NASA’s Perseverance rover had been on Mars for almost a thousand days when it spotted the odd-looking boulder on March 11. But this rock, perched on the lower slopes of Witch Hazel Hill in Jezero Crater was unlike anything it had seen before. It was a mosaic of tiny, tightly packed spheres. Each one was no more than a millimeter across, clustered together like a clutch of spider eggs or spilled ball bearings.
Scientists named it St. Pauls Bay.
It’s a weird rock. In fact, it’s so weird that researchers now really want to know how it came to be. “There’s nothing else like it in the surrounding area,” wrote Alex Jones (no, not that one), a planetary scientist with the Perseverance team at Imperial College London. “What quirk of geology could produce these strange shapes?”
A Floating Mystery
To understand why St. Pauls Bay is creating such a stir, it helps to know how most rocks on Mars — or Earth, for that matter — are studied. Typically, geologists start by looking at the context. To understand what created a rock, you need to look at the geology around it. But this particular specimen is a “float rock” — meaning it was formed someplace else and moved around.
That complicates things. Scientists don’t know if the rock originated from a nearby outcrop, tumbled down the hill, or was flung to this site by a meteorite impact. Any of those scenarios would have enormous implications for the region’s geological history.
But the surrounding area does offer some clues. Witch Hazel Hill contains distinct light and dark rock bands visible from orbit. The darker layers, in particular, could be important. They may be volcanic, laid down by ancient lava flows. Or they may be impact ejecta — material vaporized and reshaped in the heat of a meteorite strike. They might even hold signs of groundwater — the kind that once supported Mars’ wetter past. But none of this is a smoking gun pointing at what created the rock.
Fire, Water — or Something Else?
The unusual spherules resemble a kind of mineral structure found on Earth known as botryoidal formations, often created in watery environments. They’re typically associated with minerals like hematite or agate and take shape over time as minerals crystallize in fluid-filled cavities.
But St. Pauls Bay doesn’t quite fit that mold. The spherules are irregular. Some are fractured. Others have tiny holes. They don’t look quite like grape agate or terrestrial hematite. In watery environments, you’d expect much more regularity.
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That opens the door to other possibilities. One is volcanism: molten rock sprayed into the air during an eruption and cooled into tiny spheres as it fell. Another is impact: when a meteorite slams into the surface, it can vaporize rock, then recondense it into droplets that harden into spheres as they rain down. Both processes have been seen before — on Mars and Earth.
There’s also precedent on Mars itself. In 2004, NASA’s Opportunity rover discovered “blueberries” — hematite-rich spherules — in Meridiani Planum. Later, Curiosity found similar features at Gale Crater. Even Perseverance has seen spherical textures before. A few months ago, it documented popcorn-like formations in the Jezero inlet channel.
But this rock is different. It’s not just the size or shape of the spheres. It’s the way they’re fused into a single, coherent mass, the randomness of their breaks and holes, and the sheer contrast between St. Pauls Bay and the bland rocks around it.
Clues for Life?
St. Pauls Bay’s spherules may be chemical fingerprints of Mars’ ancient past. If they formed in water, they could reveal the chemistry of ancient groundwater — and whether it supported microbial life.
The broader goal of Perseverance’s mission is to gather rocks that can be returned to Earth — a journey planned for the 2030s through the Mars Sample Return program. Those samples may answer whether life ever existed on Mars. And St. Pauls Bay, as bizarre and isolated as it is, might help select the right ones.
For now, scientists are mapping the local geology, hoping to match the rock’s chemistry with the dark layers spotted from orbit. If the match is found, it could rewrite the story of Jezero Crater.
Whether it formed in a violent flash or a slow seep, St. Pauls Bay is a quiet reminder of how little we still know.
And how much there is yet to discover.