Lesson 6: The Terrestrial Planets - Surfaces [Solar System Astronomy with Joseph DalSanto]
Now. That we have an understanding of the interiors, of the, terrestrial worlds, we, can move on and begin examining their, surfaces, we. Want to see a wide variety of terrains, and features, but. Amazingly these. Can primarily be explained. By, four, processes. That occur on the, terrestrial worlds. We'll, be examining each of these in some detail, for. Now let me list them impact. Cratering. Volcanism. Tectonics. And erosion. Each. Of the trustor worlds has a varying, degree of each of these and, of course they operate under various, conditions and. Environments. But, primarily all that we see on the surfaces, of the planets is shaped by these four processes. When. We consider, impact. Cratering we need to go back to the beginning of the solar system, we. Had explained that the planets were formed by impacts. Of planetesimals. Accreting. Into larger bodies and the, final stages of this definitely, left their mark on the terrestrial worlds. Two. Of these worlds are moon. And Mercury. Still. Bear the scars of this early stage of impact, cratering. But. Over time the, rate of impacts greatly declined until, it's very very small, today, the. Result was that small craters greatly outnumber, large craters, and interestingly. The, size of craters depends, greatly on, number. One the size of the impacting, object, number. Two the, velocity, of the impacting, object and number, three the, composition, of the impacting, object, you. Can imagine that as these objects would strike the surface matter. Would be expelled, you. End up with a bowl-shaped depression. And. Again we see exactly, that on the surfaces, of these, worlds. However. Planetary, scientists, can make use of the, fact that craters. Normally. Would. Remain, on a planet, unless they, are reworked. By, another process, of the floor so. In other words on the moon and Mercury where. The other processes, don't operate, very much at all the. Creators are still evident. And therefore, this is considered a very old or ancient surface. Many. Of the original creators are still there today not, having been changed. What. About on Earth Venus, and Mars, well. Here the other three processes, again.
Volcanism. Tectonics. And erosion. Begin. To slowly gradually. Erase. These craters, and. As a result scientists, can use this, technique, of looking, at the craters to see how much. They have been changed to get, a relative, idea of the age of the surface, if. The craters again are still clearly, pristine, we know it's a very ancient, surface they've, not been changed much but. If the craters are gradually, or completely, obliterated. And reworked, and covered, over we. Know that's a younger surface, and. We see this very clearly again. On earth venus and, especially mars. Let's. Move on now and discuss volcanism, for a few minutes we're all probably familiar with perhaps, pictures, or videos of volcanoes. Here on earth erupting, lava out onto the surface air. Is where again deep within the earth great amounts of heat melt the rock that. Heat creates, great pressure and finally. That molten rock finds. A weak spot in the crust interrupts. Onto the surface. This. Of course can have great effect on the surface filling. In lower areas, building. Volcanoes. On some planets, but. There's another feature that's very very important, for us to understand, and that, is that volcanoes, not only give off liquid. Rock or lava but. They also expelled, gases, into the atmosphere, and. This can have tremendous long-term. Consequences. On a planet. Moving. On to tectonics. As the third type. Of process. That modifies. The surface of a terrestrial world. Tectonics. Really refers to tremendous. Forces. Beneath, the surface. That. Affect the surface. We. See in our diagram here an image of the, interior, of Earth and, you notice there are some. Circular. Cells. Beneath, the surface, reaching. Down deep into the earth near its core where the rock becomes very very hot and, gradually, moving towards the surface. Rising. Notice. That some of the cells may push the crust together. Whereas. Other cells may stretch. It apart well. This we call convection. When the rock is moving towards the surface, releasing. Some heat and then. Descending, back deeper, into, the earth, again. This convection, creates, tremendous, forces. Or stresses on, the crust sometimes. Pushing it together as. You see here in a diagram, oftentimes. Mountains will be pushed up because, of this tremendous. Force. Other. Times the crust may be stretched apart and we. May get a valley or a. Large crack in the Earth's