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NASA / JPL-CALTECH
NASA’s Perseverance rover, which will launch to Mars this summer, will look for signs of past lives in an ancient crater lake. But if biology ever emerged on the red planet, how will scientists recognize it? Here, mission scientist Ken Williford explains what they are looking for.
Today, Mars is hostile to life. It is too cold for the water to remain liquid on the surface, and the thin atmosphere lets high levels of radiation pass through, potentially sterilizing the upper soil.
But it was not always like this. About 3.5 billion years ago or more, water flowed on the surface. He sculpted channels that are still visible today and clustered into impact craters. A thicker atmosphere of carbon dioxide (CO2) would have blocked more harmful radiation.
Water is a common ingredient in biology, so it seems plausible that ancient Mars once offered a foothold for life.
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In the 1970s, Viking missions carried out an experiment to search for current microbes in Martian soil. But the results were considered inconclusive.
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NASA / JPL-Caltech
Previous missions have found evidence that Mars had the right ingredients for life billions of years ago.
In the early 2000s, NASA’s Mars Exploration Rovers were tasked with “following the water.” Opportunity and spirit found extensive geological evidence of the past presence of liquid water.
The Curiosity rover, which landed in 2012, found that the lake that once filled its landing site at Gale Crater could have supported life. It also detected organic (carbon-containing) molecules that serve as building blocks of life.
Now, the Perseverance rover will explore a similar environment with instruments designed to evaluate biology signatures.
“I would say this is the first NASA mission since Viking did that,” said Ken Williford, the mission’s project assistant scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
“Viking was the search for existing life, that is, the life that could be living on Mars today. While NASA’s most recent focus has been to explore ancient environments because the data we have suggested indicates that the earliest history of the planet it tells us that Mars was more habitable during its first billion years. “
Perseverance’s target is Jezero Crater, where the signs of a watery past are even clearer, when viewed from orbit, than those of the Gale Crater.
The rover will drill through Martian rocks, extracting cores that are about the size of a piece of chalk. These will be sealed, cached, in containers and left on the surface. These will be collected by another rover, dispatched at a later date, launched into Mars orbit and delivered to Earth for analysis. It is all part of a collaboration with the European Space Agency (Esa) called Mars Sample Return.
Jezero presents one of the best-preserved Martian examples of a delta: layered structures formed when rivers enter open bodies of water and deposit rocks, sand, and potentially organic carbon.
“There is a river channel that flows in from the west, penetrating the rim of the crater; and then, right inside the crater, at the mouth of the river, is this beautiful fan of the exposed delta. Our plan is to land right in front of that delta and start exploring, “said Dr. Williford.
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The delta contains grains of sand that originate from upstream rocks, including a basin to the northwest.
“The cement between the grains is very interesting: it records the history of the water that interacts with that sand at the time of the delta deposition in the lake,” says Ken Williford.
“It provides potential habitats for any organisms living among those grains of sand. Chunks of organic matter from any organisms upstream could be washed away.”
Jezero is located in a region that has been of interest to science. It is on the western shoulder of a giant impact basin called Isidis, which shows the strongest Martian signals from the olivine and carbonate minerals measured from space. “Carbonate minerals are one of the key targets that led us to explore this region,” says Ken Williford.
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Stromatolites in Shark Bay, Australia
A survey of minerals in Jezero Crater by Purdue University’s Dr. Briony Horgan, Western Washington University’s Dr. Melissa Rice (both mission scientists) and their colleagues revealed carbonate deposits on the rim. west of the ancient coast. These “marginal carbonates” were compared to a bathtub ring, the buildup of soap scum that remains after the water is drained.
Terrestrial carbonates can enclose biological evidence within their crystals. They can even help build structures that are strong enough to survive as fossils for billions of years, including seashells, corals, and stromatolites.
Stromatolites are made up of many millimeter-scale layers of bacteria and sediments that accumulate over time in larger structures, sometimes domed. On Earth, they occur along ancient coasts, where sunlight and water abound.
Billions of years ago, the Jezero coastline was exactly the type of place where stromatolites could have formed, and been preserved, making the carbonate-rich bathtub sound like a primary objective for the mission.
