hello and welcome to General astronomy lecture 10 introduction to planetary geology it's easy to take for granted the qualities that make earth so suitable for human life a temperature neither boiling nor freezing abundant water a protective atmosphere and a relatively stable environment but we need to look only as far as our neighboring terrestrial worlds to see how fortunate we really are the Moon is airless and Barren and Mercury is much the same Venus is a searing hot house while Mars has an atmosphere that's so thin and cold that liquid water cannot last on its surface today how did the terrestrial worlds come to be so different and why did Earth alone develop uh conditions to permit life on its surface we'll explore these questions through careful comparative studies of the planets focusing on the geology of the terrestrial worlds in this lecture and we will see um later on the histories of the terrestial worlds uh have been determined largely by properties that were endowed at their births and then uh we'll also talk about the atmospheres of these planets as well in a coming lecture so Earth's surface seems solid and steady but every so often it offers us a reminder that nothing about it is permanent if you live in either Alaska or California you've probably felt the ground shift beneath you in an earthquake in fact we have quite a few minor ones here in Oklahoma in Washington state you have probably witnessed the rumbling of Mount St Helens uh in Hawaii a visit to the active Koo volcano will remind you that you are standing on a mountain of volcanic rock protruding from the ocean's floor volcanoes and earthquakes are not the only processes acting to reshape Earth's surface they are not even the most dramatic far greater change can occur on rare occasion when an asteroid or Comet slams into Earth more gradual processes can also have spectacular effects the Colorado River causes only small changes in the landscape from year to year but its unrelenting flow over the past few million years carved the Grand Canyon the Rocky Mountains were once twice as tall as they are today but they have been cut down in size through tens of millions of years of erosion by wind rain and snow entire continents even move slowly about completely rearranging the map of Earth every few hundred Mill ion years Earth is not alone in having underground or undergone tremendous change since its birth the surfaces of all five terrestrial worlds that being Mercury Venus Earth the moon and Mars have looked uh much similar when they were they looked much similar when they were young all five were made of Rocky material that condensed in the solar nebula and all five were subjected to early uh impacts of the great um bombardment the significant differences in their present day appearance must therefore be the result of changes that have occurred Through Time ultimately these changes must be traceable to fundamental properties of the planets so this figure shows Global views of the terrestrial surfaces to scale as well as samples of their surface views from orbit profound differences among these worlds are immediately obvious Mercury and the moon show the scars of their battering during the heavy bombardment in that sense they are densely covered by craters and um there are regions where you see volcanic plains as well so these flat what we call Highlands which we'll talk about soon bizarre bulges and odd volcanoes dot the surface of Venus Mars despite its middling size has the Solar System's largest volcanoes and a huge Canyon cutting across its surface along with numerous features that appear to have been shaped by liquid water Earth has surface features similar to all of the other terrestrial worlds and more including a unique layer of liing or living or organisms that covers almost the entire surface of the planet which we call the biosphere our goal in this uh lecture is to understand how these differences among the terrestrial planets came to be uh and this type of study is what we call Planetary geology all the teral worlds have layered Interiors we divide the Interiors into three layers according to density first we have the core the central most region this is the high density material consisting primarily of metals such as nickel and iron that resides in the central core or the center of the planet next we have the mantle which is Rocky material of moderate density uh it's mostly minerals that contain silicon oxygen and other elements that forms uh the thick mantle that surrounds the core beyond that we have the this is the low density Rock region which consist of uh rocks along the likes of granite and basalt which are Basalt is a common form of volcanic rock um that forms the thin crust which is essentially representing the world's outer skin so this figure here is very important it shows the layers for the five terrestrial worlds although not shown Earth's metallic core actually consists of two distinct regions a solid inner core and a molten or liquid outer core Venus may have a similar core structure but without seismic data we cannot be sure we can understand why the Interiors are layered by thinking about what happens in a mixture of oil and water gravity pulls the denser water to the bottom driving the less dense oil to the top in a process known as differentiation because it results in different layers of different materials the