Unit 3 - The Terrestrial Planets (Chapters 6 - 11)

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Earth Sciences
Earth Sciences 1086F/G
Phil Mc Causland

Introduction February-06-14 10:20 AM (Mercury, Venus, Earth, Mars) - The division of planets into terrestrial and gas/ice giants is not based on mass, physical size, or any property other thandensity - The four terrestrial planets are all quite close to Sun - They are all rocky bodies, and the heat energythey receive from Sun is simply too great for any bodyto be mainlyice (likethe four gas/icegiants) - The compositionof that "rocky" material is commonto them all: a lot of silicon (Si), oxygen (O), aluminum(AI), magnesium(Mg), sulfur (S), and iron (Fe) → Basalt is also commonto them all BASALT: An igneousrock, the primaryproductof volcaniclava, fine grained,and dark grey to black - The closer a planetis to Sun (which has a massive gravitational attraction), the more often it will be bombarded by incomingasteroids and comets Learning Objectives - Learn the different models of Moon's origin and why one is favored - Earth  Size, density, and internal zones  Continentaldrift, plate tectonics, andmagnetism  Geologicaltime and age dating  Basic properties of the hydrosphere and atmosphere  How early life developed - Moon  Only the most basic properties  Summaryof its geological history - Mercury, Venus,and Mars  Basic planetfacts  Geology  Most significantinvestigative missions  How they differ from Earth Unit 3 - The Terrestrial Planets Page 1 Chapter 6: When Two Planets Collide February-06-14 10:41 AM - Mercury has no natural satellite, Venus also has none, Earth has one (Moon), and Mars has two puny little misshapen ones - Compared to all other natural planet satellites, Moon is unusual: → Very large relative to the planet it orbits (Earth) → Abnormally low density for an object associated with a terrestrial planet → A core that amounts to 2-4% of its total mass, compared to 30% for Earth's core → An abnormally high angular momentum - There are three hypotheses explaining how a planet like Earth could be partnered with a satellite like Moon, and the origin o f Moon: 1) Moon broke off from a rapidly spinning Earth — the fission hypothesis 2) Earth and Moon formed contemporaneously from the same material — the condensation hypothesis 3) Moon formed as an independent planetary body that was later "captured" by Earth during a close pass — the capture hypothesis - The objectives of the Apollo space program was to differentiate between these hypotheses to resolve the question of lunar ori gin - The Apollo astronauts brought back to Earth all the raw data and material needed to answer this question, and this evidence " killed" each of the three hypotheses: The Fission Hypothesis - Developed by George Darwin (son of Charles Darwin), proposing that Earth spun so fast in the early days of its formation that a bulge developed somewhere, and soon a chunk broke off and was thrown into orbit  Unfortunately, this would require that Earth rotated once every 2.5 hours rather than once every 24 hours, which was difficult to reconcile with the current rotational rate, so this hypothesis was rejected The Condensation Hypothesis (or "Contemporaneous Formation Hypothesis") - The idea that both bodies formed at about the same time from exactly the same parental "cloud" of dust and gas - The first evidence to support this hypothesis would have to be chemical — the must both have the same composition  They do not: Moon has a tiny metallic core compared with Earth - The second point needed to support this hypothesis would be to find that Moon orbits Earth exactly on an equatorial plane  This is not the case The Capture Hypothesis  The probability for the exact gravitational and dynamic conditions needed for an object the size of Moon to fall into orbit about Earth is astronomically unlikely  Also, some chemicals were so similar between the Earth and Moon that there had to have been some close "genetic" relationship The Giant Impact Hypothesis - A new hypothesis was proposed: a planet that was somewhat smaller than Earth (we call this hypothetical planet Theia), in a progressively unstable orbit about the Sun, gave Earth a glancing blow sometime about 4.5 billion years ago KINETIC ENERGY: Energy a body has by virtue of its motion → Specifically, the kinetic energy (Ek) of a body in motion is one half the mass (m) of the body times the square of its speed (v), or: - The Giant Impact Hypothesis came to be the preferred mechanism to explain the formation of Moon:  Collision(s) and melting of both bodies  The glancing blow(s) gave an increased angular momentum to Earth (i.e., it increased its spin rate)  The metal core of the impactor (the Mars-size body) separated and dropped into Earth, thus giving Earth a large metal core and its remarkably high density  To some degree, the molten mantle material of both bodies mixed, and formed debris in space just above Earth  Over a relatively short period of time some of the debris fell back to Earth, but most of it collected into a single mass to become Moon - One of the challenges to this kind of model has been to find conditions where Moon ends up with a composition similar to Eart h, rather than retaining a large proportion of what was originally Theia - As more precise measurements were made with improved instrumentation, some strong constraints emerged:  Tungsten isotopes tell us that Moon started life at least 30 million years after the start