“That was a eureka moment. I was seeing the same kind of textures in the rocks of Gusev crater as those in a very specific kind of volcanic rock found here on Earth. Imagine a ground-hugging cloud of hot gases and nearly molten ash and pumice flowing through the landscape for dozens of miles and piling up in layers up to hundreds of feet thick in just a few days,” said Steve Ruff, who lead the research group, in a press statement.
After forming, ignimbrite deposits slowly cool over months or years, leading to the formation of intricate networks of fractures known as cooling joins. These form as the thick piles of ash and pumice contract. The team noticed similar fracture patterns in the olivine-rich bedrock deposits on Mars, backing up the ignimbrite hypothesis further.
According to Ruff, the olivine-rich composition is unusual for ignimbrites on Earth but there is evidence for such a composition in the oldest specimens. The hypothesis of ancient olivine-rich ignimbrites on Mars could point toward a particular style of volcanic eruption that happens early in a planet’s geological lifecycle.