In the last lecture we talked about absorption in the Earth's atmosphere and how that affected our choices of where to look through the atmosphere if we were astronomers sitting on the ground. But there was something else about that, that was equally important. Remember I told you that all of these individual absorptions in the Earth's atmosphere are due to specific things. Most of these over here were water. There's the CO2, again, water, CO2. CO2, little bit of ozone in there. It's pretty obvious that we can reverse this process and say, hey, let's go look in the atmosphere of another planet for these same sorts of things and figure out what the composition of that atmosphere is. And in fact in that same paper by Sinton and Strong that I talked about in the last lecture, they did just that. They didn't just measure the total amount of light in this region around ten microns, they actually measured the spectrum across here and realized that, indeed, they were seeing absorption due to CO2. With the detection of CO2, you might love to also know, for example, is there water? Can we detect water in there? The problem with water, particularly in any of these regions and through here, is that the water absorptions in the Earth's atmosphere is so strong. There's so much water in the Earth's atmosphere, that there's no way to see if there's water in the atmosphere of Mars, because where would you want to look, I kind of would like to look right here. At a round three microns. Right here around three microns, not a single photon makes it. So what are you to do? Well, one alternative is to find weaker lines of water which are not these big broad things like this but are individual small regions where there is perhaps one very narrow absorption line. Now you don't want them too weak because if Mars doesn't have a lot of water and you find a weak absorption line, then in Mars, they'll be very little absorption. It'd be hard to detect. So what you would really like is a moderately strong absorption line that is narrow. The problem is, of course, if it's a moderately strong absorption line, then you're not going to get any light through it. There is a nice solution though. And that solution is caused by the orbits of the planets. Again let's put the sun here in the middle. Here is the Earth going around like that, Mars let's put it right here. If the Earth is right here and you are looking at Mars, the velocity between the Earth and Mars Is zero. You're not getting closer to Mars or further away from Mars, the velocity there is zero. But what if you're over here instead? You're moving this way, you're actually moving towards Mars. What if you're over here? You're moving this way, you're moving away from Mars. If you remember, this was called opposition. These two positions are called quadrature, and you can sort of see that why it's quadrature. Very nice thing happens because of this velocity of quadrature. If the Earth is moving towards Mars or if the Earth is moving away from Mars, the spectrum of Mars is blue shifted or red shifted by a small amount. It's only a very small amount. The velocities here we're talking about are something like 30 kilometers per second. And you can figure out the fractional doppler shift, which is the change in wave length of the wave length is equal to the velocity over the speed of light. Velocity, 30 kilometers a second, speed of light is 3 times 10 to 5th kilometers per second. So that's equal to 10 to the minus 4, delta lambda over lambda. So if we were looking at say, 1 micron, the shift in wavelength would be 10 to the minus 4 microns. Remember that last plot I was showing you went from one to 30 microns. Think of one part in then thousandth of a shift of that. That seems a little bit crazy but it's not crazy. Telescopes can easily resolve wavelengths as narrow as this. What it really means is that you need to have a spectral line and absorption line that is narrower than this one part 10 the fourth of its wave length so that the absorption light on Mars is shifted away from it. And this happens not in the regions that we were just looking at in the infra red where these big broad strong absorptions lines but in the in what astronomers today call the optical. Although astronomers back at the time called tit he infrared. It's just beyond what the human eye can see. And the wave lengths are something like 0.8 microns. And again, we're looking at shifts of something like one part in 10000. Again that seems like that might be a little hard but it's really kind of a big shift. The first detection of water vapor in the atmosphere of Mars was announced in a paper by Spinrad, Munch, and Kaplan's ApT. Links to all these papers are on the website so you don't have to go track it down. And it's an interesting paper to me because first author on this one is Hy Spinrad. Hy Spinrad is best known for his studies of very, very distant galaxies, but for much of his career, he also studied comets, sort of on the side. And early in his career, before I was born, he studied Mars. And I got to know Hy Spinrad when I was a graduate student. Hy Spinrad was my PhD advisor. I went to graduate school to work with Hy Spinrad on distant galaxies. Ended up working