The Mars Exploration Rover Spirit was three years beyond its warranty period in 2007 when one of its wheel motors failed, but the trench created by the dragging wheel led to one of the most important discoveries on Mars. I think of Delta. I'm confirmed. Perseverance takes place on a surface of Mars. As I described in the previous episode, Earth and Mars are in solar conjunction, which blocks radio communications and forces some downtime for Perseverance. So I thought I'd take a look back at what was happening around this solar conjunction period for the Spirit Rover. Spirit operated from 2004 until 2010 in Goosef Crater on the opposite side of Mars from Perseverance in Jezoro Crater. It landed near Hills, named to honor the seven astronauts killed in the Columbia Space Shuttle disaster in 2003. It drove up and over Husband Hill, named after Shuttle Commander Rick Husband, and down to Home Plate in a roaded volcanic ash deposit. It was there that Spirit parked during its second solar conjunction period in 2007. McCool Hill was the intended destination after exploring Home Plate, but during the drive out, one of the six motors on this six-wheel drive Rover failed. You can see the tracks going out and the dirt it churned up after the wheel failed, including more of the sulfur-rich soil that it had seen before. Spirit had to drive back toward Home Plate to get to the nearest hill slope where it could park for the winter. By this point in its much extended mission, its solar panels had accumulated a lot of dust. Parking on a slope tilted the solar panels toward the low winter sun, which provided enough extra power to survive the winter. At this point, some of you are wondering why there was no way to clean off the solar panels. As Steve Squires, the lead scientist of the mission rights in his book, this was not possible. Here's how he described it. We looked at all kinds of ways to get dust off the solar rays, of course. We looked at windshield wipers, we looked at compressed gas that could blow the dust away. My favorite was the idea of covering the solar rays with clear plastic on rollers, and when the plastic got dirty, we'd just roll the rollers and bring in clean plastic. But none of these schemes worked. The problem with all of them was that they were too complicated and too heavy. There was a limit to how much stuff we could send to Mars, imposed by how powerful our rocket square and how good our landing system was. Our design was close to the limit already, so there was no way we could add big, heavy gadgets to keep the arrays clean without dropping something even more important. All we could do was make the arrays as big as we could get them and live with the results. What Steve and the engineers never predicted was that Martian dust devils would repeatedly scour off the dust, extending the rover's life, like I presented in Episode 11. With the return of spring and increasing solar power, spirit continued its exploration around home plate. Once again, its dragging wheel exposed some bright soil, but this time it was not just more of the sulfur-rich stuff. This was nearly pure opal in silica, and so were some of the nearby rocks. Opal is a gemstone formed from the slow percolation of water through quartz-rich sandstone, but the opal that spirit found is not the gem variety. Instead, it's the kind that forms in volcanic hydrothermal environments around hot springs and fumerals, like in Yellowstone National Park in the US. This was the first time Opal in silica had been discovered on Mars. It's a big deal because on Earth hot springs are great places for microbial life, which grows in thick mats in the outflow channels. That's where they can become entombed in Opal in silica and preserve for eons of time. Even more exciting, the shapes of some of the Opal in silica rocks found by spirit look like hot spring stromatolites, like these found at Altatio, the Yellowstone of Chile. These stromatolites are a mix of biology and geology formed when Opal in silica precipitates on microbial biofilms. Without more data or return samples, we can't prove that spirit found ancient hot spring stromatolites, but at this point it's an exciting possibility.