an atmosphere is an envelope of gases around a body and so that body in our solar system can be almost anything at this point we normally think about it is being the envelope of gases around a planet like our own planet the earth we also see envelopes of gases around most of the other planets in our solar system so there's an atmosphere around mars there's an atmosphere on jupiter it's actually more common to have an atmosphere than to not have an atmosphere there's very few planets that don't mercury would be one exception the earth's moon doesn't have an atmosphere but there are moons in our solar system that do have atmospheres the one that's most commonly thought of as titan which is a satellite around saturn that actually has a thicker atmosphere than our planet earth and even planets the very smallest planets in our solar system the planet that was demoted to a proto planet is is pluto and that actually has an atmosphere around it as well so one of the critical elements of having an atmosphere is having nuff mass of the planet or the moon to actually keep that gas around that body and so what we observe in our solar system is the very smallest bodies things like asteroids and comets don't have what we would know or think of as an atmosphere and that's because their gravity isn't strong enough to hold those gas phase molecules near the surface of the planet when you have larger bodies like planets like earth size or moons the size of titan those planets have enough gravity that they hold molecules near the surface of the planet this is also an interesting thing which is that the gases that make up atmospheres actually have very different masses so you can imagine a gas that is itself a pretty heavy molecule and  those things are held by the gravity closer to the surface of the planet on the other hand you can have very light gases things like hydrogen and helium which even when you have earth like gravity can escape into space they're light enough and they  moving fast enough that they wander up towards the higher reaches of the atmosphere and then they can actually make it into space so depending on the mass of your body you can actually have different atmospheres different compositions about misfiring you can even have stratification of having different molecules closer to the surface and different molecules further out from the planet depending on that gravity and the mass of the molecule that makes up the atmosphere there's a number of gases that make up an atmosphere in our solar system in the very from planet to planet and one of the reasons for that is that the temperature of the different planets changes in general planets are warmer as they get closer to the sun and they get colder as they move out from the sun and so you can have certain substances that when it's very cold are solids their ice is or their liquids but as you move closer to the sun and the temperature becomes warmer they can find their way into the vapor phase and they can make up the atmosphere and so there's some interesting things that this is true of so we sort of think about liquid water on the surface of our  planet which is also a vapor and a component of our atmosphere but if you move further out into the solar system liquid water would never be found it would always be frozen it would always be trapped on the surface of the planet and it couldn't actually make up the atmosphere and so there's other components that are found around the different planets as well as around the satellites the moons that aboutness fears in our solar system that this is true of as well and so if you look further and further out into our own solar system you find that you have atmospheres that have different composition so you look at  someplace like titan where there's organics that make up a large fraction of the atmosphere things that we would never think about being a substantial part of our atmosphere here at earth and they participate much like water does in our own environment so you can have precipitation of organics just like we have precipitation of water around our planet the other thing that's important to know is that atmospheres are not static they don't stay exactly the same over the course of time so the early atmosphere around earth is not the atmosphere that we have nowadays the very earliest atmosphere on the planets especially the inner ones was actually blown away when the sun gave off a large amount of radiation early in its life this is something called the t tare phase of our sun and it takes place around most stars and so on all of the inner planets was blown away when this took place very early in our solar system and so what we have around earth is something that a little bit of what remained after that but also material that found its way into the atmosphere afterwards you can think about processes like vulcanism putting material into the atmosphere in the gas phase that then remains trapped you can think about comets bringing material in that forms this new atmosphere around the planet there's other processes that take place to change the atmosphere over time so earth is a classic case because we have biology that goes on there was very little perhaps really no oxygen in the early earth's atmosphere but as life forms took over and they started doing  they started to put out oxygen and so there was an early rise in the amount of oxygen in our atmosphere of course now it's porton component and it's one that we should be thankful is there other components change over time as well and this is known not only for earth but for other planets  we have evidence that mars had a very different atmosphere early in its history it was probably a warmer planet there may have been liquid water on the surface there was