With each spacecraft we launch into the final frontier, humanity learns just how different other worlds can be from the one that we call home. Take Mercury, for example it is a simultaneously boiling hot and freezing cold world that looks vaguely like earth's moon but has plenty of weird features that make it stand out in our solar system.
One of those features is a bunch of shiny holes peppered across its surface but thanks to the closer look that NASA's messenger spacecraft gave us, these holes might actually make Mercury a bit more similar to earth  because Mercury might be covered in its own kind of sinkhole. Mariner 10 was the first probe to visit Mercury completing three flybys in 1974 and 75, and some of the images it captured revealed strange bright bluish spots on the planet's surface most were inside or near impact craters and were estimated to measure anywhere from a few dozen meters to a kilometer across. But this does not mean that Mercury looks like it's covered in glitter.
The spots only really stand out in highly processed images because they don't reflect a ton of sunlight, and the rest of Mercury reflects even less light especially blue light. In comparison, because its surface is about as reflective as the graphite in a pencil, so why are these spots slightly brighter and slightly bluer. One hypothesis is that they were filled with certain organic molecules that reflect more blue light than other visible colors, but most astronomers dismissed that idea pretty quickly. Those molecules would not last very long on the surface before being blasted away by solar radiation, and decades then passed without any new clues mariner's images weren't sharp enough to pin down one definite hypothesis, and humans were busy sending spacecraft to other planets 

So when Messenger entered Mercury's orbit in 2011, it had its work cut out for it, by looking at shadows. In Messenger's high resolution images scientists could tell that these spots were actually depressions in the surface. They were not craters because they didn't have a raised rim, and they weren't bowl shaped, so they couldn't be the result of an impact. The land just sort of drops down a few dozen meters, stays flat for a bit, and then goes right back up again. The structures also appear to be pretty young, geologically speaking, the edges are still sharp. Suggesting that there hasn't been a lot of time to wear them down, so they're either like a really new phenomenon, which seems pretty unlikely, given the billions of years Mercury has been around, or they form all the time but don't last very long. With all this new data to analyze, astronomers started referring to those shiny holes as hollows, and they are still hard at work trying to figure out what's really going on.
One hypothesis is based around sulfur specifically. Sulfur just beneath the planet's surface messenger revealed so much of the element near Mercury's surface that astronomers aren't entirely sure how it got there. Maybe Mercury formed with more sulfur than other rocky planets did. And maybe ancient volcanoes deposited a thick layer of it before they went quiet. Whatever the reason, Mercury has a lot of sulfur, so here's the hypothesis when an asteroid or meteor hits Mercury.
The impact will clear enough rock and dust out of the way to bring that underground sulfur closer to the surface. Now the extreme daytime temperatures can start warming it up because the planet has barely any atmospheric pressure. The sulfur quickly sublimates, meaning it changes directly from a solid into a gas. That gas trickles up through the remaining layers of rock, leaving behind an underground hole, and without enough support the rock above that hole eventually collapses. In other words, hollows are basically Mercury's version of sinkholes.
According to a study published in 2021, a group of scientists looked at 57 compounds and calculated which of them could sublimate quickly enough to produce a hollow. While they found three possible culprits, only sulfur was abundant enough in Mercury's crust to create the number and size of hollows that Mariner and Messenger observed. And sulfur also explains the blue tint, it absorbs blue light, so when you have a patch of ground that has lost a bunch of sulfur, it's going to reflect more blue light compared to the surrounding sulfur-rich areas, and if Mercury's hollows really are formed this way they would not last very long over the millennia they would get worn down. By solar radiation and strikes from little meteorites. So this hypothesis can also explain why the hollows look so young ,but it still has its wrinkles. For example, astronomers still don't know if sulfur compounds actually absorb enough of the right colors of light to produce the overall difference in color between the hollows and the surrounding surface messenger's mission ended when it intentionally crashed into Mercury's surface back in 2015, but new data will start rolling in after the BepiColombo spacecraft enters orbit in 2025. For now, astronomers will continue poring over messenger's data looking for any clues that might tell them once and for all why Mercury is covered in shiny sinkholes, while NASA will, of course, never be able to bring messenger home, they have crashed it into a planet.
You can bring home your own little messenger in pin form because it is this month's pin of the month. This beautiful commemorative pin will be sold for the month of September and then never again. In October, there will be a whole new pin available, so to get yours you can go to dftba.com/scishow or just click the link in the description. thank you for watching, and thank you for your support And sulfur also explains the blue tint, it absorbs blue light, so when you have a patch of ground that has lost a bunch of sulfur, it's going to reflect more blue light compared to the surrounding sulfur-rich areas, and if Mercury's hollows really are formed this way they would not last very long over the millennia they would get worn down. By solar radiation and strikes from little meteorites. So this hypothesis can also explain why the hollows look so young ,but it still has its wrinkles.
For example, astronomers still don't know if sulfur compounds actually absorb enough of the right colors of light to produce the overall difference in color between the hollows and the surrounding surface messenger's mission ended when it intentionally crashed into Mercury's surface back in 2015, but new data will start rolling in after the BepiColombo spacecraft enters orbit in 2025. For now, astronomers will continue poring over messenger's data looking for any clues that might tell them once and for all why Mercury is covered in shiny sinkholes, while NASA will, of course, never be able to bring messenger home, they have crashed it into a planet.
You can bring home your own little messenger in pin form because it is this month's pin of the month. This beautiful commemorative pin will be sold for the month of September and then never again. In October, there will be a whole new pin available, so to get yours you can go to dftba.com/scishow or just click the link in the description. thank you for watching, and thank you for your support