of the solar system, which is long time after a Moon sized body would have formed by accretion  The oldest Moon rocks were formed when a "magma ocean" cooled, so Moon must have started out with a large enough input of energy to be largely melted  The isotopes of oxygen in lunar samples have almost exactly the same proportions as Earth rocks — no other object in the solar system matches Earth composition like this, so we can be quite confident that Earth and Theia compositions must have been well mixed - Combining these three constraints with the constraints we already had (low density and high angular momentum), it still seems that an impact event is required — this area of research is still very active - In November 2012, two papers with quite different views were published in the journal Science: → One paper used a computer to simulate this kind of impact scenario and the only way to get a match for Earth and Moon compositions is to make Theia nearly as massive as Earth in a low-velocity collision → The other paper came up with a high velocity impact of a much smaller body on a quickly spinning Earth - There are now two new hypotheses to test Unit 3 - The Terrestrial Planets Page 2 Chapter 7: Earth February-06-14 11:40 AM - Earth is the only planet that could currently support Earth-type life because of it's temperature, atmosphere, supply of liquid water, and stable climate Here are some basic Earth facts:  Third planet from Sun [Distance Earth to Sun = 1.0 Astronomical Unit]  Fifth largest planet  Densest major body in the solar system  One permanent natural satellite (Moon) — the first planet out from Sun to have any natural satellite  Plane of orbital (called the ecliptic) is only 7 degrees from Sun's equatorial plane  Orbital path is very nearly circular. The tilt of the rotational axis (called the angle of obliquity, or the tilt angle) is primary responsible for seasons  When Earth rotates, its rotational axis moves, making a cone-like pattern — a process called "precession". Because of this, the positions of the celestial poles change  Earth has a strong magnetic field generated by electrical currents in the iron-rich core. Interaction of the liquid metal of the outer core flowing past solid metal of the inner core establishes an electrical current which in turn produces a magnetic field (Earth's magnetic field is uniquely strong of the terrestrial planets)  Unique to all planets is Earth's atmosphere — today it consists of: 78% nitrogen, 21% oxygen, and 1% trace elements and compounds Earth's Earliest History ACCRETION: Growth by accumulation of smaller bodies, dust, and gas - Earth grew by accretion: it attracted others by simple gravity, and as it grew larger, it attracted more and more material toit faster and faster - Soon it became a protoplanet, and then a real planet - As Earth was growing by the accretion of millions of impacts, it got very hot and at least the top 500 km of Earth melted during this period The Iron Catastrophe - Iron accounts for roughly 30% of Earth's composition; iron was a significant component of early Earth - Iron is the heaviest of the components of Earth, and as Earth melted from impactorkinetic energy being transformed into heat energy, hot liquid iron from the upper layer would being to pool and sink under its own great weight - As the iron sank to the planet's core, more energy was released causing the whole planet to melt - This chaotic event is know as the iron catastrophe - As Earth continued to grow, the confining pressure on the innermost core became so great that it was transformed from a liquid to a solid — giving Earth a hot central core of solid metal inside a hot outer core of liquid metal → As the heavy metals preferentially ended up in Earth's core, the lighter elements (lots of silicon, aluminium, and oxygen) ended up at the surface Impact - After these events, the Mars-sized planet, Theia impacted Earth; reshaping it and giving it a satellite A Differentiated Earth DIFFERENTIATION: A general zonation of elements from heaviest at the core to lightest at the top → The zonation was not complete, so there are still many heavy elements at the surface - The zones of differentiation are called inner core, outer core, mantle, and crust, with oceans and atmosphere sitting on top Unit 3 - The Terrestrial Planets Page 3 - The first, and older, scheme used to label the interior divisions of Earth is:  Crust  Mantle  Core - The newer scheme used to label Earth which involves the discussion of the primary active processes (i.e.,plate tectonics) is:  Lithosphere  Asthenosphere  Mesosphere  Outer core  Inner core *Remember this figure and the proportions of the zones - The subdivisions on the left are based primarily uponphysical properties, while the layers on the right are based on chemistry - From surface to core, both pressure and temperature increase, and from surface to core, density increases LITHOSPHERE: The outer 100 km of Earth, encompassing both Earth's crust and the uppermost portion of the mantle (in the first scheme), is a solid, relatively strong, rocky layer ASTHENOSPHERE: Underlying the lithosphere, a layer of heat-softened, relatively weak, slow-flowing (almost plastic) rock located about 100 to 350 km beneath Earth's surface → Most volcanoes get their liquid from the asthenosphere → Within the lithosphere and asthenosphere such large-scale geological processes such as mountain building, volcanism, earthquake activity, and the creation of ocean basins