The nice thing about Mars, is that we have so many data sets to examine. So many different ways of probing the history of water and geology on the surface, that we can learn an incredible amount of detail. The down side to all that incredible amount of detail is that we have so much data. It's sometimes hard to step back and remind ourselves of the big picture. So I want to take this last lecture on Mars and step back and think about what we've learned. And think about what this is telling us for the history of water on the surface of Mars. First, let me put the martian timeline up. And I'll just quickly remind you what these periods are. We are sitting up here at now, and most of the history of Mars is this Amazonian period back to maybe 3 billion years ago. These are times in billions of years and they're all approximate. We don't know these numbers very well. We'll look through it what's happening in that Amazonian period, but as you remember the Amazonian is the period where very few impact craters, very little geology, very little water, just like now. That mid-period, Hesperian period, some of the main action was the large outflow channels that we saw. Noachian, that old ancient crater terrain that's up at the highest elevations and finally, Pre-Noachian, we don't really see any Pre-Noachian terrain, because it was all obliterated by the large impacts. So let's look at Mars, and think about these different time periods and see if we can reconstruct what happened. We're going to use my favorite data set. This is that website, Jmars.mars.asu.edu that I showed you about before. And this is the colored MOLA topography once again, and we're going to start looking first at Pre-Noachian terrain. because, what is Pre-Noachian terrain? It is giant impacts. What do giant impacts do? They make big basins. Here's the Hellas basin. Is one of the largest late impacts that occurred on Mars during the Pre-Noachian. And the biggest impact that occurred on Mars, that still leaves a signature, is probably this entire northern basin. If you look at it from a North Polar view, instead of just from the side view, it looks very much like a big crater basin. And simulations have suggested that it really was just a giant impact carving away the crust out of the Northern Hemisphere. So we'll put that down for the Pre-Noachian, as the Hellas space in this large scale global dichotomy. Okay, so what happened with water during the period of the Pre-Noachian? We have no idea. This is the period where water was presumably being delivered to Mars, by some of these giant impacts, out-gassing of water from the materials that went into making Mars. But given that no geology survives from this period, we really can't say what was going on with water. The first surfaces that we have intact, are the Noachian surfaces. And if you remember, the Noachian surfaces are these heavily impacted highlands in these regions. In fact, we're just going to take a random look at this region close to Hellas basin and zoom in on it. It didn't actually choose anything in particular, and we will see, because it is so common in these regions, as we zoom in you'll see that these regions are dissected by drainages. Look at all of these drainages, everywhere you look. These are dissected by drainages. And if you look at even higher resolution than we can do with this, look at that cute little one over there. If you look at even higher resolution, then you can, with this image, you see that many places are completely covered with these drainages, not just sporadically, as you can see here. Let's look right next door to where we were just looking, and again, you see these channels all over the place and what you also see, are these eroded impact craters. You see some very fresh looking craters with nice crisp rims, central peaks. But you also see a lot of these things that look like there's a crater there too, and there's a crater there too clearly, but they have been eroded away. There's a nice eroded crater up here at the top. And sometimes you see these eroded craters often have things like drainages going into them. Sometimes drainages coming out of them this one looks like it has a drainage coming out and going into here, but ending there. Here's another case where you have drainage going into a crater, not too badly eroded and then the drainage coming out of the crater again. And going out into what is best described as just an alluvial plain of some sort. The fact that there are so many river valleys that the Noachian terrains are highly dissected. The fact that the craters are eroded. Some of the craters are eroded, I should say. And the fact that many of craters are intersected by these valleys, and sometimes the valleys go into the craters and don't come out, sometimes they go in and do come out. It's all of this is a signature of long-term precipitation, but and here is an important point, not a lot of long-term precipitation. It's not enough, for example, that would hit the developed well-drained river valleys. I'm going to zoom out here. And you'll see that, unlike, say on the earth, if you looked at a place where there were drainages and river valleys, you would have things draining into this huge basin over here. This may well have been a large lake, in the Noachian. But