Here’s what the latest Mars rover has learned so far

It’s easy to take it for granted, but we’re driving around on freakin’ Mars right now.

We’ve done this a few times before, sure, but it remains one of humankind’s most impressive technological feats. The latest rover to continue our presence on the red planet is Perseverance, the star of the Mars 2020 mission that launched in July of that year and landed in February of 2021.

It has now been busy roving for over two years. News of what we’re discovering—beyond the stream of photos—tends to come in discrete bits that can be hard to connect into a bigger picture if you aren’t following closely. Consider this your wide-angle recap.

Like other rovers, Perseverance is bristling with science instruments. It has cameras of multiple kinds used both for general imagery and spectral analysis that can identify minerals. That latter function is supplemented by an additional X-ray instrument. Perseverance also has a ground-penetrating radar instrument that can reveal layering hidden below the surface. More invasively, there is a drill on the end of the rover’s robotic arm. This is used to grind clean (what geologists call “fresh”) spots for analysis, but it can also core out small cylindrical rock samples—hopefully to be retrieved and returned to Earth by a future mission.

It's not all about the rocks, though. Perseverance has a weather module tracking atmospheric conditions and airborne dust. And it has a friend—the Ingenuity helicopter has wildly exceeded its pilot-testing goal and is still flying in short hops to keep up with the rover.

This mission set down in Jezero Crater, which was chosen because rocks resembling a river delta are draped over its rim—indicating that flowing water might have met a lake here in the past. It’s the perfect environment to study the history of water on Mars and the possibility of life associated with it. There’s only so much science you can do from orbit. To untangle the forensic clues that remain here, you need to get down on the ground.

First stop: Crater

The first years on Mars were spent investigating the floor of Jezero Crater. The type of rock that would be found here was actually somewhat ambiguous from orbit. There was clearly some igneous rock, either from volcanic magma or a molten pool created by the meteorite impact that formed the crater. But some also expected to see sedimentary rock representing the bottom of a lake that called the crater home.

It turned out to just be igneous basalt under the blanket of wind-blown dust, and any lake-bottom sediments that existed here must have long since eroded away. You might think that’s disappointing—like the pharaoh’s tomb was already cleaned out by grave robbers—but this is actually one of the better looks we’ve gotten at Mars’ igneous bedrock. Missions have often targeted pockets of notable sedimentary rock, with only scattered bits of the much more common igneous rock on display.

The Martian meteorites we’ve found on Earth—chipped off the red planet during large impact events—have only given us literal fragments of the big picture. If we successfully return the eight rock samples collected from the crater floor, this opportunity to cruise around on intact igneous bedrock could answer a lot of questions raised by the meteorites.

In this case, the science team has divided the crater floor rocks they observed into two major layers. The upper one, called the Máaz formation, appears to have formed from lavas. Some portions exhibit a texture like the wrinkled (or “rope-like”) lavas we see in Hawaii. In other areas, the rock happens to stick up through the red dust as flat polygons resembling pavers in a garden, or as taller, boulder-sized blocks.

The lower Séítah formation is distinct in both texture and minerals. It stands out from its surroundings due to its thin layering and visible, closely packed crystals. And while the Máaz rocks contain lots of the mineral feldspar, Séítah's rocks are dominated by olivine, instead.

This looks like what geologists call “cumulate”—the magmatic equivalent of the gritty dregs in your coffee cup. Because different minerals crystallize at different temperatures (yes, molten rock has a freezing point), minerals like olivine that crystallize early can settle to the bottom of a magma body and accumulate. On Earth, this pattern can be seen in magma chambers that cooled underground or in some sufficiently thick lavas.

This leaves space for a few possible interpretations. It could be that a thick lake of lava formed from the impact in this crater or another nearby; alternatively, this is a single, thick volcanic lava flow. But it’s also possible that these layers represent separate volcanic events in a sequence, with erosion uncovering an old magma chamber before new lava flows buried it again. These are the kinds of details that returned samples could help us pin down. Dating the rocks could show if they formed at the same time, and finer chemical analysis could reveal how closely related they are.