originate MESOSPHERE: Below the asthenosphere, a very wide zone whose confining pressure is so great that the rock has to be solid, but the temperature is high enough that the material acts like a very stiff plastic CORE: Below the mesosphere and divided into a liquid outer core and a solid inner core (the inner core is hotter than the surface of Sun); both are nearly pure metal Magnets and Magnetism - Earth can be thought of as a dipole (2-pole) magnet - Magnetic field lines radiate between Earth's north and south magnetic poles (just as they do between the poles of a bar magnet) DYNAMO: A mechanical device that converts physical energy to electrical energy — the most practical mechanism by which to generate a magnetic field → Anywhere that electrical energy flows, there's a magnetic field surrounding it - The solid inner core spins just a bit faster than the whole Earth → Because it is surrounded by liquid, the inner core is slower to respond to any outside forces → The rotation of one metal inside another is the basis of a dynamo's construction - The occasional turbulence in the hot liquid of the outer core periodically disrupts the process to the point that the dynamogets thoroughly confused - Earth's magnetic poles have sometimes been exactly as they are now (which we call "normal") and at other times completely the opposite, with our north Unit 3 - The Terrestrial Planets Page 4 - Earth's magnetic poles have sometimes been exactly as they are now (which we call "normal") and at other times completely the opposite, with our north magnetic pole then becoming south magnetic pole (which we call "reverse") - Currently, Earth's magnetic north pole is not coincident with Earth's geographic North Pole Plate Tectonics - Earth's lithosphere is divided into a number of segments we call plates - These segments, or plates, move as a result of processes that occur beneath them — these processes are grouped together under the heading tectonics (thus "plate tectonics") - London, ON is located on the North American plate The Hypothesis of Continental Drift - Leonardo da Vinci recognized that the seashell fossils he had found in the mountains meant that a seafloor uplift must have taken place - Charles Darwin recorded evidence of an uplift on the coastline of Chile as a result of a great earthquake while on a voyage - Alfred Wegener was fascinated by the jigsaw parallelism of the coastlines on either side of the Atlantic Ocean → Further analysis showed that a number of features (like mountain chains, belts of particular fossils, etc.) could be connected across the gap of the oceans PANGAEA: Meaning "all lands"; one enormous continent that included present day North and South America, Europe, and Africa - Wegener wrote that Pangaea had somehow broke and spread apart creating the Atlantic— he called the process "continental drift" The Rock Magnetic Pattern - Instruments developed by the Allies, called magnetometers, were used to find and examine sunken German submarines and were housed inside water-tight pods while being hauled behind ships - The magnetic patterns detected by the instruments (by complete accident) showed a pattern of magnetization of Earth's crust that rapidly revived Wegener's hypothesis - The study of magnetic properties of rocks is called paleomagnetism - Magnetite (an iron oxide) is one of the relatively few natural materials that can become permanently magnetized but is a verycommon mineral; particularly in volcanic rock called basalt (the single-most common rock on Earth) - The mineral magnetite forms early in the cooling history of the hot lava from which basalt forms - Above a temperature called the Curie point, atoms are very active, but below that temperature, much less so - The iron oxide ions (i.e., charged atoms or molecules) that go to form the mineral magnetite, respond to the direction of the regional magnetic field below the temperature of 580°C (below its Curie point) → For example, when basalt lava spills out somewhere on the ocean floor and begins to cool to a temperature below the Curie point, all the magnetite grains in the rock become tiny permanent magnets having the same polarity as Earth's magnetic field at the location of eruption - Thus, the pattern of magnetism found by those magnetometers must have been the pattern of the magnetite crystals within the solidified rock - On either side of a central ridge running down the Atlantic defined a symmetrical pattern of bands of basalt with "normal" and "reversed" magnetic polarity - The ridge was determined to be a center of the spreading of plates - As magma from the asthenosphere erupts along the ocean floor rift, the spreading of the lava down both sides of the ridge provides a means of estimating the speed with which the spreading activity occurs Plate Margins - If we are creating new surface at spreading centers, then we must be destroying an equal area of surface somewhere else - In fact, in order to move one segment (or plate) on a sphere like Earth, without actually increasing or decreasing the volumeof the sphere, we end up with three different types of motion of the boundaries: either we pull the boundaries apart, push them together, or slide the plates past one another along the boundary Unit 3 - The Terrestrial Planets Page 5 different types of motion of the boundaries: either we pull the boundaries apart, push them together, or slide the plates past one another along the boundary - The three different kinds of plate margin are: 