Although NASA has enjoyed a string of high-profile successes on the red planet, sometimes Mars throws scientists a curveball. The Perseverance rover came up empty during its first attempt to collect a sample of Martian rock last week, and NASA thinks the rock likely crumbled into rubble and dust rather than remaining in one piece.
Now the rover is trekking southward to its next sample target, where the team plans to try drilling again in early September.
During the first attempt, the multi-step sampling process initially seemed to progress smoothly. The rover bored into the red planet, closed the sample tube with an airtight seal, and safely deposited the tube into a module in the rover's belly on August 6. "The system itself worked perfectly," says Jennifer Trosper, project manager for the Perseverance mission.
But as the team teased through the data, they realized the tube was empty. During tests conducted on Earth, some of the cores were smaller than others, but "we've always gotten some sample in the tube," Trosper says. The rocks near the next sample site may be more similar to some of those they tested on Earth, hopefully improving the odds of success.
The failed sample attempt joins several past missions that have struggled to dig into the red rocks of Mars as planned. “This is kind of par for the course” when it comes to Mars missions, Trosper says. A self-hammering heat probe on Mars's InSight lander, for example, only managed to sink an inch or so deep into the surface before popping back out.
"Once more, Mars shows us that it is not Earth," Trosper says.
The mystery of the missing core
While Mars is now a frigid desert, signs of past water abound, from winding stream channels to sprawling river deltas. Perseverance touched down on Mars in February 2021 to search for clues to ancient life in a 28-mile-wide crater that was probably once filled with a freshwater lake.
A key part of this search is collecting the first pristine samples from Mars's surface. Equipped with 43 ultraclean sample tubes, the Perseverance rover should collect dozens of samples across the crater floor and up through an ancient river delta. The rover would then cache the samples in a yet-undetermined place so that a future mission can scoop them up and return them to Earth.
The sampling process is a "choreographed and coordinated" series of events that requires a total of 11 days, explains Vivian Sun of NASA's JPL, who is co-leading the mission's first science campaign. The scientists begin by abrading a patch of the surface, which clears away any dust or coatings and allows them to study the makeup of the underlying rock.
The final stage of Perseverance’s first sample attempt began the evening of August 5, after the team issued commands for the automated process to start. They awoke the following morning at 2 a.m. PT to check Perseverance's progress, discovering it had successfully drilled into the surface. An image showed exactly what they expected: a hole surrounded by a ring of sand known as tailings. "It was beautiful," Trosper says.
Around 8:30 a.m., the team received images of a successfully sealed tube inside the rover. But then they looked at the rest of the data.
Before the tube was sealed, an arm inside the rover pushed it upward into a sensor to measure the volume of material—revealing nothing inside. The team then downloaded images looking down into the tube to confirm that it was empty.
"We started to scratch our heads," Trosper says.
Over the next two days, the team set to work figuring out what went wrong. Snapshots peering down the sample hole revealed a suspicious pile of dust at the bottom. Measurements of the hole's depth and images of the surrounding area further suggested that the rover hadn't simply dropped the small cylinder rock. The drill, it seems, instead ground the unexpectedly crumbly rock to bits.
Mars's uncooperative rocks
This isn't the first time that Martian rocks haven't acted the way scientists expected. NASA's Phoenix lander, which touched down on Mars in 2008, initially struggled to scoop the planet’s rusty red regolith into a device that heats the rocks to sniff out their components. The material was more “sticky” than expected, and the rocks didn't easily fall from the scoop. Scientists still debate why, Trosper says.
Another issue involved the self-hammering heat probe, or “mole,” on NASA's InSight lander, which is currently studying the interior of Mars. Each time the mole attempted to dig itself in, it would pop back out again. NASA finally gave up in January 2021 after several hammering attempts, citing a lack of friction that prevented the probe from penetrating more than an inch or so underground.
During the months of preparation before Perseverance launched, the team took more than a hundred practice samples on Earth to try and ensure the process would go as planned. But when designing missions to other worlds such as Mars, Trosper says scientists often talk about the “unknown unknowns”—situations when something totally unexpected happens. The ground-up sample is just that.
Such surprises provide an opportunity to discover something new. "One of the beauties of the work that we do,” Trosper says, “is making some of those ‘unknown unknowns’ known."
Editor's Note: This story has been updated to include NASA's new analysis of the first sample attempt.