Hidden in the minerals and textures that make up rocks are clues to how and when they were formed and later changed. These changes may occur due to the presence of water-rich fluids and may also be affected by biological processes.
We are planetologists (rock scientists) and participating scientists on the Perseverance Mars 2020 rover mission. Our research involves exploring and interpreting data sent back by the Perseverance Rover from its landing site at Jezero Crater.
A mysterious lake
Orbital images show that Jezero Crater was once the site of a stagnant body of water. It held a lake fed by water from a ~170 km long river channel, 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 boulder-rich layers that indicate that the river flow ranged from relatively calm conditions to large floods.
More mysterious, however, were 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 in its spectral signatures (measurements of the amount of radiation it reflects). .
Olivine is a glassy, green mineral (its gemstone 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 about 70,000 square kilometers, and is exposed within the crater just north and west of the Perseverance landing site, in an area called Séítah.
Séítah (which means “into 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 could preserve evidence of ancient life.
Perseverance entered Séítah in September 2021 and easily confirmed the occurrence of olivine with its remote sensing instruments. The microscopic cameras saw olivine grains two to three millimeters across, but their origin was unknown.
On Earth, olivine grains of this size and shape can be collected in a variety of geological ways, such as as air- or water-borne sand from olivine-rich areas, explosive volcanic eruptions, material ejected from a meteorite impact, or they can form as crystals in magma cooling.
Additional information was needed to interpret the olivine history, but engineering challenges initially hindered the mission’s ability to use the X-ray fluorescence (XRF) spectrometer on the Séítah rocks.
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) on Pathfinder, the two Mars Exploration Rovers Spirit and Opportunity, and the Mars Science Laboratory rover Curiosity provided ∼1.5 cm circular spot chemistry that aided geologic interpretations.
But for some Martian rocks, there are uncertainties about the small-scale features and fine rock textures that are critical to interpreting the minerals present, whether they are igneous or sedimentary, or their alteration histories.
The built-in PIXL Perseverance is a big improvement in this regard: PIXL generates ~120 micron grid maps that not only provide rock and mineral chemistries, but also textures that can be used to infer the origin, processes and relative timing of various minerals and other ingredients present.
The first PIXL scan of a rock surface on an outcrop of Séítah 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 an olivine accretion that formed when olivine crystallized near the top of a magma and precipitated and accumulated downwards due to its higher density. Clumps of olivine are known to form on Mars because they are found among Martian meteorites, consisting of a group known as chassignites, that were ejected from Mars by an impact event and eventually fell to Earth.
On Earth, olivine accumulations occur in large layered intrusions, such as the Skaergaard intrusion in East Greenland, and in dense lava flows, such as at Abitibi, Ont. region.
Prediction 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 Earth-based laboratories to examine features down to the nanoscale, which could provide information about the crystallization history, water activity in the rock, and how long the rock was exposed. This could provide clues to the history of life on Mars.
Radiometric isotopic analyzes would help pinpoint the time of crystallization. Stable isotopes (H, C, N, O) would tell us about the history of fluids on Mars. The list goes on and on!
The returned samples will allow us to answer the questions implied by the recent PIXL results. We could then provide a fuller history of the olivine- and carbonate-rich rocks at Jezero, and what they tell us about the history and potential for life on Mars.
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