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The dust on Mars is small enough to get past your lungs and into your bloodstream, and NASA just found some of it still clinging to spacesuit fabric after four and a half years of a rover driving around in it. Perseverance has a little square of Teflon bolted to it. Its X-ray instrument can't detect anything Teflon is made of, so to the X-ray that square is basically invisible. Which means for 1,645 Martian days, every signal it picked up off the Teflon came from dust that had landed on top. NASA also strapped five spacesuit swatches onto the rover before it launched in 2020, each about the size of a postage stamp. There's a piece of helmet visor in there, some glove fabric, and a few other suit layers. The original plan was to see how suit materials hold up on Mars, and measuring dust came along as a bonus. Before this, nothing on Mars had ever measured dust smaller than about 3 micrometers. This time they saw grains down to around 100 nanometers, which is roughly 700 times thinner than a human hair. Plenty of it comes in under 2.5 micrometers, the same PM2.5 reading your phone warns you about when wildfire smoke rolls into town. Static is what keeps it stuck. Grains bang into each other during Martian dust storms and build up a charge, and each material on the rover pulled in a different size of grain. Years of driving and drilling shook the bigger clumps loose while the tiniest particles stayed put the entire time. That's a real problem for anyone planning to wear a suit up there, since Teflon-type materials are common in the outer layers of spacesuits and they grab exactly that fine stuff. The chemistry makes it worse. Mars dust carries perchlorates, which can mess with your thyroid, plus reactive iron that damages cells. Apollo astronauts got hay fever symptoms from Moon dust after just a few days on the surface, and a Mars crew would be living around this for months. You might've seen that this dust can cross the blood-brain barrier. The study never tested that, it's borrowing from Earth research where ultrafine particles have been tracked traveling from the nose into the brain along the smell nerve. Also, zoom in on that calibration target. There's a tiny geocache on it stamped 221BBAKER, because the instrument is called SHERLOC and its camera is called WATSON. NASA engineers have jokes. The likely fix is something called a suitport. Your suit stays latched to the outside wall of the habitat, you climb in through a hatch in the back, and the dust stays outside with the suit. Engineers now know exactly what particle sizes they're designing against, all because of a square of Teflon the rover's own X-ray can't see.