Mars Exploration Rovers Update: Opportunity Reaches Top of Endeavour Rim, Wraps Cape Tribulation

Driving the Mars Exploration Rovers (MER) mission further into its fourteenth year of surface operations, Opportunity continued hiking the slopes of Cape Tribulation in February and finally reached the crest of Endeavour Crater’s western rim. There, the robot field geologist looked around with her Panoramic Camera ‘eyes,’ took dozens of pictures of her view, and was about to hit the road – until the scientists saw something too intriguing to pass up in the images the rover was sending home.

“It’s kind of an important time for Opportunity,” said MER Deputy Principal Investigator Ray Arvidson, of Washington University St. Louis (WUSTL) during the final days of the month. “This is it for Cape Tribulation and so we stopped short of going over the crest to acquire a good, parting shot.”

But in the images the MER scientists saw an outcrop of finely striated bedrock that looks like it had been etched or carved by something. So they decided to have Opportunity also conduct one last scientific campaign before departing. “We were about to exit the ‘door,’ but suddenly we discovered this ‘door’ is pretty exciting,” said MER Project Manager John Callas, of the Jet Propulsion Laboratory (JPL), the birthplace of all NASA’s Mars rovers.

The outcrop is more of the same breccia bedrock that formed during the impact that created the 22-kilometer (about 13.7-mile) diameter Endeavour, which the team calls Shoemaker Formation. Opportunity has been roving on and around this breccia since pulling up to the crater’s western rim in August 2011. “But we realized it would be our last full science campaign in Cape Tribulation and of Shoemaker breccia, maybe forever,” said Arvidson. More than that, this sample of Shoemaker is so visually distinctive the scientists were compelled to command the rover to take a closer look.

“We decided to take some detailed Pancam imaging of the area and are still debating whether the geomorphic expressions, the striations were formed by wind or ice or something else that occurred during the impact,” said MER Deputy Project Scientist Abby Fraeman, who first worked on the mission in 2004 while still a high school student and one of The Planetary Society’s Red Rover Student Astronauts.

After Opportunity completes her final science campaign, the plan is for the rover to exit Cape Tribulation and drive as quickly as possible to the mission’s next major science attraction, a gully, about 710 meters (0.44 mile) down the road, a place just named Perseverance Valley, where the mission will make more history with the Martian history it uncovers.

Extending down the big crater’s western rim at Cape Byron for approximately 200 meters (about 656.17 feet), the gully is just a few tens of meters wide and in orbital images looks to have been carved by water, in part because of seemingly telltale channels that branch from it. A small nearby crater superimposed on the rim indicates the gully is a very old geological feature, one that, like Endeavour, most likely dates to the Noachian Period 3-4 billion years ago when Mars was more like Earth, and featured lakes and rivers, and perhaps even an ocean.

On average, the gully’s slope is fairly shallow, about 15 degrees, a slope that should be easy for Opportunity to navigate, especially with her experience negotiating steeper slopes in Marathon Valley and Cape Tribulation. “It’s just a matter of getting there,” said MER Principal Investigator Steve Squyres, of Cornell University.

Once Opportunity crosses over Endeavour's rim at Cape Tribulation, she will head caddy corner down the outboard side of the crater to the southern part of Cape Tribulation and then onto the flatter Meridiani plains that lie in between the capes. That terrain will enable the rover to fast track her trek. “We’ll take an express route to get to the head of the gully on Cape Byron,” said Arvidson.

JPL Rover Planner Paolo Bellutta, who years ago developed a novel method to assess rover traversability on the Martian surface, used his algorithm to compute MER’s 10^th extended mission route and the “express route” to the gully. He has computed Opportunity’s routes since 2012 when the rover began exploring Cape York, the first segment of Endeavour Crater’s western rim that the MER mission checked out.

With the aid and input of what he calls “difficulty maps,” which he creates from images taken by the HiRISE camera onboard the Mars Reconnaissance Orbiter (MRO), Bellutta’s software takes into account the Martian season, the rover’s tilt, the slopes and types of terrain, as well as visible or known geological obstacles, and then automatically computes the safest efficient route.

