Focus on Arts and Ecology

The Perseverance spacecraft, due to land this week, aims to scour Jezero Crater and collect the first rocks from the red planet. 

By Alexandra Witze, 16 February 2021

Perseverance will explore Jezero Crater (shown in this artist’s impression), the oldest terrain yet explored on the red planet. Credit: NASA/JPL-Caltech

About 3.9 billion years ago, a wayward rock slammed into Mars, punching a 45-kilometre-wide hole into its surface. On 18 February, NASA plans to land its latest rover inside that pit, named Jezero Crater.

The goal is to explore an area of Mars that was once much warmer and wetter, and perhaps even liveable. Scattered throughout the crater are geological formations hinting at its watery past, including the remains of a lake and a river delta. Studying the make-up of these rocks — in a region where no spacecraft has gone before — will give NASA its best chance yet at answering the age-old question of whether life ever existed on Mars.

“Jezero is very special,” says Briony Horgan, a planetary scientist at Purdue University in West Lafayette, Indiana, and a member of the mission’s science team.

NASA’s six-wheeled rover, named Perseverance, will explore Jezero as a robotic geologist — examining outcrops and then scooping dirt and rock into a carousel of tubes stored in its belly. If all goes well, by the end of its first Mars year on the surface — a little under two Earth years — the rover will have travelled more than 15 kilometres. And it will have collected a precious set of samples that it will drop onto the Martian ground, where future spacecraft might one day retrieve them — the first attempt to bring Mars rocks back to Earth.

Destination: Jezero

Two other missions reached Mars this month. The United Arab Emirates’ Hope spacecraft started orbiting the planet on 9 February, followed a day later by China’s Tianwen-1 mission — a first for both nations. But Perseverance aims to become the first rover to land since 2012, when NASA’s Curiosity spacecraft arrived in Gale Crater, a dry lake bed about 3,700 kilometres from Jezero. (Tianwen-1 is slated to send a rover to touch down as early as May, and Hope will continue orbiting.) Only eight spacecraft have successfully operated on the red planet’s surface (see ‘Touch Down’).

Touch Down

Only eight spacecraft, beginning with the twin Viking landers in 1976, have successfully landed and operated on the red planet. Perseverance is heading to a previously unexplored region on 18 February.

Scientists are excited about Perseverance because it could answer questions about Mars’s past that they have only guessed at. Jezero’s ancient, fractured landscape will be the oldest terrain ever explored on the red planet. “This region is a very old part of Mars,” says Vivian Sun, a planetary geologist at the Jet Propulsion Laboratory in Pasadena, California, and a member of the Perseverance science team. “That’s important because the oldest periods of time are when we think Mars was most habitable.”

Ancient landscape

Perseverance aims to land within a 7.7 x 6.6 kilometre ellipse in Jezero Crater, either next to or on top of the ancient river delta it aims to explore.

From around 4.1 billion to 3.7 billion years ago, volcanoes belched gases into Mars’s atmosphere, thickening it, trapping heat and allowing clouds to form and rain to fall. Flowing water carved out river valleys and filled up lakes — and scientists see evidence of these on the surface today. Eventually, however, Mars became colder, drier and much more inhospitable.

Images taken by orbiting spacecraft show that one river that formed during this time flowed through and over the western edge of Jezero Crater. The river must have carried tiny particles of sediment scoured from far upstream. When it entered the crater, it dropped this sediment into the lake there, creating a delta that fanned out to the east. In and around the delta and lake, conditions would have been ripe for microbial life to thrive. Later, the lake drained away and left behind the formation, now turned to rock.

Mining the delta

Perseverance is heading to Jezero to explore this ancient delta. One goal will be to look for organic matter in the delta’s rocks, which could indicate whether microbes or other life forms once thrived in the region. On Earth, deltas such as that of the Mississippi River hold huge amounts of organic material, the decaying remnants of plants and other matter from farther upstream. If the Jezero delta trapped this type of organic matter, the rover could find it by rolling across the formation and drilling, says Linda Kah, a geologist at the University of Tennessee in Knoxville and a member of Perseverance’s science team.

A watery past

Billions of years ago, a river flowed into Jezero from the west, spilling into a large lake that filled the crater. Sediment exited the river to form a fan-shaped delta.

