When NASA's Perseverance rover landed on Mars back in February 2021, it brought along a small friend from Earth, the Ingenuity helicopter. With the little drone off flying around, making headlines of its own and generally wowing us all, the rover has been exploring all by itself — or so we thought.
As seen in images from the rover's hazard avoidance cameras (hazcams), Perseverance has had a constant companion since early February 2022.
This left front hazcam image, captured on Feb 6, 2022 (Sol 343 of the mission), shows a sizeable rock caught inside Perseverance's left front wheel. The inset image reveals what may be this same rock beside the rover's wheel on Jan 31 (Sol 337) Credit: Scott Sutherland/NASA/JPL-Caltech
Perseverance picked up a Martian pet rock!
According to the mission team, this rock has been hitching a ride with Perseverance for more than 8.5 kilometres now. This journey has carried it from south of the Séítah region, north through the Octavia E. Butler Landing site, and then circumventing the rougher terrain of the area towards the base of the river delta.
This map shows the path of Perseverance from February 18, 2021 to June 17, 2022, along with inset images of the 'pet rock' that has accompanied it since at least Feb 6, 2022. The yellow lines are the flight paths of the Ingenuity helicopter to date, and 'pins' indicate the June 17 (Sol 471) location of both rover and helicopter. Credit: Scott Sutherland/NASA/JPL-Caltech
"This is not the first time a rock has hitched a ride on a Mars rover mission," Eleni Ravanis, a student collaborator with the Perseverance mission from the University of Hawai'i at Mānoa, wrote in a mission update on June 2.
"Some 18 years ago, a 'potato-sized' rock found its way into the Spirit rover's rear right wheel, and had to be dislodged," Ravanis explained. "On the Curiosity rover, the front right wheel has periodically picked up its own travelling companion. While it's unclear exactly how long these rocks stuck around, they tended to hop off after a few weeks. Perseverance's current companion is therefore on its way to setting Mars hitch-hiking records!"
Perseverance's 'pet rock' on March 7, 2022 (Sol 371), as the rover drove through the Octavia E. Butler Landing site. Credit: NASA/JPL-Caltech
Although the rock is sizeable, perhaps 20-25 cm across, Ravanis says that it is not causing any damage to Perseverance's wheel. Instead, it's likely just making a lot of noise as it bumps and slides around while the rover drives.
Update (July 26, 2022): Percy's pet rock is still there!
In the latest images Perseverance sent back to Earth, dated July 25, 2022 (Sol 506 of the mission), the rock is still clearly visible inside the rover's wheel (although cloaked in shadow).
Perseverance's Left Front Hazcam took this photograph on Sol 506 (July 25, 2022), at around 1 p.m. Jezero Crater time. Credit: NASA/JPL-Caltech/Scott Sutherland
This view from Perseverance's camera was captured on July 21. The image has been enhanced to highlight the features of the rock. Credit: NASA/JPL-Caltech/Scott Sutherland
Perseverance hasn't moved very far in the past month or so. The rover is currently exploring a strip of exposed rock near the base of the river delta, officially named Hogwallow Flats. Due to its appearance in imagery from space, though, the mission team nicknamed the area 'the Bacon Strip'.
This map from the NASA website shows Perseverance's path, and its current location at Hogwallow Flats, aka The Bacon Strip. Credit: NASA
"The rocks at Hogwallow Flats appear to be very fine-grained, which is exciting to scientists on the mission as fine-grained rocks may have the best chance at preserving evidence of life," Lydia Kivrak, a student collaborator at the University of Florida, wrote in the mission blog on June 23.
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The future for the pet rock is uncertain. It may continue hitching a ride with Perseverance for a day, or maybe another week, or it could be months. When it does finally get dislodged, though, it may present a puzzle to any Mars explorer who finds it in the future.
The rocks and sediments on the floor of Jezero crater likely formed under very different conditions than those in the delta. Unless the pet rock remains with Perseverance indefinitely, wherever it may be found in the future, it will be an anomaly — a rock very different from those around it, that simply doesn't belong there under any normal circumstances.
This composite image shows Jezero crater's river delta, as seen from Perseverance's Navigational camera (navcam) on June 7, 2022 (Sol 461), while the rover sat at the ridge base. The minerals and rocks contained within the layered sediments here are from upstream of the crater, washed here by river flow billions of years ago. They are expected to be quite different from those found in the crater floor. Credit: NASA/JPL-Caltech
Also, depending on how far into the future this is, the rover could have travelled a significant distance away, with the wind eventually erasing its tracks through the dust and sand. Thus, there may be no context for how the rock got there.
"So: if you're a Martian geologist from the future reading this, maybe a Martian graduate student tasked with mapping the historical site of Jezero crater: take heed," Ravanis wrote. "If you've found a rock that looks out of place, you might just be looking at the former pet rock of Perseverance!"