NASA’s Perseverance rover has been on Mars for fifteen months now and is about to begin its trek into the fossil delta of Jezero Crater. It’s a big deal for the mission, because the delta is where water once flowed long enough and strongly enough to smooth, round and move large rocks.
Since proof of the long-ago presence of water means the area was potentially habitable — especially a delta that spreads out into what were once calm rivulets — this is where the astrobiology goals of the mission come to the fore.
Or so the Perseverance team thought it would play out.
But the big surprise of the mission so far has been that the rover landed on igneous rock, formed in the Martian interior, spewed out and crystalized and solidified on the surface.
That Perseverance would land on igneous rock was always seen as a possibility, but a more likely outcome was landing on sedimentary rock as in Gale Crater, where the Curiosity rover continues its decade-long explore. Sedimentary rock is laid down in layers in the presence of water.
As explained last week at the Ab-Sci-Con 2022 conference in Atlanta, the deputy program scientist for the mission — Katie Stack Morgan of NASA’s Jet Propulsion Lab — from the mission’s perspective the presence of both igneous and nearby sedimentary rock offers the best of both worlds.
While sedimentary rock is traditionally where scientists look for signs of ancient life, igneous rock can date the site more exactly and it can potentially better preserve any signs of early microbial life.
And in the context of Perseverance, the presence of accessible and compelling igneous formations provides for the diversity of rock samples called for in the Mars Sample Return effort — another central part of the rover’s mission.
“We did a lot of work with our different instruments to come to the conclusion that we landed on igneous rock,” Stack Morgan later said in an interview. “And not only was it igneous rock, but we found two different quite formations of igneous rocks — including a kind of surface deposit that is rare on Earth.”
“This second unit remains a mystery to us, especially since the mineral that makes it unusual — olivine — is also found in small deposits around Jezero, she said.
Olivine is a green mineral and makes up a good percentage of Earth’s mantle, but it weathers away quickly on the surface. Discoveries such as the unexpected surface olivine deposits are precisely the guideposts — albeit at times mysterious ones — that can lead to a much fuller understanding of the makeup and history of Jezero.
The rover has just recently arrived at an entrance of the delta section of Jezero and will begin driving on sedimentary rock and collecting some samples for ultimate return to Earth for laboratory testing.
The Perseverance science team already knows that organic compounds can be found Jezero — a major conclusion since they are building blocks of life — and that the area was “habitable” on early Mars. That’s when temperatures were substantially warmer, liquid water was sometimes present and the atmosphere was much thicker than today.
It took many months for the Curiosity rover team to conclude that Gale Crater was similarly habitable, but now much more is known about Mars and so those conclusions can be made more quickly.
The Perseverance team also found salt in the crystalline structure of the igneous rocks, a signpost that there was once briny water in the delta. This is important because salts keeps the water captured within the rock in a phase that could allow it to support life.
But when it comes to looking for signs of ancient life in Jezero, the sedimentary delta ahead is the jackpot. It is where many holes will be drilled and material collected and stored for potential sample return to Earth. That has already been done nine times on the igneous rock unit where the rover landed, with eight finger-sized rock cores collected.
Of equal value is the data collected by Perseverance instruments to better understand the geochemical content and geological context of Jezero. As an example of how the instruments are being used, the intense debate within the mission team over whether the landing site consisted of igneous or sedimentary rock was ultimately settled using a first-of-its-kind for Mars instrument called PIXL, the Planetary Instrument for X-ray Lithochemistry.
PIXL has a tool called an X-ray spectrometer that can identify and map chemical elements at a tiny scale. Stack Morgan said that when PIXL identified interlocking crystalline structures in those initially collected rocks, it was clear that they were igneous. This is because crystalline structures of the sort identified by PIXL don’t exist in sedimentary rocks.
The presence of the delta — which once had a large and forceful river leading into it — was what led to the selection of Jezero for the Perseverance mission. Not only does it provide an environment that surely was wet and potentially habitable, but it also offers a wide sampling of rocks and rock grains from far away that were picked up by the river and ultimately delivered into the delta.
From soon after landing, the Perseverance cameras could spot layers of clearly sedimentary rock on ridges in the crater, as shown in the opening image above. The rover cannot climb and study those outcrops, but it will have plenty more on the pathway through the delta.
Stack Morgan said that one of the most surprising discoveries so far is the presence of quite large round and smooth boulders on nearby ridges, from 1.5 feet to 3.2 feet in diameter. She said their shape means many of the boulders were carried into Jezero by water that was flowing with substantial force.
“What kind of flood was needed to get boulders that round? ” she asked. “Clearly they were knocking around in an energetic flow, but when did that happen and what were conditions like in the Martian atmosphere? These are the kinds of questions we’ll be trying to answer.”
The boulders are today about 60 meters (190 feet) above the crater floor, Stack Morgan said. “At the time the boulders were deposited, the underlying sedimentary rocks were probably much more extensive than they are today.”
“So the boulders would not have been deposited directly on the present-day crater floor, rather they would’ve been deposited on the underlying sedimentary rock as we see them in this particular outcrop.”
And in the billions of years that followed that sedimentary lying over the igneous rock would have been washed away or weathered away. Such are the geological and geochemical clues that over time allow scientists to piece together an understanding of what was happening long ago on Mars, or at least some of what was happening..
Cameras play a large and essential role in Perseverance science and also in providing images that can put us all digitally on Mars.
This NASA tour of sorts of Jezero Crater, guided by Perseverance Project Scientist Kenneth Farley of Caltech, is made from images taken during the first year of the rover’s time on Mars.
The Ab-Sci-Con conference in Atlanta offered numerous presentations and papers about the possibilities, the difficulties, the logic of searching for signs of past life on Mars.
One compelling session focused in detail on potential habitability inside igneous rocks. Based on recent advances in understanding subsurface habitats on Earth, the National Academy of Sciences directed NASA to focus on exploring subsurface habitability in relation to both understanding Jezero Crater and collecting samples for later return to Earth.
The paper, authored by Lisa Mayhew of the University of Colorado and colleagues, reported on the many ways that water had interacted with igneous rock at what was named the Roubion sample. That was the first drill site for Perseverance and it ultimately turned out to not produce a rock core to be cached for the sample return mission because the sample crumbled before it could be properly stored.
But Perseverance instruments found that the rock contained a variety of salts in it, some clays and sulfates — all signs of rock being altered again and again by water. The SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument also found organic compounds using its spectrometers and laser.
The paper concludes that the mineral assembly suggests the long-ago occurrence of serpentinization, a rock/water reaction that on Earth produces energy in the form of hydrogen and methane that can be used by lifeforms. In other words, it produces food for potential microbial life inside a rock, and therefore protected from radiation. Scientists know that such lifeforms inside rocks exist on Earth, and might have existed on Mars.
Just one of many possibilities to consider as Perseverance continues its explore.