Perseverance rover perseveres

A year after its dramatic landing on Mars on 18 February 2021, NASA’s Perseverance rover is wrapping up the first phase of its mission and preparing for the next one.

The rover’s ultimate mission is to explore sediments in the remnants of a river delta on the edge of 45-kilometre-wide Jezero Crater – sediments that might contain signatures of ancient life from a time when Mars was warm and wet and Jezero Crater held a lake fed by waters from the river that created the delta.

But in science, patience can be critical, and before speeding off to the delta, Perseverance has spent the past 13 months on the crater floor.

Part of that time went into unlimbering its instruments and making sure they were in operating shape. A bit more went into testing the tiny 1.8-kilogram helicopter Perseverance had brought with it in an effort to find out if it was possible to fly in the thin Martian atmosphere. (It was.)

But the remaining 10 months went into exploring the crater floor near the landing site, before moving off to the remnants of the delta, however tempting it might have been to move straight for them, beckoning on the horizon only a few kilometres away.

Happily, it was time well spent. The rover now has at least seven samples collected and stowed for eventual return to Earth, and has learned much about the crater into which the delta-forming river once spilled.

“Even before landing, our science team understood that understanding the crater floor is a very high priority for our mission,” Vivian Sun, a planetary scientist and systems engineer at NASA’s Jet Propulsion Laboratory, in Pasadena, California, said at this year’s Lunar and Planetary Sciences Meeting, held last week in The Woodlands, Texas. “The goal is to piece together the overall story of how the lake in Jezero Crater formed—putting together the ancient history of what happened at Jezero Crater.”

Part of these discoveries centre around the fact that the rover landed near the boundary between two different types of rocks now named the Máaz and Séítah formations.

Máaz was the one on which the rover landed, chosen because its surface was flat, firm, and easy to drive on. Not crashing onto a boulder or getting bogged down in sand was one of the mission’s top landing-zone priorities.

The Máaz unit (whose name means “Mars” in the Navajo Native American language) appears to be the younger of the two, Sun says.

Séítah appears to be a bit older (how much older won’t be known until samples are returned to Earth and examined in laboratories experienced in the use of trace isotopes to determine the age of rocks). Its name means “amidst the sand” in the Navajo language—an apt description of why NASA flight engineers had marked this region on landing maps with the equivalent of a skull-and-crossbones flag.

Both regions, Sun says, appear to be volcanic. And while they appear to be from separate lava flows, it is also possible that they are simply different layers in a single large lava flow, cooling and crystalising at different rates, with Máaz being the top layer, and Séítah the bottom. “Those are our two leading hypotheses,” she says.

Séítah’s rocks also show strong signs of being altered by water, says Eva Scheller, a graduate student at California Institute of Technology, Pasadena, who studied them using the rover’s SHERLOC instrument (which examines their fluorescence spectroscopy under ultraviolet light).

Not that this is surprising, since the existence of the delta is a strong indicator that Jezero Crater was once a giant lake. But it’s nice to find confirmation that the rocks on its floor were there when the lake existed, as opposed to being formed later on, when everything was dry.

There are even signatures of aromatic organic compounds like naphthalene (commonly used in mothballs), Scheller adds. What that means isn’t clear, she says. But such compounds are “astrobiologically relevant,” she says—and hopefully rocks containing them are now included in one or more of the rover’s growing collection of samples.

Getting samples from the Séítah formation, however, was tricky because the region really is full of potentially fatal sand traps. But living up to its name (and with help from the helicopter), Perseverance was able to find a path 200 metres into the Séítah formation, locate suitable rocks for study, and safely retreat to the safer terrain of the Máaz formation, across which it now plans to drive to the still-beckoning base of the delta.

In the interim, Perseverance scientists have been studying the delta sediments from afar—especially those in a rock formation called Kodiak Butte, which appears to be an eroded remnant of the delta, 2.2 km from the rover’s landing site.

Kodiak is about 50–60 m high, and appears to be composed of three basic layers, says Sanjeev Gupta, a sedimentologist at Imperial College, London. These layers, he adds, are of the type normally found in what is called a Gilbert delta.

Named for 19^th century geologist Grove Karl Gilbert, these deltas are formed by the flow of coarse-grained sediments into a lake.

They are different from deltas produced by the flow of finer-grained sediments like mud, into an ocean, such as the Mississippi River delta, in Louisiana.

The Mississippi River delta, Gupta says, is formed of gently sloping layers. Gilbert deltas, in cross-section, look more like an “S,” and are composed of three rock units: the bottomset, the foreset, and the topset.

The bottomset is the base of the “S.” Being on the bottom means that is comes first as the delta advances. It is composed of a thin layer of fine-grained sediments washed out into the lakebed, ahead of the main mass of the encroaching delta.

As the delta advances, these are buried beneath a steep wall of coarser-grained sediments moving ever farther into the lake. These are the foreset. The term “fore” means “front,” but that’s a bit misleading because what they are the front of is the steep wall of the delta. The bottomset, onto which they are piling, is already there first. In the cross-sections of Gilbert delta sediments, however, the foresets are the middle, sloping, part of the “S.”

Above them comes the topset: another nearly flat layer of sediments, these deposited in the shallow waters atop the maturing delta.

This cycle can, of course, repeat, as lake levels change, building one Gilbert delta on top of another.

Kodiak Butte isn’t on the rover’s planned route, so all that scientists can do with it is pore over high-resolution images, like earthly geologists studying the strata in cliffs or roadcuts.

But Perseverance is already gearing up for the next stage of its mission, which will begin by circling the impassible Séítah terrain toward the main part of the delta, aiming for a breach in it called Hawksbill Gap, through which it can climb into the upper layers beyond. “We will drive around Séítah as fast as possible to get there,” Gupta says.

Already, he says, long-distance images show that this is also a Gilbert delta, complete with bottomsets, foresets, and topsets, formed at a time when the delta was building out into about 45 meters of water depth.

Knowing the water depth represented by these deposits is important in piecing together the overall story of how the water level in the lake varied over time, Sun says.

But understanding how the delta formed is important for another reason, because it tells scientists where best to look for the type of sediments that might preserve signatures of ancient life. “The fine-brained bottomsets are where to search,” Gupta says.

Meanwhile, Ingenuity will continue to help out, looking not only for the best places to drive, but the most interesting locations for the rover’s next set of samples.

After Hawksbill Gap? Who knows? If all goes well, the rover, and its trusty helicopter, will continue to climb, perhaps moving all the way out of Jezero Crater and into the high plains beyond. If so, they wouldn’t be the first Mars explorers to vastly exceed their design life.