Why does the professor say this:

Let me remind you that ours is not the only planet with a moon. In fact, lots of the other planets in the solar system have one, and some have more than one. Take Mars, for example. The large red planet actually has two, Phobos and Deimos. Now, Phobos is the focus for this lecture. Well, to begin with, it is much closer to Mars than our moon is to us. In fact, of all the moons in the solar system, it’s the closest to its planet. And it moves rather quickly as it completes its small orbit. When we talk about planets and moons there is something called the synchronous orbit radius, which is the size and speed of the orbit a moon needs in order to spin around its planet at the same speed as the planet spins around itself. Think of it this way, when a moon reaches synchronous orbit radius, you can stand on the planet’s surface and the moon will always stay in the same place in the sky. If a moon is below this orbit, like Phobos is, then it actually rises and sets more than once a day. OK, so we know that Phobos is close to Mars and therefore seems to move quickly across its sky.
    I guess the next question that needs to be answered is related to what the moon is made of? Well, we’re not really sure, but we have some good guesses. First, most of the material is likely similar to what you would find in a C-type asteroid, which is nothing more than different types of carbon. So, it’s like ninety percent carbon. Covering the moon is a one-meter-deep layer of something called regolith. Now, the fact is that it’s just a fancy scientific word for a loose layer of material covering rock. A lot of time regolith is just dust or sand, but if it has a lot of organic material in it, we call it soil, just like you would use in a garden.
    Phobos is made up of a dust covering and rock underneath, but it is not dense enough to be solid rock to its core. So, it’s also likely that the center is made up of ice. In fact, the Russians managed to send a probe to Phobos and observed off gassing, which means that they could detect gases being released from the moon. Unfortunately, the probe stopped working before they could identify the gases, but scientists generally believe that it was water vapor. If you look at these characteristics all together, you can start to see a good argument for the idea that Phobos is just an asteroid that got caught in Martian gravity. I forgot to mention this, but now would be a good time. Phobos is not really that big. It’s only 27 km by 21.6 km by 18.8 km. That makes it smaller than Mars’s other moon, in fact, smaller than most moons we know of. I wanted to include that because it supports the argument that it’s just an asteroid pretending to be a moon. It’s not really big enough to be anything else.
    We know where it is and what it might be made of. Now let’s look to the future. Phobos loses about 1.8 meters of its distance from Mars every one hundred years. Eventually, it’s going to hit the planet, right? Well, that’s not the case. The moon will fall apart before it gets to the surface. Actually, in about fifty million years, the moon is going to dip down below something called the Roche limit. When that happens, the gravity that holds the moon together will be overcome by the tidal gravity of the planet. The moon will basically fall apart and the smaller pieces will become a planetary ring. Think about the planet Saturn and the rings it has around it. Mars will have the same thing, made up of little bits of Phobos.
    So, it’s like ninety percent carbon. Covering the moon is a one-meter-deep layer of something called regolith. Now, the fact is that it’s just a fancy scientific word for a loose layer of material covering rock. A lot of time regolith is just dust or sand, but if it has a lot of organic material in it, we call it soil, just like you would use in a garden.
    Why does the professor say this:
    Now, the fact is that it’s just a fancy scientific word for a loose layer of material covering rock.