certainly more water vapor in in the atmosphere of mars at that time it was thicker and then over the last many millions of years hundreds of millions of years mars is going into a colder and colder and colder state and a lot of its atmosphere from that time point has been frozen out on the surface of the planet so it's gone from being the atmosphere to being a component of the surface so atmospheres in general are very thin compared to the size of the planet so if we use or things and example the distance from the core to the surface of the planet is some thousands of miles but the atmosphere itself is only some miles or tens of miles thick  and we take that distance and we can actually separated into different layers so instead of thinking in miles we can think in kilometers which scientists more often uses as a unit of distance the entirety of our atmosphere is really thought to be within about the lower hundred kilometers roughly sixty miles of the surface and then we can break it up into a number of layers is usually about five that we think of the most important to us is the troposphere which is the lower most layer we talked earlier about gravity so most of our atmosphere is held close to the planet we have the thickest atmosphere at the surface that's why it's harder to breathe as you go up on tall mountains the atmosphere is becoming thinner the pressure is going down there's less oxygen that we're pulling into her lungs so you can see this change in our atmosphere as you move up nonetheless this lower part of our atmosphere which is about it's about ten kilometers thick is where all of the things like weather go on and so it's very important to us as human beings it's where all the rain takes place precipitation takes place and this is this is where we live the next layer up of our atmosphere in effect of all atmospheres is normally called the stratosphere in our case it's actually still an important layer of our atmosphere this is where ozone exists and ozone is very important for our own planet because it absorbs the incoming radiation from the sun that would be damaging to us so this would be like ultraviolet rays these are things that would cause skin cancer so the ozone layer exists in this part of the atmosphere the second layer of the stratosphere this is what filters out that radiation and that's a good thing for us this is also interesting lee what we worried about about twenty thirty years ago which was the destruction of ozone because of chemicals c f c s or chlorofluorocarbons that we were putting into the atmosphere that were catalytically destroying the ozone you remove that ozone you remove your filter the thing that's taking out these damaging cosmic rays you get much more sunburns and unfortunately in some instances you end up with things like skin cancer these layers are believed to occur in essentially all planetary atmospheres you can sort of delineate them into a trope is fear the slower thicker layer of the atmosphere and then you can think about the atmosphere changing in pressure in structure and so on in temperature as it moves upwards as it approaches space and you can't tell anymore that there's really gas face molecules there anymore so so the different layers have a variety of names you know in this case we weather around that planet is taking place and then a higher part of that atmosphere which is almost a borderline between the lower atmosphere and space there is a common thinking that atmospheres or our gas face substance but that they bear remarkable similarities to something like a liquid like you would of been a cup of coffee or a cup of tea it turns out they in general they're thought of as really being only a gas face substance the way that it's differentiated in science is something that's called the mean free path is how far a molecule can travel before knocks into its nabl neighbor so you can sort of think about a ball on a pool table how far does a ball travel before it hits the next ball on the table when you have a gas the distance between those things is very large it's many times the radius of the molecule and so if you have a gaseous molecule it's going to travel many many many times its radius before it strikes its neighbor when you have a liquid the neighbors are very closely packed together so you would travel much less than your own radius before you strike your neighbor your essential is sitting right next to them it's like our pool table but it's completely covered with pool balls and so although atmospheres have properties that are very liquid like waves and things like that we do think of them as being entirely gas we only think of things like oceans and lakes as being a liquid on earth we're biased towards water and so we think of our oceans and lakes and rivers  being covered in water as i mentioned earlier there are other planets and other satellites in our solar system that can have lakes and liquid it's that are on their surface that serve the same purpose but that are not made out of water and so again titan the satellite around saturn is one classic example where it has lakes and we have images of these beautiful lakes and they're not water at all of they're filled with organics that at that temperature and pressure are liquid one of the critical things to understand about atmospheres is that they're a balance and what i mean by that is that they're a balance between the incoming energy from the sun from the star that we all orbit around and the outgoing read ation the energy that's put off by our planet so let me explain a little bit further the sunlight that comes in warms the planet up but it doesn't continue to warm up and warm up and warm up that energy has to go somewhere and so what happens is a planet will warm up to some