The program also automatically computes alternate routes or “Plan B’s,” as Bellutta calls them. That’s just in case the rover encounters obstacles the software could not or did not account for in the initial computed route. In fact, Opportunity took a Plan B route in January when she couldn’t make it up slippery, soil-covered bedrock on steep slopes she encountered in Cape Tribulation.

When the rover gets to the Meridiani plains, she will follow the express route and in effect make a beeline for Cape Byron. Opportunity will then re-enter Endeavour’s western rim at the top of the gully and at long last begin the highly anticipated, first-ever close-up surface study of an ancient gully on Mars.

So far, all systems and circumstances are “go.” The Martian summer has been a good one for Opportunity. The rover worked under “storm free skies” throughout the month of February, according to the Mars Weather Reports, which use orbital data from the Mars Color Imager (MARCI) onboard MRO, and are produced by Malin Space Science Systems (MSSS). But “a large regional dust storm,” as Callas described it, did whip up a significant amount of dust into the atmosphere some 500 hundred kilometers away and darkened the skies over Endeavour, and that did impact the rover’s power.

Martian dust storms can be life threatening for solar-powered rovers. No rover knows that better than Opportunity. After all, she nearly lost her life in the planet-circling storm of 2007. This time around however, something scientifically fortuitous happened.

“In one of the Navcam panoramas we can actually see [the storm] in the distance,” said Callas. “This is the first time we've actually seen one of these dust storms from the ground.”

As Opportunity was wrapping her final science campaign in Cape Tribulation, the storm appeared to be moving away. "But the latest Pancam images show dust moving in the north, about 200 kilometers away," said Bellutta.

“Based on the reports from Bruce Cantor at MSSS, I think we've past the peak of it right now,” Callas said. “So we believe the risk is decreasing at this point, but we're keeping our eyes on it.”

As Mars giveth, it also taketh away. On at least two occasions in February, wind gusts cleared some of the accumulated dust on the robot’s solar arrays, giving Opportunity a nice little boost in power. Under the hazy skies typical of Martian springs and summers, the rover began the month producing a respectable 466 watt-hours, which is less than half her capability after landing in 2004 with sparkling clean solar arrays. By month’s end, she was producing 559 watt-hours of power, nearly two-thirds her full capability, and was utilizing more of the sunlight hitting her solar arrays than at the beginning of the month even though the sky overhead was even hazier.

What goes up however must come down. “We have to be careful,” said Callas. “I use the analogy that it’s like cleaning your house with a leaf blower, where you blow all the dust into the air and eventually it comes settling back down on you.” But for now, the newfound energy is something no one is complaining about.

Meanwhile, on Earth, Opportunity roved back into the spotlight for another 15 minutes of fame at the 33^rd Mars Exploration Program Analysis Group (MEPAG) meeting that convened on February 22-23, in Monrovia, CA. Chartered by NASA Headquarters, these regularly scheduled two-day gatherings assist the space agency in planning scientific explorations of the Red Planet and offer the Mars community a chance to catch up on current missions and ongoing efforts underway for future missions.

During that meeting, Fraeman presented a jam-packed MER summary that primarily focused on Opportunity’s recent results at Endeavour Crater and the major science attractions down the road. All in all, the last year and a half of work in Marathon Valley and in the Cape Tribulation part of Endeavour’s western rim has been a textbook informing success. “Opportunity's recent observations continue to increase our understanding about the range and diversity of aqueous environments in Mars' history from the Noachian Period through the Hesperian Period,” she summed up.

As February came to an end, Opportunity and the MER team were ready to rove. Although Perseverance Valley really isn’t all that far away, it’s going to take “quite a few weeks” to complete the journey, Bellutta said. “There are some challenging points, including some sand dunes, and there are also some science targets along the way.”

Nevertheless, the MER team and the rover too, it seems, run on optimism and are as dedicated as ever to keeping the first overland expedition of Mars roving along into new places. “Opportunity is doing wonderfully,” said Chief of MER Engineering Bill Nelson, of JPL. “The rover's systems are all operating as expected with every indication of continued good performance.”