Since these rocks are billions of years old, their origin story is not the only one they can tell. Data collection is also focused on how they have been altered by time and the elements—most notably water. While lake sediment is nowhere to be seen, there is still some evidence hinting at interactions with water.

Alongside the minerals that are characteristic of a basalt lava, Perseverance has spotted minerals that form as the original rock reacts with chemical visitors. Magnesium and iron from crystals of olivine, for example, have ended up in carbonate—a common product of weathering facilitated by water. Similarly, some small open spaces in the basalts have been filled in with sulfate and perchlorate salts. These could precipitate in areas where shallow, salty waters are evaporating or seeping into the ground.

But while it’s true that clues like these are present, it’s also true that they aren’t as common as they could be. So, as one paper notes, either the lake was too short-lived (and the climate too cold) to alter these rocks further, or the erosion that removed lake-bottom sediments also removed the most altered bedrock. That’s often an important forensic question in geology—do you have all the evidence, or has some been erased by later events?

Either way, the evidence that does exist is interesting enough, and samples that return to Earth would be examined for any signs that life existed in these potentially habitable conditions.

Movin’ on up

In April 2022, the mission turned to its second phase, referred to as the Delta Front Campaign. As Perseverance made its way toward the edge of the crater, its eyes also shifted to the rocks above the crater floor. These are what looked from orbit like river delta deposits, built up as sediment dropped from flowing water that slowed to a halt.

Photos taken en route showed apparent layering in the nearest outcrops. In fact, there was a familiar-looking sandwich of layers typical of a rocky river delta. As these deltas grow over time, they push farther into the body of standing water, building over the top of sediments laid down earlier. At any one point in time, you can divide the area between the river’s mouth and the open water into three zones. Starting from the land side, there's a braided pattern of flowing channels that breaks out from the river’s mouth, where the largest pieces of sediment are dumped. At some point, this reaches a drop-off at the shoreline, depositing sediment on a submerged slope. Finally, the finest remaining sediment settles out to blanket the flat lake bottom. These are (sensibly) called the topset, foreset, and bottomset layers, respectively.

When you drill down through the sediment or look at a cross-section in an outcrop, you’ll see these layers stacked in order. Fine sediment of the bottomset layer forms the deepest deposits. Higher up, slanted foreset layers appear, built on top of pre-existing lake bottom sediments. Horizontal, coarser topset layers of sediment cap off the sequence. And that’s the pattern Perseverance saw on Mars.

“One of the things that we were looking for when we rolled up to the front of the delta fan here were the very finest sediments, preferably mudstone,” University of Florida Professor and participating scientist Amy Williams told Ars, “because that's made up of one of the finest grain sizes, which is clay, and clays are really, really good at preserving organic matter[…] And when we got up to the Jezero delta front, we basically jumped into effectively fine to medium sandstones, which is coarser than clay.”

It could be that the finer bottomset sediments have also eroded away in this area. But it could also be that our Earth analogs are an imperfect match for the conditions on Mars.

“Something that I found really interesting to consider, though—when you picture deltas as we recognize them today on Earth, they're so influenced by the biology of Earth that you can't really deconvolve in some cases the [ways] that biology has made an impact on the geology,” Williams continued. “So for instance, there's been a discussion that some deltas that would have formed before land plants evolved wouldn't have necessarily had [as much] really fine clay because there were no land plants to facilitate the breakdown of rocks in that way. So maybe you don't see a ton of fine grains in the Jezero delta because we know there's no land plants.”

Sediments in these environments also don’t form simple, continuous layers like you see in the Grand Canyon. Streams wiggle around over time, with different kinds of sediment deposited inside the channel compared to just a few meters away from the channel. Additionally, the lake level could have fluctuated considerably, moving the shoreline back and forth. This means the sedimentary record they leave beyond is pretty complex, changing over short distances.

“What's really striking to me is that if you tried to ascend the delta at a bunch of different places, you would probably get different sequences that would finally give you the full story for the delta,” Williams said. “But that's not our mission to do that. And so we won't have the entire perfect history of this delta structure.”