level and then it will begin giving radiation back off to space i don't mean radiation in the senses nuclear energy i mean radiation in the term of photons that are given off by that planet so earth is constantly giving off what we term long wave radiation it's somewhere in the infrared part of the spectrum whereas the sun is giving off radiation predominantly in the visible part of the spectrum that's why we can see at the wavelengths that we do that's why we call it the visible we can see because the sun is giving off energy at that level so this balance ends up being very critically important for atmospheres in the weather that goes on around the planet you have this incoming sunlight that tends to warm you have this outgoing radiation that tends to try to cool the planet off or or find some equilibrium state so it's not continuing to warm up it's not quite that simple because you can imagine that one side of the earth is facing the sun and it's being warmed the other side of the earth is in darkness and it's cooling off the poles are cooler than the equator and so there's this constant transport of energy moving around the planet so your sunlight coming in and trying to warm and the planet is trying to redistribute that energy to the colder parts of the planet is trying to move energy towards the poles it's trying to move energy towards the dark side and this is what we commonly think of as as weather so this can be in the form of wins this can be in the form of cells that set up motions in our atmosphere this can also be in terms of convection something that we all experience during the summertime where the surface is warm air starts rising very often leads to cloud formation and thunderstorms and precipitation this is all part of the weather of the planet it's driven by that sunlight that solar energy that's coming into the planet and the redistribution of that to different temperatures different pressures within the atmosphere since we talked about atmospheres evolving over time a question the comedy comes up is can we as humans do something to change atmospheres and this actually has a term it's called terra forming the idea here is the word implies is to make something more earth like and making something more earth like is often this idea that you change something about the atmosphere if the planet's too cold you warm it up if the planet's too warm you cool it down and you make it more earth like so that people can someday inhabit that planet and so this is not a new idea this sounds like science fiction it certainly is at this point but it's not something that science fiction from the last ten or twenty years it actually has an incredibly long history and you can think of the first book involved in terror forming as not being terra forming but mars forming and so war of the worlds came out in the late eighty's hundreds from h g wells and the idea here was that these aliens these beings from mars were trying to come to the earth they were trying to make earth more mars like so that they could take it over and live there happily so i guess we could call that mars forming it was sometime in the one nine hundred forty s that people started offering books and thinking about science fiction situations where we could do things like terra form other planets so that we could send colonists there and it's sort of transferred from science fiction to science fact a little bit because groups even like nasa are looking quite seriously at terror forming mars and so mars is one of our closest neighbors in the solar system we know that there's a considerable amount of water on the surface of mars and sort of frozen in the polar caps as well as in what would be like permafrost in the surface and so we've asked the question of something we can do to mars atmosphere to warm it up could we put a powerful greenhouse gas into the atmosphere so that the temperature of the planet would rise the ices would melt maybe the atmosphere with yet become thicker you could have things like liquid water on the surface and then people could run around happily group plants and things like terra forming isn't as simplistic as just getting the temperature of the planet right or having something like liquid water on the surface there's a lot more that goes into having a habitable planet than just temperature one of the things that we talked about earlier was the ozone layer around earth which is a consequence of us having substantial oxygen around the planet and so this molecule which is zero three three oxygens bound together is very good at filtering out this ultraviolet radiation is very hard radiation that the sun puts out that's very damaging to us as living beings and so if you tear or form a planet one of the things you have to think about is if you filter out these harmful things not just have the temperature able to subsisting a person maybe have liquid water on the surface but can you also have the radiation that you need or not have the radiation that you don't want to taking place and so there's a really a lot that goes into terra forming it's not simplistic it's just sort of setting a knob for a temperature of the thermostat you really have to think through all of the other things that might be involved that might be detrimental to life on the surface at the same time and so terra forming is this this concept this idea that we're actually starting to think of how we would go about doing it there are of course ethical questions about changing other planets for our uses shouldn't we get things straight on our plan first and so on at this point we haven't attempted to do any terra forming it's just the thoughts of what we could do in order to change other planets around to get them to be more earth like it isn't a