The Mars Exploration Rovers Update Special Report: Perseverance Science So Far, New View of Endeavour at LPSC 2018

For the 15^th year in a row, Opportunity and the Mars Exploration Rovers (MER) mission drove into the spotlight during an afternoon session at the 49^th Lunar & Planetary Science Conference (LPSC), held in The Woodlands, Texas, in March. Known by its acronym, LPSC is where this niche of the space community regularly communes and where this MER-class robot field geologist is treated with reverence worthy of a veteran.

Drawing scientists, professors, and students from around the world, this annual conference is where the latest, greatest research in planetary science is presented and the craziest, most far-reaching, thoughtful theories flaunted. Under the banner Mars Rover Results I: Depositional and Environmental History, six scientists took to the podium on March 22^nd to present Opportunity’s findings in Perseverance Valley so far, as well as their analyses and the evolving thinking about the strangely unique formation that defines Endeavour’s west rim at Cape Byron.

Although the MER presentations followed six talks on Curiosity’s recent findings, Opportunity’s starbot power and the science delivered kept the room filled to the end. By the time the session was over, they succeeded in leaving the audience in a cloud of metaphorical dust, rapt in the fog of another Martian mystery.

Opportunity has been exploring the western rim segments of the 22-kilometer (13.7-mile) diameter Endeavour Crater since August 2011, racking up some 12 kilometers—more than 7 miles—climbing, driving, and sliding around, in, and through several of them. With Perseverance however, the robot is blazing another new trail into ancient Mars, charting Earth’s first on-site look at a feature that from orbit looks like it was carved by water flowing downhill. The objective is straightforward: study the valley’s morphology or shape and its compositions, and determine how, and if possible approximately when, it formed.

From the crater rim crest, or notch as it appears in images, Perseverance extends downhill at about a 15-degree angle for about 200 meters (about 219 yards) toward Endeavour’s interior. Along the way, it breaks into an anastomosing pattern or, in other words, it branches off in different directions that look like a river system. 

In the first images taken of it by the HiRISE camera onboard the Mars Reconnaissance Orbiter (MRO), this place screamed water. “That’s why we went there,” said MER Project Scientist Matt Golombek during a recent interview.

Moreover, whatever first created this feature was generally thought to have occurred at some point during the ancient Martian epoch 4.1-to-3.7 billion years ago called the Noachian Period. That’s the timeframe when Mars is believed to have been wetter and more like Earth, with rivers, lakes, streams, and perhaps an ocean. Sometimes, however, impressions from orbit are not so easily, not so clearly confirmed when viewed up close by a robot field geologist.

The MER scientists are scientists though and they assume nothing, as a rule. Long before Opportunity even pulled up to Perseverance in May 2016, the approach to researching this distinctive place was declared,decidedly unbiased and intentionally simple: “We’re going about this with multiple working hypotheses,” said MER Principal Investigator Steve Squyres, of Cornell University. That has been the mantra ever since.

It’s an approach, geologically speaking, that is well traveled and long proven, as Rob Sullivan, a MER Athena science team member, also of Cornell, pointed out. The Method of Multiple Working Hypotheses, written by American geologist and educator Thomas Chrowder Chamberlin (1843-1928), was first published in 1890 in the journal Science.

“Basically, Chamberlin argued that when approaching a complex problem, carrying multiple working hypotheses encourages greater objectivity,” said Sullivan. “He also theorized that many geological problems are best explained by several different processes acting at different times or simultaneously over history to produce landscapes as we find them in our times.”

Chamberlin went on to found and edit the Journal of Geology, serve as President of the University of Wisconsin, President of the American Association Advancement of Science (AAAS), and a director of the Walker Museum at the University of Chicago. During the last 125 years, it has become a classic, published many times in different formats, and its principles guide and influence research integrity to this day.

“One of the biggest mistakes you can make as scientists is that [attitude]: ‘I wouldn’t have seen it, if I hadn’t believed it,’” noted Squyres. “You don’t want to fall into that trap. The thing to do at this point is keep an open mind – multiple working hypotheses – and let’s just let Mars tell the story.”

Opportunity is now halfway down Perseverance and Mars is still talking. Beyond the sometimes-slippery slopes of the gravelly terrain on which Opportunity is roving, the team’s challenge lies in the fact that Perseverance is the only geological feature of its kind at Endeavour. There is nothing anywhere else in the rim that looks anything like it. 

The valley’s “singularity” is “an outstanding characteristic,” Sullivan said. That’s the good news, right? The bad news? “This poses important challenges to all candidate origins, including water,” he said.

So uniqueness is making the task of uncovering the story of Perseverance one of if not the greatest Martian mysteries that Opportunity and the MER science team have undertaken. Nevertheless, in tight, on-point presentations, the six lecturers of MER research at LPSC this year reviewed some solid geologic studies,laid out a number of key findings that reveal an evolution in the team’s thinking– and proved once again that there’s nothing quite like a good Martian mystery. 

