NASA's Opportunity rover has traveled its last mile on Mars. Trapped amidst darkness and sandstorms, the rover has finally met its match on the Red Planet. While Opportunity's time actively roving the planet is over, its Martian discoveries have been fundamental in shaping human understanding of Earth's closest neighbor. Here are some of Opportunity's greatest contributions to humanity.
Originally part of a pair with the rover Spirit, Opportunity began making history shortly after landing in the Meridiani Planum on Mars. The Planum was chosen for its low elevation and potential richness in minerals. Two days after touchdown on January 24, 2004, scientists were stunned at what they found.
"We scored a 300-million-mile interplanetary hole in one," Steve Squyres of Cornell University, principal investigator for the science instruments on Opportunity and Spirit. "We are actually inside a small impact crater."
During its descent, Opportunity detected an even bigger impact crater nearby. This remarkable landing, seen here, would lead to the first of Opportunity's many finds.
In February 2004, weeks after its hole in one, Opportunity's microscopic imager, attached to its robotic arm, made an astonishing discovery. In his , Dr. Sqyuers wrote,
"we saw some very strange things … we see these strange round objects we’re calling ‘spherules’ embedded in the outcrop, like blueberries in a muffin. The outcrop erodes away as it gets sandblasted, and the spherules (which seem to resist erosion better than the rest of the outcrop does) fall out and roll down the hill. Weird."
These blueberries, actually made of the mineral hematite, were formed in the ground by water. Months later, scientists in Utah showed how an on Earth. Finding an Earth–Mars connection within a month of landing is pretty good, but Opportunity wasn't done yet.
Opportunity achieved all of its stated goals within three months, so NASA began expanding the parameters of its ambition and set its sights on Victoria Crater. The journey would be long—21 months. But the opportunity to study the massive crater, five times wider than a football stadium, offered a chance to explore the geologic history of the planet. They had to try.
"This is a geologist's dream come true," said Dr. Steve Squyres on September 27, 2006, when Opportunity finally got there. "Those layers of rock, if we can get to them, will tell us new stories about the environmental conditions long ago. We especially want to learn whether the wet era that we found recorded in the rocks closer to the landing site extended farther back in time. The way to find that out is to go deeper, and Victoria may let us do that."
Once there, Opportunity circling the rim, just trying to figure out how to get inside. But it was worth it—Victoria offered consistent proof that Mars in fact had once been covered with water.
Here on Earth, meteorites are highly valued and at auction. But there's something even more rare than a meteorite: a meteorite discovered on Mars, the first ever discovered on an alien planet.
"This is a huge surprise, though maybe it shouldn't have been," Dr. Squyres at the time in a press statement.
Opportunity's spectrometers were able to determine that the basketball-sized meteor was mostly made up of nickel and iron. That surprised scientists further.
"Mars should be hit by a lot more rocky meteorites than iron meteorites," he noted. "We've been seeing lots of cobbles out on the plains, and this raises the possibility that some of them may in fact be meteorites. We may be investigating some of those in coming weeks. The key is not what we'll learn about meteorites—we have lots of meteorites on Earth—but what the meteorites can tell us about Meridiani Planum."
For years, scientists had been looking for evidence of water on Mars. In 2011, Opportunity found these bright veiny minerals, which were made up of a material commonly used in fertilizer known as gypsum.
"This tells a slam-dunk story that water flowed through underground fractures in the rock," said Steve Squyres in a .
"This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can't be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It's not uncommon on Earth, but on Mars, it's the kind of thing that makes geologists jump out of their chairs."
The veins were small, around the size of a human thumb, and easy to miss—nothing like them could be seen in the surrounding 20 miles. NASA scientists informally named the find Homestake, considering how much it reminded them of Earth-bound phenomena like the White Sands National Monument in New Mexico.
"It is a mystery where the gypsum sand on northern Mars comes from," said Opportunity science-team member Benton Clark of the Space Science Institute in Boulder, Colorado. "At Homestake, we see the mineral right where it formed. It will be important to see if there are deposits like this in other areas of Mars."
Clark also speculated that Homestake "could have formed in a different type of water environment, one more hospitable for a larger variety of living organisms."
In 2012, Opportunity rode out its eighth year and fifth winter on Mars. The rover spent its winter months at Greeley Haven, located at the northwestern tip of a vast crater named Endeavour. Opportunity had spent years trying to get here, driving further in its eighth year than in any other.
Unusually stationary, Opportunity spent its winter vacation studying what JPL's Diana Blaney, deputy project scientist for the mission, "the most active process on Mars today"—wind.
"It is harder to watch for changes when the rover is driving every day," Blaney said. "We are taking advantage of staying at one place for a while."
While the wind study and an accompanying geophysical study of the Martian interior were important, it's hard to get past that gorgeous view. This picture is composed of 817 images taken by Opportunity's panoramic camera, known as Pancam.
"The view provides rich geologic context for the detailed chemical and mineral work that the team did at Greeley Haven over the rover's fifth Martian winter, as well as a spectacularly detailed view of the largest impact crater that we've driven to yet with either rover over the course of the mission," said Jim Bell of Arizona State University, Tempe, Pancam lead scientist.
Named after Ron Greeley, a professor at Arizona State who passed away in 2011, it's hard to think of a more peaceful spot in the solar system.