it sure doesn't look like there were any streams that spent much time flowing, and eroding a shoreline in through here. If you look down on the other side of the Hellas basin, you certainly see some things that look like they are flowing into it. And certainly down through here small scale things flowing in, but no large scale rivers, no giant drainages that developed over the entire region. Zooming back out again, one of the things that you do see about the drainages in the Noachian, is that they drain if we look at the drainages over here, for example, on the edge of the northern plains. You can see that they are, even in the Noachian, they are draining in this direction. Even though they're a little bit hard to see, they're here. Not major drainages, but certainly drainages in through there. The other direction that you see things draining at the same time is away from the big Tharsis bulge. It's showing you that the Tharsis bulge was forming or formed at the time of the late Noachian, at least. And let's look in here and find some Noachian drainages. Which you see by zooming all these small-scale drainages in through here. They all are going down away from, there are also cracks, of course. So these small-scale drainages are going away from the Tharsis region, and they're flowing on the southern regions either into this big depression or of course the even larger Hellas basin. But the Noachian also has, I showed you, degraded craters. Let's go back to this middle section that you can see really well, of the Noachian. We see some degraded craters in through there. Here's some more of these degraded craters, a lot of degraded craters, slow channels outflowing. But you also see some very fresh craters in the Noachian, these craters have not been eroded. Something happened from the beginning of the Noachian when these craters got eroded away, to not much later. When crater is formed and did not get rerouted. So, I think it's safe to say that in the Noachian, there was precipitation and the only way that you're going to have precipitation was to have a warm and wet Mars. Warm and wet Mars from maybe 400 million years during this Noachian period, it's not a super long period of time, but it's enough to have left significant imprint on the surfaces that we see. Of course I'm just looking at the geology right now, but as you remember from the mineralogy for the geochemistry, that these Noachian surfaces, these Noachian craters, could well have been filled with water at the time, would have been filled with water. They're closed depressions if it was raining, if the rivers are draining into and there is evidence in many of these craters for clays. For things where rocks interacted with water. The story for this period of Mars is pretty consistent. We'll call this precipitation. To be clear though there's still considerable uncertainty about what exactly this precipitation and wet warm Mars might have actually been like. If you were paying close attention you probably would have noticed that John Grotzinger, implied that perhaps all of Mars was warm and wet and went through a long period like this. Bethany Ehlmann, on the other hand, suggested that much of the clay formation that she was seeing took place underground in hydrothermal systems. And that maybe there was a little bit of precipitation now and then. But it was local and short lived. These are the sorts of questions about the Martian climate of the past that scientists are still trying to answer today. What else happened in this time period? Well, we know that the Noachian terrains have the remnant magnetism, the Hesperian and the Amazonian, the magnetic field shut off around this time period. Was this the final straw that led to the removal of the atmosphere and led to the collapse at the end of the Noachian? Unclear, it's also a possibility that impacts occurred in this late, heavy bombardment period that blew off some of the atmosphere. Whatever the case, by the Hesperian, there's very little evidence for any sort of of wide scale precipitation. Instead, the Hesperian is the time of these massive outflows. So remember that Tharsis had already been forming by the end of the Noachian, but it must have really gotten going in the Hesperian. You certainly can tell by the ages of the craters, at least on the surface, that a major building occurred in Hesperian. And as the major building occurred, you can see that it had a dramatic impact on an entire side of the planet. Cracks and ridges forming throughout here. You can imagine that as this giant bulge started forming up and cracking surfaces across here a gigantic crack developed across here. And perhaps magmas were in contact with, what might have been ice, what might have been pressurized water in groundwater systems. Now, there is no longer precipitation, but there still would be groundwater in through there. And occasionally, those groundwater systems would break through. Here's a particularly beautiful example of an outflow channel. You can see how it goes all the way up here to the North, this outflow channel. And where does it start? What does it come from? Well, the answer is a little bit of nothing. It comes from this wall right here, and this material that looks like it perhaps collapsed. The idea here is that