Welcome back. Today we're going to talk about what is probably the second most interesting planet in the solar system after Earth, Mars. Mars is a really intriguing place. The interesting thing about Mars is it probably was a nicer place to live in the past, and it's certainly one of the most interesting places for us to visit in the solar system, and as we'll discuss at the end of this lecture, it is the obvious target for human exploration. It is going to be potentially a very interesting place to visit and perhaps someday to live. Let's begin by talking about some of the basic properties of Mars. Mars, of course, is more distant than the Earth from the Sun. It has- and we'll talk more about this- a very elliptical orbit. At closest distance to the Sun, it's 1.38 AU. That's 1.38 times the Earth's distance from the Sun. At maximum distance, it's 1.66 AU. Its orbital period's a bit more than two years. And it's a planet that sort of intermediate in size between Earth and Mercury. Its radius is about half the radius of the Earth, and its mass is much smaller than the Earth. It's only about tenth the mass of the Earth. This combination means gravity on the Martian surface is much weaker than Earth's. That means Mars is less able to hold onto atoms in its atmosphere and molecules in its atmosphere, so its atmosphere is much thinner than Earth's, and that will end up making a very big difference in its climate. Mars has two moons, but unlike the Earth's moon- the subject of the next lecture- Earth's moon is pretty big. Mars' moons Phobos and Diemos are quite small. Mars does have a tenuous atmosphere. The atmospheric pressure is about 0.6 percent of the Earth's atmosphere, and its atmosphere is mostly made up of carbon dioxide, nitrogen, and argon. There's some wonderful materials on the web on Mars. And let me encourage you to go to three sites, and they'll be linked off our webpage. One is the Mars Google site. Google's done a wonderful job, of course, in providing a map of the Earth that lets you explore roads and maps of the Earth from above where you can see topography. They've done the same thing with Mars. And let me encourage you to go to this website, explore the Martian surface. You can put in famous sites on Mars like Olympus Mons, the great Martian mountain that's higher than any mountain on the Earth's surface. You can look at the landing sites of curiosity or Viking or some of the sites that people talk about as potential places that once had water or potential sites for future human landing. And that's really fun to play with; I encourage you to go look at that site. NASA maintains a wonderful website that describes a lot of the missions I'll touch on here at mars.jpl.nasa.gov, and I encourage you to go look at that site. And we'll also provide a link to the Seven Minutes of Terror website, a video which NASA put together describing the tremendous challenges associated with landing on Mars. And as part of that interview, they'll talk to some of the really fabulous engineers at the Jet Propulsion Lab who designed the entry system that enabled Curiosity to land on the Martian surface. So, Mars is a planet we studied in many different ways. There are a number of orbiters that have mapped the Martian surface, taking images from space and providing really detailed pictures of Mars. There are landers that have gone to the surface, and the most famous of these landers are the rovers that are now probing along the surface looking at samples. We'll talk about Curiosity and how it's actually doing really detailed geology on the Martian surface and giving us a lot of new insights. Another important way we learn about Mars is through Martian meteorites that reach the Earth. What happens is, a meteor comes in, hits the Martian surface, throws off rocks, those rocks fly through space, and a small fraction of them fall inwards in the solar system and hit the Earth. And by looking at the composition of certain meteorites, we're able to determine that these meteorites have Martian origin. And one of the things Curiosity has done is confirm that inference. And this means we have Martian samples on Earth where we can take these samples to our lab and study the properties of these meteorites in detail. Now, this is very useful, but what we'd really like to do is not have samples that were randomly thrown off from random parts on Mars– we don't know where, we don't know how they've been affected in detail by transit. What we'd love to be able to do is have a rover identify some of the most interesting samples, pick those up, and then have a future mission, take those samples back to Earth, where we could study them in detail, and at the end of the lecture, we'll talk about the future of Mars exploration, and a very important part of that future will hopefully be sample return, the ability to bring rocks back from the Martian surface, bring them back to laboratories here on Earth where we can study them in detail and learn much more about Martian history and whether Mars once held life or perhaps even holds life today. And finally, we'll turn to talk about the possibility of human exploration. I think there's a broad consensus that the next big step in human exploration beyond the Moon is going to Mars. And we'll talk a bit about some of the challenges of traveling to Mars and settling on that planet. The Martian surface, we've had wonderful images of the Martian surface, and again let me encourage you to go to the Google website and explore the surface yourself. The surface mostly looks broadly desert-like, like this, near the equator. The north and south