Hidden in the minerals and textures that make up rocks are clues to how and when they formed and were later changed. 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 participating scientists on the Mars 2020 Perseverance rover mission. Our research involves exploring and interpreting data returned 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 fed by water from a river channel about 170 km long, and images show a delta – a fan-shaped platform of sediment – at the mouth of the channel. This delta is composed of layers of finer sediments mixed with rock-rich layers that suggest that the river flow fluctuated between relatively calm conditions and large floods.
More mysterious, however, were the exposed rock units in the floor of Jezero Crater, where Perseverance landed on February 18, 2021. An enigmatic unit, identified by the presence of olivine and its spectral signatures (measurements of how much radiation it reflects) .
Evidence of 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 at high or low temperatures, usually from melting or fluids that may have been supportive of life.
The olivine-rich unit is widespread in the region beyond Jezero, covering approximately 70,000 square kilometers, and exposed in the crater just north and west of the Perseverance landing site, in an area dubbed Séítah .
Séítah (meaning “in the middle of the sand” in Navajo) is covered in a network of sand dunes, making it difficult for the rover to navigate. However, it was considered a key target for understanding the history of this region of Mars and because its carbonate minerals could retain evidence of ancient life.
Perseverance entered Séítah in September 2021 and easily confirmed the presence of olivine by its remote sensing instruments. Microscopic cameras saw two to three millimeter grains of olivine, but their origin was unknown.
On Earth, olivine grains of this size and shape can be concentrated through a variety of geological pathways, including as aeolian or water-borne sands from olivine-rich regions, explosive volcanic eruptions, material ejected by the impact of a meteorite, or they may form as crystals. in the cooling of the magma.
Additional information was needed to interpret the olivine story, but technical challenges initially hampered the mission’s ability to use its X-ray fluorescence (XRF) spectrometer on the Séítah rocks.
Sophisticated equipment
XRF spectrometers have been important instruments for determining the elemental compositions (from sodium to iron and some trace elements) of rock surfaces on Mars.
Alpha Particle X-ray Spectrometers (APXS) on board Pathfinder, the two Mars exploration rovers Spirit and Opportunity, and the Mars Science Laboratory Curiosity rover provided bulk chemistries of circular spots of approximately 1, 5 cm which helped the geological interpretations.
But for some Martian rocks, uncertainties remain about the fine-scale features and fine rock textures that are key to interpreting the minerals present, whether igneous or sedimentary, or their weathering histories.
The onboard Perseverance PIXL is a great improvement in this respect: PIXL generates approximately 120-micron grid maps that not only provide rock and mineral chemistries, but also textures that can be used to infer origin, process and relative timing of various minerals and other components. present.
The first PIXL scan of a rock surface on an outcrop of Séítah called Brac finally identified 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 which formed when olivine crystallized near the top of a magma, and was deposited and accumulated downward due to its higher density . Olivine cumulates are well known to form on Mars as they are found among Martian meteorites, including a group known as chassignites, which were ejected from Mars by an impact event and eventually fell to Earth. .
On Earth, olivine cumulates occur in large layered intrusions, such as the Skaergaard intrusion in eastern Greenland, and in thick lava flows, such as those found in Abitibi, Ontario. Region.
Anticipate carrots
As remarkable as the PIXL scans are, Perseverance is equipped with a very sophisticated sampling tool, which it used to collect cores from Brac. At least one of these baseline samples will likely be brought to Earth in the early 2030s as part of the Mars Sample Return effort.
Mars Sample Return would allow researchers in terrestrial laboratories to examine features down to the nanometer scale, which could provide information on the history of crystallization, water activity in the rock and the duration of exposure of the rock. This could provide clues to the history of life on Mars.
Radiometric isotope analyzes would help determine the timing of crystallization. Stable isotopes (H, C, N, O) would tell us about the history of fluids on Mars. The list is lengthened increasingly!
The returned samples would allow us to answer the questions suggested by recent PIXL results. We could then provide a more complete history of the olivine- and carbonate-rich rocks of Jezero, and what they tell us about the history and potential for life on Mars.
Mariek Schmidt, Associate Professor, Earth Sciences, Brock University and Chris Herd, Professor, Earth and Atmospheric Sciences, University of Alberta
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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