On Mars, how do you pick a rock? You don’t always select it; sometimes it chooses you. Perseverance has had an unexpected traveling partner for the past four months. A boulder found its way inside the rover’s front left wheel on sol 341—over 100 sols ago, in early February—and has been moved more than 5.3 kilometers since then (8.5 km). This boulder isn’t causing any damage to the wheel, but it has stuck to it and made periodic cameos in our left Hazcam photographs throughout its (undoubtedly bumpy!) ride.
This isn’t the first time a rock has joined a rover mission on Mars. A potato-sized boulder got stuck in the Spirit rover’s back right wheel 18 years ago, and it had to be freed. The front right wheel of the Curiosity rover has occasionally picked up its own travelling partner. While it’s unclear how long these rocks stayed, they usually disappeared within a few weeks. As a result, Perseverance’s current companion is on its way to breaking hitchhiking records on Mars.
On its journeys, Perseverance’s pet rock has seen a lot. We were still in our Crater Floor Campaign on sol 341, where we looked at rocks from the Máaz formation, which we believe is formed up of lava flows. If this pet rock could talk, it may tell us about the changes it saw as we drove north past the Octavia E. Butler landing site and then west to the western Jezero delta, passing the stunning ruins of the delta’s former extension, Kodiak. We’ve entered the Delta Front Campaign and have just abraded our first sedimentary rock. Perseverance’s pet rock has travelled a long way from its original location.
Where might this pet rock’s adventure come to an end? It’s possible that the rock will fall out during our ascent of the crater rim in the future. If it does, it will crash on rocks that will be radically different from itself. “We might confuse a future Mars geologist who discovers it out of place!” joked one of our team members this week.
So, if you’re a future Martian geologist reading this, or a Martian graduate student entrusted with tracing the historical site of Jezero crater, pay attention. If you come across a rock that doesn’t seem to belong, it could be Perseverance’s old pet rock.