The rover’s time up close with these rocks has certainly added to that story, though, as have the cache of rock samples. For example, there is a stack of delta deposits capped with a rubbly layer containing smoothed boulders over a meter in diameter. It takes energetic floodwaters to move rocks that large—a different picture from a lazy-stream-meets-lake delta kind of environment. As the rover climbs upward, it may eventually find out where those boulders came from.

The highest lake level implied by these layers is also a fair bit lower than the height of the channel on the far side of the crater, which had been considered an outlet. That would make the crater into a closed system where water flows into a lake but can only leave by seeping into the ground or evaporating.

And there are once again minerals present that point to the presence of water even after these sediments were deposited. Some fractures in the rock are filled with veins of a relative of gypsum, for example. “There's clearly been water that has interacted with these rocks at a variety of periods in its history,” Williams said.

Dates from these rocks could tell us more about exactly when water was present in this area—and whether our current understanding of Mars’ climatic history needs to be revised.

On top, and just getting started

Since February 2023, Perseverance has been cruising upslope, exploring the top of the delta and aiming to climb out of Jezero Crater entirely when that’s done. It takes time for the science team to analyze data and publish interpretations of what they’re seeing, so there isn’t as much to say here yet. But the images and mission updates do show us what the rover has been encountering.

There’s definitely more of the chunky stuff—pebbly sedimentary rock layers and rounded-off boulders scattered about the surface. Lobes of this sediment built by stream channels that wiggled back and forth over time point to that faster-flowing stream environment. “It looks like it's got a more complicated history than maybe we would have anticipated from orbit,” Williams said.

The rover spent time on several attempts at drilling a core of this pebbly rock, frustrated at first by its crumbliness. But the sample it eventually acquired could be an interesting one for several reasons. First, each pebble is a piece of some other rock, so it’s a sample with bonus samples inside. Additionally, a rock like this could provide clues about whether Mars’ magnetic field was still active when it formed—whenever that was.

Perseverance’s route also took it past a notable landmark called Belva crater. This conspicuous divot must be the result of an impact event that occurred after these rocks were already here. That punches a nice hole that gives the rover a view of the layers beneath it. But it’s also shallower than most craters of this diameter, and a closer look may reveal why. Did something fill it in, or has the rim eroded down?

The upcoming itinerary will send Perseverance up to the rim of the crater—and hopefully beyond. The delta rocks will transition to whatever the surrounding bedrock is, which should be a particularly interesting area to explore. One simple question the rover will answer is whether the rock around the crater rim is the same stuff it saw on the crater floor. But it’s also apparent that there’s something different about the ring of rock just inside the crater walls.

“I’m really excited about the marginal campaign because we see a carbonate signature from orbit,” Williams explained. “And we've seen little bits of carbonate here and there in our mission so far. But again, it's that reconciling [of what we see] from orbit with [observations] on the ground. I'm really keen to see what we're going to encounter when we get up there.”

The rest of the rover’s rock samples will be cached up on the crater rim, completing Perseverance’s primary mission. The Mars Sample Return mission that will hopefully retrieve these caches is years away, but the plan for the mission represents an almost unbelievable degree of difficulty.

There’s going to be a lander, some kind of errand-rover to scoop up all the samples and load them into the lander, a small rocket to lift them into orbit, and an orbiting spacecraft to catch that relay baton and fire it back to Earth.

“It's never been done before,” Williams noted. “And I think it's going to revolutionize what we know, not only about Mars, but I think it's gonna give us context for all the rocky inner planets. I think it's going to be one of the most incredible things that we can do for exploration and planetary science—to return these samples and be able to explore them with the suite of instruments that we just can't send to Mars.”

Until then, Perseverance is doing its part. Once it drops this next sample cache, it is expected to get an extended mission to do some more exploring. The rover is in great shape, so hopefully it will follow in the wheel tracks of its predecessors by carrying on well past the goal it was designed for—as part of the active Earthling presence on Mars.