layered Interiors of terrestrial worlds tell us what that they underwent differentiation at some time in the past which means all of these worlds must have once been hot enough inside for their internal Rock in metal to melt which was during formation dense materials like iron sank toward the center which drove the less dense Rocky materials toward the surface so you'll not it's a lot of interesting features here and this is another one of those images that's good to reference uh if you ever need it um but you'll see that for example Mercury has a really large core compared to the rest of its um M uh interior and then the Moon is kind of the opposite as a very very small core compared to the rest of its uh internal um structure so we're going to learn a little bit about how all these things came about so um comparing the trust worlds Interiors provides important clues about their early histories models indicate that the relative proportion of metal and rock should have been similar throughout the inner solar system at the time that they formed which means we should expect smaller worlds to have correspondingly smaller metal cores we do indeed see this General pattern in the figure on the previous slide but as I mentioned it's not perfect Mercury's core seems surprisingly big while the moon's core seems surprisingly small these surprises are a major reason scientists suspect that giant impacts affected both of those worlds in Mercury case a giant impact that blasted away its outer Rocky layers while leaving its core intact could explain why the core is so large compared to the rest of the planet we can explain the moon's small core by assuming that the moon formed from debris blasted out of the Earth's Rocky outer layers the debris would have contained relatively little highdensity material and therefore would have ACC Creed into an object with a small metal core so you can see an example of those two again pulled up here in terms of rock strength a planet's outer layer consists of relatively cool and rigid rock called the lithosphere where lithos is Greek for stone that essentially floats on the warmer softer rock beneath as shown by the dash circles in the previous figures the lithosphere encompasses the crust and part of the mantle of each World notice that lithospheric thickness is closely related to a World's size smaller worlds tend to have thicker at uh thicker Li lithospheres excuse me the two largest terrestrial planets that being Earth and Venus have thin lithospheres that extend only a short way into their upper mantel the smaller worlds Mars Mercury and the moon have thick lithospheres that extend nearly to their cores the thickness of the lithosphere is very important to geology a thin lith lithosphere is brittle and can break easily while a thick lithosphere is much stronger and inhibits the passage of molten rock from below making volcanic eruptions and the formation of mountain ranges less likely so if you want to go back you can see this so the lithosphere is defined by the dashed line and the region from the surface to the end of that dashed line so again for Earth and Venus we have small lithospheres but for the other planets uh well and the moon the lithosphere extends almost all the way to the core and we'll learn about how that happens later on the fact that rock can deform and flow also explains why large worlds are spherical while small moons and asteroids are more potato-shaped or randomly shaped the weak gravity of a small object is unable to overcome the rigidity of its Rocky material so the object retains its shape it had when it was born for a larger World gravity can overcome the strength of solid rock and slowly deforms and molds it into a spherical shape gravity will make any rocky object bigger than about 500 km in diameter into a sphere within about a billion years larger worlds become spherically more quickly especially if they are molten or gaseous at some point in their history the most interesting aspects of planetary geology are those that cause the surfaces of the terestrial worlds to change with time we use the term geological activity to describe ongoing changes for example we say the Earth is geologically active because volcanoes uh earthquakes erosion and other geological processes are continually reshaping its surface in contrast however the moon and Mercury have virtually no geological activity which is why their surfaces today look essentially the same as they did billions of years ago interior heat is the primary driver of geological activity for example volcanoes can erupt only if the interior is hot enough to melt at least some rock into molten lava but what makes some planetary Interiors hter than others well to find the answer we must investigate how Interiors heat up and cool off as we'll see we can ultimately trace a planet's internal heat and hence its geological activity back to its size so size is the key here so before we get into the uh details of that let's take a look at both of these types of processes the heating processes first and then we'll look at how planets cool a hot interior contains a lot of thermal energy and a law of conservation of energy tells us that this energy must come from somewhere although you might first guessed that the sun would be the heat Source this is not the case sunlight is the primary heat source for the surfaces of the terrestrial