 Greetings and welcome to the Introduction to Astronomy! One of the things that I like to do in each of my introductory astronomy classes is to begin the class with the astronomy picture of the day, from the NASA website that is apod.nasa.gov slash apod. And today's picture, for July 31st of 2023, well it is titled Phobos over Mars. So what do we see here? Well this is, the surf the planet in the background is a portion of the planet Mars. And in the foreground we see one of its two moons. So Phobos is the largest of Mars's two moons, although still quite small. And it is the innermost, closest to the planet. Now Phobos is also very interesting in that it is a doomed moon, meaning that it is eventually going to be destroyed by the Martian gravity. Why? Well it's in a very low orbit. So it's very low, orbiting very quickly around Mars, in fact so fast that you would actually sometimes see it rise and set twice in a day. That's in how low of an orbit it is. And it's also will eventually be drawn down into Mars. Now is that going to happen next week? No. It'll happen soon by astronomical standards, but still we are talking about about 50 million years and at that point Mars would be down to just a single moon. Of course we don't know if Mars ever had any other moons. Phobos and its other moon Deimos are also believed to have been captured asteroids. Mars is right on the inner edge of the asteroid belt. So there are a lot of small objects there and it's quite possible that some of them could wander into its gravitational field and be caught into orbits. And that may explain why this one is caught into such a low orbit. Just the physics of how it happened to approach Mars could have put it in an unusual orbit like that. Now we also see on Phobos itself a number of craters, but very interestingly Phobos also may be covered with a good amount of dust on it. Now when we look at footprints on the moon we see that there is a good footprint there but it's not very deep. Here we see that the dust could be almost a meter in depth based on current studies. So very interesting also there. We're also missing in this image the largest crater on Phobos, which is on the other side of it at this point. And that is one that was almost the size of the moon itself and would have come very close to shattering this moon when that impact occurred. Now of course we don't know when that impact occurred, whether it was before this moon might have been captured by Mars perhaps or maybe it was something that happened while it was in orbit that actually put it into a different orbit that got it into this very low orbit. Although when we see we also see some similarities here. We see craters on both of the two objects. Mars having very distinct craters there down on its surface and craters on Phobos. But we do notice a distinct difference in the color. Phobos is one of the darkest moons in the solar system, reflecting very little light and making it very hard to be able to detect. Now this image was taken a couple of years ago by the Mars Express spacecraft, which is in orbit around Mars, which is the only way to really be able to see these moons in detail. While they were detected from Earth, they really cannot be seen from Earth as anything other than tiny points of light around Mars to really see them in detail requires travel to the red planet by craft such as the Mars Express. So that was our picture of the day for July 31st of 2023. It was titled Phobos Over Mars. We'll be back again tomorrow for the next picture, previewed to be Monster at the Sun's Edge. So we'll see what that is about tomorrow. And until then, have a great day, everyone, and I will see you in class.