In this lecture, we'll take a tour of the surfaces of the worlds of the solar system. Our focus will be on rocky planets, yet we'll also visit many of the larger moons. We'll do this in the context of comparative planetology, where we'll learn about other worlds by comparing their features to those of earth. Yet we also learn about earth by exploring the range of possibilities available in the solar system, and where our planet fits in.

Lets just jump right in and start with mountains, which are found on several worlds, each forming in a different way. Here on earth, if we look at the global map, we can see several very long mountain chains, such as along the west coast of the US in North America, and along the west coast of South America. If we look closer at South America, we can see the very long chain of the Andes mountains, running all along its west coast.

This is a result of plate tectonics, where the oceanic plate of the Pacific has run into the continental plate of South America. This is not the highest mountain chain in the world, which is the Himalayas. They're pictured here, north of India, where two continental plates have collided. The Indian plate has moved northward, smashing into the Asiatic plate, and lifting up the Himalayas. The highest mountain in the world is part of that, Mt. Everest. We see it rising high over the Tibetan plateau, reaching 28,000 feet.

Mountains are not only on earth, but also exist on other worlds. We see an example of one mountain on the moon. It looks somewhat like the picture of Mt. Everest, yet this is Mons Hadley Delta, and this is the Apollo 15 mission. Mons Hadley Delta rises about 15,000 feet above the lunar mare, where the astronauts landed. So mountains on the moon are similar in height to those of earth.

As a matter of fact, there's even a mountain chain. If we back off a bit, we can see running right across the center of this image, the Apennine chain of mountains on the moon. or the Alps of the moon. These are probably the mountains that Galileo saw, when he first turned his telescope to the heavens, and was able to see that not all heavenly bodies were perfect spheres. He could see the mountains on the moon, which violated the principle of Aristotelian philosophy. However, if you look even further out, to the full moon, you can see that these mountains are not the result of plate tectonics, but are actually just part of the rim of a very large crater. The mountains on the moon are the result of impacts.

On Venus, we also have mountains. We see a topographic view from its north pole. Blue represents the lowland regions, and white the high regions. It's rather easy to pick out the highest region, and if we zoom in this area called Lakshmi Planum, the white region is called Maxwell Montes, the highest point on Venus, its tallest mountain.

It's also coincidentally, the only feature on Venus that is named for a man, named after James Clerk Maxwell, the physicist who derived the equations of electricity and magnetism. Yet every other feature on Venus, is named for a woman, as we might expect! Maxwell Montes also did not form by plate tectonics, nor is the result of impacts. Rather, it's the result of compressional tectonics. Venus is a single plate planet, so instead of having multiple plates like earth, it has one plate over its entire surface. Yet compression within that plate, is what probably caused Maxwell Montes to rise up so high over the surface.

Next we'll explore volcanoes. This activity is abundant in the solar system, and on earth the most common form of volcano formation is what an oceanic plate hits another place, and subducts underneath it. The pressure created in the subduction zone, sensed magma to the surface, and creates a volcano. We see a very recognizable volcano in Mt. St. Helens, caused by the subduction of the Juan de Fuca plate, underneath the North American plate. This images show it before its 1980 eruption, while another images show it afterward. This is the definition of blowing your top.

These volcanic explosion can be incredibly powerful. There are maybe a few thousand volcanoes across the surface of the globe, generally located on these plate boundaries. When we think of volcanoes, we often think of a ring of fire around the Pacific Ocean, where the Pacific plate meets the continental plates on either side. Volcanoes are also observed on Venus, and we see Maat Mons, one of about 1000 volcanoes on that planet. There is a ton of such volcanic activity there, because 85% of its surface is covered by volcanic plains.

Venus also features a unique volcanic feature that appears nowhere else on any other planet. This is Bahet corona, an oval shaped volcanic uplift. Apparently there are these volcanic uplifts that take entire regions of Venus' surface, up and then compresses them back down. We can see all these fractures that form this oval shaped region of the corona, about 200 km across.