planets but virtually none of his solar energy penetrates more than a few meters into the ground internal he heating is a product of the planets themselves not of the sun three sources of energy explain nearly all the interior heat of the terrestrial worlds first we have heat of accretion so accretion deposits energy brought in from afar by colliding planetesimals as a planetesimal approaches a forming Planet its gravitational energy is converted into kinetic energy which causes it to accelerate so basically it accelerates As It Gets nearby upon impact much of that kinetic energy is then converted into heat adding to the thermal energy of the planet the next is heat from differentiation so we've been talking now a couple times of this process of differentiation where heavy material sinks to the bottom and lighter material floats to the top well when a world under goes differentiation the sinking of dense material and rising of less dense material means that mass moves inward losing gravitational potential energy this energy is converted into thermal energy by the friction generated as material separates by density so basically you have material moving up some moving down they are rubbing past each other and just like if you were to rub your hands together right now you would feel them warm up a bit so that friction generates uh some of that heating the same thing happens when you drop a brick into a pool as the brick sinks to the bottom friction with the surrounding water heats the pool though of course the amount of heat from a single brick is far far too small to be noticed and then the last here is heat from radioactive decay so the Rock and metal that builds the terrestrial worlds we know now contains radioactive isotopes of elements such as uranium potassium and thorium when radioactive nuclei Decay subatomic particles fly off at high speeds colliding with neighboring atoms and heating them so the combination of the three heat sources explains how the terrestrial Interiors ended up with the core mantle cross structures the many violent impacts that occurred during the later stages of accretion deposited so much energy that the outer layers of the young planets be began to melt this started the process of differentiation which then released its own additional Heat this heat along with the substantial heat from early radioactive decay made the Interiors hot enough to melt and differentiate throughout all right so those were the three heating processes we also have three cooling processes cooling a planetary interior requires transporting heat outward just as there are three basic heating processes for planetary Interiors there are also three basic cooling processes the first is convection convection is the process by which hot material expands and Rises while cooler material contracts and falls it therefore transfers heat upward and can occur Whenever there is a strong heating From Below you can can see convection in a pot of soup or a pot of boiling water on a hot burner and you may be familiar with it in weather warm air near the ground tends to rise while cool air above tends to fall next we have conduction conduction is the transfer of heat from hot material to a cool material through contact it is operating when a hot potato transfers its heat to your cooler hand when you pick it up conduction occurs through the microscopic Collision of individual atoms or molecules molecules of materials in close contact are constantly colliding with one another so the faster moving molecules in the hot material tend to transfer some of their energy to the slower moving materials of the cooler material I meant the slower moving molecules of the cooler material excuse me and then the third cooling process is radiation planets ultimately lose heat to space through radiation remember that objects emit thermal radiation characteristic of their temperatures this radiation which often is light carries energy away and therefore cools an object because of their relatively low temperatures planets radiate primarily in the infrared so this is something we'll learn about in a couple lectures from now um but there's more than just visible light there's infrared ultraviolet and so on so planets radiate mostly in infrared which is why we don't see it for Earth convection is the most important heat process in the interior hot rock from deep within the mantle gradually Rises slowly cooling as it makes its way upward by the time it reaches the top of the mantle The Rock has transferred its excess heat to its surroundings so it is now cool and begins to fall back down this ongoing process creates individual convection cells within the mantle shown as small circles in this figure here uh with arrows indicating the direction of flow mantle convection stops at the base of the lithosphere where the rock is too strong to flow as readily as it does lower down from the base of the lithosphere to the surface heat continues upward primarily through conduction although some heat also reaches the surface through volcanic eruptions that directly carry The Hot Rock upward when heat finally reaches the Earth's surface it then is radiated away into space so I told you a moment ago that all of this comes back to size of the planet to the size of the planet so we'll look at that a little bit now size is the single most important factor in planetary cooling just