Opportunity’s Exploration of Perseverance Valley

Steve Squyres, PhD
MER Principal Investigator; Athena Science Team Lead;
James A. Weeks Professor of Physical Sciences, Cornell University

“It’s a work in progress.” Steve Squyres cut to the chase. Then he launched the MER talks with a “high-speed” walkabout of the evidence the Athena science team members have found to date at Perseverance Valley and the implications that data may have for solving the mystery of what it was that created this unique geological feature.

Showing an overhead view of the valley as it cuts through the rim of Endeavour, Squyres described Perseverance as having “a braided sort of appearance” and “a very distinctive look that is the consequence of some kind of fluid flow.” 

At the approximate halfway point on her journey however, Perseverance is still a real mystery. “We have multiple working hypotheses,” said Squyres. “When we started we had three hypotheses – dry avalanche, debris flow, some kind of fluvial river transport. We can test these hypotheses by looking at the features on the ground.” 

Since last May, Opportunity and the team have been looking at all the features on the ground, as well as imaging the morphology. While they have found evidence that seemed to point to past water, they also uncovered other potential contributors to the original working hypotheses that would be detailed during the conference. 

“Out on the plains above the top of Perseverance Valley, west of the crater rim, are some very broad shallow troughs that more or less seem to terminate at the top,” [the rim crest], “as if it they would be the kinds of things through which fluid flows, and then spilled down,” Squyres said, pointing to the top of the notch in another overhead image. “But there’s a serious challenge for this hypothesis. The top of Perseverance actually slopes up,” he said.“Mars is a weird place, but not so weird that water flows up hill.” 

Still, it has been a few billions years or so, give or take. You could solve this if this surface was originally horizontal and then [part of] it went through some compaction –” he said. [Tim Parker, a MER Athena Science Team member and a geology lead, would expound on that as he offered up his lake spillover theory later in the afternoon.]

Once Opportunity got into the valley, the scientists found the topography to be “really very subtle,” Squyres said. “A lot of it is in-filled with soil.” The rover has also found plenty of visibly scoured outcrops. 

At a site named La Bajada, where two distinctly different outcrops are separated by the valley path, the scientists even found breccia outcrop with telltale wind tails that “really caught our attention,” said Squyres, recalling how Opportunity popped two wheelies to be able to hunker down over this outcrop to and check it out up close. “This wind tail or flow tail indicates the direction of flow,” he said, pointing to an image of the flat rock at La Bajada. “And that direction is – uphill.” 

Since wind blows radially out and up the crater rim, most of the science team members view this as “pretty strong evidence of wind erosion,” Squyres said. “This doesn’t mean the valley itself has been carved by wind, but it certainly at least points to aeolian erosion overprinting after the valley itself formed.” [Aeolian erosion was one of the multiple working hypotheses Rob Sullivan would focus on later in the afternoon.]

In the troughs where the soil is, the scientists found “distinct linear arrangements less than one meter across,” Squyres said, what geologists refer to as stone stripes, “You can see these parallel ridges across and the ridges are composed of coarser grains with finer grains in them. We immediately think these could be some kind of periglacial features like the stone stripes found in a number of places on Earth where there is shallow subsurface ice, like on Mauna Kea [Hawaii]. But when you look at these things closer, it doesn’t hold together so well,” he said.

The science team needed to find out if Perseverance Valley is “a place where ice could be stable or if there is something unique or special about Perseverance Valley that would cause these features to be here,” Squyres explained. “Because we don’t see stone stripes a lot of places.”

Mike Mellon, a planetary geologist and geophysicistand associate research professor in the Applied Physics Laboratory at Johns Hopkins University, stepped in at MER Deputy Principal Investigator Ray Arvidson’s request to help with some thermal modeling. The objective was met. “The answer is no and no,” Squyres said. 

So what are they? Sullivan suggests these stone stripes could be megaripples, an extensive undulation of the surface typically tens of meters from crest to crest and tens of centimeters in height or larger. Somewhat like the ‘frozen waves’ on sandy beaches or seabeds on Earth. “You’d think something really big, but ‘mega ripples’ simply means they’re developed from material that hasfine fractures and saltation,” said Squyres. “We may in fact be seeing something new here.”

Another key observation the scientists have been making is the number of trough areas where there is a distinct difference between one side of the trough and the other side. “This is suggesting there may be a fault here and that the erosion that formed Perseverance Valley may be in large measure structurally controlled,” Squyres explained. “It could be wind erosion, as opposed to any kind of water erosion. We’ve got to take that fairly seriously actually.” [Crumpler’s presentation on the structure of the crater and the fault zones in Perseverance and Sullivan’s talk on the potential contributions of reactivated faulting and wind processes would soon expound on how seriously.] 

