Explanation of Why Most of the Satellites & Planets Have Many Craters

I'm Eric Loberg with the Taylor Planetarium at the Museum of the Rockies. I'm going to discuss why most planets and moons have craters on them. And that's because of when they were formed. All of our things were formed about four point six billion years ago. All the planets started to coalesce and come together, and clump together out of this big gaseous form. The sun forms together, all the planets start to form too. The planets start to move around the Sun. And that takes oh a couple a hundred million years. And then a little bit later about four point one billion years ago scientists planets actually started to shift. And so what's then left behind is our planets Mercury, Venus, Earth, Mars. And then a little further out the giant gas planets, Jupiter, Saturn, Uranus and Neptune. Except the thought is Neptune was actually inside of Uranus. And these planets were all probably clumped a lot closer to the sun then they are today. And what happened is planetary migration. We see this with other solar systems out beyond our own. We see planets that aren't where we think they should be. And we think that's because they either have moved closer to the Sun or farther away. And scientists think that Neptune actually started out closer then Uranus to the Sun. And Neptune slowly moved out. But what happened in the meantime is there's lots of leftovers, leftover in the solar system. All sorts of rocks that are spread around here. The planets mostly collect these comets, these asteroids and they put them into moons. Or they are in a nice asteroid belt between Jupiter and Saturn. But there's probably an asteroid belt out farther and as Neptune started to move out all these little rocks started to interact with Neptune's gravity. And Neptune started shooting these in, into the inner solar system about four point one billion years ago. And they started to collide with these other planets. These have a lot more mass then the asteroids that were being shot in. They start to hit those, those inner bodies. They start to hit those rocky bodies. And that's why we see on the moon's of Uranus, Saturn, Jupiter, we see all these moons with asteroid spots. We also see them in all the planets, all the inner planets and our moon. We see lots of collisions. And where we get that date from is our moon. When the astronauts land on the missions to the moon they scoop up rocks. They bring them back to earth and we figured out how to date those rocks. Most of those rocks were melted about four point one billion years ago. And that's when the moon was being impacted by these collisions over and over again. The rocks were remelting, we have these big craters left behind. The other reason we still see them on most of the planets we don't see very many on Earth because Earth has a lot of volcanoes. We have earthquakes, we have tectonic plates. So things are shifting. Our mountains are going up. Our oceans are going lower and we have a lot of water on Earth as well which we don't have on these other planets. And so most of Earth's craters have been washed away either by our waters or our volcanoes. We see less craters here then we do say on Mercury, which doesn't have any volcanoes and no water. There's no water on Mars so we see quite a bit of craters still on Mars but there are volcanoes. So some of those have gone away. There are lots of volcanoes on Venus. We see some craters but they've gone away again. Where we see lots and lots of craters, lots of impacts left behind especially on those little moons. Which don't have plate tectonics, they don't have volcanoes, they don't have running water. And so they have been scraped away. And that's why we have most of our crater impacts today. And that's where they came from. I'm Eric Loberg with the Taylor Planetarium at the Museum of the Rockies.