The rover’s science load will help investigate these and other questions.
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Science Photo Library
The layers in a stromatolite are irregular and wrinkled. Perseverance will look for similar ways
Two cameras with zoom on the rover’s mast, part of the Mastcam-Z system, will examine the landscape and send information on colors, structures and textures.
Several meters away, an instrument called Supercam will fire a laser at the rocks to measure their elemental and mineral composition.
This will help scientists select “parking spots” where they can deploy the robotic arm. The arm drill is used to wear down and flatten a 4.5 cm circular rock patch. The turret, or the end, of the arm is turned.
An instrument called Sherloc captures images of the flat area and produces a detailed map of the minerals present, including organic ones. Another instrument called Pixl will give scientists the detailed elemental or chemical composition of the same area.
Within this dataset, scientists “will search for concentrations of biologically important elements, minerals, and molecules, including organic matter. In particular, [it’s] when those things are concentrated in ways that potentially suggest biology, “says Ken Williford.
Gathering many lines of evidence is vital; Visual identifications alone will not be enough to convince scientists of a biological origin, given the high level of claims of extraterrestrial life. In the absence of a big surprise, the findings are likely to be described only as possible bio-signatures until rocks are sent to Earth for analysis.
Referring to stromatolites, Dr. Williford explains: “The layers tend to be irregular and wrinkled, as expected for a group of microbes living on top of each other. All of this can be fossilized in a way that is visible even to cameras. “
“But it is when we see shapes like that and, perhaps, one layer has a different chemistry than the next, but there is a repetitive pattern, or we see that organic matter is concentrated in specific layers, those are the final biological signatures that we hope to find . ”
However, Mars may not easily reveal its secrets. In 2019, mission scientists visited Australia to familiarize themselves with the fossil stromatolites that formed 3.48 billion years ago in the country’s Pilbara region.
“We will have to look for more [on Mars] that when we went to Pilbara … our knowledge of its location comes from many decades of many geologists going year after year and mapping the territory, “says Ken Williford.
On Mars, he says, “we are the first.”
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Briony Horgan
Dr. Briony Horgan and colleagues have been working on Lake Salda, Turkey, where carbonate terraces and beaches can be found. The rocks in the area are similar in composition to what is seen in the Jezero crater
A mineral called hydrated silica also appears to be present near the delta. Its properties also make it good at preserving signs of life, including “microfossils,” the tiny remains of bacteria, fungi, or plants.
On Earth, we can detect fossilized microbes at the level of individual cells. But to see them, scientists have to cut a piece of rock, grind it to the thickness of a sheet of paper, and study it on a glass slide.
No rover can do this. But then, you might not have to.
“It is very rare to find an individual microbe that is alone,” says Dr. Williford.
“When they were alive, if they looked like Earth’s microbes, they would have joined together in small communities that accumulate in structures or groups of cells that are detectable by the scout vehicle.”
After exploring the crater floor, scientists want to bring the vehicle to the edge. The rock cores taken here, when analyzed on Earth, could provide an age for the impact the crater forged and a maximum age for the lake.
But there is another reason to be interested in the rim of the crater. When a large space object collides with rocks that contain water, the enormous energy can establish hydrothermal systems, where hot water circulates through the rocks. Hot water dissolves minerals from rocks that provide the necessary ingredients for life.
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NASA / JPL / JHUAPL / MSSS / BROWN UNIVERSITY
The Jezero Delta is one of the best preserved examples on Mars
“If that happened, that would have been the first habitable environment in Jezero Crater,” says Ken Williford. The evidence, along with signs of any life that colonized the environment, could be kept on the edge.
The current mission scenario envisions the rover driving the nearby Northeast Syrtis region as an “aspirational goal.”
It is even older than Jezero and also promises exposed carbonates, which may have formed differently than those in the crater.
If, at the end of this quest, there have been no signs of past lives, the search will not be over. The focus will be on those cores, awaiting delivery to Earth.
But the exciting prospect remains that the mission will not only throw more questions, but also answer. That result could shake the planet. Whatever is awaiting courageous perseverance, we are on the brink of a new phase in our understanding of Earth’s close neighbor.
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