While the origin of Perseverance is still “a work in progress,” Squyres personally ruled out the dry avalanche theory. The downhill slopes are “too gentle,” at 15-degrees, and the rover has seen “no deposits of anything” indicating a past avalanche. That leaves water or fluvial transport, wind, and debris flow, as he viewed it, still among the multiple working hypotheses. 

There was more to come, more to go, and new observations about how planetary forces, like people, sometimes work together.

Fault Control and Regolith Diversity

Larry Crumpler, PhD
MER Athena Science Team; Research Curator Volcanology, Space Science, New Mexico Museum of Natural History and Science; Associate Professor, University of New Mexico

On the first geologic traverse ever conducted by a rover within the rim of a large impact crater, Opportunity has driven approximately 12 kilometers (more than 7 miles) in the western segments of Endeavour. Along the way, Larry Crumpler has been studying the crater’s structure, as well as the rocks, outcrops, and troughs to understand how the structure “agrees with the actual physical characteristics” the rover is finding on the ground. In his talk, he laid out evidence that the west rim segments and Perseverance Valley are “strongly” controlled by faults and fault zones created at the time of impact.

As Opportunity descended from the Cape Tribulation rim segment and roved to Perseverance, Crumpler studied geological characteristics documented in the images the rover sent home. “One of the large-scale characteristics is the segmentation of the crater rim,” he said. 

“When you actually look at the rim, it consists of these little segments that are on the order of a couple of hundred meters long and maybe one hundred meters wide,” Crumpler continued. “They tend to be offset from one another; they tend to have different orientations. And when viewed together, one gets the impression one is looking at some sort of structural division in the make-up of the crater rim.”

Moreover, as Opportunity has traveled from one rim segment to another, “the nature of the outcrops, the basic fundamental geology was changed in very subtle ways, chemically and structurally,” he said. On those treks, the rover has crossed transition areas, where the terrain changes and where the scientists have seen “intense alteration characteristics and the most interesting geology,” another indication that a major structural change happened, or perhaps “a major fault the bounds the segment.”

Until recently, the MER scientists have seen very little evidence in the outcrops that show unequivocally a fracture or fault was/is associated with these transition areas. “It’s been somewhat frustrating,” Crumpler said. Since entering Perseverance Valley however, Opportunity has been returning images that show distinctive lithological divides and linear formations that, he noted are the “characteristics of linear fractures that are controlling things on an outcrop scale.”

Different kinds of rocks near each other, and especially those with a sort of boundary between them, often indicate the presence of a fault to geologists. As Opportunity has descended the approximate 15-degee slope of Perseverance Valley, the rover has also found the juxtaposition of different rock types have aligned downhill.

The strip of terrain that separates the outcrops of La Bajada – where bright, flat rocks on the northern side contrast with lumpy, darker rocks on the southern side, where Opportunity found the wind tails that point up hill – “looks suspiciously like some sort of fracture,” said Crumpler.

“There is some sort of zone here that’s very disturbed looking and when we sat on it, we really could not say what it was,” he said. “But off to the side there was another very linear feature that is several centimeters wide characterized by a half-meter wide, very long zone of parallel red and grey materials.”

Like a fracture or alteration zone, this linear feature cuts through the impact breccias and moves downslope. “As we moved farther down the valley, it continues for some 50, 60 meters, and this feature is trending parallel to the same La Bajada feature,” he pointed out.

When Crumpler mapped the potential fractures and faults of La Bajada based on the physical characteristics at the site, he found that a fault bounds the margins of the south trough. “That says perhaps the valley itself has actually got significant fractures or faults and is actually following the current track,” he said. 

With the data in hand, Crumpler postulated: “There could be a fault running through here and it could be wind erosion that formed the valley.” But, he added: “The real question is: Is Perseverance Valley just a graben?” 

A graben, by definition, is an elongated, depressed block lying between two faults. An example of a graben on Earth is a rift that cuts across land. Crumpler was quick to note that he thinks the formation of Perseverance is “more complex” than that. However, he concluded: “There is evidence that the valley structure was controlled by some sort of major fault that likely influenced fluid movement in the crust and may have controlled trough margins.”

Multiple Working Hypotheses at Perseverance Valley: Fracture and Aeolian Abrasion

Rob Sullivan, PhD
MER Athena Science Team member; Senior Research Associate, Cornell Center for Astrophysics and Planetary Research,Cornell University

After an introduction underscoring the value of the multiple working hypotheses approach the team is taking despite the understandable interest in the possibility of flowing water, Rob Sullivan focused his presentation on evidence showing the potential contributions that fracturing and aeolian or wind abrasion made in the formation of Perseverance.

Sullivan's presentation began with a quick familiarization "tour" of aeolian abrasion features on Earth, including wind-altered rocks in Coachella Valley, California. “Wind-driven sand impacting rocks can completely change rock shapes by eroding away a great deal of material while applying special, diagnostic textures to rock surfaces that remain,” he said.

