| | >>Note: For the medieval astronomer/astrologers the Universe was a small place, the Earth was the center, and events in the heavens were orderly and designed to benefit humanity. The only change that was deemed appropriate was cyclic change such as the (mostly) orderly motion of the planets on the sky or the daily travel of the sun around the heavens, for cyclic change returns one to the starting point and so is not really change at all. In Europe of the Middle Ages this belief was elevated to the level of religious dogma, and one dared challenge this worldview at considerable personal peril. |
| | However, the Copernican revolution began a long process that changed completely our perception of the heavens and humanity's place in the Universe. Beginning in the 16th and 17th centuries and continuing until today, observations and increased theoretical understanding demonstrated that the Universe is enormous, that it has existed for periods that dwarf human lifetimes, and that we do not occupy the center of the Universe (for there is no center). Probably less appreciated is a change with antecedents in events observed hundreds of years ago, but that has accelerated at breathtaking pace over the last 30 years. As observational astronomy at wavelengths other than visible light (Radio-Frequency, X-Ray, Gamma-Ray, Ultraviolet, ...) has become more commonplace, we have begun to appreciate that the Universe is party to scenes of unimaginable violence. Far from an orderly stage for stately and gentle physical processes, the Universe at various times and various places undergoes violent cataclysms releasing energy on a scale to numb the mind of even the most analytic physical scientist. |
| | The medieval natural philosopher would perhaps have had even greater difficulty accepting this insight than accepting the Copernican hypothesis that the Earth was not the center of the Universe, for it would have destroyed the strongly held belief that the Universe existed as a nurturing cocoon for humanity. However, it is supremely ironic that these violent processes that on the surface seem hostile to the place of humanity in the Universe are in fact essential to the production of the present Universe. In particular, our modern understanding is that there would be no matter as we know it, no life as we know it, and no humanity to contemplate these questions, in the absence of violent processes that would, of themselves, destroy all life within countless light years. |
| | The development of these ideas has been a truly remarkable odyssey in the history of human thought. These lectures represent an introduction to how this modern worldview has come about, and a survey of the often beautiful, sometimes astonishing, but never dull, Universe described by these evolving ideas. |
| | A Sense of Time and Scale in the Universe |
| | Precursors to Modern Astronomy |
| | Overview of the Sky and Planets |
| | The Development of Modern Astronomy |
| | Timekeeping and the Celestial Sphere |
| | Overview of the Solar System |
| | êîíåöôîðìûíà÷àëîôîðìûThe Earth is certainly the most familiar planet, though it has only been a few hundred years since we fully realized it was a planet. We begin our study of objects in the Solar System with the Earth because it is interesting in its own right, and it provides a test of many observing techniques that we wish to use for other objects in the Solar System. |
| | Geological Differentiation |
| | The Earth did not have the interior structure described in the preceding section when it was formed. The geological process by which the Earth came to have its present interior structure is called differentiation, and is illustrated in the following figure. |
| | êîíåöôîðìûíà÷àëîôîðìûWithin about 1 billion years of its formation the Earth was melted by heat arising from a combination of sources: |
| | While the Earth was molten, gravity acted to concentrate more dense material near the center and less dense material nearer the surface. When the Earth solidified again (except for the liquid outer core) it was left with a layered structure with more dense material like iron and nickel near the center and less dense rocks nearer the surface. As the outer layers cooled and solidified, large cracks developed because of thermal stress, leaving the lithosphere broken up into large blocks or plates. |
| | As we shall see, this has enormous implications for the subsequent geological history of the Earth because it produces conditions favorable for plate tectonics. One of the crucial questions that we will have of all solid bodies in the Solar System is whether they have ever been differentiated. |
| | Gravitational energy left from the formation of the planet |
| | Decay of radioactive material trapped in the body of the Earth |
| | The Interior of the Earth |
