Perseverance is dropping off a batch of samples for Mars Sample Return robotic team to retrieve.

While the impressive toolkit aboard the Perseverance rover provides earthbound scientists with a trove of data on the geological and biological history of Mars, the ultimate goal is to get rock and dirt samples collected by the robotic geologist back to Earth for closer examination. Toward this objective, Perseverance is wrapping up its prime mission by creating a depot of samples on the Martian surface that will later be gathered by a robotic retrieval crew.

Since landing on Mars in February 2021, Perseverance has been busily collecting samples of interesting Martian geology with a drill at the end of its robotic arm and a sample preparation facility built into the rover.

The Martian exploration robot has collected a pair of samples from each of 10 targets and stored them in sealed titanium tubes that will allow the rocks and dirt inside to be transported back to Earth without being contaminated.

Half of every pair of samples will be deposited at Three Forks, a specially designated location to leave backup samples for collection if needed. The twins to these samples will remain inside Perseverance, which will hand them directly to a team of Mars Sample Return robots being sent to Mars in about five years to retrieve the rocks and dirt samples collected by Perseverance.

"The samples for this depot – and the duplicates held aboard Perseverance – are an incredible set representative of the area explored during the prime mission," said Meenakshi Wadhwa, the Mars Sample Return program principal scientist from Arizona State University. "We not only have igneous and sedimentary rocks that record at least two and possibly four or even more distinct styles of aqueous alteration, but also regolith, atmosphere, and a witness tube."

First Martian sample depot

Building the first geological sample depot on another planet is not as simple as just dropping the tubes of rocks and dirt in a pile and moving on. Instead, each sample must be dropped in a location that can be recovered by the team of robots that will be sent to Mars to collect the samples and send them back to Earth.

This ensemble will include a lander carrying a Mars Ascent Vehicle, basically a rocket that will launch the samples into Martian orbit, where they will be collected by an orbiter that will transport them back to Earth.

While it is expected that Perseverance will hand off the samples it is carrying directly to the lander, which will load them into the ascent vehicle, the sample return team will also include two Mars helicopters similar to the successful Ingenuity that could collect samples from the depot for transport to Earth.

To ensure these samples are collectible by the pair of Mars copters designed to carry one sample at a time, the depot location must be level and rock-free, with enough room to individually drop each sample tube.

"Up to now, Mars missions required just one good landing zone; we need 11," said Richard Cook, Mars Sample Return program manager at NASA's Jet Propulsion Laboratory in Southern California. "The first one is for the Sample Retrieval Lander, but then we need 10 more in the vicinity for our Sample Recovery Helicopters to perform takeoffs and landings, and driving too."

To give the sample robots ample room to retrieve the sample without disturbing the others, Perseverance will drop each sample about 16 to 19 feet apart in an intricate zigzag pattern at the Three Forks cache site.

"You can't simply drop them in a big pile because the recovery helicopters are designed to interact with only one tube at a time," said Cook.

The depot's success will depend on accurate placement of the tubes – a process that will take over a month.

While dropping each sample, Perseverance will send back images and precise location data that the Mars Sample Return team can use in the event that they need to send the helicopters to retrieve the duplicate samples.

Perseverance for extended mission

By the time all 10 samples are dropped, Perseverance will have completed its one-Mars-year (about 687 Earth days) primary mission. However, much like the rover's name suggests, the tenacious explorer will be continuing with an extended mission that begins on Jan. 6.

"We will still be working the sample depot deployment when our extended mission begins on Jan. 7, so nothing changes from that perspective," said Art Thompson, Perseverance's project manager at JPL. "However, once the table is set at Three Forks, we'll head to the top of the delta. The science team wants to take a good look around up there."

This extended mission will be the first time the rover investigates the Martian geology beyond the Jezero Crater, where it landed.

Called the Delta Top Campaign, this new science phase will begin when Perseverance completes a roughly one-month climb of the delta's steep embankment and arrives at the expanse that forms the upper surface of the Jezero delta. During this approximately eight-month campaign, the science team will be on the lookout for boulders and other materials that were carried from elsewhere on Mars and deposited by the ancient river that formed this delta.

"The Delta Top Campaign is our opportunity to get a glimpse at the geological process beyond the walls of Jezero Crater," said Katie Stack Morgan, deputy project scientist for Perseverance at JPL. "Billions of years ago a raging river carried debris and boulders from miles beyond the walls of Jezero. We are going to explore these ancient river deposits and obtain samples from their long-traveled boulders and rocks."

Given the NASA scientist's plans for Perseverance to hand off its payload to the Mars Sample Return robotic team when they arrive on the Red Planet in around 2030, it is expected that the geological rover will have plenty of time for several other exploratory missions over the coming years – providing us with a broader understanding of Mars' geological and biological past.