With Earth-based examples as a basis for comparison, he turned to Mars, showing examples of aeolian abrasion along Opportunity's traverse at Meridiani Planum. Beyond the pervasiveness of this process at Meridiani Planum, Sullivan’s main point was that aeolian abrasion happens fast. That’s fast geologically speaking.

“Ejecta blocks thrown out by recent impact craters on the wind-rippled plains that the rover crossed to get to Endeavour have been eroded on timescales” of hundreds of thousands of years, “similar to the rate of ripple migration,” Sullivan pointed out. [These timescales were quantified to less than one million years in “Constraints on ripple migration at Meridiani Planum 
from Opportunity and HiRISE observations of fresh craters,” a paper written by MER Project Scientist Matt Golombek, and for which Sullivan was a co-author, published in the Journal of Geophysical Research in 2010.]

Well, that raises a couple of pertinent questions – and would seem to dam the flowing water hypothesis. If aeolian abrasion is that effective and this valley presumably formed in the first half of Martian history, how could it have survived looking like a river system more than two billion years to today under the onslaught of the constant winds? Alternatively, if water carved Perseverance Valley more recently – so that the trough system remained recognizable as it is today despite degradation by aeolian abrasion – is it possible that there was flowing water in just this one location, and if so, then where did that water come from?

Sullivan illustrated these issues withclose-up pictures of rock surfaces within Perseverance Valley that showed diagnostic aeolian abrasion textures, indicating wind-driven sand blowing up and out of the crater. “This is inconsistent with any signs for water erosion acting downhill,” he said.

From Marathon Valley, another low-lying site that cuts into Endeavour’s west rim in the Cape Tribulation segment, which Opportunity explored in 2015-2016 before heading farther south along the rim to Perseverance, Sullivan showed rather dramatic examples of aeolian abrasion. The wind processes have reshaped exposed rocks there, as well as planed off most of the valley floor, which, unlike Perseverance, is not soil-covered, he noted.

Interestingly, the bare rock surface of Marathon Valley's floor is marked with“a complex network of fractures at all orientations, many of them soil-filled, others not,” Sullivan pointed out. “Some of the largest fractures are oriented downslope, toward the center of the crater–these are important features for comparison with Perseverance Valley,” he added.

Opportunity’s views from within Perseverance Valley reveal that the potential traces of stream [water] flow seen from orbit turn out to be rather unimpressive features of very low relief. Essentially, “these features are very shallow, soil-filled troughs between similarly low-relief outcrops,” he said, noting that these troughs share characteristics with sand-filled fractures seen elsewhere, particularly the features interpreted as fractures at Marathon Valley.

Sullivan proposed that the shallow, interleaved trough system of Perseverance Valley might have been formed by “a network of ancient fractures and faults that reactivated after the potentially wet, warm climate of the Noachian Period, conceivably multiple times, with long hiatuses in between,” he said. “Minor surface relief resulting from each reactivation, like from any upward displacement of one block relative to another, would be followed each time by erosion and reduction of the newly-formed relief by aeolian abrasion and other weathering processes."

The problem of Perseverance Valley's "singularity" remains, of course. Of all the fractures and faults caused by the impact that riddle the bedrock all through the Endeavour region, “why would only those in the area of Perseverance Valley reactivate from time to time?” Sullivan wondered.

He wasn't sure, but he pointed the location of Perseverance Valley in a low area of Endeavour's rim between higher rim segments on either side. Sullivan speculated that a highland on one side eroding down faster through the ages than the highland on the other side might periodically focus some stress-adjusting fault reactivation in the more vulnerable area in between, Perseverance Valley. Still, “the heart of the issue,” he pointed out, really is: “How rare is fault movement at Meridiani Planum during the last half of Martian history?”

To explain the singular nature of Perseverance Valley, the team is looking for a process – perhaps faulting/fracturing, perhaps something else – that is very rare, “but yet not entirely absent,” Sullivan said as he clicked to one of the final slides of his presentation.

On the screen was a Navcam picture that Opportunity took more than 3000 sols ago that showed something indeed very rare: a fracture that had reactivated recently enough to offset a large wind ripple. One side of the ripple had become misaligned with the other side, with the fracture slicing in between. "Displacement fault activation does occur, very rarely, and on the same timescale as ripple migration,” Sullivan said. “Therefore, it is comparable with the survival time of minor surface relief, such as recently emplaced impact ejecta blocks, against aeolian abrasion.”

While this observation seems promising, Sullivan did not seem entirely convinced. “Opportunity is only half-way down Perseverance Valley, with many more observations to make,” he reminded the audience. 

It was a closing comment that emphasized the point that the “multiple working hypotheses” approach is what’s still driving the MER mission objective, something Sullivan had already summed up in his last slide:

"NO conclusions about PV origin yet. Multiple working hypotheses!! Collect more data."

