The Perseverance Mars Rover:
NASA’s Next Giant Leap in the Search for Signs of Life Beyond Earth
On February 18, 2021, the most ambitious mission to Mars yet will land on Mars: the aptly named Perseverance rover. Perseverance will trek through ancient lake beds and river channels in Jezero Crater to search for signs of past microbial life on Mars preserved in the rocks. Along the way, the rover will collect samples of martian rocks, regolith, and atmosphere. These samples will be picked up by a future mission and brought back to Earth, where laboratory scientists will scrutinize them for signs of life and clues to the history of Mars for decades to come.
The Perseverance rover on the Mars 2020 mission is likely the best chance within our lifetimes for NASA to create a scientific revolution in astrobiology. Decades of Mars exploration have clearly shown that Mars has hosted a variety of environments with conditions suitable for life as we know it, and NASA has made the case that Jezero Crater in particular had the right combination of timing, geologic processes, and water chemistry to preserve signs of ancient microbial life, if it existed.
This means that there is a chance that Perseverance will collect the sample from Mars that answers the question: “Are we alone in the universe?” This question is especially relevant right now. During the coronavirus pandemic, the mission has stayed on track in spite of disruptions and delays, and we have been reminded that all life is vulnerable and precious.
Welcome to Jezero Crater
On February 18, Perseverance will enter the martian atmosphere at 21,000 kilometers per hour (13,000 miles per hour), and seven nerve-wracking minutes later, will be gently lowered onto the surface by a jetpack and tether combination known as the Sky Crane. The rover will land in Jezero Crater, a site that NASA hopes will provide a window to a time when rain fell and rivers flowed on ancient Mars.
Over the past 30 years, a fleet of rovers and orbiters have built a picture of an Earth-like ancient Mars. Three to 4 billion years ago, Mars hosted vast river networks as long as the Mississippi, deep lakes that contained the building blocks of life, and hot springs that bubbled with potential for life. These watery environments were able to exist because ancient Mars had a thick atmosphere. However, that atmosphere has been leaking away, leaving the surface today cold, dry, and inhospitable.
After five years of debate, Jezero Crater was selected as the site on Mars that is most likely to preserve signs of life that might have inhabited Mars billions of years ago, when microbial life was first starting on Earth. Satellite images of Jezero show a river leading into the crater and ending in a large delta, which must have formed in a long-lived ancient lake.
The Muddy Search for Organic Molecules
The delta in Jezero is the first major target for Perseverance’s search for life. When a river flows into a body of water and slows down, the bigger particles like sand tend to drop out right away around the mouth of the river, creating deposits like sandbars. However, the smallest particles like mud and organic molecules tend to travel farther, creating thin layers at the bottom of the lake that can be preserved underneath deltas as they build.
On Earth, these muddy layers are one of the best places to look for concentrated organic molecules derived from life in the watershed and the lake itself. Using orbital data, scientists like Prof. Tim Goudge at the University of Texas–Austin have argued that these thin layers are exposed in the now eroded front of the Jezero delta and could preserve signs of ancient life in the lake and beyond.
But other craters on Mars also contain beautifully preserved deltas; what makes Jezero special is that it also contains unique mineral deposits. These minerals, known as carbonates, provide a totally different mechanism for preserving life in an ancient lake in Jezero Crater.
Microbes: The Original Beach Bums
Orbital spectrometers have shown that carbonate is present throughout Jezero Crater and in the watershed. On its own this is exciting since carbonates on Earth most commonly form due to interactions between atmospheric carbon dioxide, rain, and rocks. That means that carbonates on Mars might provide answers to important questions like “How much rain fell on ancient Mars?”
However, the orbital data also show that carbonates are particularly concentrated around the inside edge of Jezero Crater. The carbonates create a partial “bathtub ring” of mineral deposits right at the elevations where the shorelines of an ancient lake might have reached. In a paper earlier this year, we suggested that these carbonates might have formed along ancient beaches as lake waters evaporated.
Stromatolites can fossilize the microbial colonies that helped form them and even preserve textures of the microbes themselves. Ancient stromatolites preserve some of the earliest signs of life on Earth, and Perseverance will search for similar signs of past life along the ancient beaches of Jezero Crater on Mars.
