I want to digress a little bit and ask a simple, but important question, which is, how do we tell how old a surface is on Mars. Okay, first you might ask yourself the question, what does it mean to be an old surface. Well, we can think about it in the example of the earth. There are places on the earth that have recently been resurfaced by erosion, by flows, by mountains coming up. There are places on the earth that have been basically intact for a large chunk of the history of the earth. These are the ages of those surface. Well, how do we figure out the age of a surface on the earth? Sometimes it's at least easy to figure out relative ages of surface because we can see layers. And sometimes we see those layers nicely stacked on top of each other and can say, oh yes, this thing at the bottom is older than this thing on the top. The greatest example of that is the Grand Canyon of course. Not only can you see the layers, the colors of the same layers all the way across, you can also see the shapes of the same layers. Some layers are particularly hard to erode away and so they make very straight cliffs like this. Some layers are very easy to erode and they erode very nicely like this. And you can follow these everywhere. You can see this line, it goes all the way here, it goes all the way here. It's the same line all the way across here, all the way across here. These layers go for miles and miles and miles through the canyon and it's pretty obvious that these on the bottom are very old and these on the top are much younger. And the Grand Canyon is a nice case where it's also good to remember that you can't simply say that there has been a nice time progression from here to here. There's something in the Grand Canyon and in fact you can see it right in here, this dark stuff versus the sort of grey stuff. This is an unconformity, it's called in geology. An unconformity is where rocks of a certain age are touching rocks of a very different age. And that can happen where you could imagine that you had layers stacking up on each other like this, and then something happens that washes away some of the layers and then new layers stack up like this. There's an unconformity right here where two layers are in contact that aren't the same age. The other interesting thing that could happen and it doesn't happen here, which is why this is such a nice region to look at. The other thing that could happen is that layers can get warped. They can get tilted. They can be entirely upside down and try to confuse you in a lot of different ways. That's great. Can we do it on Mars? Well, not really. Not in a global view. We'd like to have a general global idea of where the old surface is on Mars, where the young surface is on Mars. We can't really count layers all the way around the planet, stack them up and see what's on top and what's on the bottom. One thing that we can do on Mars though that we can't do on the earth very well is to simply count the number of craters in different regions on Mars. Now this makes sense. If you imagine that craters, that asteroids have been impacting Mars since the beginning of the creation of Mars, then a region that has a lot of impact craters will be very old. A region that's new, that has been resurfaced in some way, will have only a few impact craters. And even if we don't understand what the ages mean from these craters, we can at least make a relative idea of what's young and what's old by looking at a plot like this. Let's look at one. This is from a paper published in 1973 based on the original Mariner 9 data, and it's a nice one because it shows just the biggest craters and it shows the whole planet. And it gives you a very good overview of where the old regions are and where the young regions are. Okay. Where is the youngest regions? Well, that's pretty obvious. There's a whole swath up through sort of like this, where there are very few craters. This is the youngest part of the planet. What's the oldest? Well, if you look really closely, again you might choose to separate into a very cratered region in through here, and maybe sort of over in here, versus a less cratered region all around the rest of the planet. And if you chose those boundaries or something close to those boundaries, you have basically done what people do for Martian history. You have divided Mars into three periods. And that's really the main thing that we know about Martian history and how old the surfaces are, are these three periods. And they are named after the regions where these things are. So this is the Amazonian. This is the youngest. The current era is called the Amazonian. This is the oldest and this is the region of Noachias. So this is called the Noachian. And this middle region is named after this region of Hesperia and so it is called the Hesperian. And when you hear people talk about Martian rocks or Martian landforms or something, you will hear them using these three words over and over again, Amazonian, Noachian, Hesperian. And the most important thing to remember is that Noachian is the oldest stuff that's around; Hesperian is the middle stuff; Amazonian is new, new up to right now. Currently, it is the Amazonian period. These are the equivalent to the names that terrestrial