Origins of Perseverance Valley by Spillover of a Small Lake

Timothy Parker, PhD
Athena Science Team member; Geophysics & Planetary Sciences Researcher, Jet Propulsion Laboratory(JPL); Originator of the Mars Ocean Hypothesis

Tim Parker has been following the water, or at least doing everything possible to find evidence for flowing water in Perseverance Valley’s past. Matching high-resolution orbiter views with projections of images Opportunity has taken on the ground, he presented his show-and-tell visual analyses conjecturing how this unique geological feature could have been formed by a short-lived flood that spilled over from an ancient lake to the west of the crater rim. 

“The notch in the rim of Endeavour Crater is located at exactly the point where Perseverance Valley originates in the crater rim,” he said, pointing to an image showing the notch at the very top of the valley. “The anastomosing channel suggests that discharges were too high and flow durations were too short to allow development of a single, narrow, graded channel.”

If Perseverance Valley does represent a short-lived flood from a lake source, it would not necessarily require precipitation to form, Parker noted. “This might also provide an explanation as to why there aren’t other channels elsewhere at Endeavour.” If not a short-lived flood, then it could be, “something more like an ephemeral stream,” with a modest discharge channel,” he suggested. 

Throughout his presentation, Parker defined Perseverance as “presumably a channel.” Classically defined, a channel is “a type of landform consisting of the outline of a path of relatively shallow and narrow body of fluid like a river, river delta or strait.”

Since the channel begins at the rim crest, “the source is likely surface ponding,” Parker continued. That ponding would likely be to the west, in the area atop the valley, where Opportunity conducted a walkabout. The water would have flowed “from this proposed lake into Perseverance” at the notch in the topography, “which is only about a meter deep and right at the head of the valley,” he said.

“If you flood the terrain in our HiRISE DEM map to an elevation of -1455.15 meters, you get a connection between the interior and the exterior of the crater right at the proposed notch,” Parker showed with a visual. “But the depression here isn’t closed, so the plains are open to the west,” he said. 

There’s the rub: there is no catchment, nothing to contain the lake. And there is nothing there today in the modern topography above Perseverance to indicate there was anything there billions of years ago that would have served to enclose the ponding water. “You need to tilt it [the area atop Perseverance to the west of its entrance] by somewhere between .5 and .8 degrees to the east,” toward the crater rim,” Parker explained. “Then you do get a closed depression, a two-meter deep closed depression.” 

Alternatively, he added, “an ice sheet west of the crater rim could have also closed the embayment.” Still, there are issues to resolve. 

The compaction models apparently don’t support the tilting.Addressing that, Parker was frank: “If the models that predict compaction are missing something and you can get that much rotation in the sediment in the plains out here, then this might be a reasonable suggestion.”

Considering this lake spillover hypothesis, the fact that Perseverance is a truly unique feature begs the question: ‘Where does the water come from to feed this lake spillover theory and why doesn’t it channel other places?’ 

“That’s a problem,” Parker admitted. “Possibly mitigatable by having an ice sheet out here and the ice is providing a dam on the west side for that feature. But then same problem, where did the ice/water come from?”

The source of the water is, of course, at the core of the lake spillover theory, but that part of the equation is still missing. Opportunity’s imaging further muddles the problem as Parker sees it. “I’ve put a really hard stretch on the NavCam and Pancam orthomosaics, so you can see fine detail when you zoom in – looks really salt and peppery at this scale, but it shows the channel reallywell. That lookslike a channel to me,”he said. “If it’s not, how come it looks like a channel to me?”

Parker’s hope now is pretty much the same as the other scientists’ hope – that Opportunity finds “more glorious detail” as the mission heads farther down the valley.

Degradation of Endeavour Crater Based on Orbital and Rover-Based Observations Together with Landscape Evolution Modeling

Madison Hughes
PhD Candidate, Department of Earth and Planetary Sciences
Washington University St. Louis

Madison Hughes took the conference audience way back, to the Noachian Period some 4.2 to 3.7 billion years ago and more specifically to the time shortly after the big impact that created Endeavour Crater. Using a landscape evolution model known as MARSSIM,she investigated the degradational history of this 22-kilometer (13.7-mile) diameter crater to get an overview of how this crater has “aged” over the last few billion years. 

Hughes wanted to find out how erosional processes on Mars have changed since those bombardment days of the Noachian and how they have shaped the geomorphology of Endeavour – and what all that has to implyabout the environmental conditions that have been in play in Meridiani Planum throughout the last few billion years. 

Studies into the densities of impact craters on the Martian surface have defined three broad periods in the planet's geologic history: 

• Noachian: 4.1 to 3.7 billion years ago. Since arriving at Endeavour, Opportunity has generally been roving around in the ancient surfaces of this period, whenthe oldest extant surfaces of Marswere formed. Asteroids, meteors, and comets regularly pummeled the planet, and liquid water produced river valley networks, lakes, perhaps an ocean, and extensively eroded everything.

• Hesperian: 3.7 to 3.0 billion years ago. Defined by the formation of extensive lava plains and catastrophic releases of water that carved extensive outflow channels around Chryse Planitia and elsewhere, this time period also harbored lakes and/or seas in the northern lowlands.