Perseverance: A Robotic Astrobiologist
The payload that each Mars rover has brought to the Red Planet has varied depending on the kind of science the mission aimed to achieve. Spirit and Opportunity were robotic field geologists, searching for signs of past water by looking at the surfaces of rocks with a hand lens and mineral identification instruments. Curiosity is a roving geology laboratory analyzing the habitability of these ancient watery environments by grinding up rock samples to search for trace amounts of organic molecules. Perseverance is a robotic astrobiologist searching for signs of ancient microbial life, also known as biosignatures, which requires a totally new set of instruments.
Convincing microbial biosignatures are tricky to nail down. For example, the presence of organic molecules isn’t enough on its own, as they can also be produced by processes other than life. Instead, concentrated organic molecules must be present in a context that suggests they were emplaced by life, such as in mineralized layers that might have once been a microbial mat. These types of detections are even better if they’re in concert with other potential biosignatures, such as microbial textures, mineral deposits, or isotopic signatures.
Perseverance will look a lot like Curiosity — the two rovers share the same chassis and landing system. However, Perseverance has received a major overhaul under the hood in order to not only search for possible biosignatures but to also place them in fine-scale context. Perseverance packs two Raman spectrometers, SuperCam and SHERLOC, which use lasers to stimulate emission from rock surfaces. SuperCam will be able to detect organic molecules in rocks from meters away, along with their chemistry and mineralogy, while SHERLOC and the elemental mapper PIXL will be able to map organic molecules, compositions, and rock textures at the scale of a grain of salt. Perseverance also carries powerful set of cameras (Mastcam-Z), a ground-penetrating radar (RIMFAX), and a three-dimensional weather station (MEDA).
However, finding definitive evidence of microbial life is extremely difficult. Ultimately, we will need to look at samples from Jezero with advanced instruments on Earth. This is why Perseverance will also collect up to 38 pencil-sized rock cores that will be returned to Earth by a series of missions in the late 2020s.
Paving the Way for Future Mars Exploration
Perseverance will have many new capabilities that will transform how we explore Mars. The rover carries Ingenuity, a small helicopter that will be the first aircraft to fly on another planet. Because Mars’ atmosphere today is so thin — only 1% of Earth’s — Ingenuity has to be extremely lightweight [1.8 kilograms (4 pounds)] with very large blades [1.2 meters (4 feet) tip-to-tip] to get off the ground. Ingenuity will take images of the distant landscape and help us scout the rover’s traverse; future Mars missions could adopt this model of rovers and aircraft working in tandem.
Looking even further ahead, Perseverance will help prepare for future human missions to Mars. One of the many challenges for astronauts will be the packing list for a two-year roundtrip journey, which includes air, water, and rocket fuel to get home. If these resources could be harvested on Mars, human missions would be much more feasible. The MOXIE instrument on Perseverance will test a process for creating oxygen from Mars’ carbon dioxide atmosphere. In the future, similar instruments could be sent ahead of astronauts, so that breathable air and liquid oxygen rocket propellant are waiting when they arrive.
Planning for the Long Drive
Since Jezero was selected as the landing site two years ago, the Perseverance science team has been poring over all the previous research on the crater, analyzing all the data collected by NASA satellites and coming up with strategies for what the rover will do once we land.
Perseverance will land either on the delta or on the crater floor and then spend a month or so checking out and testing all the instruments and the sampling system. The rover will then drop the helicopter off to conduct a series of flight tests lasting another month. During this time, the science team will be working hard to collect as much data as possible to decide a path forward.
No matter which exact path we choose, we know that it will include a detailed investigation of the bottommost layers of the delta to search for biosignatures in ancient mudstones. Perseverance will then travel across dry riverbeds on top of the delta to the edge of the crater, to search ancient shorelines for signs of stromatolites and other biosignatures in the carbonates.
After this first trek, which should be about 15 kilometers (9 miles) long and take about 1.5 Mars years (3 Earth years), the rover will deposit an initial cache of samples in a safe location that a future rover can access. At this point, the mission will have fulfilled all the key goals that NASA has set, but that won’t be the end of Perseverance’s Mars journey.
Up and Out of Jezero Crater
Another compelling property of Jezero Crater that helped lead to its selection as the landing site for Mars 2020 is that the terrain just outside of the crater is an entirely different and nearly equally exciting geological wonderland.
