Hidden in the minerals and textures that make up rocks are clues about how and when they were formed and then altered. These changes can occur due to the presence of water-rich fluids and can also be influenced by biological processes.
We are planetary petrologists (rock scientists) and scientists participating in the Mars 2020 Perseverance rover mission. Our research involves exploring and interpreting data sent by the Perseverance rover from its landing site in Jezero Crater.
A mysterious lake
Orbital images show that Jezero Crater was once the site of a stagnant body of water. It contained a lake that was fed by water from a river channel about 170 km long, and the images show a delta, a fan-shaped sediment platform, at the mouth of the channel. This delta consists of finer sediment layers mixed with boulder-rich layers suggesting that the river flow fluctuated from relatively calm conditions to large floods.
More of a mystery, however, were the rock units exposed on the floor of Jezero Crater, where Perseverance landed on February 18, 2021. Of particular interest was an enigmatic unit, identified by the presence of olivine, its signature spectral (measurements of the amount of radiation). reflects).
A panorama of Brac, captured by the Mastcam-Z camera system aboard NASA’s Perseverance Mars rover between November 6 and 17, 2021. The panorama is made up of a total of 64 images stitched together after being sent back on earth. (NASA/JPL-Caltech/ASU/MSSS)
Evidence from history
Olivine is a glassy, green mineral (its gem variety is peridot) that usually crystallizes in high-temperature magmas. In contrast, carbonate minerals can form from high to low temperatures, usually from melts or fluids that may have been favorable for life.
The olivine-rich unit is widespread in the region beyond Jezero, covering approximately 70,000 square kilometers, and exposed within the crater just north and west of the Perseverance landing site in a region called Séitah.
Séítah (which means “in the middle of the sand” in Navajo) is covered by a network of sand dunes, making it difficult for the rover to navigate. However, it was considered a compelling target for understanding the history of this region of Mars and because its carbonate minerals might preserve evidence of ancient life.
Perseverance entered Séitah in September 2021 and easily confirmed the occurrence of olivine using its remote sensing instruments. Microscopic cameras saw grains of olivine two to three millimeters across, but their origin was unknown.
On Earth, olivine grains of this size and shape can be concentrated by a variety of geological means, including wind- or water-borne sands from olivine-rich regions, explosive volcanic eruptions, material ejected by impact from meteorites or they can form in the form of crystals. in the cooling of the magma.
Additional information was needed to interpret the history of the olivine, but engineering challenges initially prevented the mission’s ability to use its X-ray fluorescence (XRF) spectrometer on the Séítah rocks.
A close-up of a rock called Dourbes, taken by the WATSON (Wide Angle Topographic Sensor for Operations and Engineering) camera at the end of the robotic arm aboard NASA’s Perseverance Mars rover. (NASA/JPL-Caltech/MSSS)
Sophisticated equipment
XRF spectrometers have been important instruments for determining the elemental compositions (sodium to iron and some trace elements) of rock surfaces on Mars.
The alpha particle X-ray spectrometers (APXS) aboard Pathfinder, the two Mars exploration rovers Spirit and Opportunity, and the Mars Science Laboratory rover Curiosity provided bulk chemicals from circular spots of about 1, 5 cm that helped geological interpretations.
But for some Martian rocks, uncertainties have persisted about the small-scale features and fine textures of the rocks that are critical to interpreting which minerals are present, whether igneous or sedimentary, or their alteration histories.
The onboard PIXL Perseverance is a big improvement in this regard: PIXL generates ~120 micron grid maps that not only provide rock and mineral chemistries, but textures that can be used to infer origin, processes and relative time of the different minerals and other components. present
The first PIXL scan of a rock surface in a Séítah outcrop called Brac finally nailed down the origin of the unit as igneous. Olivine grains are well-formed crystals with straight edges. Other high-temperature minerals, including feldspar, and larger minerals enclose or occur in the spaces between olivine crystals, indicating slow cooling of a magma.
Brac is a type of rock called olivine cumulate that formed when olivine crystallized near the top of a magma and settled and accumulated downwards due to its higher density. Olivine cumulates are known to form on Mars because they are among the Martian meteorites, which comprise a group known as chassignites, that were ejected from Mars by an impact and eventually fell to Earth.
On Earth, olivine cumulates occur in large layered intrusions, such as the Skaergaard intrusion in East Greenland, and in thick lava flows, such as that found at Abitibi, Ont. area
Video recorded of Mars by the Perseverance rover.
Anticipation of core samples
As remarkable as the PIXL scans are, Perseverance is equipped with a very sophisticated sampling tool, which it used to collect Brac cores. At least one of these core samples will likely be brought back to Earth in the early 2030s as part of the Mars sample return effort.
Mars Sample Return would allow researchers in laboratories on Earth to examine features down to the nanoscale, which could provide information about crystallization history, water activity in the rock, and how long the rock was exposed. This could provide clues about the history of life on Mars.
Radiometric isotopic analyzes would help determine the time of crystallization. Stable isotopes (H, C, N, O) would tell us the history of fluids on Mars. The list goes on and on!
The returned samples would allow us to answer the questions that PIXL’s recent results hint at. We could then provide a more complete story of Jezero’s olivine- and carbonate-rich rocks, and what they tell us about the history and potential for life on Mars.