• Amazonian: 3.0 billion years ago to present. Characterized by low rates of meteorite and asteroid impacts, the planet evolved during this period to feature cold, hyperarid conditions essentially similar to those on Mars today. 

Clearly Endeavour Crater has undergone extensive erosion over the last few billion years. “The deposition of the Burns Formation, sulfate-rich sandstones in the late Noachian to early Hesperian Period, buried most of Endeavour,” Hughes noted. “Still, some rim segments are exposed.” 

For the simulation, Hughes used the nearby, fresh-looking Bopolu Crater, because of its similar diameter, younger age, and relatively fresh appearance as an analog for Endeavour. With a diameter of 19 kilometers, Bopolu, scientists believe, formed sometime after the Burns Formation buried much of Endeavour. She input elevation and other pertinent topographical data of Bopolu into the MARSSIM model and began running the simulations. 

Endeavour’s still-exposed rim segments provided Hughes with constraints as to what the final crater morphology should look like, including such defining aspects as the slope of the crater rim and the final thickness of regolith present on the rim. 

With MARSSIM, software developed by Alan Howard of the University of Virginia, a co-author of the work, Hughes was able to “tweak the parameters” asa function of time in a series of forward models. Matching what is already known about past environments and their timeframes in Meridiani Planum,she simulateddifferent erosional environmentsto advance the research. 

After running the fluvial version of the model, she took the output and input it into a more hyper-arid environment, and then took the output from that and ran it through the aeolian evolution model.“I’m trying to create a certain morphology, so basically doing I am running hundreds of models to get the environment that produces what Endeavour looks like now,” Hughes said. 

From the work, Hughes was able to conclude that the morphology of Endeavour was likely produced during an initial period of fluvial or water erosion in a semi-arid to arid (dry) environment. “The simulations significantly infill the crater interior with sediment sourced from the inner crater wall, and erodethe crater rim,” she said. “The morphology produced is a rounded crater rim with little to no regolith cover and minimal channeling on the crater interior.” 

This initial period of water erosion was “followed by a drier period of erosion without water, dominated by lower rates of weathering, diffusion, and mass-wasting, also known as slope movement or mass movement,” Hughes continued. “This smooths out the crater walls and further fills in the interior of the crater. It also causes the exposed bedrock on the rim to be more subdued.”

Hughes then modeled the subsequent period dominated by aeolian erosion and deposition that continues to this day. “This smoothed out the rim further, and deposited aeolian sediment in the interior of the crater,” she said.

The plan forward, Hughes said, is to continue this research to better understand the degradational history that shaped Endeavour Crater and determine the implications for paleoclimatic conditions. “I want to model the deposition of the Burns Formation and the subsequent backwasting and erosion of the rim to try to create a final morphology that is even closer to how Endeavour is expressed today,” she said. 

While her work already indicates that farther into the crater, Opportunity will most likely find more Burns Formation exposures, continued research,” Hughes said, “could “illuminate the timing of Perseverance Valley’s formation on Endeavour Crater’s rim.”

Rock Suites of Endeavour Crater: Comparing Perseverance Valley to the Floor of St. Louis Crater

Michael Bouchard
PhD Candidate, NASA Earth and Space Science Fellow
Department of Earth and Planetary Sciences, Washington University St. Louis

Michael Bouchard searched for clues to the origin of Perseverance by comparing the composition of rocks within the valley to other rock suites observed along the rim of Endeavor Crater. He found an intriguing similarity between the Perseverance rocks and rocks Opportunity previously studied inside Spirit of St. Louis, the small, approximately 30-meter crater located to the north, at the head of Marathon Valley, which Opportunity studied in May 2015. 

Using data that Opportunity gathered with her chemical detecting Alpha Particle X-ray Spectrometer (APXS) along with close up Microscopic Imager (MI) pictures and color images the robot took with her stereo color Pancam, Bouchard compared the rocks of the two sites using fairly standard rock identification tools. These tools included a hierarchical cluster analysis, an error-weighted reduced-chi-squared similarity index, which is a kind of regression tool to detect the geochemistry, and a multi-component mixing model. 

For this research, Bouchard got inventive and modified and adapted the standard chi-squared index, creating a special, first of its kind “similarity index.” His combination of these two techniques with informed image analysis into a multi-prong statistical grouping model is novel and served the research well. 

From Perseverance, Bouchard took Opportunity’s APXS/MI results on Zacatecas (Sol 4787), Albuquerque (Sol 4854), Durango (Sol 4916), Carrizal (Sol 4943), Parral (Sol 4794), Mesilla (Sol 4895), and Bernalillo (Sol 4861, 4865) and compared them to data on more than 150 rock targets the rover had analyzed with the APXS elsewhere on the crater rim. Of all those, four – Donald A. Hall (Sol 4023), Harold M. Bixby (Sol 4013), Lambert Field (Sol 4003) inside Spirit of St. Louis (SOSL) Crater, and Roosevelt Field from Lindbergh Mound (Sol 4009) – “jumped out as highly similar,” Bouchard said. 