crust, perhaps. That crust me filling with ocean water but, the point is the, surface, is greatly affected by this. Force. Two. Famous illustrations, as you see here on the left the Himalaya, Mountains are. Being pushed up as, India. Is pushing, into Asia, they. Are growing taller each year on. The. Right we see the Red Sea region here. The Red Sea is, an area where the crust is being stretched, apart and we have a much lower depression which again has, filled in with seawater. Our. Fourth process, is erosion. And we're probably somewhat, familiar with this we. See some images here of the Colorado River over. Long periods of time carving. A channel. Which, we know is the great can't Grand, Canyon, we. Think too of great glaciers, in different parts of the world and how they have resurfaced. Different. Areas, it's. Also important, to realize that wind, can, act as an erosional, agent by gradually. Slowly wearing, away rock maybe. Piling, up in some areas or moving in from other areas. So. I think you can see that erosion, requires. An atmosphere, number. One an atmosphere. Is required, to have sufficient, pressure at the surface so that, liquid water can exist that's. A fundamental, requirement, it. Also is important, of course for we and so. Erosion, really only occurs on a planet, that has an atmosphere. Well. Now that we've seen the, basics, behind, the four processes, let's. Go ahead and apply what we've learned towards. Each terrestrial, world we'll, take a brief glimpse at each we'll see how each process, has affected that world and we'll get a very nice big. Picture or context. In which we can see each, individually. Starting. With our moon probably. Familiar and just looking up at it there's many many craters there so, as we mentioned at the outset, here's. Where cratering, was dominant and really. The other processes. Were not, why. Is that well. You may recall from our previous lecture, that the moon being a very small world lost. Its internal, heat and. Therefore, really doesn't have the energy to. Drive volcanism. Or tectonics. And so, there's really no features on the surface that indicate these, processes. Played, any great role, certainly. There could not be any erosion so, we know there's no atmosphere on, the moon as well so. The moon has a very ancient surface heavily, covered in craters some. Areas a little more than others some. Regions there was a bit, of lava that flowed over the surface, when.
A Large impact, melted. The surface and this. Lava then would fill in the, craters and we see a little smoother region today or a little. Younger region, but. Again this is primarily due to the heat of a huge impact, and of. Course being a small world, the, moon again has lost its heat and is now geologically. Dead. Some. Of onto Mercury which, has. An appearance of very much like the moon once. Again a heavily cratered surface, tells it's a very ancient surface. Many. Of the original creator, still, in place there they've simply, not been altered over time, we. Do see a few. Mild. Features. We might call them that, indicate some, motion. Beneath the crust mercury. As you see has been known to have large. Long cliffs. Indicating. Some movement. Beneath the surface this. Was almost certainly very long ago when there was still a little bit of heat left, and, drove this little bit of activity. Again. With no atmosphere we certainly wouldn't expect any erosion there on Mercury and, with, very little heat it. Really wouldn't be much in the way of volcanism, there. May have been a little bit early in its history and, we do see some areas where there is some lava flows but. Again this would have been very early and by, now has long since died away, so. Once again a smaller, world having, lost its internal, heat mercury. Is quite geologically. Dead. Let's. Move on now to Mars a fascinating. Planet one. That displays a great variety of features. In. This false, color image that you see here we see again, regions. Of great diversity, in the. Top we see a false, color blue lowland. Region, it was relatively, smooth, which, tells us of course that some of the original craters, have been erased, but. Notice in the left hand image these southern, highlands higher. Elevations. Heavily. Cratered and so. Again we have just said this would be considered a much older region, with, all of these craters here and, so there's quite a variety some. Regions of Mars more. Heavily cratered than others. Mars. Is well known for, huge, volcanoes. Which, clearly indicate, that this was a very, important, process but. Even in addition to the huge ones there's a number of smaller ones as well. How. About tectonics.
