So far, we've used some very tangible evidence to look at the presence or the possible presence of water on Mars in the past. We've used things like images of drainage systems in canyons. We've used measurements of elevations that show where things would have flowed into. As missions to Mars have gone on, increasingly sophisticated instrumentation has flown to Mars and investigated these questions in even more detail. One that I think is really important but takes a little bit of explaining is the use of gamma ray spectroscopy and neutron spectroscopy. These aren't things that most people are intimately familiar with or find intuitive right away, so let me take a second to explain how they actually work. Okay, well here at least is NASA's attempt to explain how it all works. The process starts when the surface of a body is hit by these cosmic rays. What are cosmic rays? You probably heard the term cosmic ray before, and maybe have not really thought about what it means. Cosmic rays are very high energy ionized particles that are coming streaming from space. From the sun, from the center of the galaxy, from many other places, and they are penetrating through everything all the time, including your head right this very minute. Cosmic rays are the sorts of things that maybe can cause long term damage to your DNA by breaking apart some of the, the bonds, and mutations. I see cosmic rays a lot when I use a digital camera to take pictures of the night sky, and if you, you look very carefully at those dark regions of the night's sky, you see, you see bright spots every once in a while, and those bright spots were cosmic rays that have gone through and lit up the detector, not the sky, but the detector and that's bad spot. Cosmic rays are sufficiently energetic that they can hit a nucleus of an atom and knock off a neutron. So a lot of this that we're going to discuss here, we're going to talk about the, the nuclei atoms. And we often think about atoms, they're a bunch of neutrons, bunch protons, and surrounding all of these are a bunch of electrons at different energy levels. And most of the time when, when we think about the interaction of these atoms with radiation, we think about things like electrons changing energy levels, or electrons getting lost and ionization occurred. When an electron changes energy levels, a photon is emitted, and the photon has a wavelength of that particular energy level. Same thing happens within the nucleus, that we don't usually think about it this way unless we're nuclear physicists, but protons and neutrons also have energy levels. Protons and neutrons can be removed from the nucleus, and when that happens, and when protons and neutrons change energy levels, they emit photons. The photons that they emit in this case are extremely high energy photons because these things are bound very, very tightly. And these are gamma rays. There is the symbol for gamma right there. These are gamma rays that are emitted when these processes occur. Okay, so again, let's restart. Cosmic ray comes in, hits a random nucleus, we don't care what it is, and it knocks off a neutron. This neutron goes blasting around and a couple different things can happen to this neutron as it goes blasting around. First thing that can happen is a collision with another nucleus and when this collision happens with the other nucleus, first the, the neutron bounces off essentially but as it bounces off it excites the nucleus to a higher energy state. Nuclei don't like to be in high energy states for very long. So that energy state decays back down. Boom, the gamma ray is emitted. The gamma rays that are emitted through this process, just like the photons emitted when electrons move in energy states, the gamma rays that are emitted are very distinct for each nucleus. So by looking at the gamma rays that come off, you can not only tell that things have been happening, but you can tell exactly what the collision was that occurred down here. Another thing that can happen is, after a few collisions and it slows down a little bit, the, neutron can actually get captured by the nucleus and again, the proton, gamma ray photon, will be emitted. The other way to get gamma ray photons is from radio active decay. Gives off these gamma ray photons, you have to figure a way of figuring out which is which, but these come at specific energies also. So this is the gamma ray half of the gamma ray spectrometer. But remember there's also a neutron spectrometer that is that you actually don't just watch the gamma rays come up, you measure the neutrons themselves. How does that work? Again, you have the cosmic ray collision here and again a nuetron can go blasting off and sometimes, that neutron can simply escape the surface without interacting with anything, when it does it will go off into space. It can however have this series of collisions like we looked at over here and looked at over here, and these collisions slow the neutron down until it is finally going at a speed that's similar sort of to the background speed of neutrons, the temperature, the, the thermal energy of that surface. So, if you were looking at neutrons coming off the surface due to these ga, cosmic ray impacts, you'd see fast neutrons, and then you would also see a series of much slower neutrons coming up through here. One of the really important things that we can measure is, the ratio of fast neutrons to slow neutrons. What is that ratio going to tell you? It's really going to tell you how much slowed down these neutrons get. Why do we care how much slowed down the neutrons get? Well for one really good reason which is the thing that slows down neutrons the best, is hydrogen. Why is that? Well neutrons can hit any thing that's in there, any, any nucleus that's in there, but if it hits a heavy nucleus a silicon nucleus, an oxygen nucleus, things that are fairly common. If it hits one of those very heavy nuclei, it just bounces off with the same speed. It gives a little bit of speed to the other nucleus, but not much happens. It's like taking a superball and throwing it at a bowling ball. That bowling ball doesn't move. Super ball just bounces off at exactly the same speed. If the neutron however hit something that it's about it's same mass, now imagine taking your super ball and throwing it at well, a super ball. What happens? The super ball bounces off sure, but that other super ball comes off at about the same speed. The overall energy of the super balls has been decreased. And then, each of those is going to collide and turn, and it's going to create this thermal cloud of hot neutrons, but not these very fast neutrons in through there. So, the amount of cooling, in through here, depends critically on the amount of hydrogen, in particular. What has hydrogen? Well I'll give you a hint. You can drink it. Although not in the case of mars, cause it's going to be mostly frozen. Let's look first at some of the gamma ray data that came down from the very first science