First ever look under a Mars rock

At the end of April, Curiosity's drill bit got stuck in a rock, leading to unprecedented efforts to free it and an unprecedented look at a surface hidden from view for millions or maybe billions of years. Spoiler alert, nothing crawled out. On this episode of Mars Guy, we last left Curiosity in what could have been a cliffhanger, or maybe that's a rock hangar, if late-breaking images hadn't arrived on Earth. Here's Mars Guy for scale after the rock crashed to the ground. It started with a routine drilling operation to collect rock powder for analysis by on-board science instruments. The routine ended when the drill bit actually lifted the rock instead of lifting out of the rock. At that point, the scientists had to take a back seat to the engineers tasked with getting the rock off the drill, so no looking under the rock. When vibrating the bit using the percussion action of the rotary percussion drill failed to free it, more extreme measures were employed. These included lifting the rock up and away and putting it through various moves including both rotary and percussion action on the drill bit. The rock can be seen to reorient in the minutes before it finally came off, which was captured in color by Mastcam. It looks like when the drill was subsequently angled down, the rock fell off and crashed to the ground, fracturing it in the process. It's noteworthy that the hole is still intact. This shows that the rock actually slid off the drill bit rather than breaking apart while on it. This is emphasized by subsequent close-up views with the arm-mounted MOLLE camera showing an intact hole with fractures surrounding it, which formed after the rock smashed upon impact. The texture appears to be that of a fine-grained sandstone or maybe a mudstone that has been weirdly etched on its surface by wind and sand and time. Nighttime views with MOLLE's LEDs allow a look inside the hole, which nicely show the bottom, revealing that the drill bit did not actually go all the way through this thin slab of rock. Instead, it looks like the bit carved out a space along part of the circumference, which probably trapped the bit under a slight overhang and prevented extraction. I suspect that the drill bit drifted as it penetrated, as can be seen during testing of the feed extended drilling mode that I described in the previous episode. If it encountered a slightly softer rock layer at depth, the bit could possibly carve it out in response to a slight lateral torque caused by the drift. In the Sauls or Martian days afterward, the team investigated the drill bit, which is really more like the tip of a jackhammer. The images appear to show that the drill bit and its sleeve were not damaged by all the unprecedented activity they were forced to endure. Note, though, that if there had been damage, there are two spare drill bits available that can be swapped in if needed. So what about the place where the rock came from? The team got some great observations of the spot where the sun hasn't shone for millions or maybe billions of years. If we're looking at a surface where a layered rock parted when Curiosity lifted its drill, then it's certainly been a few billion years since the layers of fine sand or mud were first deposited before lithifying into sedimentary rock. Obviously, though, we're not looking at a pristine fractured surface. Instead, there's brown to dark gray basaltic sand spread across the entire surface. The close-up views reveal small ripples of sand and a range of variably colored grains, many of which are well-rounded. These observations show that this is windblown sand that's managed to infiltrate the rock. I suspect that some combination of chemical and physical erosion attacks the rocks in this place, creating planes of weakness, which allowed Curiosity's stuck drill bit to detach upper rock layers from lower ones. Although literally revealing, the exposed surface shows nothing notably different than what can be seen on adjacent surfaces. If you were hoping to see bugs or worms, sorry to disappoint.