Well. We. See in the right-hand image a vast, canyon, system the Valles, Marineris. 2500. Miles long, this. Enormous. Crack, in the surface we believe was caused by. Tremendous, upwelling, of material. Beneath the surface and so, this is clearly a sign of tectonics, and, finally. We're gonna see a couple pictures in just a moment of how. Erosions, taken place on Mars in. Two different ways. Here. You see an image of some, of the large volcanoes, on Mars, four of the largest in the entire solar system so, this clearly tells us Mars has had an active past weather. Periods. Of great volcanic, activity, now. Being a. Medium-sized, world Mars. By now has lost much of its heat and, we don't believe it is active today but. The activity may have lasted quite a long time until fairly. Recently and. In, terms of our solar system, 200. Million years ago is fairly, recent. We. Mentioned the great Valles, Marineris Canyon. System here, you see an image of that in more detail showing, us again this tremendous. Rift in the surface of Mars more. Than likely caused by again, a great upwelling, of material, it, cracked the surface open over. Time different parts of those cliff. Walls might slump, inward lijing kind of see in the right-hand image there and that, would gradually enlarge the, canyon system even. Further, so. Really some very dramatic, very huge. Tectonic. Features and volcanic. Features here. On Mars. How. About cratering on Mars while. Mars is quite a few more craters an earth, but. Not nearly as much as the moon or mercury so, somewhat of an in-between case. And. As we mentioned there, are regions of Mars that are heavily. Cratered and others that are not heavily, created, so a great, variety as, we just mentioned in the Highland. Or lowland. Regions, great. Amount of variety. But. Interestingly as, we look at different regions that have some craters, we, can again use that technique we mentioned before. Carefully. Looking at the craters to see their condition, are they still. Fresh, so to speak and pristine and unmodified. Or on. The other hand have they actually been modified. Changed. Worn. Away, here. You see a couple images showing regions. On Mars with some craters at various, stages, of decay. If. You look down in the extreme lower left we see a fairly young fresh, crater this is what we would expect it to look like, but. Notice, the other two images show creators. That have begun, to fill, in with. Dust, and other soil that's, been transported. There by, wind. So. This is a good example of, wind, erosion. Although. Mars has a fairly thin atmosphere it. Is enough to enable the transport, of this material, and, these, creators can gradually, be. Filled in and in. This sense then be considered as eroding. Our. Right hand image shows a vast, dune, field on Mars where the wind has sculpted the surface here and we see very few if any craters, so. Again we do see this erosional, processing, on, Mars not. Nearly as strongly. As on earth but. Over long periods, it certainly has a tremendous, effect.
So. This is the first manner. Of erosion, on Mars, but. We and I are probably more familiar with erosion in terms of water, well. There's strong evidence that, at one time Mars, in the past had. Liquid water on the surface but. No longer does, here. You see in them it's showing just. Such a region that gives us this impression. We. Look down with orbiting spacecraft and, we see ancient riverbeds, channels. That, almost certainly were carved by water they. Have the exact same appearance, as such, features here on earth, but. How could we really verify that, that's exactly what's going on here. Well. You may have heard that in recent years nASA has been able to place. Spacecraft. On the surface of Mars in, particular. Roving. Spacecraft, that are able to then travel, to different regions and examine. The rocks close up in detail some. Even able to drill into the rock get a pristine, sample and analyze its composition. What. Have they found they. Found rocks that, only form, in water and, so. This does confirm, that, here on Mars we find regions, where, there had to have been liquid water in the past this. Famous Spirit, and Opportunity Rovers. Did, a tremendous job beginning. 2004. And more. Recently NASA's. Curiosity rover, continues this task as it, investigates, a particularly rich area, on. Mars. From. Orbit we can even look down with spacecraft and, map, different regions, composition. And see. Where some of that water may have gone, not, much of it was lost from the planet but, there is still some that is basically, buried, within the soil as frozen, water or ice and so. This confirms for us that again at one point in the past Mars, was. A wet world rocks. Formed, in the water and. We see that left over today. You. May recall we explained what happened to Mars. Remember. Mars he cooled off its. Magnetic, field weakened. And that. Left its atmosphere, vulnerable. To the sun's radiation. When. The atmosphere was gone, Mars could no longer support, liquid water on the surface. Finally. Let's go ahead and take a look at Venus another. Fascinating, world with a rich variety of landscapes, and, terrains, and features, now. You recall the Venus is more difficult to investigate. Because. Of that very thick atmosphere.