From those comparisons, Bouchard found the rocks in Perseverance to by and large be “members of Endeavour Crater impact breccia” or Shoemaker Formation, although he found the Parral and Mesilla cobbles had distinctly different compositions when compared to the rest of the Perseverance rocks. Turns out, Parral is “highly similar” to the Matijevic Formation targets Fullerton and Azilda. While Parral does not contain any of the “newberry” concretions, Bouchard noted, “it does contain resistant angular clasts,” which one would expect to find if it were an impact breccia like the Shoemaker Formation. 

“In addition to being similar to Matijevic Formation, the Parral cobble is also similar to a class of ‘blue’ rocks we’ve seen along the rim of Endeavour crater,” he said. [These rocks appear blue in false color composite Pancam images.] “They could be impact melt, but I suggest in other work that they are volcanic rocks that both pre-date and post-date the Endeavor Crater impact. The interpretation of these Perseverance cobbles as blue rocks was strengthened when we found an entire outcrop of blue rocks (San Miguel) in Perseverance,” Bouchard noted later.

Parral and Mesilla, he suggested, “may represent an allochthonous population of cobbles that have been deposited in Perseverance.” In other words, these cobbles probably came from someplace else, “most likely somewhere up-valley,” Bouchard said, “and then were transported downhill by some mechanism, like gravity, water, or wind.” Interestingly, mixing models of the Perseverance floor rocks (Zacatecas, Albuquerque, Carrizal, Durango) show that this type of “blue cobble” (Parral) makes up “an estimated 25% the valley fill material,” he added.

The similarity of the Perseverance rocks to the rocks located in the Spirit of St. Louis Crater to the north of Perseverance is noteworthy. Both locations are features that are younger than Endeavour Crater, have Shoemaker-like bedrock covered with soil and pebbles, and, significantly it would seem, both boast alteration zones that may have once been conduits for aqueous fluid flow, what the team scientists call “red zones.”

Spirit of St. Louis Crater is surrounded by a quasi-circular zone of “red” material found to be enriched in silica and oxidized, sure signs of past water. In Perseverance Valley, Bouchard said: “We see ‘red’ lineations oriented subparallel to the down-valley direction, but have yet to visit or do in-situ analysis on one of these zones.”

There is every good reason to do that now. “One of the hypotheses for the origin of the valley is that weakening along Endeavour crater radial fractures might have made Perseverance Valley a favorable location for downslope mass wasting, or erosion by ice, or flowing/seeping water, however supplied,” he pointed out. 

“The fact that we see evidence of alteration along such fractures and the similarity of compositions of Perseverance materials to those within Spirit of St. Louis Crater perhaps points to a similar post-Endeavour crater processing or sourcing of materials, yet to be discovered by ongoing exploration of the valley and its bedrock by the Opportunity rover,” said Bouchard in closing.

The second part of the Mars Rover Results in Depositional and Environmental History took place in the Poster Session just a couple of hours following the talks. There, Opportunity’s science and engineering featured in three poster presentations. 

Ray Arvidson was on hand to discuss Hill Driving with the Opportunity and Curiosity Mars Rovers, an abstract and poster he produced with MER/MSL RP Paolo Bellutta and othersto investigate the rovers’ mobility limits associated with hill driving. Although neither Opportunity nor Curiosity, which share the same rocker bogie drive system, were designed to hike Martian mountains, drive up steep, gravelly slopes, or dive into craters, each is effectively being used as a virtual instrument to retrieve geomorphic and surface properties. 

When one of the rovers is negotiating a path up hill, terrain type makes a big difference obviously. Everyone agrees that sand or a surface slick with pebbles should be avoided. But, the primary take-home lesson learned? That depends on who you ask. 

“Don’t drive up steep hills,” said Arvidson. 

“Actually,” Bellutta said via email later, “descending is more hair raising for me (if I had any hair left on my head). Skid is a much more serious hazard since it can lead touncontrolled rover path.” 

Another major exploration hazard on Mars is dust, because it’s everywhere. It blankets the landscape and covers just about every rock with an exterior veneer or coating. N.J. Bradley and colleagues from Brock University in Ontario, Canada, studied the MI pictures of a selection of rocks, both ‘as is’ and brushed with the RAT, analyzed the dust coverage, and then compared their dust coverage values to the rover’s APXS data. The goal: to see if they could use this approach as a method to constrain bedrock composition. 

In their Analysis of Martian Dust Coverage and Correlations with APXS of Bedrock Targets Examined by the Opportunity Mars Exploration Rover, Bradley et al., write: “Comparisons between dust coverage and element composition are a valuable tool in forming assumptions about the overall composition of the bed-rock.” Even though the brushed rock targets “are not pristine,” they “do provide a better window into interpretation of bedrock composition than dusty ‘as is’ rock surfaces.” 