huge leap to go from terra forming to changes that we want to make to our own planet and in fact we've already changed the atmosphere of our planet we've changed the climate of our planet as we add things that are known greenhouse greenhouse gases to our earth's atmosphere things like carbon dioxide nitric oxide methane we're increasing the temperature of our planet and we've actually seen this we have observations of this we've had solar satellites in orbit around our planet for the last you know forty years or so a little bit longer than that in some cases and we can measure the temperature we see that the planet is actually warmer now than it was you know forty fifty sixty years ago and so we've actually unintentionally change the climate of our planet by putting these greenhouse gases into the atmosphere there are some folks that have suggested that perhaps we do things to change the climate back to where it was before we started to change it's the nature of our atmosphere in the climate of the planet this is collectively termed geo engineering  and there's actually a number of scientists that have sort of rebelled against using this word and the reason is that engineering implies a certain level of certainty a certain level of control that's not there and so people have tried to get away from the term geo engineering and they've used things along the lines of modification modification cloud modification to acknowledge that we're very uncertain if we do something to our atmosphere what exact effect it will have in terms and so let me give you an example of that one of the common ideas would be that we could put a lot more particles into the atmosphere and so we've talked a little bit about cause and particles increasing the amount of sunlight that scattered back into space and if we were to geoengineering a planet or modify the beat of the planet the idea is that if we put more particles or more clouds into the atmosphere we get rid of some of the sunlight that's coming in and the temperature of the planet drops down unfortunately this is a bit like what we talked about when we discussed terra forming which is that it's not just setting the temperature of the planet that's important there's other factors that come into play and so those other factors are that if you start to for example put particles in the atmosphere you scatter a lot of radiation out of the  way you also are changing clouds at the same time clouds have an effect on the precipitation at the surface of the planet that's really critical because it means that you're not just changing temperature but you're changing rainfall you're going to have places that get drier going to places that get wetter and while that might not seem like such a bad thing if you're actually living in a location that gets less rainfall and perhaps your crops start failing you might not be so happy that the temperature is right but the precipitation is wrong another thing that's known to happen of adding particles  clouds to the atmosphere especially in the case of particles high in the atmosphere is that they can have a detrimental effect on that ozone layer that we talked about earlier you can search showing up that ozone and destroying it and so again you can get the temperature right but maybe you searching in that hard radiation that's causing skin cancer and again you might be happy that the temperature is ok but if you have skin cancer as a result you might not so much care that the temperature was exactly right not a direct effect of putting those particles or changing the clouds around the planet but certainly something that's not tackled is that if you allow greenhouse gases to continue to rise and you try to mask their temperature effect with particles in clouds the acidification of our ocean which is due to the increase in c o two continues unabated you continue to  the ocean at the same time there's also a number of sort of moral ethical and legal implications who controls the thermostat who decides when this is deployed when it's not deployed there's also the question of you know what ends up happening who makes the profit who decides to put these particles into the atmosphere who gets charged to do that who gets paid to do that is one country going to benefit are many countries going to benefit to spread this out over the whole planet one final thing that i mention  which i think personally is very interesting i find a lot of things about you know the so called geo engineering it's nonstarters i would i would be comfortable with making a drought in one location at the expense of another location or reducing ozone in causing skin cancer that's that's just a nonstarter for me but another interesting thing is that as soon as you start scattering radiation increase scattering you use are putting cars in the atmosphere additional particles you cut down on the sunlight reaching the surface of the planet this is critically important because that's how we generate power from things cells and so one of the ways that we get away from fossil fuels is to increase the amount of renewable energy things like solar cells if you're cutting into their margin if you're making them less and less viable less and less possible that they work putting out less and less power as a result you're actually undercutting what you need to get away from the fossil fuels in the first place we've talked a lot about the gases that make up atmospheres and in general we think about atmospheres as being the gas and below the turn on the planet but one of the things that we specialize in here is studying the other components that are found atmospheres and these are the small particles we usually call them aerosols that are found in atmospheres the other thing that's quite common