We. Can't just fly over and take a close look or even see it well from Earth but rather we've got to send spacecraft to map Venus with. Radar, so. The images you're going to see were taken with radar and they require just a bit more interpretation. Here. You see false color images in somewhat. More detail than we saw previously of, Venus. Surface, here. The pinkish. Whitest regions are higher. Elevation. And the. Blueish regions are the lower elevations. Well we, see clearly there is at least one very large pile. Of agent region, and a, few other smaller ones but. The remainder, of the surface, it's. Relatively. The same elevation, maybe. Some very mild or modest changes, but. This in itself is a clue on earth. And, on Mars and, even on the moon of mercury we see more dramatic, elevation, changes, and yet it, seems as if Venus surface doesn't have such dramatic changes, why. Is that. Well. It turns out that the answer has to do with, the volcanism, on Venus, now. When we look to our radar images we don't see very many craters. While. That implies that something clearly, must be erasing. Them. Certainly. Some of them are going to burn up the impacting. Objects will burn up on the way through the thick atmosphere but. That doesn't fully explain the, lack of craters. Instead. What. Really happened, on Venus was. That between 700. And 800 million years ago there. Was a tremendous, amount. Of volcanic, activity. This. Activity spread, lava far and wide over the surface of Venus and you, can well imagine that, most of that lava would end up in the lower regions, filling. In those lower depressions, and making. The surface seem as if much of it was of similar. Elevation. And that's. Exactly what we see, so. We don't know every detail, of this event and we don't know every reason why Venus, was, particularly, active but. It certainly seems based on evidence from its surface there. Was a particularly, active, time on Venus, when. Much volcanism, was occurring, we. See certainly. Today many many volcanoes, in, these radar images, and, other. Regions, again that are greatly filled, in with lava, so. A very interesting puzzle, for us to continue to work on in. The history, of Venus. How. About tectonics, and erosion on, Venus, well. We've seen features through the radar images showing again the crust being stretched apart the. Crust being pushed together much. As we described, here on earth clear. Evidence for tectonics, and frankly. That doesn't surprise us does it when. We consider that Venus is a large terrestrial, world nearly as large as Earth we. Would almost expect, it to retain a great amount of heat and that there, would be continue to be a great amount of activity, based on that heat so. Wouldn't be too surprising at all that Venus does have again, a significant, amount of tectonic. Activity. And. Again we do see some features indicating, that. Now. As far as erosion, this. Is again a very interesting unique. World in the sense that Venus as you might know has a very thick, atmosphere. Now. You might think at first well that should mean a lot of erosion but actually, the, atmosphere is so thick that, the. Wind, is not able to move very much not very fast at all to transfer, or material, and so, there's very little if any wind erosion. How. About water erosion. Well. Of course Venus is quite hot, on the surface, and so we wouldn't expect any frozen water but. Could there have been liquid, water on Venus. Well. Interestingly we, believe Venus started out very much like Earth probably. With what. Oceans, of water and. Yet the, few images we have on the surface today show. Absolutely no, water, on the surface of Venus other. Evidence, indicates, that there simply couldn't be any how. Can we say that.
Well. I want to take a minute and share with you a little bit of the story of how Venus got the way it is and. I'll share with you a bit of a more, detailed, slide you may wish to pause the video and examine, it at your leisure and consider, what it's really telling us. Venus. Story really begins with the fact that it was clearly, closer to, the Sun than Earth this and, naturally. We would expected, them to be warmer. Wouldn't we well. We think that early in its history that, closeness, to the Sun that higher temperature, meant that much of Venus oceans. Began. To evaporate. Well. The. High temperature, of course would cause that to happen now on earth you would say well that water, would precipitate back, out and fall, back to earth, however. At. Venus, higher temperature, that, water vapor, effectively. Held. In much of the heat and caused. The surface to get even, hotter, this. Is the greenhouse effect that, perhaps you've heard about, this. Process. Once it gets hotter then. Evaporates, more, of Venus, original, water and the. Temperature rises even more this. Process, reinforces. Itself to the point that it begins to get so hot, that. Carbon dioxide that. Is locked, into the rocks of Venus, begins. To be released or baked out so to speak, while. Carbon dioxide, is also a very powerful greenhouse. Gas, it. Also is. Going to hold the heat in and cause. The surface to get even, hotter so. Now we've got virtually. All of Venus water, evaporated. Holding. In the heat we've. Got carbon dioxide, from the rocks add, it to this to hold an even more heat that, temperature is rising and rising and rising Venus. Water is at a very high altitude in. Its, atmosphere. What. Happens then. Remember. A key point we saw in, a previous lecture. Venus, has a very slow, rotation and as. A result a very. Weak magnetic, field. We. Explained, that that magnetic, field, protects. The atmosphere. Well. With no magnetic field, Venus. Water was. Vulnerable. It. Was easily broken, apart by the sun's powerful radiation. Much. If not all of it lost to space, very. Little left today. So. We really see how this process played, out and. Resulted. In Venus not having, any liquid water on its surface today a, very. Dramatic different. History than. That of earth on earth. Although some water evaporated. The, temperature, was low enough that again it would typically, precipitate. Back out and. Remain in liquid form, on, the earth so. Again a really keel key factor, here that, lack. Of magnetic field, that. Extra, temperature. All. Of this conspired, to, basically, remove, the.