1) Divergent margins: These margins are characterized by plates moving apart — we commonly call these spreading centers Ex/ The continuous ridge of under-water volcanoes that mark the center of the Atlantic Ocean; it's appropriately known as the Mid-Atlantic Ridge 2) Convergent margins: These are the boundaries characterized by plates moving towards each other, such that the edge of one plate (whichever is more dense) sinks beneath the edge of the second plate — this action is commonly referred to as subduction, and the locations are called subduction zones Ex/ The convergent zone marking the western edge of the whole South American continent 3) Transform fault margins: These are boundaries where two plates slide past each other, with no significant vertical motion — sometimes the faults themselves that make up the margin are called strike-slip faults Ex/ The series of faults cutting across the western edge of California; the most noteworthy is the San Andreas Fault - Most volcanic eruptions occur along plate margins— but only the divergent and convergent types - Most earthquakes occur along plate margins — any of the three types How to Push/Pull a Plate - Three forces could play a role in moving the lithosphere: 1) A "push" away from a spreading center: rising magma at a spreading center creates new lithosphere and in the process pushes the plates away from each other  Pushing involves compression, but the existence of rifts along a mid-ocean ridge indicates a state of tension (the opposite of compression), thereby making this an unlikely mechanism 2) A "dragging" on the back of a convecting current beneath — proponents of the dragging idea point out that the lithosphere breaks and starts to sink through the asthenosphere because the cold lithosphere is denser than the hot asthenosphere  This means that a sinking slab of lithosphere might exert a pull on the entire length of the plate, and to compensate for the descending lithosphere, rock, in the asthenosphere must flow slowly back toward the spreading center (i.e., convection) - Both the pushing and the dragging mechanism have problems, however - Plates of lithosphere are brittle, and they are much too weak to transmit large-scale pushing and pulling forces without major deformation occurring in their middle (which there is no evidence of) 3) The whole plate sliding downhill, away from the spreading center (so gravity is the main force) — the lithosphere grows cooler and thicker away from a spreading center. As a consequence, the boundary between lithosphere and asthenosphere slopes away from the spreading center SEISMIC TOMOGRAPHY: A method of detection for identifying areas where a lithosphere plate fragment has dropped into the asthenosphere and beyond into the mesosphere — they are detected because they are cooler than the surrounding material, thus seismic waves pass through them at a different rate than through hotter material - A break in the lithosphere develops because of mantle plumes Mantle Plumes - A mantle plume rises through the base of the lithosphere because the fierce heat of the adjacent core produces large pockets of hot, and therefore less dense, material that starts to rise through the more stable mesosphere about it (this hot material continues to rise through the lithosphere) - Once the plume arrives at the base of the lithosphere it can either: (i) break through the brittle, cold layer almost immediately, forming just one or two volcanoes, Unit 3 - The Terrestrial Planets Page 6 - Once the plume arrives at the base of the lithosphere it can either: (i) break through the brittle, cold layer almost immediately, forming just one or two volcanoes, or (ii) it may pool under the lithosphere, gradually pushing it up, until one or more breaks occur, and a string of volcanoesresult Dating Rocks: Earth's Geological Time Scale - Two approaches to dating rocks: 1) Relative age dating (determine an age relative to rocks around it) 2) Absolute age dating (getting an absolute years old) Relative Age Dating - Geologists can unravel the sequence of rock formations in the field by looking at their relative relationships → When observing a sequence of undisturbed horizontal sedimentary rocks, the layers at the bottom of the sequence are older than those on top - In time, earth scientists compared rock sections from all over the world and put them into a world-wide time scale — a geological time scale - That time is divided into readily recognized blocks according to some criteria unanimously agreed upon:  Proterozoic (meaning development of life) is the big block of time just before 545 million years ago  Phanerozoic refers to all of time from the Cambrian Period until present day, where there has been abundant, complex life, and is divided into 3 main periods: • Paleozoic (early life) • Mesozoic (middle life) • Cenozoic (recent life) → 65.5 million years ago, an asteroid impact killed some 70-75% of all species, including most dinosaurs → 251 million years ago, an event (perhaps an asteroid impact) killed off about 96% of species on Earth Absolute Age Dates - The discovery of radioactivity has enabled scientists to determine numerical ages of rock units - Radioactivity provides a "clock" that begins running when radioactive elements are sealed into minerals → To say an element is radioactive, is to say that it is unstable or that it "decays" - What we need to determine the age of most rocks is:  The rate of radioactive decay — the half-life  The amount of the