poles have ice caps made of carbon dioxide and water. And Mars' surface is cold, with a characteristic temperature of about 280 degrees Kelvin or -55 centigrade. Mars' interior– remember, Mars is a much smaller planet than the Earth– but it has the same, broadly speaking, multilayer kind of structure, probably a solid, mostly iron inner core surrounded by a liquid core made of mostly perhaps iron and sulfur, maybe some silicon, and then a relatively thick crust whose thickness goes from about 30 to 100 kilometers. Unlike the Earth, it does not have plate tectonics, it doesn't have active volcanism. And, as we'll look at in a moment, it doesn't have a magnetic field. Mars has weak gravity. Remember, its mass is less than the Earth and its radius is a bit smaller, about half the size but its mass is a tenth, so gravity is much weaker on its surface. Since the height of mountains are determined by the balance between the strength of materials and the strength of gravity, it would be very hard on Earth to support a mountain much higher than Everest. But since Mars' gravity is weaker, Martian mountains are higher, and Mount Olympus on Mars is, Olympus Mons on Mars is quite large. Mars does not have a significant magnetic field, and that has an important effect on the evolution of its atmosphere. Remember, Earth has this magnetic field that serves as a shield. So when the solar wind comes in, material from the solar wind is mostly deflected away from Earth and only a small fraction comes down at the poles, where we have auroras. Mars lacks that, and that means you get to strip off even more of its atmosphere this way, and that this serves as a way of ending up with an atmosphere that's very thin, it's about 0.07 atmospheres. It's an atmosphere of lots of dust and one of the dramatic things we see are these dust clouds and dust devils. At higher altitudes, we see water and ice clouds in the Martian atmosphere, carbon dioxide clouds, and a run in temperature, where it's about 250 Kelvin at the surface and then drops as we move up and out with altitude. So, Mars has a very interesting set of seasons. Mars' seasons are driven by two effects. When Mars is closer to the sun, it's hotter, gets more radiation. Of course when it's further away, it gets less. But you're also affected, as in the Earth's case, by tilt. The northern hemisphere, when this face is away from the sun, and when you have the northern winter, when you face away from the sun, you get less sunlight. The angle, it's lower in the sky and you have fewer hours of sunlight in the northern winter. So during this season, the northern ice cap grows. You then go through northern spring, southern fall. And then, you have a very severe winter in the south because the south gets the minimum amount of radiation for two reasons. It faces away from the Sun more, and the whole planet is further from the Sun. The angle of the Martian axis relative to the ecliptic, its tilt, is very similar to the Earth's tilt at the present moment. And the Martian weather is very much driven by the CO2 cycle. What happens is, during the southern summer, the carbon dioxide ice pole in the south evaporates, goes into the atmosphere. A lot of energy is transferred into the atmosphere through latent heat of the carbon dioxide. The carbon dioxide then condenses in the north, and we can see the northern cap go through its annual cycle of shrinking, and then later growing. And as we observe Mars through the Martian seasons, we can see the polar caps in the north and the south grow, shrink, and grow again. The Earth also has its seasons, but an important difference between the Earth and the Sun is the Earth's orbit is much closer to a circular orbit, so there's much less variation in the Earth's distance to the Sun. So in the case of the Earth, our weather is driven almost entirely by tilt. Of course, in June, the northern hemisphere faces towards the Sun, which is why in June summer begins in the north and winter begins in the south; and on Earth in December, it is the southern hemisphere that is facing the sun more, so the southern hemisphere begins its summer while the northern hemisphere begins its winter. Mars has significant long-term variations in its weather as it goes through its cycles because Mars has big variations in its orbit. In the Earth's case, the Earth's tilt, its obliquity, is controlled by the Moon and doesn't vary very much. Mars' obliquity varies a lot and varies chaotically over time. So right now, the Martian tilt is similar to that of the Earth, but over millions of years, that tilt will vary and you'll go through periods where it lines up and there's almost no seasonal variation to periods of time where there are very significant seasonal variations and differences between the northern and southern hemisphere. The eccentricity of the Martian orbit also varies. There are periods of time in Martian history where Mars' orbit was more like Earth, close to circular, and other periods of time where the orbit was even more eccentric, and there are even more dramatic variations between when Mars was closest to the Sun and farthest to the Sun. And as a result, Mars has over a long time period very variable climate. And one of the many things we'd like to understand about Mars is how this cycle affects its climate and its evolution. Now, let's now turn and think about how things could have been different, and I want you to go work on this problem before we return, we'll make some estimate of what would happen if Mars had been a more massive planet and could have held on to more carbon dioxide in its atmosphere.