Perseverance makes history with Mars rock discovery
Researchers from the Queensland University of Technology (QUT) are part of the team that has collected the first datable Mars rock samples, providing a benchmark for mapping the history, evolution and changing climate of the Red Planet.
The scientists got a surprise when NASA’s Perseverance Mars rover began examining rocks on the floor of the Jezero crater in 2021. Because the crater held a lake billions of years ago, they had expected to find sedimentary rock, which would have formed when sand and mud settled in a once-watery environment. Instead, they discovered the floor was made of two types of igneous rock — one that formed deep underground from magma, the other from volcanic activity at the surface.
QUT’s Dr David Flannery, long-term planner for the NASA Perseverance mission, said the discovery, published in the journal Science, would allow researchers to date the samples that NASA will eventually bring back to Earth. This would result in a better understanding of warmer and wetter climatic periods in Mars’s past and any evidence for past life that may be discovered by the mission.
“It was a surprise that we didn’t find sedimentary rocks on the crater floor, but also ideal because finding a datable igneous sample was one of the main mission goals,” Flannery said.
“Ancient igneous rocks will allow us to date a several-billion-year-old rock with very high precision. This will provide important timing and duration constraints on the history of Jezero crater and its surrounding region.
“So effectively, we landed on exactly the thing we needed to help us with one of our other main goals, which is to find evidence of past life.
“If we find that this lake was a habitable environment, for example, we will have an age constraint on when it was habitable.”
Flannery said that on Earth, reactions of igneous rocks with water produce diverse habitats for microbial life — and that rocks on the Jezero crater floor appear to share the same characteristics, with the potential to record biosignatures of ancient habitats. The rock sample was identified by its composition and mineralogy using the Planetary Instrument for X-ray Lithochemistry (PIXL), which was developed by a team of engineers and scientists from nine institutions across three continents.
“PIXL observations were critical to proving the rock was an olivine cumulate and not just a sedimentary rock with the same composition,” said PIXL lead and QUT alumna Dr Abigail Allwood.
“The PIXL team used the maps of fluorescence and diffraction of X-rays to prove that the rock formed by cooling of a melt.
“This kind of definitive observation is rare in planetary science. Usually, it’s easier to add more alternative hypotheses than it is to be able to absolutely refute any hypothesis with confidence.”
The PIXLISE visualisation software used to identify materials on the Mars surface was developed for NASA’s Jet Propulsion Laboratory (JPL) by QUT Planetary Surface Exploration research group programmer Peter Nemere. Algorithms to allow PIXL team scientists to map the X-ray diffraction data were developed in collaboration by QUT and JPL, and proved the igneous rock formed by cooling of a melt.
“Our working hypothesis was that we were at the bottom of a lake, because sedimentary features viewed from orbit suggested the crater was under water at some point in the past,” Flannery said.
“Now we know we’re looking at olivine which, on Earth, would be pushed towards the surface from the mantle through tectonic processes — volcanos and lava flows.
“This discovery is hugely important because these types of rocks can be dated with radiometric techniques, the same techniques that we use on Earth to date very old rocks.”
The discovery of olivine inside the crater also helped solve the longstanding mystery of an olivine-rich outcrop that was known to span 70,000 km2 from the inside edge of Jezero into the surrounding region. The olivine is said to resemble Martian meteorites found on Earth and was discovered after creating an abrasion patch — grinding away the surface material — in preparation for studying rocks.
Scientists have offered various theories as to why olivine is so plentiful over such a large area of the surface, including meteorite impacts, volcanic eruptions and sedimentary processes. Another theory is that the olivine formed deep underground from slowly cooling magma, before being exposed over time by erosion.
Using PIXL, the scientists determined the olivine grains in the area measure 1 to 3 mm — much larger than would be expected for olivine that formed in rapidly cooling lava at the planet’s surface. Due to their large size and uniform composition that would require a very slow cooling environment, the likely theory is that magma in Jezero did not erupt to the surface but formed underground before being exposed over time by erosion. Their findings have been published in a second study in Science.
The Mars Perseverance mission is planning to cache about 30 rock samples for return to Earth. With planned launch dates for the Earth Return Orbiter and Sample Retrieval Lander in 2027 and 2028, respectively, the samples are expected to arrive on Earth in 2033.
“Being able to date these old rocks on Mars allows us to unravel its history,” Flannery said.
“Studying Mars helps us put the Earth in context and gives us a mirror to better understand how our planet might be special.”
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