isotope that is in the process of breaking down — the parent  The amount of the isotope produced by the breakdown — the daughter Ex/ When we apply the technique to basalt, we are dating the exact time at which the rock cooled from lava - The most common radioactive elements are uranium and thorium - The Earth's surface is younger because of the constant activity of plate tectonics Earth's Atmosphere, Hydrosphere, and the Beginning of Life The Ancient Atmosphere - The things that separate Earth from the other planets are:  The composition of our atmosphere  Liquid water on surface (our hydrosphere)  Our biosphere - The composition of Earth's atmosphere, its atmospheric pressure, and density, are all intimately related to the fact that Earth is covered by vast quantities of liquid water and is teeming with plant life that uses photosynthesis PHOTOSYNTHESIS: The process plants and a few other organisms use to combine sunlight, water, and carbon dioxide to produce oxygen and sugar (which is energy for the organism) - The oceans provide an enormous heat reservoir that stabilizes Earth's climate and keeps the planet's surface temperatures atoptimal levels for the life forms inhabiting it - Other planets have atmospheres, but none has atmosphere that nurtures a biosphere - Earth has not always had a biosphere-friendly atmosphere — it developed thought the geological ages in response to slow changes in other parts of the Earth Unit 3 - The Terrestrial Planets Page 7 - Earth has not always had a biosphere-friendly atmosphere — it developed thought the geological ages in response to slow changes in other parts of the Earth system (it changed about the time Earth's surface solidified) - Today on Earth, nitrogen dominates, making up about 78% of the atmosphere, withoxygen holding a strong second with 21%, and the remaining percentage is made up of argon and traces of carbon dioxide - The compositions used to be nearly identical, but theamount of atmosphere (i.e., the atmospheric pressure) was dramatically different among the three neighbours → Earth's atmosphere has a pressure of 1 bar → Mars has a thin atmosphere with 0.07 bars only (due to it's small mass) → Venus has an extreme atmospheric pressure of about 90 bars (Venus and Earth have a very similar and mass and size, therefore cannot use either to describe the difference)  Rain on Earth dissolves carbon dioxide which vastly lessens the amount of gas in the air, and thus the atmospheric pressure — this also explains why Earth's temperature is much lower than that of Venus - The beginnings of today's atmosphere were volcanic gases — these gases were water vapour (H O) and 2arbon dioxide (CO ) (the ea2ly volcano-derived atmosphere was devoid of oxygen (O )) 2 Add the Hydrosphere and Biosphere - The comets and asteroids that pummelled contained small quantities of water and because Earth's surface was very hot, the water stayed as water vapor in the atmosphere - When Earth's had cooled sufficiently, the water vapor in the atmosphere started to condense and rain began— with this falling rain, the hydrosphere was born - The hydrosphere is largely a closed system — it neither gains, nor loses water (water is recycled) - The appearance of life marked the origin of the biosphere (approximately 3.5 billion years ago) → This life developed in the ocean as it was sheltered from the hostile atmosphere - After this momentous life-event occurred, the biosphere slowly started to change the atmosphere in ways that made it friendlier for the biosphere to grow ever larger — which eventually enabled life to spread to the land - The biosphere changed the atmosphere in two ways: 1) Through the process of photosynthesis (by which plants combine CO and H 2 to fo2m organic matter and O ), the bi2sphere added oxygen to the atmosphere 2) Through the removal of carbon from the atmosphere to form organic matter and limestone, the biosphere lowered the CO content, a2d as a result, the temperature declined The Rise of Oxygen — The Fall of Carbon Dioxide - Earth is the only planet on which water can exist in liquid form on the surface, and liquid water is essential for life - Liquid water Is also responsible for most of the erosion and weathering of Earth's continents - Water can dissolve a certain amount of CO , 2nd thus we can bring CO out 2f the atmosphere and dump it into oceans as a component of rain - If life were to develop in an environment containing no free oxygen (i.e., ananaerobic environment), it would probably get energy from a process like fermentation (breakdown of carbohydrates) - The most ancient fossils that have been found to date are single celled organisms called prokaryotes - They lived in an anaerobic ocean but were capable of photosynthesis (i.e., produced free oxygen in their regular life process) - At this time the oceans were enriched with dissolved iron which absorbed free oxygen from the air - The oxygen produced by the photosynthesis by these most primitive organisms would have quickly combined with iron; oxidized iron is quite insoluble in water, so a red colored iron oxide would be precipitated to the ocean floor - The deposits are typically thinly banded, illustrating a life-death cycle of prokaryotes; abundant organisms means lots of oxygen produced (thus a layer of iron oxide on the sea floor) — but the oxygen soon kills most of the organisms (thus a layer of sediment lacking iron oxide), and as soon as the organisms flourish again, the cycle repeats - Any oxygen formed