the ground water was released, either through melting or through depressurization. It went exploding out, and then the surface collapsed on top. And you can see that similar sort of process, not exactly the same. But that similar sort of process looks like it might have occurred at the top of some of these other valleys. Like where did this large outflow happen? Maybe it's associated with this. Maybe it's just the side of this wall. But of course it led to that biggest outflow on Mars that we looked at in detail that goes out through here and flows n out through there. Notice most of these giant outflow channels are associated with Tharsis. It's a pretty good clue that something about the volcanism, something about the heating of the groundwater was to blame for cracking things open and spewing the water out through there. [INAUDIBLE] itself drains into the northern plains out through here. So spectacular events that occurred, and these spectacular events there's evidence that that recurred repeatedly. There's evidence that significant amounts of water flowed out through there. And, if significant amounts of water flowed out through there, the northern regions would have been inundated and there would have been seas in the North. Now, those seas might not have lasted very long. Remember, we're in the Hesperian now. It has stopped raining, because Mars is getting colder. And so, if these outflow channels go into the northern oceans, they might freeze over very quickly. So you might have a northern ocean, but really just a remnant ocean that's frozen on top and slowly dissipating. The Hesperian is this period of evaporation. When you look at the craters of Hesperian ages, and you look at their minerology, and you look at what's happening there, you see those sulfur rich deposits, those sulfates, like that we're seeing at Meridiani Planum. The sulfates are the sign of evaporation, the sign of large playa, these lakes that are going away and leaving behind their salts. These things that were once clays that were being nicely developed by precipitation and rock interacting. These things are now evaporating away, leaving dry, dead salts. By the end of the Hisparian, that's sort of it for the history of Mars. Now, it's not totally it for the history of Mars. We still have the obliquity changes that change around the polar caps. We have the formation and evolution of glaciers and glacier deposits all the way down to the equator. But really, all of that is sort of small-scale moving around of the water on a mostly frozen dead planet. So to answer the question that we started out this beginning part of the class with, where is there water on Mars? When has there been water on Mars? The answer is there's always been water on Mars. A better question might have been in what form and what location is the water on Mars? There is water on Mars today locked away in glaciers underneath rubble in the equatorial regions. There are the polar deposits. Large amounts of water in through there. That's really it for today. There's groundwater or perhaps permafrost under a lot of the high latitude regions too, but at current margin temperatures and pressures, not much happens with all of that frozen water. 3 billion years ago, though, there were these periods of massive outflow. It was the end of the water period on Mars, though the water was evaporating away out of craters and out of canyons, and leaving behind these salty deposits. And yet every once in a while massive floods would race across the planet, probably caused by the interaction of magma with the water table or the ice table. Go back in time, another 700 million years, and we get to the period of time where Mars would have been the most earth like that it would have been today. There was precipitation, clearly. There were rivers, there were lakes. There wasn't a lot, it wasn't well-developed river systems, they didn't drain into large-scale basins. It looked more like the the Western United States. The Western United States has things like great salt lakes, death valley, these things where water goes in and eventually seeps in to the water table or makes lakes, but it doesn't go to a large scale ocean system. So we say Mars, had a warm wet period at this point, but it was kind of warm and kind of wet, but compared to now, supper warm, super wet. Even earlier, well, who knows what happened even earlier. This is when Mars was getting pelted by large things. Even some so large that almost broke Mars apart, like the one that made the northern impact basin. What are all the implications of all this water? Well, when we get to the final unit of this class, we'll actually talk in detail about if you were someone who were looking for life on Mars, where would you go? How would you do it? I think you have a bit of a clue right now. Is that if you're looking for evidence that something evolved on the surface of Mars and you were trying to land somewhere, you might want to go to something in the Noachian. You certainly don't want to go to the Amazonian, where everything is cold and dry and dead. Hesperian, big outflow channels, maybe there's still some stuff hanging around. You can go back into this Noachian period. It was the most, like certain periods in the Earth, that's a good place to go start.