We get much more volcanic activity on Mars, as we see one of its most active regions called the Tharsis uplift, where there are several very large shield volcanoes. Three at the lower right and on larger one to the upper left. These are created through very fluid lava that has been spewed out over a very long period of time. That's why these volcanoes are so large. How large? The larger one is called Olympus Mons, by far the largest volcano in the entire solar system. By itself, this volcano is about the same size as the state of Arizona, 600 km across! In height it's about 27 km high, so compared to Mt. Everest, it basically dwarfs it, so it really does deserve the name of Mt. Olympus.

Yet Mars is not the most exciting place for volcanic activity in the solar system. That claim goes to a large moon of Jupiter, called Io. We see all of its pock marks, which are volcanic regions. It's literally covered with these, which are continually erupting. When the Galileo probe flew past it in April 2007, the region around the Palio volcano was images. When it came by again in September of 2007, an entirely new volcano had erupted.

This incredible amount of volcanic activity is due to Io's elliptical orbit, and Jupiter's gravity. As Io comes closer to Jupiter, its surface gets pulled in. As it goes farther away, it flexes out. This fluxion in and out of Io's surface, creates the heat that causes the volcanic activity. Now normally this process should dissipate, and the surface flexion should help to circularize the orbit. Yet Io is in a resonance orbit with the other Galilean moons Europa and Ganymede, and they keep Io in its orboit and maintain Io's volcanic activity.

One of Frank's favorite images in all of astronomy is where the plums of Io's volcanoes stretch so far up, above its surface, you can see them on the limb. Here's a closeup of it. It's just amazing that on earth, to get a plume that high, we'd have to be going 300-400 km into the air. Our volcanoes don't do anything like that. Now this was one of Frank's ultimate favorite images, but then the new Horizons mission went past Jupiter and got this sequence of 5 images of the Tvashtar volcano on Io. We can see 5 images that show it erupting, and the plume spreading out across Io. Just an amazing sequence of images.

Next we'll turn to canyons, which are carved both by water and geologic forces. On Earth, the most impressive canyon is in Arizona. We see the state viewed from space, and if we look in the upper region, we can see the Grand Canyon, from the upper-right, along down through the center, wriggling around, continuing off to the left of the image. It's about 150 km long and 2 km deep. It has been carved by the Colorado River over millions of years.

Now Venus has canyons as well, called chasma, or plural chasmata. We see an image where all the bright yellow regions across the center and curving over on the right, are these chasmata. They are very deep and narrow canyons that stretch for hundreds of km, not caused by water erosion, but geologic faulting.

On Mars we have perhaps the most impressive canyon in the solar system. We see an image of Valles Marineris, a very extensive canyon system on Mars that shows both fault scarps and water erosion. So it has both of the processes we see on Earth and Venus, occurring on Mars. This canyon is so large, that it can be seen when we take a picture of the entire planet. It's roughly 4000 miles long, which compared to the US, would stretch from California to New York. The canyon is about 7 km deep, and makes for a spectacular feature.

We have a wonderful movie that can take us on a fantasy trip through Valles Marineris. It's based upon topographical data from the Mars orbiter. We begin on the western edge of Valles Marineris, and then fly down the main canyon, originally created by earthquake faulting, many billions of years ago. As we come to the central region, called Melas Chasma, and we can see lots of sediments on the right, we imagine an old ancient lake.

As we pass to the north, we pass over an ancient landslide and into another basin. Notice the atmospheric effects on the sun. They're rendered realistically so it looks just as if we were on Mars. One thing not realistic is the vertical exaggeration, as this is the same enhanced relief that we'd find on a globe with relief on it. We pass to the northern most wall to the canyon and see large landslides, examples of water erosion, that have widened the valley overtime.

This really is a magnificent structure, extending for hundreds of km across the face of Mars. Now they say they do things big in Texas, but considering Olympus Mons and Valles Marineris, Texas ain't nothin' compared to Mars!