as a hot potato remains hot inside much longer than a hot pea a large planet can stay hot inside much longer than a smaller one you can see why size is the critical Factor by picturing a large planet and uh by picturing a large planet as a smaller Planet wrapped in extra layers of rock those extra layers of rock act as an insulation so it takes much longer for interior heat to reach the surface size is therefore the primary factor in determining geological activity the relatively small sizes of the Moon and Mercury probably allowed for their Interiors to cool within a billion years or so after they formed as they cooled their lithospheres thickened and mantle convection was confined to deeper and deeper layers ultimately the mantle convection probably stopped altogether with insufficient internal heat to drive any further movement of interior Rock the moon and Mercury are now considered geologically dead meaning that they have little or no heat driven heat driven geological activity today in contrast the much larger size of Earth has allowed our planet to stay quite hot on the inside mantle convection keeps interior Rock in motion and the heat keeps the lithosphere thin which is why geological activity can continually reshape the surface Venus probably remains nearly as active as Earth thanks to its very similar size Mars with a size between those of the other terrestrial worlds probably represents an intermediate case it has cooled significantly during its history but it is probably still warm enough to re uh to retain some of its internal heat for at least some geological activity interior Heat plays another important role it can help uh create a global magnetic field Earth's magnetic field determines the direction in which a compass needle points but it also plays many other important roles the magnetic field helps create a magnetosphere that surrounds our planet and diverts the paths of high energy charged particles coming in from the Sun the magnetic field therefore protects Earth's atmosphere from being Stripped Away into space by the solar particles many scientists suspect that this protection has been crucial to the long-term hability I'm sorry habitability of Earth and hence to our own existence so it's really amazing uh there's charged particles coming from the Sun but we have a magnetic field around the earth that invisible what I would consider a force field in the simplest of terms there's this invisible field around us that basically diverts any of this charge particle coming from the Sun if we did not have this those charged particles would just smash into our atmospheric particles and start to strip away our own atmosphere so we're lucky that we have this magnetic field so let's dive into this a little bit further to see how it works you are probably familiar with the pattern of the magnetic field created by a iron bar so this image here on the right so there's a magnet here with a North and South Pole it's a bar magnet and you might have seen this in demos and stuff before maybe in another course or maybe somewhere online um but if you sprinkle iron filings around the magnet you'll see them align in this interesting pattern it's this looping pattern from north to south so um that's a common view of a magnetic field you can't see it with our own eyes but if you sprinkle some iron filings they align with those field lines well Earth's magnetic field is generated by a process more like that of an electromagnet in which the magnetic field arises from a battery that forces charged particles that being electrons to move along a coiled wire so you can see this here this is a battery just hooked up to some wire and if you wrap the wire into a loop it will create magnetic field that creates a similar looping structure now I won't go into the physics of this U my physics 2 class does go into a lot of detail about how this electric current can create a magnetic field and things like that um but for our course here in astronomy we'll a lot of the physics but the point is a battery hooked up to a coil of wire does produce magnetic field shown by the pink lines so um Earth does not contain a battery I mean we've we've kind of been able to tell so far but charg particles move with the molten material in its liquid outer core so we do have a liquid outer core which means there is the motion of charged particles internal heat causes the liquid metal to rise and fall via convection while Earth's rotation twists and distorts distorts this convection pattern the result is that electrons in the molten metal move within Earth's outer core in much the same way that they move in an electromagnet generating Earth's magnetic field so although in the top we needed a battery the the reason that a magnetic field is created was the motion of charged particles well we have a liquid core with charged particles moving around so in much the same way way we generate a giant magnetic field around the earth we can generalize these ideas in uh to other worlds there are three basic requirements for a global magnetic field first an interior region of electrically conducting fluid that being liquid or gas such as molten metal right so you need a region of some kind of liquid or gaseous metal um convection in that layer of fluid must also occur so it needs to be moving and we also need a moderately rapid rotation so just the convection alone