Some of NASA's longest-lived projects, like the Voyager spacecraft, can outlive their creators. That was sadly the case with Opportunity, when JPL engineer Jake Matijevic passed away in 2012 due to complications with asthma. Matijevic was crucial to the U.S. space rover program, playing roles in the 1996 Pathfinder project all the way to his death in 2012, when he was working the Curiosity rover.
After his passing, NASA named a hill in his honor and sent Opportunity to explore. Matijevic Hill includes a small rock outcropping called Kirkwood. At Kirkwood spherucls were discovered resembling the blueberries found earlier, but lacking in their high iron content. A mystery presented itself.
"This is one of the most extraordinary pictures from the whole mission," said Steve Squyres in a . "Kirkwood is chock full of a dense accumulation of these small spherical objects. Of course, we immediately thought of the blueberries, but this is something different. We never have seen such a dense accumulation of spherules in a rock outcrop on Mars."
"They seem to be crunchy on the outside, and softer in the middle," Squyres continued. "They are different in concentration. They are different in structure. They are different in composition. They are different in distribution. So, we have a wonderful geological puzzle in front of us."
A puzzle worthy of its namesake.
After Matijevic Hill, Opportunity turned its eyes toward an area named Solander Point, where it hoped to ride out a rough Martian winter. This is a challenge of planetary exploration—with brutal conditions breathing down their necks, scientists had to determine what was worth studying at the expense of other objects of interest. A fractured rock named Esperance made the cut.
"Esperance was so important," said Steve Squyres in a press statement, "we committed several weeks to getting this one measurement of it, even though we knew the clock was ticking."
What made Esperance so worthwhile? The history of the planet's water appeared to present itself.
"What's so special about Esperance is that there was enough water not only for reactions that produced clay minerals, but also enough to flush out ions set loose by those reactions, so that Opportunity can clearly see the alteration," said Scott McLennan of the State University of New York, Stony Brook, a long-term planner for Opportunity's science team.
At this point, Opportunity had been on Mars for nine years—but it had still never seen a rock like Esperance, made up of aluminum, silica, calcium, and iron.
Looking at Opportunity's pictures, there appeared to be "intense alteration of Esperance along fractures that provided conduits for fluid flow," Squyres said. "Water that moved through fractures during this rock's history would have provided more favorable conditions for biology than any other wet environment recorded in rocks Opportunity has seen."
In 2014, NASA marked Opportunity's tenth year in Mars with a particular triumph. On July 27, the rover odometer showed a total of 25.01 miles. Opportunity had officially driven further off-world than any other rover in the history of space exploration.
"Opportunity has driven farther than any other wheeled vehicle on another world," said Mars Exploration Rover Project Manager John Callas, of NASA's Jet Propulsion Laboratory, in a .
"This is so remarkable considering Opportunity was intended to drive about one kilometer and was never designed for distance. But what is really important is not how many miles the rover has racked up, but how much exploration and discovery we have accomplished over that distance."
Opportunity overtook Lunokhod 2, a Soviet lunar rover from 1973. NASA scientists tipped their cap to their Communist forebears by naming a crater south of Solander Point Lunokhod 2 Crater, seen here in Opportunity's stereo view.
Opportunity continued to push itself. In January 2015, it climbed the highest peak of its mission—. And then on March 24, continuing to drive further, it finished the first Red Planet marathon with 26.219 miles under its belt.
"This is the first time any human enterprise has exceeded the distance of a marathon on the surface of another world," said John Callas in a . "A first time happens only once."
Of course, NASA doesn't just want records—it wants science to help better understand the solar system. In July, Opportunity began exploring what scientists called Marathon Valley in honor of their achievement. They discovered what NASA referred to as "streaks of red-toned, crumbly material," which they would normally study with Opportunity's Rock Abrasion Tool (RAT), which grinds away at bedrock.
But there was no bedrock to grind.
"Instead," said Steve Squyres, NASA "found one of the highest sulfur contents that's been seen anywhere on Mars. There's strong evidence that, among other things, these altered zones have a lot of magnesium sulfate."
"We don't think these altered zones are where the clay is," Squyres said, "but magnesium sulfate is something you would expect to find precipitating from water."
As the years went by, support for Opportunity dwindled on Earth. In 2015, then NASA Administrator Charles Bolden Opportunity as a mission "whose time has passed." While several scientists disagreed, Bolden's point wasn't outrageous: The rover had seen the wear and tear of multiple Martian winters. But it wasn't done yet.
In 2018, its 14th year, Opportunity discovered a curious texture on the western rim of the Endeavour Crater that reminded scientists of mountains on Earth. They called the channel Perseverance Valley.
"Perseverance Valley is a special place, like having a new mission again after all these years," Opportunity Deputy Principal Investigator Ray Arvidson. "We already knew it was unlike any place any Mars rover has seen before, even if we don't yet know how it formed, and now we're seeing surfaces that look like stone stripes. It's mysterious. It's exciting. I think the set of observations we'll get will enable us to understand it."
"One possible explanation of these stripes is that they are relics from a time of greater obliquity when snow packs on the rim seasonally melted enough to moisten the soil, and then freeze–thaw cycles organized the small rocks into stripes," Arvidson continued. "Gravitational downhill movement may be diffusing them so they don't look as crisp as when they were fresh."
It's just a hypothesis. Scientists still aren't sure what caused the stripes.
Now it seems as if Opportunity's time is up. But the rover has taught scientists so much about the planet, and its similarities with Earth, that its legacy will last dozens of lifetimes.