Welcome back, everyone. So, now that we've done a tour of the planets, it's time to talk about the other material in the solar system. And this is what I like to think about as construction debris. so the sun contains about 99.86% of the solar system's mass, and that's really led by Jupiter. which has most of the mass. But while the solar system's, you know, orbiting mass is, most of it's locked up in planets, that doesn't mean that there's not other stuff out there. And that's really important for us to understand. and so, between and beyond the planets lie smaller orbiting bodies that are essentially stuff left over from the construction of the solar system. And it's been through their study that we've really gained some profound insights into not only the assembly of our own solar system and its history, But also how other solar systems are constructed. So let's go through these briefly. The first concept I'd like to introduce here is the idea of the snow line, which is a very important theoretical concept in thinking about solar systems. And the snow line is basically the distance from a star at which you can have ice just sitting there. Anywhere inside the snowline the suns or the stars radiation is hot enough that the ice would just boil away. Beyond the snow line the the flux of stelar radiation is weak enough that water could stay in the frozen form. Now why this is important is because material that was born, Inside the snow line is going to tend to be very water-depleted. Whereas material that was born, planets, asteroids, etc, that was born outside the snow line, is going to tend to be very ice rich. And we can see this as we look at the components of the debris left over from the construction of the solar system. So the, let's first talk about asteroids. And asteroids are basically giant floating mountains. And there's a number of different kind of asteroids. But, what they share in common is that they were either material that was on its way to becoming a planet. Well during the early when the planets were being assembled. What we might call planetesimal, or its material from larger bodies that collided and broke apart. So there's too many regions that we find for the asteroid, for asteroids. The first is the asteroid belt which many people have heard, in the region between Mars's orbit and Jupiter's orbit. And that's actually inside the snow line. So what we find is that there's not a lot of ice on these asteroids, they tend to be quite rocky. and as we're going to see, this is going to be different from material that we find further out beyond Jupiter's orbit. So the asteroid belt has the four largest objects actually contain, you know, a fairly sizable fraction. About more than one half of the mass of the entire belt. And you know, if you've ever seen Star Wars or something. You know? People fly through asteroid belts. And it looks as if there is just a swarm of asteroids. That's really not the truth. They're separated by, you know, quite immense distances. but still, there's enough out there that the gravitational interactions with planets, that interactions with planets can really perturb them. and now there's a second kind of asteroids that we're interested in, that we're very interested in, which are what are called the Earth crossing asteroids. And these are asteroids that have smaller orbits, and in particular they have elliptical orbits. And that's very important because if they were on circular orbits, they would never cross the orbits of any other planet. Now, what we're particularly interested. Are asteroids whose elliptical orbits take them around the across the earth's paths, path as it goes around the sun. Why do we care about these? Because they can have tremendous impact if they ever hit the earth. So, there have been approximately five mass extinctions in the history of the earth. And we believe at least one of them was caused by an asteroid impact, so that's the dinosaurs. So the reason there's no pterodactyls flying around today or any Tyrannosaurus rexes is essentially because of an impact most likely with something like a Earth-crossing asteroid. Another two types of asteroids. The C-Type and the S-Type. The main difference has, is, is, is important for us to understanding how planets form. When you aggregate material together through gravity to form a planet test mold or a planet as planet test molds themselves join together. if you wait long enough the gravitational attraction tends to pull the material, the heavy material to the center. So, you know, the the heat and pressure is enough at the center of these objects for things to melt. And, then the heavy stuff like the iron will sink to the bottom. That's what's called differentiation. And, the difference between the two types of, asteroids is on one hand the C-Type, we don't see any evidence for differentiation. So, these tend to be rocky bodies with you know, metal sort of distributed through them. Whereas with the S-Type, we do see evidence for differentiation. In some case actually, we have completely iron meteorites which are, which are really remarkable. Okay, so that's the asteroids. Now what are the other kinds of space debris or construction debris? There are comets. Now comets in general are things that were born, beyond the snowline. Because they are, sometimes they're called dirty snowballs or, or snowy mush balls. They are you know, really very water-rich, or other kind, you know, water and other kinds of ices, rich. And most of them have come from, well there's two places where they come from. Either the short period comets, the comets whose orbital period on the order of 200 years or less. Tend to come from a region called the Kuiper belt beyond the orbit of Neptune. And in fact actually we would now consider Pluto to be a Kuiper Belt objects, one of the largest Kuiper Belt objects but we don't think of it as a planet anymore. It also is, a construction debris. It is a planet that almost made it and we would think of it as a dwarf planet perhaps. and then there are long period comments which have, you know, very orbits such that, you know, it may be thousands of years. In fact often we don't see more than one pass of a long-period comet. Short-period comets like Halley's Comet we see, you know we have recordings of it passing many times. now these, the long-period comets, actually come from a spherical region about 100,000 times the Earth's orbit away from the sun. And this is what's called the oort cloud, and it's most likely passing stars, if stars get close enough to perturb comets in the oort cloud then they will get, the gravity will kick them a bit and send them hurtling towards the sun. so now comets we think there's so much water in comets. It may even be possible that the earth's water came from collisions with comets early on in the solar system. And we certainly have seen already examples of comets hitting at least one body and that was Jupiter. comet Shoemaker Levy nine astronomers were quite happy a decade or so ago to watch Shoemaker levy nine get caught by Jupiter's gravity, pulled apart into a string of objects, and collide, one of the fragments colliding one after the other with Jupiter. It was a spectacular event for astronomers. Now, finally what we want to talk about are what we actually get to see entering our atmosphere, which are what are called what before they enter the atmosphere they are called meteoroids. And these are just sort of small rocks. Asteroids are mountain sized objects. If we're talking about things that are, you know, the size of a human being or smaller we're talking about meteoroids. And these are generally either dust that came from comets, as comets passed passed the sun. one thing we want to remind ourselves about comets is the tails, the beautiful tails. And, that comes about because of interactions as with the the sunlight. As a comet comes closer to the sun it's surface gets heat, heated up and then jets of materiel will burst out and, that will be both water, and ices, and also dust, and small grains as well. And, so meteoroids are this debris leftover either from asteroid collisions, or from comments. Some of the meteoroids can even come from impacts between asteroids and planets where material from planets gets blown outward. And then when they hit the atmosphere we call them meteoroids, right? So, meteoroids are those beautiful streaks of light we see going through the sky. Most of them are really tiny dust grain or, or pebble sized rocks. Occasionally, something much larger does manage to make it in. As in the ex, extraordinary Russian meteoroid that fell in 2013, that was recorded by many cameras. during that remarkable day. So, one interesting factoid about meteoroids, and meteorites, and meteoroids, is that there's about 100 tons of material being added to the earth every year. from space, material that has fallen into the atmosphere. A lot of it is just dust, which just comes in and settles. but some of it is meteoroids, which actually burn up in the atmosphere and then fall to the Earth as dust. And occasionally, an actual rock which makes it all the way down. So that's really quite remarkable. It means we're constantly getting space dust being added to the sol, to the, the, planet. And in fact if you were to wipe your finger across your car, there's a good chance that there's probably about, you know ten thousand grains of space dust that just fell within the last day or so. on to on to your car.