isn't going to generate that magnetic field we need that twisting and distorting caused by the rotation as well so Earth is the only terrestrial world that meets all three requirements which is why it is the only terrestrial world with a strong magnetic field the moon has no magnetic field presumably because its core has long since cooled in season convecting Mars's core probably still retains some heat but not enough to drive uh core convection which is why it also lacks a magnetic field today Venus probably has a molten core layer much like that of Earth but either its convection or its 243 day rotation period is too slow to actually generate that magnetic field Mercury however remains an enigma it possesses a measurable magnetic field despite it small size and slow 59-day rotation the reason for this may be Mercury's huge metal core which may still be partially molten and convecting the same three requirements for a magnetic field also apply to Jovian planets and stars for example Jupiter's strong magnetic field comes from its rapid rotation and its layers of convecting metallic hydrogen that conducts electricity the sun's magnetic field is generated by the combination of convection and ion gas or Plasma in its interior and its rotation now that we have discussed how Earth and other terrestrial worlds work on the inside we are ready to turn to their surfaces surface features tell us a great deal about the histories of the planets although we find a huge variety of geological surface features on earth and the other terrestrial worlds nearly all of them can be explained by just four geological processes es first impact cratering the creation of bull-shaped impact craters by asteroids or comets striking a planet's surface second by volcanism the eruption of molten rock or lava from a planet's interior onto its surface third tectonics this the disruption or uh of a planet's surface by internal stresses and fourth erosion the wearing down or building up of geological features by wind water ice and other phenomena of planetary weather so these four things together can shape the surface of a planet so we'll break each one of them down oops first we had impact cratering the scarred faces of the Moon and Mercury attest to the battering that the Tres or worlds have taken from leftover planetesimals such as comets and asteroids they also immediately reveal an important feature of impact craters small craters far outnumber large ones confirming that many more small asteroids and comets orbiting the Sun uh than large ones while the moon and Mercury bear the most obvious scars all of the terrestrial worlds have suffered similar impacts an impact crater forms when an asterid Comet slams into a solid surface impacting objects typically hit the surface at a speed between about 10 10,000 and 7 I'm sorry 10,000 and 70,000 m/s at such tremendous speeds the impact uh releases enough energy to vaporize Solid Rock and blast out a crater uh which is a Greek word for cup debris from the blast shoots high above the surface and then rains down over a large area if the impact is large enough some of the ejected material can escape into space and you can see a little graphical representation of this here so we have an impactor coming in and you can see the resulting cratering craters are usually circular because an impact blasts out material in all directions regardless of the incoming object's Direction laboratory experiments show that craters are typically about 10 times as wide as they uh as the objects that created them and about 10 to 20% as deep as they are wide for example an asteroid 1 km in diameter will blast out a CR a crater about 10 km wide and 1 to 2 km deep a large crater may have a central Peak which forms from when the center rebounds after impact in much the same way that water rebounds after you drop a pebble into uh the surface of water so the figures here on the right show two impact craters one on earth at the top and one on the moon at the bottom detail dets of crater shapes provide clues about geological conditions for example this figure contrasts three craters on Mars the crater in figure one that on the left has a simple bll shape as we expect from the basic cratering process the crater in the second figure has an extra large bump in its Center and appears to be surrounded by mud flows uh suggesting that underground water or ice melted or vaporized on impact the mud debris then flowed across the surface and hardened into the pattern that we see today the crater in figure three shows obvious signs of erosion it lacks a sharp rim and its floor no longer has a well-defined bll shape this suggests that ancient rainfall eroded the crater and that the crater bottom was once a lake studies of crater shapes on other worlds provide similar Clues to their surface conditions in history next is volc uh volcanism we use the term volcanism to refer to any eruption of molten lava whether the lava comes from a tall volcano or simply Rises to the surface through a crack in the planet's lithosphere volcanism occurs when underground molten rock typically called magma finds a path to the surface as you can see in this little cutout here on the right the same molten rock that is called magma when it is underground is called lava once it erupts onto the surface Molten Rock tends to rise for three main reasons first molten rock is generally