With the caveat that each image “must be examined individually to assess variations in lighting, shadows, and bedrock type,” the result showed “good agreement” between their dust coverage analyses and APXS determined elemental concentrations. The finding, Bradley and his colleagues concluded, “lends confidence” to this approach to gauging bedrock composition.

Crumpler’s “In Situ Mapping of Fault-Control and Regolith Diversity at the Head of Perseverance Valley Endeavour Crater, Mars” covered the primary bases that he presented earlier in the day, that Endeavour’s rim segments and Perseverance Valley appear to be fault controlled. While the study of Perseverance is ongoing, he wrote that the fractures or faults in the upper floor of the valley, and discontinuities between segments in Endeavour’s west rim, are “consistent with the development of transverse faults as deformation in the upper crust accommodated by uplift along separate blocks during crater formation.” 

In other words, whatever hit Mars and created Endeavour cracked things up a lot as the Big Thud reverberated through the planet’s crust. Moreover, the faults at the head of Perseverance Valley “support the inference that transitions between Endeavour’s rim segments are significant structural offsets that controlled both subsurface aqueous transport and observed intense bedrock alteration,” Crumpler wrote, and may well have played a key role in the development of Perseverance’s morphology and possibly “the processes from which the valley originated.”

Discovery Continues

As the scientists packed up and headed home and back to their routines, Opportunity pulled up to that long line of vesicular rocks on the floor of the valley in the south trough. During the last couple of weeks, the scientists have been scrutinizing the latest little bounty from Mars.

In the images Opportunity recently sent home, they are already seeing signs that bolster Crumpler’s working hypothesis that the crater rim segments, as well as Perseverance Valley, are strongly fault controlled. “The line of vesicular rocks are right between two fractures,” he pointed out. “And again they are parallel to all the other indications of fractures in the valley.”

As for the glassy, vesiculated rocks? They may be volcanic impact melt or a new class of Martian rocks, or not. Analysis is.

With the stone stripes now viewed as notperiglacialproducts,that all but knocks ice off the multiple working hypotheses list, for the moment anyway. The stripes do not point directly downhill and Mellon’s thermal modeling didn’t support the ice it would take to create these geological features as they occur on Earth. 

They remain a Martian mystery that Arvidson intends to solve.“The stone stripes tend to strike to thenortheast, cutting across topographic contours, he said. “Are they wind ripples? Who knows? They are enigmatic for sure. I want to map them in detail after we get all the data from Perseverance.”

As for deposits or signs of flowing water?

Elusive. The lack of evidence for any kind of ancient sea walls or acatchment to the west, the fact that the area atop Perseverance tilts away from the rim crest to the west, and the reality that the compaction models don’t support the necessary tilting to close in a lake on the western side are “too much” for some of the scientists on the team to favor the lake spill-over theory.

At the same time, Golombek noted: “So far, Tim’s data and analysis is the strongest evidence that there could have been water that spilled over the edge and carved this channel.” That doesn’t mean it was water, “it’s just consistent with water finding the low point and going downhill,” he said.

The water theory getting some weight these days seems to be rising up from the Martian ground. If underground waters moved along the fractures, they could have carved Perseverance, and then as a result of the winds sweeping up and out of the crater and preferentially eroding along fractures, the valley may have been shaped over the millennia into its current morphology. “Surely wind could shape a minor feature like Perseverance, working its way along fractures and irregular outcrops, making what appears to be the remnants of a fluvial system,” said Arvidson.  

With Opportunity’s cumulative data from Perseverance, and the research efforts of members of the science team, the thinking of the momentis that this unique formation is very much fault controlled; that the outcrops have been significantly shaped by wind; and that regolith has filled in the low areas.

“There is an important clue in the continuous topographic profile from the Shoemaker formation breccias cut by Perseverance, to the Grasberg and Burns formation deposits downhill of the valley,” said Arvidson. “Perseverance must have formed after the Grasberg and Burns rocks inside the crater were deposited, and then eroded back to form this smooth, continuous profile.”

That would date the valley’s formation to late Noachian, early Hesperian. Or two to three billion years ago. Give or take a billion years, the undeniable thing is Perseverance looks like it was carved by water. It just looks like a channel where water once flowed, like a river system.

Have you ever heard of ‘equifinality?’” asked Arvidson.

Equifinality is the principle that hold in open systems a given end state can be reached by many potential means. That takes us back to Chamberlin, the value of multiple working hypotheses, and his decree that “many geological problems are best explained by several different processes acting at different times or simultaneously over history.”

“The story is we don’t know,” said Golombek. “Yet.”

And that takes us back to Squyres’ opening: Perseverance is “a work in progress.”

What we do know now though sets the stage for Opportunity’s next discoveries. And those findings, whatever they may be, all but promise that this veteran robot hero will make another appearance in the spotlight at the 50^th annual LPSC next year.