geologists use that you've heard to describe the geological ages before, and the terrestrial geologists do it based on things like fossils and layers. And here it is based entirely on these large scale crater counts. We'd like to be able to do better than to simply say, well, we have young stuff, we have middle stuff, we have old stuff and simply from using the data on Mars, you can't do it. We really don't know the ages of these regions on Mars and in fact, this is one of the big motivations of the next NASA Rover. The Mars 2020 Rover is going to have some techniques to take rocks and actually date those rocks. It hasn't been done before. What you can do is guess how many craters should have been formed on Mars and therefore, how old things were. You could imagine, you could say, look, if I know - first off I know that Mars is the same age as the earth in general. The easiest number in the world to remember, 4.567 billion years ago is when everything formed. And if I say, look, craters have been landing on Mars at a steady state ever since then, then the age of a surface versus the number of craters should be something like this. Surfaces that are exactly formed today, something washed away the surface, then its form should have zero craters. Craters that are the oldest should have the largest number of craters. And so you would say something like there are everything either formed around in here or maybe around in here or the newer stuff formed around in here. That might not be so bad and you could try to guess what this rate is of craters by looking at the number of asteroids that cross the orbit of Mars and coming up with a number. People have done that and come up with numbers this way. But the problem is that we don't think that the bombardment on Mars, the asteroids hitting the surface of Mars have been doing so at a uniform rate. And the reason we don't think they've been doing it at a uniform rate is because we do have one place in the solar system where we have craters like this and we can count the craters like this and we can date the rocks and that one place is the moon. The reason we can date the rocks on the moon is because we have picked those rocks up and brought them back to the earth, to terrestrial laboratories where you can use traditional geochemical techniques to figure out how long ago this piece of rock was molten. If you know that this piece of rock is part of a big impact sheet that you can see came from a specific crater and you can figure out how long ago it was molten, you have aged that crater; you have dated that crater. When you do that on the moon, you find evidence that the number of impacts over a certain period of time did something like this. There were a lot at first and the number declined and then at something like 3.9 billion years ago, it spiked again and then declined again to now. Now some of this makes sense. This part, the fact that there were a lot of impacts at first and then they declined, this is to be expected. If you remember our general picture of the way the solar system formed, there was the big cloud of gas and dust. Planets built up from smaller things to bigger and bigger and bigger and that means as they're building up, they're having impacts, but those impacts eventually make the final number of terrestrial planets. But there's still stuff around, so that stuff around is still hitting the planets. In fact, something the size of maybe Mars hit the earth to form the moon itself. That happened back in this early time period. This is the cleaning out of the inner solar system and it dropped very quickly. But the evidence then suggests that there was a spike at 3.9 and again a clearing out. This spike at 3.9 billion years goes by the name of the Late Heavy Bombardment. And it's called Late Heavy. It's late because it is after the clearing out of the solar system has occurred. You would expect that this thing should have just continued like this and boom late, 600 million years after the formation of the solar system, there's suddenly a big spike in bombardment. Heavy because it's heavy, and bombardment because it's a bombardment. There is however, a lot of controversy about whether or not this is real. Some people say, no, no, it really just did something more like this. Some people say, that this is maybe a little bit of wiggle and some people say it's all just noise. The specialists in cratering will debate this for all time I suspect, but let's go on the assumption that it was actually real. You should ask yourself, what the heck was going on? Why would there suddenly, out of nowhere, be a huge increase in craters right here? There's been a lot of explanations for the Late Heavy Bombardment. We'll talk about one in particular when we talk about the small bodies in the solar system. In fact, the things that would have caused the Late Heavy Bombardment and the history of the solar system that we're learning from them. But there are many explanations. Some of them are explanations such as you know, perhaps maybe there were other things that formed with the moon. And the earth is here, the moon's over here and maybe there were some other bodies around the earth and they finally hit the moon at that point. If that's the