with him on comets because that was his second love. And then kept on going into the solar system. Hy Spinrad, however, was the first person to give a solid detection of water vapor on Mars, and I'm going to show you this solid detection now. Spinrad and collaborators used one of the other large telescopes at the time, the 100 inch Hooker telescope on Mount Wilson. And Mount Wilson is just right here I'm actually looking at it right now and if I were to go up to the top of the building I could actually see some of the historic telescopes out there on Mount Wilson. Spinrad used that telescope on Mount Wilson specifically at quadrature so that the Doppler shift would be just right that he might be able to detect the water from Mars, and here is the best detection. This big deep absorption this is transmission through the atmosphere. Think about it he looks at Mars and he sees a deep absorption line right here. This is absorption in the earth's atmosphere, the symbol for earth is that. So you often write it this way, this is absorption in the earth's atmosphere that we knew about. Where is Mars? Mars is right there. That little Doppler shift is exactly the amount predicted If there is water vapor in Mars and you are looking at the spectrum of that water vapor. Now if it were just that little divot right there, it might be not much to write home about, but there were six or seven other locations where they could see that and they were pretty convinced that that's they're seeing. Let me just show you now blown up to very high resolution what the transmission through the Earth's atmosphere looks like at this point. Remember I said in the visible part. Well this is 0.8 microns, this is a little bit beyond the visible part. But in the visible part, it's mostly completely transmissive. But as we get out here into around 0.8 microns, you start to see some absorption, some absorption, some absorption. Then a whole forest of these little tiny absorptions just like the type we were looking for and these are all, almost all due to water vapor. And these are the ones that Spinrad and his collaborators were going after. So let's look specifically at this tiny little line right here at, this is at 8189.27 angstroms or as I keep saying 0.818927 micron. So it's a tiny tiny regimen spectrum. Okay, so here's a modern spectrum not of Mars but simply of the Sun. The meaning that looking at the sun through the Earth's atmosphere you're really getting in these regions a spectrum of the transmission through the Earth's atmosphere. And here is that line 81 89.27. This is absorption due to water. In fact all of these lines are absorption due to water. And the nice thing that you see is that in the real Earth atmosphere. It's really smooth up through here. And then there is that little tiny divot there, that little tiny divot there, is right there and there a couple of other little things. You don't see them over here, but in the Earth's atmosphere, looking just at the Sun and not at Mars, you don't see this. I have to say that looking at all of the data from that initial Spinrad paper, 1963, I might have been pretty skeptical. They went on over the years to continue to detect the water, and in fact, to really detect the latitudinal variation of the water. And also the seasonal variation of the water and these days this line of water is fairly easier to detect with modern telescopes. And so this really is the very first detection of water of any sort on Mars. I think you can tell though that there's not very much water. If this is the earth atmosphere and this is the martian atmosphere it's pretty clear that mars is not like the earth in the amount of water vapor it has. In fact Spinrad estimated that there is something between five and ten microns of precipitable water. What does that mean? That means if you take all of the water out of the atmosphere and collapse it onto the surface, the total layer of water on the surface would be five to ten microns, micrometers. That's a tiny, tiny, tiny thin layer of water on there. The earth, for all of its water that it has, has more like millimeters of precipitable water in the atmosphere. Still doesn't seem like much, but it's something like a factor of 1,000 more. People have been pretty clear, from just looking at Mars, that there were no global oceans on it. But the very small abundance of water vapor in the atmosphere made it pretty clear that there aren't large reservoirs of water that are openly communicating with the atmosphere too. There's one last hope of a holdout for a major water reservoir on Mars, and that was of course, the caps. We saw those white caps, people saw them come and go with the seasons and what else could they be? And in fact, Spinrad and his collaborators found that the presence of polar caps was correlated with the appearance and disappearance of water vapor. When the polar caps were bigger, there was less water vapor in the atmosphere. When the polar caps were smaller, there was more water vapor in the atmosphere. It seemed likely to many people at this point that those big, white, polar caps must, of course, be a huge reservoir of water ice on that planet. We'll examine that idea in detail in the next lecture.