are hot particles that form clouds liquid water clouds or ice clouds in the earth's atmosphere clouds can be composed of just about anything and other atmospheres this is an especially important topic today when you've probably seen out the window it's rather green and gray out in boston and so this is the formation of this new faces condense face of water in our  so particles and clouds are very important for atmospheres in that they change the radiated balance of the planet and so what i mean by that is that we've all got outside on a on a clear day and we might have a great cloud sitting over us and the club passes over the sun and what happens we fall into a shadow and we cool off as a result so that cloud has gotten between us and the incoming solar radiation and it's scattering that radiation back into space and as a result we on the planet are cooler this is actually happening all the time unbeknownst to us at any one time there's particles in the atmosphere around us over us above us all the way to the upper reaches of the atmosphere and little by little dish scattering a little bit of that sunlight back into space and so on average not all the energy that reaches the top of our atmosphere makes it to the bottom the particles are constantly scattering that but radiation back into space and as a result the planet is a little bit cooler for those particles being there when the relative humidity gets h enough in the atmosphere those particles are able to pick up water vapor from the atmosphere if it's cold enough it's not liquid water that they pick up it's actually ice that they form and so when we have these water droplets forming one hundred percent relative humidity or ice crystals  form clouds we call the liquid water clouds warm clouds are more liquid clouds the higher clouds the ice clouds are something that we turn serious clouds and these are critically important again in maintaining the energy balance of our planet clouds are very effective at scattering radiation back into space at any one time they cover something like thirty to fifty percent of the surface of our planet one hundred percent outside on boston on an april day like we have today and as a result the planet is cooler as a result th clouds weren't there the planet would be much much warmer on average than it is right  a lot of the work that we do in our laboratory is looking at terrestrial problems so understanding how particles are changing in our atmosphere what effect they're having on precipitation how our rain patterns changing our snowfall patterns changing and so on but one of the projects that we've become very interested in recently is understanding clouds around other planets and so what we've been able to do is leverage some of the technology that we have for to cloud formation and apply it to problems around other planets so the one that's closest to us in a solar system sense is mars and so we use some of the chambers that you can see in the background from me for understanding other atmospheres and you might think that it's hard to get yourself a sample of mars but we're actually able to use data from landers and probes around other planets like mars to figure out what the atmosphere is and so we can fill these chambers not with an earth like atmosphere but instead with carbon dioxide that would mimic mars we can drop the pressure and make it more like the pressure that you would find on mars we even have the ability to use minerals that are from earth but that look much like the minerals that we know are on the surface of mars as the aerosol particles the dust particles that would be in that atmosphere we can expose them to water and to c o two and see when they form clouds and you can imagine that this is really important in understanding the water budget of mars how it forms polar caps how the permafrost there forms as well as the carbon budget of that planet because a lot of it atmosphere a lot of the c o two actually freezes out on a yearly basis and forms part of the caps so where is our polar caps are made entirely up of water ice mars is very interesting because it has carbon dioxide and water ice and it's polar caps to different  substances and we're able to understand that better because of these laboratory studies more recently even we started to realize that there's planets around other stars so called exoplanets we've known about these for a few decades now and is us we're getting more and more and more information and one of the recent findings is that most exoplanets appear to have clouds around them these are very unconstrained systems we would say scientifically that's a nice way of saying that we really don't know much about them we might know their temperature we might know a few things about their atmospheres we might know the density and the size of the planet but that's almost it and so what we've been doing here with some of these chambers is filling them up with exotic atmospheres hydrogen atmospheres helium atmospheres much much warmer for example than you might find in our solar system and trying to find what materials condense into those conditions what materials could form clouds and this is actually critically important because by knowing what those materials are it will see through that atmosphere at certain wavelengths of light so something like a thousand nanometers which is outside of our ability to see is a wavelength that's able to peer through the organic clouds in titan's atmosphere and so the orbiter cassini that surrounds saturn at this time is able to see through those clouds image the surface of titan in a way that we couldn't do with just simply looking at the planet and our hope is that we might be able to find ways to do that around some of these exoplanets as well