less dense than Solid Rock and lower density materials tend to rise when surrounded by high density materials second because most of Earth's interior is not molten the solid rock surrounding a chamber of molten rock that is a magma chamber can squeeze the molten rock driving it upward under pressure and third molten rock often contains trapped G gases that expand as it Rises which can make it rise much faster and lead to dramatic eruptions now not all volcanoes are the same we do have several types so the result of an eruption depends on how easily the lava flows around the surface lava that is runny can flow far before it cools and solidifies while thicker lava tends to collect in one place broadly speaking lava can have lava can shape three different types of volcanic features the runni lava flows far and uh I'm sorry the runni lavas flow far and flatten out before solidifying creating vast volcanic planes so you can kind of see that here in the left it looks like a fairly flat region magma just comes up to the surface and basically fills up like a pool uh in the Middle with somewhat thicker lavas they tend to solidify before they completely spread out creating what we call a shield volcano so named because of their shapes shield volcanoes can be very tall but are not very steep examples include the mountain the mountains of the Hawaiian Islands on Earth and Olympus Mons on Mars which you can actually see here in this figure this large surface here the thickest lavas cannot flow very far before they solidify and therefore they build up tall uh these create steep Strat volcanoes like you see here with Mount Hood on the right examples include Mount Fiji of Japan Mount kjaro um and Mount Hood in Oregon volcanic mountains are the most obvious results of volcanism but volcanism has had a much more profound effect on our planet it explains the ex existence of our atmospheres and oceans recall that Earth accreted from Rocky and metallic planetesimals while water and other I were brought in by planetesimals from far more distant reaches of the solar system that crashed into the growing planets again that goes back to our discussion um during the formation of the solar system where we talked about the snow line where water could exist and stuff like that so water and gases became trapped in those interior planets in much the same way that gas in a carbonated beverage is trapped in a pressurized bottle volcanic eruptions later released some of this gas into the atmosphere spere in a process known as outgassing outgassing can range from dramatic as during a volcanic eruption like you see at the top right to more gradual as when gas escapes from volcanic vents like you see in the bottom right the same type of outgassing also occurred on the other terrestrial worlds that is virtually all of the gas that made the atmospheres of Venus Earth and Mars and the water vapor that rain down to form Earth's oceans originally was released from the planet Interiors via outgassing all right our next topic was tectonics the term tectonics comes from the Greek word tecton for Builder notice the same root word uh notice the same root in the word architect which means master builder in geology tectonics refers to the building of surface features by stretching compression or other forces acting on the lithosphere tectonic features in a variety of ways for example the weight of a volcano can bend or crack the lithosphere beneath it while a rising plume of hot material can push up on the lithosphere to create a bulge however most tectonic activity is a direct or indirect result of mtle convection the crust can be compressed in places where adjacent convection cells push Rock together so you can see that right here on the left of this Central image this type of compression helped create the appalachi mountains of the Eastern United States cracks and valleys like you see on the right of this middle image uh form in places where adjacent convection cells pulled across aart examples of such cracks and valleys include the gvir plains on Venus uh the sir seran valleys on Mars pardon my pronunciation and New Mexico's Rio Grand Valley on Earth the ongoing stresses of mantle convection ultimately fractured Earth's lithosphere into more than a dozen pieces or plates these plates move over under and around each other in a process that we call plate tectonics the movement of plates explains nearly all of Earth's major geological features including the arrangement of the continents the nature of the sea Flor and the origin of earthquakes because plate tectonics appears to be unique on Earth we'll save this for a future discussion so we'll come back to this when we talk about the Earth in particular our last of the four geological processes that shapes um a planetary surface is erosion this one's pretty simple erosion refers to the simple uh I'm sorry it refers to the breakdown or transport of surface Rock through the actions of Ice uh liquid or gas the shaping of valleys by glaciers uh the carvings of canyons by rivers and the shiftings of sand dunes by wind are all examples of erosion note that although we often associate erosion with breakdown it also builds things up such as sand sand dunes river deltas and lake bed deposits all right so we'll finish off this lecture with a couple slides discussing how we can relate uh cratering on a surface to the age of a planet