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Berkeley ASTRON 10 - 16. Impact Cratering, Crust Shaping Processes

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Impact Cratering, Crust-shaping processes, 9 October 2013!Impact cratering: !1.Impact cratering used to be an area of disagreement between geologists A.and astronomers, but it was accepted in the 2010s. !Astronomers proposed impact cratering as early as 1870 and generally a.accepted it by the 1940s, but geologists didn't accept it until the 1970-80s.!Now, there is no longer any major scientific debate that impacts cause b.craters.!Craters only form if the surface of an object is solid:!B.When the surface of an object is hot/molten, impacts will cause a splash, a.but the evidence of the impacts will sink under the surface. !Because Venus's craters are fairly new, astronomers know that its 1.surface was molten until 700 million years ago (a fairly recent time by astronomy's standards). !An object solidifies from the crust down.!C.After an object is molten throughout from heat generated by impacts a.and internally, the outside layer of that object radiates heat away. It is therefore the crust which first turns solid. !Crusts start out very thin and thicken by solidifying downward. !b.After a while, the crust of an object becomes so thick that its molten c.core breaks past the surface in a fewer and fewer places. Eventually, this object would start looking more like the earth or moon.!How craters are formed through impacts:!D.Before the moment of impact, an explosion caused by the energy of a a.meteoroid's motion turning into the energy of heat generates shock waves and turns solids into gases. !Gases move outward rapidly since take up more space that solids, b.causing another explosion. !Both the explosion and the shockwaves excavate a crater.!c.The shockwaves continue outward and debris falls all around, creating d.the upraised rim of the crater and a blanket of material around the crater. !Pulverized crust and (in larger craters) molten rock cause a crater's flat e.floor.!The light stuff around fresh craters are crushed rock and little sphere of E.rock that melted, turned into globules, and solidified as glass droplets !These glass spheres are an exact mirror: they reflect the light that enters a.them from the same direction it entered. !When you look at the moon with the sun at your back, the glass beads b.reflect sunlight towards the sun and you.!Craters come in all sizes: !F.Some are the size of houses, and others are microscopic (a result of a.cosmic rays). !Some craters within a certain range of size have central peaks. !G.Craters above or below that range, as well as some craters within that a.range do not have central peaks. !This range depends on the size of the object that is being impacted.!b.The mechanism by which these central peaks are formed is still a c.mystery. !If the rock was molten you could see a splashback, but the rock the 1.centrals peaks are made out of never melted. !Any student who solves this problem to the satisfaction of experts in 2.the specialty, gets an instant A for the entire course, regardless of anything else. !The surface of the moon: !H.The craters of the moon appear vividly in telescopes, so for a time a.astronomers thought that craters were a unique feature of the moon (which was wrong).!Astronomers know from space craft that there are craters A.everywhere in the solar system. !You do not need to know all the names of the craters on the moon, but b.know that they have names. !The craters on the moon are named by astronomers for astronomers 1.and a couple other important people. !Besides craters, the moon also has broad flat dark areas called maria c.(sing. mare). !Astronomers now know the sequence of the moon's formation and have d.a good idea of the timing of when it occurred: !All of the big hits were in the first 2/3 of a billion years. !1.Most of the heavy bombardment of the moon occurred at the 2.beginning of the solar system around 4.2 billion years ago.!There was another period of heavy bombardment about 3.8 billion 3.years ago called the "late heavy bombardment."!Since then, this part of the solar system has been a in period of light 4.bombardment.!For example, the meteor last april in Chelyabinsk, Russia and June A.30th, 1908 in Tonguska, Siberia. !Individual craters on the moon: !e.On the near side: !1.Tycho: !A.This crater is 109 million years old by all 3 different methods a.astronomers use to date craters.!It is one of the most recent large craters on the moon. !1.It is not very eroded since erosion on the moon is negligible. !2.The walls of this crater are irregular, and there is a central peak. !b.There is a dark area immediately surrounding the crater and a c.whole bunch of light material.!Wide and deep. !d.All of these are standard feature of craters.!e.Langrenus: !B.Multiple/jumbled central peaks. !a.Ridged outer crater, walls have terraces. !b.Fairly large. !c.Plato:!C.It has a flat floor, meaning it was flooded by molten rock.!a.No one knows if the rock flooded immediately after its 1.formation or a much later. !Maskelyne: !b.Too small to be flooded, just an impact pit.!1.There are also lots of crater on the far side of the moon. !2.The surface of Mercury:!I.It looks strikingly like the surface of the moon.!a.However, the size range of craters which have central peaks on Mercury b.is different than the size range of craters which have central peaks on the moon.!Mercury has light stuff around its fresh craters which you cannot see at c.its first quarter or crescent phase.!The surface of Venus: !J.We do not have good images of Venus' surface in visible light because a.the planet is wrapped in clouds. !The photos which give astronomers the best idea of what Venus' 1.surface looks like are those taken in radar from spacecraft orbiting it.!Venus has craters too, and some of its craters have central peaks. !b.The surface of Mars: !K.Mars' surface is full of craters. !a.One of its craters is on a ridge, so we can see a cross section. !b.The stripes of dust in this cross section of the crater tell you the 1.layering of Mar's surface. !The dark dust layer and light dust layer are composed of different A.kinds of dust brought to the crater by Mar's wind. !The dust was trapped inside the crater, and eventually becomes a.a thick layer. !The flatlands of Mars have much less dust. !b.There are also lots of landslides on Mars.!c.The surface of other moons: !L.Moons of Jupiter: !a.Ganymede: Number 3 moon of Jupiter.!1.Largest moon in the solar system (larger than Mercury). !A.If Ganymede orbited the sun rather than Jupiter, it


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Berkeley ASTRON 10 - 16. Impact Cratering, Crust Shaping Processes

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