in the above process never leaves the oceans, but gets grabbed by iron and falls to the ocean floor - After close to 2 billion years, most of the available iron in seawater was finally consumed, and free oxygen began to appear - Virtually immediately, new multi-cellular organisms, called eukaryotes, gained an advantage in this environment:  They used oxygen for respiration  Grew rapidly in vey colonies  Were truly photosynthetic (so produced large amounts of oxygen)  Thus contributed a great amount of oxygen to the atmosphere, and rapidly changed the biosphere of Earth - As land forms rose above sea level (primarily as a result of volcanic deposits) weathering due to acidic rain and erosion proceeded — a multitude of elements ended up in solution in seawater as a result - Calcium (Ca) was carried into the seas by fresh water, and chemically combined with carbon dioxide to form grains of calcite(CaCO ) whi3h precipitate and form limestone, or may be consumed, again with carbon dioxide to form skeletons/shells for marine organisms - This ongoing process removes CO fro2 the ocean water/atmosphere "budget" - Volcanic eruptions, tectonic processes and biological processes now maintain a continuous flow of carbon dioxide from the atmosphere to the various organic and inorganic "sinks" and back again - The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of Earth'ssurface temperature - The free oxygen in Earth's atmosphere is produced and maintained by biological processes Unit 3 - The Terrestrial Planets Page 8 Chapter 8: Moon February-07-14 2:04 PM - Moon is the only nearby object we can look at and pick out features (in astronomical and geological terms, our eyes can "resolvefeatures"on Moon) without help - Moon is the product of a gigantic glancing collision between Earth and a planet we call Theia — this collision resulted in a mixing of the two bodies, with Earth getting most of the metal core of the impacting body, leaving Moon with relatively low density material richer in silicon, aluminum, magnesium and oxygen - Following the collision, the two bodies become "solar dance partners", meaning that there is a gravitational attraction between the two that dictates the physical properties of their orbital and rotational motion TIDAL COUPLING:Earth's gravitational influence on its much smaller neighbor has forced Moon into exactly the same rotational period and orbital period as Earth → The result of this synchronized pattern is that we always see only one hemisphere of Moon, and the "far side" never rotates into a position of view - The relationship between Earth and Moon is constantly changing, and eventually tidal coupling will be complete: one face of Moon will constantly point toward Earth, and one face of Earth will constantly point toward Moon - At times, Moon has had a partner: an asteroid came so close to Earth that it was captured by Earth's gravity The View from Earth - Moon is an airless world → Our understanding of gravity tells us that a world as small as Moon must have a low escapevelocity (the initial velocity any object needs to escape gravity), and gas atoms near its surface escape easily into space → We can see dramatic and sharp shadows between daylight and darkness, meaning there is no air on Moon to scatter light and soften shadows - The face of Moon turned towards us is termed the near side, and it is divided into:  Light areas called the lunar highlands — composed of a rock called anorthosite),and  Darker areas called maria (meaning "seas") — the dark material filling the maria is actually dark, solidified basalt lava from earlier periods of lunar volcanism - Both the maria and the highlands exhibit large craters that are the result of asteroid impacts — the highlands are saturated with these craters, so much so, that it would be impossible to form one new crater without destroying the equivalent of one old crater - These craters allow us to determine the relative ages of the highlands (older) and the maria (younger) - The lava flows that created the maria happened long ago and were much too fluid to build peaks — a few small domes were pushed up by lava below the surface, and we can see long, winding channels called sinuous rilles → In some cases, these channels may have had a rock roof which formed a lava tube, and when the lava drained away, meteoroid impacts collapsed the roof to form a sinuous rille - The far side has almost no maria and a large number of asteroid impact craters Craters to Learn By METEOROID:Sand size to 100 m diameter ASTEROID:Greater than 100 m diameter to 1000km - The more impact craters a surface has from impacting meteoroids and asteroids, the older the surface - Current large impacts are rare Crater Making 101 - Meteoroids and asteroids typically strike planets at 10 or more kilometers per second because of a combination of orbital speed and planetary gravity - If the planet has an atmosphere (like Earth), smaller meteoroids may be destroyed in the atmosphere or slowed so much that th ey do not make craters - The kinetic energy ( ) of high speed impacts is converted upon impact into thermal, acoustic, and mechanical energy; the latter fractures, distorts, and ejects rock from the impact site — making a crater - A shockwave, or highly compressed energy zone in front of the fast-moving asteroid or meteoroid, compresses the rock and makes it deform (almost like fluid) around the impact site - Upon penetrating the surface, this shockwave "explodes" or releases energy