and this is quite fascinating it turns out that the the smaller the terrestrial world the less internal heat it is likely to have retained and thus the less geological activity it will display on its surface the less geologically active the world the older and hence more heavily cratered its surface this rule means that we can use the amount of cratering visible on a planetary surface to estimate the age of its surface and how geologically active it is so this is a very important idea right so a smaller planet has less internal heat so we know that it must be less geologically active well if that's the case it must be older because it's been cooling for a long time and hence the surface will be more heavily cratered because you won't be seeing volcanoes and things like that reshaping the surface so notice that impact cratering is only one of the four processes with an external cause it that being the random impacts of objects from space this fact leads to one of the most useful insights in planetary geology we can estimate the geological age of any surface region from its number of impact craters with more craters indicating an older surface and by geological age we mean the age of the surface as it now appears a geologically young surface is dominated by features that have formed relatively recently in the history of the solar system while a geologic Ally old surface still looks about the same today as it did billions of years ago you can understand this idea by thinking about why the moon has so many more impact craters than Earth recall that all the terrestrial worlds regardless of their size or distance from the Sun were battered by impacts during the heavy bombardment that occurred early in our solar systems history most impact crators were made during that time and relatively few impacts have occurred since in places where we see numerous craters such as on the surface of the Moon we must be looking at a surface that has stayed virtually unchanged for billions of years in contrast when we see very few craters as we do on Earth we must be looking at a younger surface one on which the scars of ancient impacts have been erased over time by geological processes such as volcanic eruptions and erosion careful studies of the Moon have allowed planetary scientists to be more precise about surface ages the degree of crowding among craters varies greatly from place to place on the moon in the lunar Highlands which is the brighter higher areas uh craters are so crowded that we see craters on top of one another but in the lunar maria which happens be the dark regions that you see the lower regions we'll get into this later um we only see a few craters on top of generally smooth volcanic PLS radiometric dating of rocks brought back by the Apollo astronauts indicates that those from the lunar Highlands are about 4.4 billion years old telling us that they that the heavy cratering occurred early in the Solar System's history but rocks from the Maria dates to 3 to 3.9 billion years uh ago telling us that the lava flows that made these volcanic planes had occurred by that time because the Maria contains only about 3% as many craters as the highlands we conclude that the heavy bombardment phase must have ended by about 4 billion years ago go and relatively few impacts have occurred since that time so I just am going to leave you now with a couple interesting slides so um a planet's fundamental properties of size distance from the Sun and rotation rate are responsible for its geological history so these next three slides uh show the role that each key property sep uh each of those Key Properties separately um but a planet's overall geological Evolution depends on a combination of these three effects so we've talked about this one already the size of the planet matters right so basically um if it's smaller it cools more rapidly which means there's less geological activity and then the surface is going to be older and vice versa well when you talk about distance from the Sun we also mention that at the surface the sun does affect uh the temperature so if you're close to the Sun the surface is too hot for rain snow or ices to occur um and it allows as gases to escape more easily so the atmospheres are probably more thin if at all uh planets at intermediate distances from the Sun like our own Earth we have everything in moderation and then planets far away are cooler at the surface so it can allow for I and snows to form um that limits erosion because you won't have any liquid uh precipitation um atmospheres could exist as well U but they more easily condens to form IES and we'll see that when we talk about Mars later on on what carbon dioxide does in the atmosphere and last but not least rotation we know rotation uh is a primary reason for magnetic fields uh and weather as well we know most of our weather comes from the rotation of the earth as well so basically slow rotation means less wind and weather and a weak magnetic field but if you have a stronger rotation you have more of all of those things so I just wanted to throw these in at the end for a nice General um reference and from this point we will start to look at a lot more detail in how geology works on each of the different terrestrial worlds so I'll dig a bit deeper and then after that we'll get into the atmospheres as for as always thanks for watching and have a great day