[MUSIC]. Welcome to this third session on exo-planet atmospheres. In this session we will learn about the chemical composition of these atmospheres. Let's first have a look at stellar atmospheres. In this case we will have mostly atoms and ions, individual atoms and ions. Because of the high temperatures that that you will find in, in Stellar atmospheres. So molecules disassociate and only atom, and new atoms and ions are there. While the situation's quite different for planets, which are mostly at lower temperatures. In this case most of the matter is in the form of molecules usually. And so it's a completely different chemistry of course. And it's a bit more complex, to, to say the least. In particular, you can have not only molecules, but also condensates that will form hazes in clouds. And as we will see, this is also a major component of exoplant atmospheres. And not only is easy to interpret and to model. How do we know which molecules we'll find in exoplanets? Well we can start again from the concept of thermodynamic equilibrium. You can extend this concept to the chemistry of the matter you are interested in. And here you will have a similar concept of chemical equilibrium that applies, and that dictates which species, which chemical species are the most abundant, under given temperatures and pressure conditions. But we still need a starting point, we need to start somewhere. And the starting point would be the elemental abundances that you will find on average in the galaxy. Which correspond roughly to the composition of the sun. And which is expected to also be the composition of the proto-planetary disk from which the planets formed. So you could take the sun, you could take other stars in the solar neighborhood. The, the, the main pattern of elemental abundances won't change that much. So, let's now have a look at solar composition. In this plot here, you will see the abundance of each element as a function, as a function of atomic number. And I would like to point out that the vertical axis is on the logarithmic scale. So differences in abundances are here on an logarithmic scale. So this means they are, there are huge differences from one level to the next. If we first look at Hydrogen and Helium, the two simplest elements, you will see that they are, of course, by far the most abundant in the universe. As in the cell and also it is the case in giant planets. The next most abundant elements, are the trio carbon, nitrogen and oxygen. That's are on average something like a 1000 times less abundant than hydrogen and helium, but that we will still find in large amounts in exoplanet atmospheres. And then you, you see that elements decrease quite fast with automic number. With a very specific patterns that are the result of the, of how elements are produced in stars that shape these, these plot actually. Now if we try to move further, to understand what molecules we can find in exoplanet atmospheres, it's not an easy task, because chemistry is very complicated. I tried on this plot to summarize the situation. On this plot we'll show you on the X axis the expected or the likely abundance of some species. Not in the sense of being quantitative, but more in the sense of getting a qualitative understanding of which species we can expect to find in large amount, and which species will be there only as trace gases. So that's the meaning of the x-axis. On the y-axis, I put temperature. Again, it's not a quantitative measurement of temperatures, more it's to be understood in a more qualitative way. Now, following what we saw in terms of cosmic abundances. We can expect that molecule hydrogen and helium will be the most abundant species in an exoplanet atmosphere. And this is indeed true, under one condition. One condition is that the Exoplanet was able to accrete these gas from the protoplanetary disc, but also to retain it, to keep it. We know that planets which do not have a high enough mass will lose to space, hydrogen and helium. Because these, these atoms can just escape from the planet. And so we'll have a big pictures were giant planets will have a major constituent hydrogen and helium, while tinier planets like earth, mars and venus won't have any hydrogen or helium in their atmosphere. So in this case the major constituents will be completely different, of course. But based on the cosmic abundancies patterns that we've seen before, we can still say something about what should be the most abundant molecules in general. And this is what I tried to represent here. After hydrogen and helium, the most important molecules you can expect to find in the atmospheres are the ones that are highlighted in red here. Mainly water vapor carbon monoxide CO methane at lower temperatures and so on. Why is that? Just because this molecules would, would be the, the main bearers of oxygen, carbon, and nitrogen. Which are the most abundant atoms after hydrogen and helium. So that's why we can exect in general to have a lot of water vapor in exoplanets. Now depending on temperature, you will find different species. For example carbon you will find carbon in in different forms, whether you are in a high temperature atmosphere, or a low temperature regime. in, in high temperatures like on close in exoplanets, you expect carbon to be in the form of carbon monoxide, CO. While on cold atmospheres like Jupiter, or Saturn, in our solar system, we would expect carbon to be in the form of methane. [BLANK_AUDIO] So why are, why is the knowledge of chemical composition so important? Well, for many reasons, but one of the main reasons is that these molecules are the main sources of opacity in the atmospheres. So, knowing the abundances and the cross section of these molecules will allow us to solve their relative transform equations, and compute the emergent spectra and then we'll be able to compare observations to models. So what are these opacity sources? Well they are, of course, the electronic transitions in atoms and molecules, where you would have an electron jumping from one energy level to the next. By absorbing a photon, and then you could have this photon re-emitted when the action gets back to its original energy state. But of course you also have now many more transitions in molecules. These are the rotational vibrational transitions. Molecules have many other degrees of freedom compared to an individual atom. Molecules are free to rotate and to vibrate. And these are also quantized states of energy, which will generate hundreds of thousands, or even millions and billions of. Different transitions that will be sources of opacity in atmospheres. And then we also have some continuous opacity sources mostly Rayleigh scattering. Which is the scattering of light of electromagnetic radiation when incident on particles that have a size much smaller than the wavelength of light. Light for example, hydrogen molecules in gas chained atmospheres or oxygen and nitrogen in the Earth atmosphere, which explains why the sky is blue. And there is also a nation source of opacity which we call Mie scattering which is scattering but this time on condensates, particles that are larger than the wavelengths of light. Typically hazes and clouds, which will scatter light. So finally we can move on to a look at more details what these opacity sources look like as a function of wavelength. In these two graphs here, we are showing the molecular and atom cross sections, absorption cross sections, and also sketching cross-section in some cases. For the major species that we can expect to find in atmospheres. These are quite crowded plots of course. But we can try to look at some of the major features that are interesting to us here. On the left plot you can for example follow the opacity of water vapor in blue. And you will see that there are several bands of absorption. These are these rotation vibration transitions, that span the whole near infrared region of the spectrum. So you can expect to see spectral signatures of water, in larger quantities in this spectral range. You can also see some atomic cross section like the one of sodium and potassium. These are the two the two very high opacity's that you can see on the left of the plot. Because these have very strong absorption lines in the visible region, you can expect to see sodium and potassium in the spectrum of exoplanets. Because with such an odd cross-section even a very tiny amount of these molecules, with these atoms by the way, will produce a detectable spectral feature. If we look at the right plot there the wavelength range extends more to the red to the infrared. And this plot is useful to see for example the opacity of carbon dioxide. That's the brown curve that, that you, where you see relatively big bump on the right of, of the plot, at around 15 micron of wavelength. Where a carbon dioxide is actually very opaque I mean the cross-section is very, very high there. And that explains, for example, the green house effect on earth. That's where carbon dioxide will absorb the thermal light radiated by the Earth's surface. And will prevent these lights from escaping to, to, to space. It will re-radiate it towards the ground which will trap heat at the surface of the Earth, and increase the Earth's temperature by some degrees. So now, now that we have a better understanding of all these opacity's and chemical composition in the next session we move on to study in more detail the temperature profile of exoplant's atmospheres. Thank you. [MUSIC]
