According to a series of four papers by the Perseverance science team, the floor of Jezero crater had eroded more than planetary researchers expected. The team also found that the rocks collected from four sites in the crater are igneous cumulate rocks — that is, they were formed by the cooling of molten magma and are the best rocks for precise geochronology once the samples have returned to Earth; they also show evidence of having been altered by water. The scientists originally expected that sedimentary lake or delta rocks would lay on top of these igneous rocks; it’s likely that the softer sedimentary rocks wore away over eons, leaving the tougher igneous rocks behind.
This artist’s concept depicts NASA’s Perseverance rover. Image credit: NASA / JPL-Caltech.
The Perseverance rover, part of NASA’s Mars 2020 mission, landed on the floor of Jezero crater on February 18, 2021.
The rover was designed to characterize the geology, investigate records of past climate, and seek evidence of possible ancient life contained in rocks laid down when Martian surface conditions were more habitable than today.
The multi-year mission is intended to select, collect and document about 35 rock and soil samples, for potential transport to Earth by future missions for laboratory analysis.
“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,” said Dr. David Flannery, a researcher at the Queensland University of Technology.
“If we find that this lake was a habitable environment, for example, we will have an age constraint on when it was habitable.”
Jezero crater, with a diameter of 45 km, lies on the western flank of the 4-billion-year-old (Noachian age) impact basin Isidis and northeast of Syrtis Major, a younger shield volcano.
The crater was formed by an impact onto an early Noachian surface that includes Isidis impact deposits.
It later hosted an open basin lake, fed by a large system of river valleys that were active between 3.8 and 3.2 billion years ago.
Sediments delivered by those rivers into the crater lake produced a clay and carbonate bearing delta, which appears to be well preserved.
“The fact that we have evidence of aqueous alteration of igneous rocks — those are the ingredients that people are very excited about — is huge,” said Caltech Dr. Kenneth Farley, lead author of one of the new papers.
In their paper, published in the journal Science, Dr. Farley and colleagues described the geologic formations on the floor of Jezero crater, and the rover operations.
They found that the crater floor consists of igneous rocks altered later by water.
The rover collected core samples of these rocks, which are now stored aboard Perseverance for potential return to Earth.
“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,” Dr. 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.”
“We’re excited to see even better results about organics and ancient habitable environments,” said team member Dr. Briony Horgan, a researcher in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University.
“I think it’s really setting the stage that Mars is this watery, habitable place, and all the samples we’re getting back are going to help us understand the history of ancient microbial life on Mars.”
In the second paper, also in the journal Science, Dr. Yang Liu of NASA’s Jet Propulsion Laboratory and colleagues investigated the petrology and composition of the Séítah formation, the lowest exposed geologic unit in Jezero crater.
They found that these igneous rocks are mainly made of coarse-grained olivine (an igneous mineral), indicating that they formed during slow cooling of a thick sheet of magma. The rock composition was similar to some Martian meteorites.
Orbital images of the Jezero crater floor study area. Image credit: Farley et al., doi: 10.1126/science.abo2196.
In the third paper, published in the journal Science Advances, University of Oslo’s Dr. Svein-Erik Hamran and his colleagues analyzed a continuous radar scan captured by the Perseverance rover during its initial 3-km journey, revealing the crater’s bedrock stratigraphy and electromagnetic properties to a depth of about 15 m below the surface.
They suggest that the layered structure below the crater floor may indicate a history of igneous activity and of repeated exposure to liquid water.
“We were quite surprised to find rocks stacked up at an inclined angle,” said University of California, Los Angeles Professor David Paige, one of the lead researchers on Perseverance’s Radar Imager for Mars Subsurface Experiment (RIMFAX).
“We were expecting to see horizontal rocks on the crater floor. The fact that they are tilted like this requires a more complex geologic history. They could have been formed when molten rock rose up towards the surface, or, alternatively, they could represent an older delta deposit buried in the crater floor.”
“Most of the evidence gathered by the rover so far points to an igneous, or molten, origin, but based on the RIMFAX data, we can’t yet say for certain how the inclined layers formed.”
“RIMFAX obtains a picture of underground features by sending bursts of radar waves below the surface, which are reflected by rock layers and other obstacles.”
“The shapes, densities, thicknesses, angles and compositions of underground objects affect how the radar waves bounce back, creating a visual image of what lies beneath.”
In the fourth paper, also in the journal Science Advances, Dr. Roger Wiens of Los Alamos National Laboratory and colleagues analyzed observations from the rover’s remote-sensing instrument during its first 286 Martian days on Mars, revealing a volcanic terrain with layered rocks stratified below the surface by density and composition.
They found that lower stratigraphic rock layers are richer in normative pyroxene and that the lowest observed layer is olivine-rich with the highest observed density.
_____
K.A. Farley et al. Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars. Science, published online August 25, 2022; doi: 10.1126/science.abo2196
Y. Liu et al. An olivine cumulate outcrop on the floor of Jezero crater, Mars. Science, published online August 25, 2022; doi: 10.1126/science.abo2756
Svein-Erik Hamran et al. 2022. Ground penetrating radar observations of subsurface structures in the floor of Jezero crater, Mars. Science Advances 8 (34); doi: 10.1126/sciadv.abp8564
Roger C. Wiens et al. 2022. Compositionally and density stratified igneous terrain in Jezero crater, Mars. Science Advances 8 (34); doi: 10.1126/sciadv.abo3399