Jezero Crater is nestled within the outer rings of the Isidis Basin, a massive 1500-kilometer-diameter (932-mile-diameter) impact crater that was created by the last planetesimal-sized body to hit Mars around 3.9 billion years ago. This impact dug up and flung out the depths of the crust and maybe even the upper mantle of Mars, and created extensive hydrothermal systems that could have supported life for long after the impact itself.
After exploring Jezero Crater, Perseverance will climb up onto the rim of Jezero and out onto the plains beyond, known as Nili Planum. This area has been eroded by wind and rivers to expose Isidis ejecta, including massive blocks of crust thrown up to 1000 kilometers (631 miles). Some of these blocks appear to be made of layers, perhaps preserving river, lake, or ocean sediments from a time before the Isidis impact. These most ancient materials might trap remnants of the now-defunct magnetic field on Mars, helping us understand how Mars formed and how the interior cooled down.
But what is most exciting about Nili Planum is that is gives Perseverance yet another shot at searching for signs of life, but in environments totally different from the lakes and beaches of Jezero Crater, such as ancient hydrothermal systems and deep aquifers in the crust. One of the challenges of searching for life on ancient Mars is that while we know that the surface was habitable, we don’t know for how long or what exact conditions prevailed. It’s possible that the subsurface of Mars may have been a more clement place for life for longer than the surface. Perseverance will use its suite of instruments to search for biosignatures in the ancient crust of Nili Planum, most likely trapped in minerals deposited by water in fractures and veins that were once deep underground.
After driving more than 45 kilometers (28 miles) and operating for more than 6 Earth years on the surface, Perseverance will deposit a final cache of samples on Nili Planum, including duplicates of all the critical samples collected in Jezero. NASA has already certified a landing site for future Mars sample return missions on Nili Planum, so if Perseverance can make it that far and deposit that final, complete cache of samples, that will be the cache that is returned to Earth.
Timing is Everything
Perseverance will have work to do beyond just searching for biosignatures. Our first big campaign might be on the crater floor beyond the delta, which is mostly covered by a dark bedrock unit with sinuous margins. This unit might be a lava flow or some other type of volcanic deposit, and if so, this is an extremely important place to collect a sample to send back to Earth. One of the big outstanding questions about Mars is, “Just how old are the rocks?”
On Earth, we date rocks using isotopes, but we’ve only managed to do this for one place on Mars: Curiosity used its mass spectrometer to determine that at least some of the grains in the lake sediments there are over 4 billion years old. But for most of Mars, we don’t have that kind of data, so instead we use the density of impact craters on the surface to get an idea of the age o
f that surface. The idea is simple — because asteroid and comet impacts are random, the more craters on a surface, the older it must be.
However, turning the number of craters into an age is extremely difficult and imprecise. Right now, the only method we have to link crater densities to specific ages is by comparing rocks collected on the Moon by Apollo astronauts to the number of craters at those sites. But we don’t know how well this relationship holds for Mars — for example, we don’t know if Mars was hit by the same number of objects over time. So to really figure out how old everything is on Mars, we need a sample of dateable material from a surface with a clear cratering age. In Jezero, we hope that the dark material on the crater floor, with a cratering age of around 2.2 billion years, will provide this sample.
NASA Takes the Next Giant Leap for Life
By laying the groundwork for sample return with Perseverance, NASA is taking the next giant leap in its exploration of Mars. The rocks collected by Perseverance may be our only shot in the foreseeable future to search for signs of life with samples from another planet. This mission, therefore, is not just “go big or go home” — it is “go big and go home.”
Acknowledgments. This article first appeared in short form in The Conversation on July 29, 2020, originally co-written with Prof. Melissa Rice at Western Washington University.
About the author: Briony Horgan is an Associate Professor of Planetary Science at Purdue University. She is a Participating Scientist on NASA’s Mars Science Laboratory rover mission and a Co-Investigator on NASA’s upcoming Mars 2020 rover mission, the first step toward Mars sample return. Her research program uses data from NASA satellites and rovers, along with laboratory and field work back on Earth, to understand the surface processes that have shaped Mars and the Moon.
Cover photo: An illustration of NASA’s Perseverance rover landing safely on Mars. Hundreds of critical events must execute perfectly and exactly on time for the rover to land safely on February 18, 2021. Credit: NASA/JPL-Caltech.