Hidden in the minerals and textures that make up rocks are clues as to how and when they were formed and later modified. These changes can result from the presence of water-rich fluids and can also be influenced by biological processes.
We are planetary petrologists (rock researchers) and participating scientists on the Mars 2020 Perseverance rover mission. Our research involves examining and interpreting the data sent back by the Perseverance rover from its landing site in Jezero Crater.
A mysterious lake
Images from orbit show that Jezero Crater was once the site of standing 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 consists of layers of finer sediments mixed with layers rich in boulders, which indicate that the river flow has fluctuated from relatively calm conditions to major flooding.
Even more mysterious, however, were rock units uncovered in the floor of Jezero Crater, where Perseverance landed on February 18, 2021. Of particular interest was an enigmatic entity identified by the presence of olivine and its spectral signatures (measurements of the amount of radiation it reflects).
testimony of history
Olivine is a glassy, green mineral (its gem variety is peridot) that normally crystallizes in high-temperature magmas. In contrast, carbonate minerals can form at high to low temperatures, usually from melts or liquids that may have been favorable to life.
The olivine-rich unit is widespread in the region beyond Jezero, covers about 70,000 square kilometers and lies within the crater north and west of Perseverance’s landing site in a region called Séítah.
Séítah (meaning “among the sands” in Navajo) is covered by a network of sand dunes, making it difficult for the rover to navigate. However, it has been viewed as a compelling target for understanding the history of this region of Mars, and because its carbonate minerals may harbor evidence of ancient life.
Perseverance entered Séítah in September 2021 and willingly confirmed the presence of olivine through its remote sensing instruments. The microscope cameras saw grains of olivine two to three millimeters in size, but their origin was unknown.
On Earth, olivine grains of this size and shape can be concentrated in a variety of geological ways, including as wind- or waterborne sands from olivine-rich regions, explosive volcanic eruptions, material ejected by meteorite impact, or they can form as crystals in cooling magma.
Additional information was needed to interpret the olivine’s history, but technical challenges initially hampered the mission’s ability to use its X-ray fluorescence (XRF) spectrometer on Séítah rock.
Sophisticated equipment
XRF spectrometers have been important tools for determining the elemental composition (sodium to iron and some trace elements) of rock surfaces on Mars.
X-ray alpha particle spectrometers (APXS) onboard Pathfinder, the two Mars Exploration Rovers Spirit and Opportunity, and the Mars Science Laboratory rover Curiosity provided bulk chemistry of ~1.5 cm circular spots that supported geological interpretations.
For some Martian rocks, however, there are uncertainties about small-scale features and fine rock textures that are critical to interpreting the minerals present, whether igneous or sedimentary, or their history of change.
The PIXL onboard Perseverance is a major improvement in this regard: PIXL generates ~120 micron grid maps that provide not only the rock and mineral chemistry, but also textures that can be used to explain the origin, processes, and relative timing of the various minerals and other components present.
Finally, the first PIXL scan of a rock surface at a Séítah outcrop called Brac revealed that the unit was magmatic in origin. The olivine grains are well-formed crystals with straight edges. Other high-temperature minerals, including feldspar, and larger minerals trap or occur in the interstices between olivine crystals, indicating slow cooling of a magma.
Brac is a type of rock called an 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 found among the Martian meteorites, which comprise 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 the Abitibi, Ont. Area.
Anticipating drill core samples
As remarkable as the PIXL scans are, Perseverance comes with a very sophisticated sampling tool that was used to collect Brac cores. At least one of these core 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 Earth-based laboratories to study features down to the nanoscale, which could provide information on crystallization history, water activity in the rock, and the length of time the rock was exposed. This could provide clues to the history of life on Mars.
Radiometric isotope analysis would help pinpoint the timing of crystallization. Stable isotopes (H, C, N, O) would tell us something about the history of liquids on Mars. The list goes on and on!
Returned samples would allow us to answer the questions suggested by the recent PIXL results. We could then provide a more complete history of the olivine- and carbonate-rich rocks at Jezero and what they tell us about the history and life potential of Mars.
Mariek Schmidt, Associate Professor, Geosciences, Brock University and Chris Herd, Professor, Earth & Atmospheric Sciences, University of Alberta
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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