paper that was written showing what the gamma ray data looked like, and here you see an actual spectrum, gamma ray spectrum, energy in the kiloelectron volt range. That's very very high energy photons. And you see three spectra from collected gamma rays over different regions of the planet. First, you see the North Pole, and basically, you see a little bit of nothing. You see something right here for aluminum, and a little peak right there where hydrogen should be. And maybe nothing else. That's North Pole raw of 75 degrees latitude. The middle, nothing, nothing, nothing, maybe a little bit of hydrogen right there, nothing, nothing, nothing. And the South Pole, south of 60 degrees, nothing, nothing, nothing, one big signal in through there, and, and out. Ton of hydrogen emission. Gamma rays that came from hydrogen being emitted at the South Pole. It's important to know that the gamma ray spectrometer is only sensitive to the first, maybe, meter of the surface. It doesn't penetrate any more deeply than that. And this was northern winter. So the North Pole, had a CO2 frost covering the surface of the otherwise water ice cap. So you don't see the water that we actually know is up at the cap there. But in the south where there's no CO2 frost you see a ton of that. Let's compare now the strength of that gamma ray emission as a function of latitude on the planet. And you'll see just as I was talking about, here is the gamma rays on this points here. Ton of gamma ray emission from that hydrogen line, starting at something like 45 degrees south. Sort of flat across through here, and the beginning of a rise and then a big drop. This big drop is because this is where the C02 ice cap is, frost cap is. But it was starting to go up again perhaps it might continue up like this. And in fact, at later seasons it would seem that it does continue up like that. Let's compare that now to the neutrons. Now, remember what the neutrons do. The neutrons are attenuated by hydrogen nuclei. They are slowed down by hydrogen nuclei. So if you look at the number of these thermal neutrons, and epithermal, don't worry about what those two mean, but they're not the fast ones. The fast ones just come straight out. These are ones that have been cooled down by interaction with hydrogen, and look what happens. Relatively flat, relatively flat. And then at the same place, this goes up, this go plummeting down through here. Same place, this starts to go up, this goes plummeting down, and then who knows up through here. This is again the problem with the polar cap, and again, observed over many different seasons. You could see that there's plenty of water here. You can take these measurements, and now turn it into a map. We call it water on Mars in the upper meter of Mars. But it's really of hydrogen atoms in the upper meter of Mars. And it looks like this. Again, favorite regions. Here's the Tharsis bulge, our favorite three volcanoes. Here's Valles Marineris. So now you sort of know where you're looking. Here is the northern plains, southern highlands, and a ton of water from sort of 50, 60 degree north all the way up. A ton, by a ton let's see what this means. This means something like greater than 30% of the material in this layer. 60 degrees. Greater than 30% of the materials is hydrogen, is presumably water. Down here again 60 degrees. There is a lot of water. In the crust, even in these regions, in through here, which are never covered in polar caps, never, never get very cold at all. We still see regions with, sort of 8-10%. All these blues, 10% water. 10% in through here, in through here. Some of the areas are very dry. These 2% values. Some of these high lands in through here. Some of these equatorial regions. But, there is a lot of water, a lot of, some type of water in the upper meter of the surface of Mars. What type of water is it? Well, it's not liquid water. The upper meter of the surface of Mars is still too cold for liquid water to be stable. The answer is probably some surface ice. We're used to the idea that the north polar cap is a big mound of ice, but we're not used to the idea currently, that this ice extends all the way down through here into these regions, and that, that some of that subsurface ice could even be equatorial. Is it true? Well, how would you find out? I will give you two ways that it was found out that, yes, indeed, this is true. First way, fly to a polar region dig, see what you get to. The phoenix lander landed at about 65 degrees north in this region here, in one of these very blue areas, where there's something like more than 30% water in the upper meter, and it landed and it dug, and here's what it saw. When this picture came up from the Lander, scientists involved hemmed and hawed and argued about the different possible things it could possibly be. But I tell you, you don't necessarily have to argue too much, you can look at that and, if your first guess is that it looks like ice just below the surface of this dusty cover service, your first guess is right. Let me zoom in a little so you can see a little bit better. I mean just looking at it right here you can see regions that are, that look like hard scraped snow in through here and even a little bitty layer of ice that is maybe even melting the thin part through here melting through here since it's been exposed, that's water ice and it is just below the surface over a big chunk of the planet. I promised you I would show you two, here is another way we know how extensive that water ice is. We have enough space craft in orbit around mars these days staring down that we can find things like fresh impact craters, regions where we had images and there was no impact crater, and come back and boom. There's a brand new impact crater that wasn't there before. Here's a couple of craters that are curved at about 45 degrees north latitude. 45 degrees north latitude is not that region where you're in the 30% water, you're in the ten's of percent, not very much necessarily in that first meter, but slam a meteor into the surface and what do you see? You see these bright spots right here, these are the first images, these bright spots. The crater is right here, the bottom of the crater has these nice bright spots. What are they? One guess. Yeah, look, look what happens over time. This is about a three month sequence and, at the end of three months, you see the craters. You don't see those white spots at all anymore. You have seen that they've faded through time, like this. And, that's because, the ice that's in the bottom of these craters. Ice is not stable on the surface, it evaporates. But if you can cover it with a little bit of dust, that ice is stable. So the gamma ray spectrum graph, the neutron spectrometer and these other sorts of images have given conclusive evidence that sub surface ice is pervasive, even at moderately low latitudes, on Mars. In the next lecture, we'll look at some evidence to see really how pervasive that subsurface ice is.
