In the very place it might be reasonable to expect, perseverance discovered circular rock structures resembling ones formed by microbial communities and some lakes on Earth. This exciting possibility, called for a closer look, on this episode of Mars Guy. In a recent episode cleverly titled, Where's the Beach? Okay, not so clever. I presented the arrival of perseverance in terrain thought to represent the near-shore setting of ancient Lake Jezero before the river deposits spilled in. What I failed to notice at that time are the slabs of rock in the scene with what appeared to be concentric curves. Here's Mars Guy for scale. I did note that scientists would be on the lookout for structures resembling the roughly circular features known as stromatolites that are found along the shoreline of some lakes on Earth, like Great Salt Lake in Utah. That's because they are formed by a combination of biology and geology, specifically bacteria and calcium carbonate. They resemble a reef, but they're not actually coral, and they're better referred to by the more generic term, microbialites, because they don't typically show the internal laminations that are a key feature of stromatolites. The circular features in Jezero crater have obvious differences, but the view with the zoom lens of mass cam z added to their intrigue, nicely emphasized in this 3D rendering from the mass cam z team posted to Sketchfab. You could almost start to imagine that the circular features are remnants of microbialites after the water disappeared and erosion took over. Meanwhile, perseverance was making its own circular features in the slab of rocketed parked in front of, a braiding spot for the science instruments to investigate, and collecting a core sample for possible return to Earth. But the larger natural ones had indeed drawn the attention of the team. After finishing operations at this location, perseverance was commanded to drive just under 10 meters, to get a closer look at the nearby circular features. This beautifully rendered nav cam panorama from Simeon Chmoush shows the start of that short drive, just behind the slab with the two new holes in it, and ending with a great view of the slabs with circular features. They looked even more intriguing from here, but this is Mars, and you always need to look for an explanation that doesn't involve biology. As a geologist, that's something I can do. In this case, I was struck by what looks like a transitioning from a fairly squared off corner to a circular interior, which is not the appearance of microbialites. Instead, this reminded me of rock features on Earth known as core stones, which are nicely shown in these examples in volcanic andesite from Argentina. They result from a process known as sphyroidal weathering, which starts with water percolating down through existing fractures known as joints. In the case of the andesite, it was found that growth of calcium carbonate and other minerals in pore spaces created stresses that helped further fracture the rock, leading to core stones. In Cesaro Crater, the rocks may have first been deposited by volcanic activity, and fractured into joints during cooling, and sometime later infiltrated by water, maybe from Lake Cesaro. This could have led to precipitation of carbonate minerals that contributed to sphyroidal weathering and the formation of core stones. Since then, they've been plained off by sandblasting, conveniently represented by the sand ripple parked on top. The fact that this location is known to host carbonate minerals adds way to this geologic story. And the fact that perseverance only spent one saw here shows that there just wasn't enough evidence to support a microbialite hypothesis. It backed away and drove off to explore other features of the margin carbonate terrain, propelled by circular features that were, without a doubt, formed by life.