below surface, any rock layers that may initially have been flat are heaved upward and outward, bent back, and folded back over surrounding rock - The rim around the crater is built partly from the up-thrust rock and partly from excavated debris dumped around the edge - For Earth impacts, we use a "rule of thumb" that says a crater will be roughly 15-20times the size of the impactor - For Moon, that factor must be larger (perhaps 50 times) because there is no atmosphere to slow objects Cratering Curves - Surface areas of Mars and Moon that have been bombarded by impactors for long periods of time can be quite accurately dated by counting craters Unit 3 - The Terrestrial Planets Page 9 - The kind of plot above is called a cumulative crater size frequency distribution → When we plot the numbers of craters of different sizes on the graph, they fall on a straight line (the axes do not increase linearly) → As the exposure time of the surface increases, the total number of craters increases correspondingly, and the straight line relationship on this plot shits from the bottom left outward toward the top right — very old surfaces eventually get to a point of saturation when every new crater obliterates an old one and this is represented by the line labelled "crater saturation" - The cratering rate for Moon was much higher farther back in time Lunar Exploration - Apollo 11 - On July 20th, 1969, Neil Armstrong became the first human to set foot on Moon - The first step onto the lunar surface from the Apollo 11 Lunar Module, the Eagle, fulfilled the promise of President John F. Kennedy that the U.S. would land a man on Moon before the end of the decade - The scientific return from the lunar mission included high-resolution imagingof the lunar surface, lunar samples, topographic, seismic, and gravity data, and information on the lunar environment - Luna 1 - The Soviet Luna 1 (1959)was the first spacecraft to reach Moon, and the first of a series of Soviet automatic interplanetary stations successfully launched in the direction of Moon - A large cloud of sodium gas was released by the spacecraft and left a glowing orange trail allowing astronomers to track the spacecraft - A record of other "firsts" in lunar exploration:  The first successful USA mission toward Moon was Pioneer4 — it carried a lunar photography experiment but passed too far from Moon's surface to activate it  The USA then had a long streak of successes with their Ranger series (Ranger 3, 4, 6, 7, 8, and 9) — this series took a stack of excellent photos and scientific instrument readings before they crashed in the lunar surface (they were designed to do so in preparation for soft landings)  The USSR won that race: Luna9 was the first ever Earth ship to soft-land on another planetary body • After landing in the Ocean of Storms, the four petals, which formed the spacecraft, opened outward and stabilized the spacecraft on the lunar surface • Spring-controlled antennas assumed operating positions, and the television camera rotatable mirror system, which operated by revolving and tilting, began a photographic survey of the lunar environment • When assembled, the photographs provided a panoramic view of the nearby lunar surface  Within months the USA also made a soft landing with Surveyor  The USSR sent Zond5 to orbit Moon while carrying animals, insects, plants, seeds, and bacteria, then brought the capsule back for a successful landing — this was a planned precursor to a manned trip which they completed with Zond6  The USA sent Apollo 8 (the first manned lunar orbital mission) (1968) around Moon, brought the crew back successfully, and set the stage for landings • With Apollo 10, they did a complete "dry run" except for the landing itself • After being pushed into a stable Earth orbit, "Charlie Brown", carrying 3 astronauts, separated from their thruster rocket and took a cruise to the Moon were they entered lunar orbit • The next day, two of the astronauts climbed into "Snoopy" (a Lunar Module), separated from their CommandService Module, andcompleted 31 revolutions of the Moon— during which time they took an abundance of photos of the proposed landing site for the next mission • The two small modules rejoined each other and returned back into stable Earth orbit where they performed a parachute-controlled splash- down in the Pacific  Apollo 11 was the first mission in which humans walked on the lunar surface and returned to Earth — Neil Armstrong and Edwin E. "Buzz" Aldrin Jr. set up scientific experiments, took photographs, and collected lunar samples during their stay on Moon Moon Rocks - All the rock samples that the Apollo astronauts carried back to Earth were igneous - Every single solid rock on Moon's surface is igneous in origin → They formed by the cooling and solidification of molten rock - Some 95% of Earth's outer few kilometers is composed of igneous rocks - No sedimentary rocks were found by the astronauts, which is consistent with Moon never having had liquid water on its surface - Moon rocks are very dry — only the smallest trace water (detected with very sensitive instruments) have been found in Moon rocks - Rocks from the lunar maria are dark-colored, dense basalts — these rocks are rich in heavy elements which give them their dark color - Some of the basalts are vesicular,meaning that they contain holes caused by bubbles of gas in the molten rock that form when the rock flows out onto the surface, where the pressure is low Unit 3 - The Terrestrial