We've seen evidence of groundwater sapping on the surface of Mars from those early Mariner images. We've seen evidence of huge floods in outflow channel. And we've seen that there have been large scale changes on Mars in the last 4 billion years. Question that we might still ask is was there ever water on the surface, freely flowing? Was there ever precipitation? The answer to that question might come from finding things that look like river channels that were formed from things like rain. And it looked like from some early mariner images and then from much better viking images, that such a thing was exactly what was found. Look at this viking view, for example. Look at these beautiful little channels that we have ensizing this region and through here. These don't look like those sort of blocky channels that we saw before with the obvious sapping going off on the sides, the sort of things that were typical of the American southwest. This look, when you look at this, they look a lot like they might be some sort of little river channels. They're smaller, they're much smaller scale than the things we were looking at before only tens of kilometers, and then they disappeared. They either disappear into something that has been resurfaced. Or they disappear into a sort of, this looks like a kind of flat region. Why might they be disappearing? Well, you could imagine water flowing down these things, and then finally ponding in these regions like this. Let's look at a few more examples. I love this one because it reminds me of flying over, again, the American Southwest. You fly over the American Southwest, it's desert train, you're looking out over the ground and you see that the entire, every slope is dissected with drainages going from the smallest drainages collecting into bigger drainages. And you see that right here, you see large drainages out through here, then you see networks of the small ones coming in to smaller, smaller, growing and growing, collecting together into these dendritic channels. Again, you remember that word from before, and I showed you that picture of the Tibetan mountains, where you see major channels. Each major channel is fed by medium channels, which are fed by small channels, which are fed by tiny channels. And critically, every single region of this map is drained by some channel. The only places that you don't have channels are where you have ridges. In fact, you could use these channels to measure out the ridge lines. The ridge lines must go something like this. Smaller ridges go here. Big ridges go through here. And if you looked at the topography of these mountains, you would find exactly such a thing. This is a classic feature of rainfall driven drainage systems. So the question to ask, does Mars do that? Well, we'll take a look at that in a minute. And I just want to show you a couple more really spectacular pictures. And first, I want to, like usual, orient you. Notice, we're sitting here staring straight down at Valles Marineris. I always like to start from here just to sort of get a feel for where on the planet I'm looking. And I'm going to look for something called the Eberswalde Crater. Let's go fly there, and take a look at what it looks like. There he comes, coming in, coming in, let's stop right there. And what I want to show you are these features in here. See these right now, it looks almost like a cauliflower or something. This is the crater, the crater is fairly large, and in fact bigger than this field of view. But what's interesting is that these channels are coming in to the crater. And if you zoom out just a little bit, you can see them a little larger scale crater flows coming in through here, ending up inside of a crater. Seems a little strange, what are they going to do? Well, when they get inside the crater, let's look what happens. So this is that cauliflower region they are getting. I'm going to now show you pictures of that cauliflower region, and see what that really looks like. Looks like this, remember the channels were coming in from this side, looked like water was flowing in through here, there's another big one where you can see this channel in through here like this. And then this is the crater. The crater is huge off in here even all the way over to here, and actually much bigger than that. And the channel comes in, and then does this funky pattern, what is this funky pattern? This is a pattern that you would get from something like a delta, what is a delta? A delta is when you have a river or flow coming in through here. And the water stops because it has hit water in through here. It hits the water in through here, it dumps its sediment, then makes channels flows one way for a while, flows another way for a while. Instead of a network of rivers coming together, it's a distributary network. It goes this way, then this way. Then this way, then this way. This looks to all the world like a river flowing into a crater that was, at the time, full of water and forming a delta in through there. So is it true? Was there a time when Mars was full of these rivers flowing and precipitation causing these sorts of things? Well, one interesting question to ask ourselves is when did all this happen? How do we figure out when all this happen? Well, the one way to do it is to go look at all of these valley networks, as they're called, and see how old they are. See how old they are, by which I really mean see what sort of terrains, what age of a terrain they sit upon? Here's a map from a 1995 paper, which tried to go through and find every single one of these valley networks, and map their locations. This is about 45 degrees south, about 45 degree north. This has the same orientation that we typically see. So you should start to, by this point, recognize some of the regions. There's Tharses, this is the big volcanic region. This is the region that is generally Amazonian in age, the young stuff. Remember, look sort of like this, Amazonian is all right here. And of course, all the stuff above here is Amazonian. The Hesperian was all the stuff that was down in through here, and in fact, the stuff that you don't see. And this stuff, the ages of this stuff in through here is generally Noachian. Most of these valley networks are appearing in Noachian regions. That means that they formed in that one period, between something like 4 billion years ago, and 3.7 years billion years ago. And then in general, they stop. You don't see these in these middle aged units down here, and you certainly don't see them in the Amazonian aged units up there. So what was happening? Was it raining 3.7 billion years ago on Mars? The debates are still out. There are still some people who interpret these value networks as actual rainfall collecting and going down river. And some who believe that these valley networks are really just that same sort of seepage stuff that we saw when we saw the really stubby things that looked like this. You can tell the difference if you look very carefully at the rivers themselves. Then, for example, maybe you can see they're the u-shaped valley heads at the start of these things. Some of them look like they have it. Some of the river valleys look like they are coming down from ridges. A ridge across here, river valleys, river valleys, river valleys, and then smaller ones, and then big ones. Some of them look like they are when we get the topography. Some of them look like they aren't. There's enough variation across all of these that there's room for debate as to what really happened. But there really are sort of two or three major categories of what could have happened. Category one is that it was wet. How do I make it wet? Well, we'll talk about Martian climate, in particular, ancient climate in some subsequent lectures. But it was wet at the Noachian period, and it was raining. Option two is that it was never wet, it was never raining, and all of this is just ground water seepage. Much in the same way we saw these blacky channels that looked very much like ground water seepage. The problem with ground water seepage is that these valley networks carve out such big valleys that there needed to have been recharge of the ground water. You can't have just had water that was there breach somehow, flow, and boom, that's it. There's too much carving. Valleys are too deep. There has to be recharge. How do you get more water up into those aquifers? Well, one way is with rain. But if you have rain, you have this too. So it could be ground water with rain and wet, so you can combine these two. Another way to do it is to originally have had water underneath the surface of Mars. Ground water on Mars. And over time, that ground water sinks lower and lower. And eventually, it hits some magmatic regions, big hot regions in through here. And those magmatic regions cause it to rise back up, geothermal fluids of the type we get on Earth. Those hot fluids flow back up and then can come out and flow on the surface, and continuously recharge these regions up through here. That means that Mars could have been cold and dry the entire time with groundwater that just kept on recharging. These valley networks are clearly telling us something, they're clearly telling us that water was flowing. And the big question that still remains is, how long was that water flowing on the surface during that maybe 300, 600 million year period? Was it flowing very intermittently every time a little recharge happened, it just went, was it raining? Continuously, was it hot, was hit humid? These are the questions that we're still going to try to answer about what Mars was like.
