 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the planet Mercury. Now of the planets, especially the inner planets, Mercury is probably the least well explored even though it is not that far away from us. It's not that much further away than Venus or Mars but it is much less detailed, has been explored in much less detail. So let's take a look a little bit about what we do know about Mercury. So some of the basic properties to start off with here its orbital period is 88 days meaning it takes about three months to orbit around the Sun once. Its semi-major axis or average distance from the Sun is about four-tenths of an astronomical unit so it is less than half the distance of the Earth from the Sun much closer. Its diameter is a little more than a third the Earth's diameter. Its density however is very similar to the Earth's and that is going to tell us that it is a very high concentration of metallic materials. Its orbital eccentricity or how squashed its orbit is is about 0.2. Among the major planets that is the largest eccentricity and its rotational period is 59 days so it takes 59 days for it to spin on its axis once relative to the stars. Now let's look a little bit about the interior of Mercury and what we know about it and before let's actually take a look at the image here first that we have this was taken by the Mariner spacecraft that flew by Mercury back in the 1970s and we can see that overall it looks a lot like our own moon very heavily cratered. The one thing we notice that's missing is the maria we do not see any maria on Mercury. So let's look a little bit about the interior structure of Mercury and our studies of the interior are very difficult because we have never landed on its surface so we don't really know exactly what the interior is like we have to make estimates by comparing it to what we know about the Earth and other planets so what we understand of it is that it is a very large core and if you notice here the core is a very big chunk of the interior and it has a much smaller mantle compared to the Earth. The Earth had a smaller core and a larger mantle and Mercury has a larger core and a smaller mantle and this would be iron and nickel that would be at the central portion again. The core in this case is actually about seventy percent of the diameter of the planet so the core dominates Mercury why it has such a high density and such a high concentration of metal so in a way it's a large ball of iron and nickel with a little bit of a rocky crust on top of it. We also know that that core is still partially molten and that's because Mercury does have a weak magnetic field nowhere near the strength of the Earth but it does have a magnetic field and in order to generate a magnetic field especially in something rotating so slowly it must be partially molten. Now Mercury, as I mentioned, it does not rotate very quickly so let's take a look at how Mercury rotates because it has an interesting pattern here. In fact until the nineteen sixties it was thought that Mercury might be the hottest in the coldest planet that it might be tidally locked to the Sun much as the Moon is tidally locked to the Earth and that would mean that one side always faced the Sun and one side always faced away. It would have made it simultaneously the hottest planet on the side facing the Sun and the coldest planet in the solar system on the side facing away. However radar observations in the nineteen sixties showed otherwise and it turns out that its rotational period of fifty nine days is two-thirds of its revolution period of eighty eight days and it means that Mercury is locked into a, what we call a resonance with the Sun but it's not a one-to-one resonance where it rotates once for every time it revolves it's actually a two-to-three. So that means that two years on Mercury are the equivalent of three days. So the diagram here shows that at position one Mercury has rotated and then it moves to two. It's now rotated halfway around. So now two at this point is pointing in this direction in space one day later it's pointing in this direction in space the second day later it's around here and then four now it's pointing back down again five six and then if we put in number seven up here it would be back to where it started. So once it's had three days occur that is the equivalent of two years and that is another example of a resonance that occurs in this case with Mercury. That happens with the moon in terms of a one-to-one resonance where its day and its year, its orbital period around the earth are exactly the same. Now let's look a little bit about what we do know of Mercury. We can't really see it from the earth very well. We can't see surface features so it wasn't until 1974 when the Mariner 10 spacecraft flew by and got us the first images of the surface. So that was the first time we could actually see it. It's always too close to the sun to be seen from the earth. We have to look through lots of atmosphere and we can never get any kind of clear images even with very powerful telescopes. We also had the messenger spacecraft which orbited around Mercury for several years giving us a complete detailed surface map of the planet. And what we find is that it does look a lot like the moon. We see lots of craters. We also see large impact basins. What we don't see are the Maria. The Maria darker surfaces on the surface of the moon, large flooded basins. We don't see those on Mercury and not just on one side but on any side of it. Now a little more detail on the surface features if we look at some of the messenger images close up. We do see that there is a very large basin which we call the Caloris Basin. It is 1300 kilometers in diameter and is comparable to the larger Maria on the moon. So what that means is that if we look at the whole basin here that is somewhat similar but it has not been flooded by the lava flows that we had on the moon. It still has a decent number of craters around it and inside of it so we still see a lot of cratering around it. So while large impacts did occur on Mercury we do not see any of the Maria like we see on the moon. If we look at the part of the planet's surface opposite to this, opposite to the Caloris Basin it's a very jumbled up terrain. And if you can imagine a very large impact occurring that seismic waves would travel through the planet and would reach the other side. So if the large impact occurs here as those waves travel through the planet they then come together on the other side and it jumbled up the terrain on this side. So if the Caloris Basin is here then the jumbled terrain is here on the opposite side of the planet. And that is just really all of those seismic waves focusing their way through the planet. Now one of the other features that is unique to Mercury is what we call a lobate scarf. So we can take a look at an image here of this and you can see the scarf not very not straight kind of winds its way through a couple of different craters here. It doesn't look very big but these are actually cliffs that are kilometers high. So not just little bulges but actually things that if you were looking at this if you were trying to climb that cliff you might have a mile to climb or even more. These are believed to be wrinkling a wrinkling of the crust. Essentially what would have happened is that the planet would have started to cool now the interior, the exterior would have cooled and the interior would still have been molten. And as we know most objects as they cool tend to get smaller. So as the interior continued to cool it would shrink down leaving empty space in between and the crust would have collapsed down back onto that giving us these scarps and they are unique as far as we know to Mercury. That is really the only place we see the specific type of scarf we call a lobate scarf. So a very distinctive feature for Mercury. And as I've said Mercury has been explored by just those two spacecraft although the messenger craft in spending a long time in orbit was able to give us a complete map before it finally ran out of fuel and crashed into the surface of Mercury giving it one more crater. So let's finish up as we do with our summary and what we find here is first of all Mercury has surface features much like those of our own moon but a very different interior structure. Remember that Mercury has a lot of metal associated with it. It can almost be a metal planet 70% of its diameter is the core which is solid metal. We saw that Mercury's rotational period is two-thirds of its period of revolution. So it does have a what we call a resonance in terms of its orbit and its spin. And finally we looked at the lobate scarps which are a feature that are distinctive to Mercury and probably formed when the planet shrank early on in its history. So that concludes our lecture on the planet Mercury. We'll be back again next time for another topic in astronomy. So until then have a great day everyone and I will see you in class.