Next, lets take a look at riverbeds, which provide proof for the existence of flowing liquids on many worlds. Earth's riverbeds provide distinctive and recognizable patterns. We've already seen one example with the Colorado River and Grand Canyon. We also see images that look something like dendritic channels, which are drainage patterns coming from a river channel.

It's very similar to the same features we see on Mars, with these same dendritic-like channels, showing evidence of water flow. Not current flow, but flow in the past. Valles Marineris, as you follow it down to its end, you can see that water once flowed along it too. There are many other features on Mars that show water flowing in the past, as well as catastrophic releases, and miniature floods. Yet liquid water cannot exist on Mars' surface today, since its atmosphere is too thin and there's just simply not enough pressure to hold water in its liquid form.

One place that's very surprising where we see these same riverbed channels, is Saturn's moon Titan. These are methane channels on Titan, carved by liquid methane. We have also seen methane lakes on its surface, although one wouldn't want to go swimming in them, since methane is liquid at a temperature of -200 degrees Centigrade!

There are other riverbed-like features that appear on other planets, and here is one on Venus, that we call a sinuous channel. It looks like a river, but its really been created by a lava flow. There is another one on Earth's Moon called the Hadley Rille. It was observed by the Apollo 15 astronauts, and as a matter of fact, if you follow it to the north, right around its bend, is the location of the Apollo 15 landing site. The mountain we see next to it, is Mons Hadley Delta that we saw before. The astronauts took the Rover out to explore the Hadley Rille, and we can see what it looks like from the ground. Basically it's a collapsed lava tube that once flowed across the moon.

The next topic to take a look at are geysers, which can spew a variety of substances. On Earth, the most famous geyser is Old Faithful, in Yellowstone national Park. It erupts for a few minutes on average every hour and a half, one of about 300 geysers in the park. Nearly half of all the geysers on the planet, are in Yellowstone.

Geysers are geologic features that release heated water from beneath the surface. Ground water makes its way down through the rocks of the crust, reaches the magma underneath it, heats up, and is periodically released. These are rare features that exist only in very specific spots on the planet.

We also see geyser-like features elsewhere in the solar system as well. Neptune's largest moon is called Triton, and in the upper half of the image of it, we see what they call the cantaloupe terrain. Triton is extremely cold, at the edge of the solar system, so its surface is covered with ice.

The bottom half shows streaks of black, which appear to all be streaming away from the south pole of Triton, which does have a very thin atmosphere. So we've determined these black streaks are methane fountains, liquid methane spewing up from the interior of Triton and being blown back by very thin winds that move away from Triton's south pole. These methane fountains rise nearly 8 km into the air, before being streamed back, carried by the thermal currents.

There is another moon that has even more impressive fountains. This is Saturn's moon Enceladus, a rather small, icy, and not particularly important moon. Except for these fountains. It orbits about twice the distance from Saturn, as its main rings are from the planet. Its important surface features are these cracks in the south, called the tiger strips. They are very deep crevices, which for some reason, seem to be heated. We're not sure why exactly, but so seem to have excess heat associated with the strips, which appears to power some ice fountains.

We see another image of Enceladus that is back lit, and down the bottom you can see some wispy fountains coming off. Perhaps these tiger strips are heated by tidal heating like Io, yet we're really not sure. The interesting thing is that this may indicate the presence of liquid water, just below its surface. Again, we're not sure. One thing we are sure of, is that these fountains contribute to, and may even be the main source of the faint E ring of Saturn. The planet has some very bright main rings, yet also some very faint outer rings, and Enceladus may be one of its sources.

The next thing to view are polar ice caps. These exist on both earth and Mars, growing and shrinking with the seasons. Now earth has ice covered poles, simply because they receive less sunlight. It's colder at the poles. Earth also has a 23.5 degree tilt, so in some months the poles get no sunlight, while in some they get continuous sunlight. It then makes sense that the ice caps would shrink in the summer, and grow in the winter.