“We are all very forward looking now,” said Callas. “The entire team is excited about zooming down the outside of the crater to get to the gully.”

During the first two weeks of February, it was all about roving for Opportunity. On February 1^st, not too long after the Sun rose on Endeavour Crater, the robot woke up on her Sol 4631 and got moving. She climbed 26 meters (85.03 feet) heading southwest toward her exit at the crest of Endeavour Crater’s rim and then, like always, took the routine images with her Pancam and Navigation Camera (Navcam) after the drive.

Following a sol of recharging and taking more images of her surroundings, the rover focused on a target the team named Wilton, after the North Dakota site that Lewis and Clark passed through on the Corps of Discovery Expedition commissioned by President Thomas Jefferson. The rover drove again on Sol 4633 (February 3, 2017), following the computed route to southwest and upslope, putting another 22 meters (72.17 feet) behind her. “There was no IDD work at the location of Wilton,” said Ed Guinness, Senior Research Scientist at WUSTL. “It was a Pancam imaging target.”

Opportunity finished off the first week of the month with a 13-meter (42.65-foot) drive on Sol 4636 (February 7, 2017). Along the way, the ‘bot established a new Mars rover distance record when her odometer clicked past 44 kilometers (27.34 miles), a record the rover drivers had been looking forward to achieving.

Mars seemed to applaud with a gust of wind that cleared some of the accumulated dust from Opportunity’s solar arrays as the first week of the month gave way to the second. The fresh dusting translated to an energy boost of an estimated 70 watt-hours.

In addition to taking the usual images of her surroundings at this pit stop, the robot also checked out “a relatively small patch of outcrop,” as Guinness described it. In keeping the current naming theme, the team christened it Rulo, after the Nebraska site that the Corps of Discovery visited in July 1804, just a couple months into the first American expedition to cross what is now the western portion of the United States.

It was a quick study that entailed the standard MER science protocol, taking the needed close-up pictures with her Microscopic Imager (MI) for a mosaic of the surface, followed by placement of her Alpha Particle X-ray Spectrometer (APXS) to glean the outcrop’s chemical composition. Opportunity completed the last of the in-situ or contact measurements on Rulo on Sol 4638 (February 9, 2017), and then continued her hike upslope.

The exit from Cape Tribulation had proved to be challenging, but Opportunity continued to show her MER mettle throughout February as she followed the “Plan B” route Bellutta computed. On Sol 4640 (February 11, 2017), the rover put 28 more meters (91.86 feet) in the rear view mirror and pulled up to the top of Willamette Valley and the crest of Endeavour’s rim.

“Realizing the rover would soon be leaving Cape Tribulation forever, we stopped and just started acquiring the panorama,” said Arvidson. When the images began arriving at JPL, the team saw a vista filled with views the likes of which neither they nor Opportunity had seen in months of hiking the slopes – and they also zeroed in on the striated or etched bedrock.

For the rest of the month, Opportunity spent quality time taking the images needed for the mission’s "parting shot" panorama. The MER team quickly named it Rocheport, after a town along the Missouri River that Lewis and Clark and crew passed through on June 7, 1804.

On Earth, Rocheport features “a vertical cliff formed of limestone bluffs that is highly fractured,” said Arvidson. “It was full of rattlesnakes when Lewis and Clark visited and so they killed three snakes and then left.” The Corps of Discovery would pass through Rocheport one more time however, retracing their steps and visiting again on September 19, 1806 on the return trip from the Pacific Ocean.

Despite its distinctive look, the striated bedrock, which exhibits “extensive scours,” is “Shoemaker breccia for sure,” said Arvidson. Even so, the scientists were bent on checking it out. “This is such an interesting site in terms of these carvings, these striations, that we felt we better document it. And since we’re there doing the Rocheport Panorama, it made sense to also do some IDD work,” he explained.

Since Opportunity would soon be saying ‘adios’ to Cape Tribulation, the segment of Endeavour’s western rim that she had roved down, around, and through for more than two years, the scientists also realized this would be the last chance for a final science campaign in this neck of the Martian woods.