Water From, a Venus, quite. An interesting story when we consider, how, that played out long ago and then, the volcanism of Venus also to. Shape the surface that we see today. Well. We learned a great deal about the, surfaces, of the terrestrial worlds haven't we let's, go ahead and do our summary, we. Saw that there were four main processes. That. Shaped the surface of a terrestrial world. Impact. Cratering. Volcanism. Tectonics. And erosion. We. Saw that as far as the moon and Mercury were concerned, really. The dominant process there was cratering, wasn't, it, original. Impacts, from the final stages of accretion are. Still in evidence today why. Because. The other three process simply, didn't, operate on these worlds, there. Really was no atmosphere, to cause erosion, there's. Very little internal heat to. Cause volcanism, or, tectonics. However. When we came to Mars we saw a great variety in the landscape, didn't we some. Regions were heavily. Cratered, indicating. They were still quite old but. Other regions, were far. Less cratered, indicating, something had resurfaced, those, areas, what. Was that something well again we saw number one numerous, volcanoes, including some giant ones, so. Mars held onto its internal heat longer. And. It was able to then have, this tremendous amount of Welkin ism no, doubt much of that surface, covered. In lava to erase, the craters, we. Also saw evidence of tectonic activity and. Again that didn't surprise us did us when we said what Mars would, hold on to its heat for some time perhaps, not all the way to stay but. It would hold on to that we, saw at least one huge. Canyon. That. Occurred. When the surface was split open and. So. Again, this tells us there was much geologic. Activity there. Finally. What about erosion on Mars well. We did see of course even today wind, erosion as, dust. And soil are transported. In the weak Martian, atmosphere. But. What was real significant. Was, we do see evidence of. Past. Liquid. Water on Mars there's. None there today because the atmospheric, pressure is insufficient. But, there's clear evidence not, only in the features that we see but. When we've landed spacecraft. Picked, up the rocks and examine, them we, find rocks that only could, form in water so. Clearly early in Mars history it. Was a warmer, wetter world much, more hospitable and, yet. It changed dramatically. Finally. We saw that Venus has. Few craters for. Several reasons number one many objects would burn up in that thick atmosphere, but. Perhaps more importantly, the, fact that Venus had. A tremendous. Period, of, active. Volcanism and, much. Of the surface. Has been filled in with lava. There's. Really not much erosion there on Venus that Emma sphere is almost too thick very. Little wind can move any material about, and. We also explored. This whole. Issue. With, Venus water didn't we nina started, out with water but. Its proximity to the Sun meant. That this higher temperature, evaporated. That water that. Water now held in much of the heat and a. Greenhouse effect driving. The temperature even higher. Baking.
Co2 Out of the rocks. Intensifying. The. Greenhouse, effects still further till. All the water and I was in the atmosphere. Why. Isn't it there today because. Again the slow rotation of Venus means that. There's very little magnetic field to. Protect the water from. The sun's radiation, and, therefore, that water was broken apart long ago. So. I think you can see the, great variety of surfaces. That we see among the terrestrial worlds and yet. Interestingly we. Saw we can explain, pretty much everything we see with. Those four major processes. Didn't we so. These are really the key to understanding. The. Terrestrial, worlds their. Surfaces, their features everything, we see in them we. Can usually attribute, back to, those four processes. So. Hope you've enjoyed our two-part look at the terrestrial, worlds when, we pick up next time we'll examine some smaller bodies, in the inner solar system the. Asteroids.