We see the northern ice cap on earth in March of 1998, which is largest right after the northern winter. We then see what it looked like by September of 1998, smallest right after northern summer. It's quite a change between these two images. The same thing occurs on Mars, and we see the Hubble images of the planet with its polar caps. Yet they are mostly carbon dioxide ice, instead of water ice like on earth. Its atmosphere is 97% carbon dioxide, and as we've said, you can't have liquid water on the surface.

Mars similarly has a tilt of 25 degrees, so its very much the same type of tilt which earth has. You would expect to see similar seasonal changes in its ice cap, and we do see them. We see Mars over the course of many months. During the Martian winter, it has a very large polar ice cap, while as it progresses towards the summer, the cap shrinks.

Yet this shrinking is not melting as it is on earth, where the caps melt into the polar ocean, but here it's a process called sublimation, where the carbon dioxide ice in the cap is going from ice, directly to gas, which adds to the atmosphere of Mars. It's actually kind of interesting because we see the cap shrink on one pole, so the ice goes into the atmosphere and actually freezes back out at the other pole!

We'll finish off this planetary comparison with somewhat of a non-comparison. Earth does have one unique feature, and if you just look at the planet, it stares you straight in the face, its oceans. Nearly 70% of its surface is covered with oceans. Their importance is that water is a necessary ingredient for life, as we know it. When you look for life in the universe, we are looking essentially for the presence of water. Life requires carbon chemistry, water, and a source of energy.

Well carbon chemistry and a source of energy are relatively abundant throughout the solar system, yet water is the main ingredient we're looking for. Now its kind of interesting to note that only recently have we found another world in the solar system, with liquid on its surface. That is Titan, which has methane lakes. So while earth has been unique for having water on its surface, its no longer unique for have a liquid on its surface.

Lets take a look at a global overview of the rocky planets, comparing their overall surfaces. Each shows a dichotomy that has rougher upland regions, and smoother lowland plains. Earth and Venus are very similar, as we see in a diagram where they are dominated by the blue color representing volcanic plains. Venus has had many epochs of volcanic resurfacing, so its surface is almost entirely covered by volcanic plains.

Earth has been dominated by plate tectonics, so we can see the dichotomy between the ocean bottoms, which are volcanic plains, and the continental plates. We also see on earth and Venus, the pink regions that represent mountains. Venus has rather strange shaped mountains compared to earth, as we are used to the long mountain chains that you do not see on Venus, where the mountains occur due to compressional tectonics.

Venus also has very small orange circles in this diagram, the coronae we mentioned earlier. The colors you see only on earth, are green and tan, representing deposits on top of the continental plates and the cratered regions, respectively. Although we have a lot of erosion on earth, we still have a few regions with significantly more craters. This occurs, of course, on the continental plates.

When you look at Mars and our Moon, you see volcanic plains, the blue that we saw on Venus and Earth. This is very small on the Moon, yet rather dominant on the northern half of Mars. The dominant feature on both of them is the tan that represents the cratered regions, as well as yellow that represents the heavily cratered regions. The Moon also has red regions that represents the ejecta.

The Moon is a geologically inactive body and almost all of its surface features are due to the heavy bombardment era and the cratering that has occurred since then. Mars has a very strong dichotomy between the volcanic plains in the north, and the heavily cratered regions in the south. We don't know why this strong dichotomy exists, and its something that the planetary scientists are studying.

It is natural to marvel at the diversity of surface features across the solar system. We have looked at mountains, volcanoes, canyons, riverbeds, geysers, ice caps, and oceans. We've seen that the seven wonders of the solar system, far outpace the seven natural wonders here on Earth. Yet one learns more by identifying the similarities, rather than looking at the differences. When you look at similar features, you recognize that similar forces are at work, geologic and hydrologic, across the solar system.

If you think back to what we just discussed on the Moon and Mars, and think of something that doesn't appear that strongly on Earth, it's the covering of craters. The Moon and Mars are full of them, while it's not so apparent on Earth. It's a very important surface feature across the solar system. They are so important and prevalent, that they get their own lecture!