Planets Page 10 surface, where the pressure is low - Absolute ages of the mare basalts have determined that the lava flows that formed these rocks must have happened some time af ter the end of the heavy bombardment - The highlands are composed of anorthosite:low-density rock containing calcium-, aluminium-,and oxygen-rich minerals that would be among the first to solidify and float to the top of molten rock - The rocks of the highlands, although badly shattered by impacts, represent Moon's original low-density crust, whereas the mare basalts originated from magmathat rose from the deep crust and upper mantle - A large fraction of the lunar rocks are breccias,rocks that are made up of fragments of earlier rocks cemented together by heat and pressure - Evidently, meteoroid/asteroid impacts have broken up many of the rocks and fused them together time after time - Both the highlands and the lowlands of Moon are covered by a layer of powdered rock and crushed fragments called the regolith Surface Heat Flow - The rate at which heat escapes from a planet and the rate at which temperature increases as we go down into the interior of the body are important parameters for judging the interior characteristics that we can't see directly - As proven by measurements, Moon's small size means that the body never reached the relatively high temperatures of Earth Seismic Activity SEISMOLOGY:The study of a series of vibrations that move the ground - Moon is seismically much quieter than Earth - Records for Moon show that there are some deep quakes from the interior and surface quakes from impacts - This evidence indicates Moon has a core that is hot and perhaps still partially molten, but is small Geologic History of Moon - The surface of Moon has been completely mapped (by the US probe called Clementine)which has greatly helped to understand its geologic history - The four-stage history of Moon is dominated by a single fact — Moon is small, only one-fourth the diameter of Earth Stage 1: - The Apollo Moon rocks show that Moon must have formed in a molten state - As Moon cooled, denser materials sank to form a small core, and low-density minerals "floated" to the top to form a low-density crust - The low-density measures of lunar surface rock implies that the entire Moon is deficient in iron and other heavy metals relative to our planet - There is no evidence for any lunar magnetic field - The radioactive ages of Moon rocks tell us that the surface solidified from 4.6 to 4.1 billion years ago Stage 2: - This was a period of cratering that began as soon as the crust solidifies - The older highlands show that the cratering was intense during the Late Heavy Bombardment at the end of planet building - Moon's crust was shattered to a depth of 10 kilometer or so during that bombardment Stage 3: - Such intense cratering led to lava flooding - Although Moon cooled rapidly after its formation, some process heated material deep in the crust, and part of the material melted - Molten rock followed the impact cracks up to the surface and flooded the giant basins with successive lava flows of dark basalts — these lava flows formed the maria - The place on Moon opposite the Imbrium Basin (a crater caused by an asteroid the size of Rhode Island) is now a strangely disturbed landscape called jumbled terrain - Note: Studies of Moon show that its crust is thinner on the side toward Earth, perhaps due to tidal effects — consequently, while lava flooded the basins on the earthward side, magmawas unable to rise through the thicker crust to flood the lowlands on the far side Stage 4: - This was a final period of slow evolution - Most surface changes during this period have been produced by the more or less constant bombardment by small meteoroids Unit 3 - The Terrestrial Planets Page 11 Chapter 9: Mercury — A Sun-Scorched Rock February-07-14 8:24 PM - Mercury is the first planet from the Sun, has no natural satellites, and is small - It stays in the same region of the sky as Sun which makes it hard to see from Earth— even with powerful instruments, scientists have only seen pale smudges of Mercury's surface from Earth - Nearly all our detailed knowledge of the planet was collected during the three passesMariner 10 made of Mercury (before the craft's communication equipment failed) - This was until the launch of MESSENGER, whose main objectives were to determine surface material composition, investigate reflective materials at poles, analyze magnetic fields, and determine if there is a liquid outer core— after 6.5 years, MESSENGER successfully entered orbit around Mercury - Mercury looks very similar to Moon: for both, rotation has been altered by tidal attraction, their surfaces heavily cratered,their large craters are flooded by ancient lava flows, and both are small, airless, and have ancient, inactive surfaces Planet Facts  Diameter - 4878 km (roughly about one and one-half that of Moon)  Density - 5.4 grams per cubic centimeter  Mean surface temperature - 350°C day; -170°C night  Rotation period - 58.65 days (compared to 24 hours for Earth)  Orbital period - 88 days (compared to 365 days for Earth)  Orbital speed - 47.87 km/s  Orbital eccentricity - Most eccentric of all terrestrial planets  Orbital inclination - 7 degrees to the plane of Earth's orbit (the ecliptic)  Axial tilt - Almost too small to measure  Magnetic field - Present (at about 1.1% the strength of Earth's field) Orbital Chaos - M
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