So, during the second half of February, in addition to taking the dozens of images needed for both Navcam and color Pancam panoramas, Opportunity worked on the investigations of striated bedrock and small rocks. Using her APXS and MI to conduct the regular MER scientific research, the robot field geologist, for example, checked out a 2-centimeter (0.78-inch) dark pebble, the team named Boonville, after an area of wooded bluffs overlooking the Missouri River, through which Lewis and Clark passed.

Back in the old frontier days of the early 1800s, Daniel Boone, the legendary explorer who has gone down in history as one of America’s first folk heroes, lived right on the river. “But Lewis & Clark never stopped to say ‘hello’ to Mr. Boone,” Arvidson pointed out. “They floated and walked right past his house and then kept going. I guess they were in a hurry.”

On Mars, Opportunity not only said “hello” to Boonville, she examined it and it’s surroundings, taking offset measurements in order to determine how the dark pebble differed, or not, from the geological stuff beside it.

Then, during the final weekend in February, Opportunity bumped 1.29 meters (4.23 feet) on Sol 4654 (February 25, 2017) to reach another part of the striated bedrock, where a clast or small rock fragment was embedded within the breccia matrix, said Arvidson. That bump brought the rover’s odometer to 44,033.87 meters (44.03 kilometers, 27.361 miles) and would be her final drive of the month.

Following the bump, the robot then did an IDD salute, raising her arm out of the way so she could image the ground before her. “We looked at the small, 2-centimeter black clast and the surrounding matrix in this part of the bedrock,” said Arvidson. The team christened this final Cape Tribulation science target Waverly, after a site across the Missouri River from where Lewis and Clark camped in June 1804, and which is now a town.

While the robot field geologist was beginning work on Waverly, a regional dust storm several hundred kilometers away had successfully lofted a significant amount of dust into the atmosphere that darkened the skies over Endeavour Crater. The increased opaqueness of the atmosphere above, in turn, reduced the amount of sunlight Opportunity could utilize for power production.

But, the robot lucked out, or Mars smiled down again, as a wind gust or two presumably generated by that storm, whisked off more dust from Opportunity’s solar arrays.

“The Tau or measure of atmospheric opacity peaked at 1.56, up from its previous baseline of around 0.8,” reported Nelson. “As a result the direct sunlight dropped from around 45% to about 21%, partially cancelling the rover’s improved dust factor and leaving Opportunity’s energy only slightly better, at around 527 watt-hours.”

With storm-related events like this, the rover’s increase in energy tends not to last long because the dust that the storm whipped up into the atmosphere eventually begins raining back down. Within a couple sols, the Tau quickly dropped to 1.04 or in other words improved,” said Nelson. “But the dust factor got worsened, dropping from around 0.92 on Sol 4654 to 0.84 on Sol 4655 (February 26, 1017). 

The MER ops engineers prepared a backup plan for Opportunity, just in case the dust storm moved closer to Endeavour and the situation deteriorated, said Bellutta. If they saw a further increase in the atmospheric opacity, they would command the rover to immediately cross over the crest, drive a few meters and position herself on more a Sun-friendly locale, where she could soak up as much sunlight as possible. “It's not a bad situation to be at in the moment,” he noted as February began to fade. “We hope there isn't going to be a lot more dust accumulation. For the time being, we're happy the solar deck has been cleaned.”

Meanwhile, on Earth, the MER scientists were debating how this distinctive striated bedrock might have formed. JPL’s Tim Parker, a MER Science Team member, is theorizing that the striations were formed by ice. Arvidson opined that “the preponderance of evidence is for wind erosion.” Another possibility is that it was scoured by a spillover. And the debate continues.

Climbing up the slopes had been a long haul, but the rover that loves to rove accomplished the achievement with aplomb. Since beginning the descent into this general section of Cape Tribulation, from Spirit of St. Louis Crater through Marathon Valley, down to Spirit Mound and then back up to the rim crest, the rover logged 127 meters of uphill hiking in 95 sols of driving, informed Bellutta. “That is a really fast climb in my opinion especially for a vehicle on Mars.”