case - which I don't actually think it is. But if that were the case, then the Late Heavy Bombardment would be a moon-specific. Most of the other ideas involve more solar system wide cataclysms in which case, if this happened on the moon, then the same thing happened on Mars. If you take those ideas of how much bombardment happened on the moon and you apply those to Mars, you can start to get dates. They're still rough dates, but you can start to get dates for what these periods were on Mars. And the dates look something like this. Here's our general timeline. The earliest stuff is Amazonian. This is time going in this direction. Right here is now. Right here is 4.567 billion years ago. And there's the Amazonian that's been happening right now, Hesperian from the middle periods, Noachian. There's also this mystery time over here where we don't see anything. This is generally called pre-Noachian. And these are the things for which there is no cratering record preserved, no surface preserved from here. And the times - I'm going to give you some very rough numbers because the numbers are not known very well. But let me just put you some in here. This transition into the Noachian is something like 4 billion years ago. But the Noachian didn't actually last very long. The end of the Noachian, we'll call it something like 3.7 billion years. So that's only 300 million years in a 4-billion-year history, the very heavy cratered regions. The Hesperian ended maybe 3 billion years ago. So we have about 700 million years of this. And the Amazonian with very few craters is the longest. This is something like 3 billion years and if you look at those regions they look very little cratered. And it's because if you remember those cratering rates have declined incredibly. There are very few things left around that are creating craters these days. Now if you hear Martian geologists talking about dates with numbers, you should ignore them. And if they talk about things that are Amazonian or Hesperian, then you should listen and say, aha, I know exactly what they're talking about. Nobody knows if it's really three billion years to 3.7 billion years, but everybody generally agrees if you call something Hesperian, these things are in that middle time period. Whether or not this this extends another 200 million years this way or this way is less important. And with that, let's go back and look at those craters that we see on Mars and think about what might be going on here. We have very, very old stuff. The Noachian stuff again is in this region, maybe a little bit here and maybe a little bit here. What does it take for something to be really old? Nothing. Nothing can have happened in something like four billion years. We don't have very much four-billion-year crust left over on the earth because we have things like weather and plate tectonics. But if there is four-billion-year-old regions left on Mars, it's a pretty good indication that things like weather and plate tectonics are not actually happening. Just thinking about the Amazonian stuff, that's the stuff up through here. The Amazonian stuff, well okay, so it's maybe only three billion years old, but something happened between 4 and 3 billion years that erased all of the craters on the surface. What could it have been? Well, here's one clue. All of our favorite volcanoes are there in the Tharsis region. In fact, the three big volcanoes at Olympus Mons are somewhere right around in here and much of this material in through here is volcanic in nature. It's not that it's a volcanic eruption that came from Olympus Mons and spread down. But it's more like there were volcanic fissures all throughout there and that lava spread throughout all these fissures and then lava spread on top of them and closed them up. So this whole region of Tharsis, the Tharsis Bulge is young-ish material because lavas have covered things over. And there's more volcanoes over here in Elysium, that you can see here. Up through here though, it's not clear that it's volcanic material, so this is something we'll have to think about in particular. Finally, though let's think about the Hesperian stuff. Here's the Hesperian region down in through here. It's not as old as here. Something happened, something important happened between maybe about, we'll call it 3.7 and 4 billion years although we don't really know what those numbers are. Something happened which erased all of the craters 4 billion years ago down through here, but by 3.7 new craters had formed and nothing new was happening to erase them. So this surface has been preserved since 3.7 billion years old even though it was erased at 4 billion years. So even just looking at these craters, we can see historically something interesting has happened on Mars. Not that Mars was dead enough that there are regions that have not been touched. Something happened 4 billion years ago that erased probably the craters everywhere else and then craters started re-forming here. And something has happened at something like 3 billion years ago that erased all the craters up through here. Some of that's volcanic; some of it is probably not volcanic. And figuring out that history is going to be one of our major tasks for the rest of these lectures.