Another place to look for signs of past life on Mars is along the ancient shores of the lake. In particular, Perseverance will study an arc of rocks that run alongside the delta and parallel to the rim of the crater. Earlier observations from orbiting spacecraft suggest that these rocks could be made of carbonate minerals, which are relatively rare on Mars but common on Earth. Carbonates, including limestone, are often associated with living organisms such as coral reefs, and with fossils that preserve past organisms such as microbes. “Carbonate excites the hell out of geobiologists like me,” says Kah.

A path forward

Once it lands, Perseverance’s biggest challenge will be to quickly and efficiently explore all these target areas and collect samples. NASA has allotted only one Mars year (equivalent to nearly two years on Earth) for Perseverance to carry out its initial mission.

So scientists on the team have been drawing up routes that the rover might take soon after it touches down. “A lot will depend on where we land” in the 8-kilometre-wide target area, says Kah.

A rover’s journey

One possible route would see Perseverance landing on the crater floor, travelling across and along the ancient river delta and collecting rocks from multiple locations before driving out of Jezero to lay those rock samples down for future missions to bring back to Earth.

In one scenario, Perseverance will land on the crater floor, just in front of the delta’s cliffs. Scientists aren’t entirely sure what the crater floor is made of, but the rock appears to be dark and smooth and could be a volcanic flow. If so, picking up a sample will be a top priority for mission scientists: when that sample eventually gets back to Earth, researchers will be able to analyse it and obtain a precise date for the volcanic eruption that created it, which will help them to understand the timing of geological events in Jezero.

From the crater floor, the rover could drive up and onto the delta, grabbing clay-rich samples and other rocks. It could then head onto the potentially carbonate-rich rocks, drilling into those as well, and afterward drive up onto the rim of Jezero.

If the rover takes this route, it will have travelled more than 15 kilometres and collected at least 15–20 tubes of rocks of different types.

Rock collector

To obtain rock and soil samples, Perseverance will use a drill on its robotic arm (see ‘Tools To Explore’). It will collect duplicates of the most important samples, filling two tubes with the same rock. When the rover reaches the rim of Jezero Crater, it will lay one set of these tubes on the ground and keep the other stashed in its belly. This is to ensure that there is an initial collection of Mars samples available for a future mission to pick up easily and return to Earth.

If the rover is still healthy after its first Mars year, it will head from the crater rim out to the surrounding plains, called Nili Planum. This part of its journey will explore the most ancient terrain yet, including enormous blocks of jumbled-up rock that were blasted from deep inside Mars when another huge asteroid hit, nearly 4 billion years ago. These rocks could come not just from the Martian crust, but from a deeper layer of Mars known as the mantle — which scientists have never been able to look at directly before. “They’re just sitting there, perhaps for more than three billion years, waiting for us to look at them,” says Horgan.

Tools to explore

The car-sized Perseverance carries multiple instruments to probe the Martian landscape. The drill that it will use to collect rock samples is at the end of its robotic arm.

Also up on Nili Planum, according to past observations, are the remains of ancient hydrothermal systems, where hot water once percolated through the Martian crust. On Earth, in places like Yellowstone National Park in Wyoming, these hydrothermal areas are teeming with microbial ecosystems. The same might once have been true on Mars. So Nili Planum offers a type of potentially habitable environment entirely different from that in Jezero Crater.

If everything is still going well in this ‘extended mission’, Perseverance will drill and collect samples until it runs out of tubes. With one set dropped off at the rim of Jezero Crater, the rest of the tubes will be laid down somewhere out on Nili Planum, awaiting retrieval.

Perseverance will perform other tasks during its time on Mars, including testing the first helicopter placed on another world — as a way to scout the safest path across the landscape — and experimenting with ways to extract oxygen from the Martian atmosphere, as a resource for future human explorers. But its top priority is to collect those samples for eventual return to Earth.

The European Space Agency and NASA are working on plans for two missions needed to retrieve the samples. Launching no earlier than 2026, they would send a rover to Jezero Crater or Nili Planum to retrieve the sample tubes laid down by Perseverance, as well as an orbiter to loop around Mars. The rover would put the tubes on a rocket and launch them into Mars orbit, where the orbiter would grab them and fly them back to Earth no earlier than 2031.

Perseverance is the first step in that lengthy process — a step scientists have been anticipating for decades. “Perseverance will be the first rover to seek the answer to the outstanding questions that we haven’t been able to address with other rovers — was there life on Mars, and can we find evidence that there used to be life?” says Sun. “I’m super-excited.”

(Sources: Nature)