At month’s end, Opportunity was ready already to rove over crest and hit the Meridiani ‘highway.’ If all goes as planned, the robot field geologist will be making her departure as the MER team members gather at Caltech for the annual MER meeting on March 1^st and 2^nd.

“Once we get on the Meridiani plains, then we’ll turn left and go as fast as possible,” said Arvidson. “Except end-of-sol for imaging, we probably won’t be stopping much. We want to get to the head of the gully on Cape Byron sooner rather than later.”

The team will be initiating a new naming theme, said Arvidson. But what that theme was not yet determined at post time.

Later in March, several MER science team members will launch their veteran robot field geologist back into the spotlight at the 48^th Lunar and Planetary Science Conference (LPSC) to be held at The Woodlands, Texas.

When Opportunity does pull up to Perseverance Valley, the mission will transition from a rock mission to a mission focused on the geomorphology and sedimentology of the site. “What we really want to do with Opportunity is use mainly Pancam stereo imaging to acquire some detailed, textural images, and also to look for compositional information that can give us some evidence for how this feature may have formed,” Fraeman said at the MEPAG meeting.

The MER scientists will be testing three hypotheses. Was the gully carved into the rim by a fluvial feature with a relatively low sediment load, such as a lake spillover or perhaps even a river? Was it formed by a wet debris flow that had a higher sediment load and was driven by rainfall or snowmelt? Or, unlikely as it seems, could it have been created by a dry avalanche?

The favored hypotheses involve water and tantalizing hints for those theories appear in the orbital imagery. “In the HiRISE images, we can almost see these anastomosing channels a few meters wide that arise from this break in the slope,” Fraeman noted.

On Earth, anastomosing channels are multi-threaded links or branches that radiate from and occur at lower gradients of streambeds and often feature thick clay and silt banks.

If the scientists discover that the gully was formed by one of the two water hypotheses, they will then seek to uncover and characterize the source of the water. With the rover’s detailed imaging, the MER scientists will look for specific sedimentary structures like levees and bedforms that may point to one of these processes. They will also be scrutinizing characteristics of deposits they find, such as roundness, sorting, grading, and grain size of the sediments. One other important question they hope to answer is why this is the only gully feature they see on the rim of Endeavour Crater. 

Considering that the team’s research will represent the first-ever surface study of a gully on Mars, whatever the rover and the scientists discover will add a new chapter to the story of water on Mars.

Winter solstice in the southern hemisphere of Mars, where Opportunity’s site is located, occurs on November 20^th of this year. In order for the rover and the MER scientists to have time to do some work before the freeze of the season slows the mission down, the rover really needs to be there several months in advance of that date.

“It's kind of like you don't want to arrive at Disneyland two hours before it closes,” said Callas, using one of his hallmark analogies. “You want to get there early enough so you have a full day of fun and excitement. So it is desirable for the rover to get there as soon as possible so we can do as much as we can before we have to possibly slow down because of the limited energy during the winter.”

Time passes just as quickly on Mars as it does on Earth. So the push to get to the gully is really on now and there is a sense of some urgency among the team members. Beyond sand dunes and other obstacles that may slow Opportunity on her journey south, the rover will also have to hunker down incommunicado for two weeks in July for the solar conjunction. That’s when the orbits of Mars and Earth position the two planets on opposite sides of the Sun and disrupts communication.

This regular celestial event occurs this year on July 26^th, but for a two-week period around that date, communications between the rover and her mission ops team will be sketchy at best, completely blocked at worst, and are not advised. Typically, missions at Mars stand down and wait it out. In any case, Opportunity won’t be able to rove during the two-week blackout period of solar conjunction.

Despite the push, MER has always been and remains a science-driven mission and there are some waypoints that the scientists have identified along the way. “Like we always do, we'll decide when we get to those waypoints if there’s anything compelling,” said Callas. “If not, we move ahead.”

After Opportunity’s three-month march up the slopes of Cape Tribulation, the ops team is both “jazzed and cautious as one terrain and driving regime gives away to another,” said Nelson. “Even more than the science team, at least some of the engineers are becoming more focused on winter needs and the efforts it will take to get the rover in the right place for winter. It may end up becoming a race to get there in time.”