| | êîíåöôîðìûíà÷àëîôîðìûThe study of the Earth's surface and interior is the domain of geology. We know little directly about the interior of the Earth. Most of our information in that regard has come from seismic waves, which are vibrations in the body of the Earth. |
| | There are two general categories of seismic waves: |
| | êîíåöôîðìûíà÷àëîôîðìûP-waves, which are longitudinal pressure waves and can propagate in both solids and liquids |
| | êîíåöôîðìûíà÷àëîôîðìûS-waves, which are transverse waves that can propagate in solids but not in liquids |
| | Structure of the Interior |
| | êîíåöôîðìûíà÷àëîôîðìûAccumulated and detailed seismic studies, coupled with theoretical speculation, suggests the interior structure shown schematically on the left (the figure is not to scale). The Earth is believed to have a solid inner core, made mostly of iron and nickel. This is surrounded by a liquid outer core, also mostly iron and nickel. The diameter of the core is estimated to be 7000 km, compared with a 12,700 km diameter for the entire planet. The crust is only a few tens of kilometers thick. The region between the core and the crust is called the mantle. The upper part of the mantle and the crust together are called the lithosphere. Sitting just below the lithosphere is a region of plastic consistency called the aesthenosphere. We shall have more to say about the lithosphere and aesthenosphere shortly. |
| | êîíåöôîðìûíà÷àëîôîðìûIt is now uniformly agreed that the crustal plates of the Earth are in horizontal motion. This is called continental drift colloquially, and plate tectonics in technically more precise language. This is newly won knowledge. Although the idea has been around for almost a century, it was not generally accepted (indeed, was often considered crackpot) until the last few decades. |
| | The Drift of the Continents |
| | We now believe that the surface of the Earth looked very different 200 million years ago from its present appearance. In particular, the continents have changed because they sit on blocks of the lithosphere that are in horizontal motion with respect to each other, and indeed they continue to change because the horizontal motion continues. The following figure illustrates. |
| | The present continents separated from two supercontinents called Laurasia and Gondwanaland through this process of plate tectonics. The two supercontinents may have once been united in a single supercontinent called Pangaea, though this is less certain. |
| | The Origin of Plate Tectonics |
| | What is the origin of plate tectonics? The continents drift slowly (the timescale for substantial change is 10-100 million years), but that they drift at all is remarkable. The following figure illustrates the structure of the first 100-200 kilometers of the Earth's interior, and provides an answer to this question. |
| | The crust is thin, varying from a few tens of kilometers thick beneath the continents to to less than 10 km thick beneath the many of the oceans. The crust and upper mantle together constitute the lithosphere, which is typically 50-100 km thick and is broken into large plates (not illustrated). These plates sit on the aesthenosphere. |
| | The aesthenosphere is kept plastic (deformable) largely through heat generated by radioactive decay. The material that is decaying is primarily radioactive isotopes of light elements like aluminum and magnesium. This heat source is small on an absolute scale (the corresponding heat flow at the surface out of the Earth is only about 1/6000 of the Solar energy falling on the surface). Nevertheless, because of the insulating properties of the Earth's rocks this is sufficient to keep the aesthenosphere plastic in consistency. |
| | Very slow convection currents flow in this plastic layer, and these currents provide horizontal forces on the plates of the lithosphere much as convection in a pan of boiling water causes a piece of cork on the surface of the water to be pushed sideways (following figure). |
| | Of course, the timescale for convection in the pan is seconds and for plate tectonics is 10-100 million years, but the principles are similar. Thus, we see that differentiation is crucial to plate tectonics on the Earth, because it is responsible for producing an interior that can support tectonic motion. |
| | Evidence for Plate Tectonics |
| | êîíåöôîðìûíà÷àëîôîðìûThe original conjectures concerning plate tectonics were based on circumstantial evidence like the shapes of continents being such that they would fit well if pushed together. Today, we have a much broader set of evidence in favor of the hypothesis. |
| | Indications of Tectonic Activity |
| | êîíåöôîðìûíà÷àëîôîðìûAmong the classes of evidence for continental drift and the underlying plate tectonics we may list. |
| | The shapes of many continents are such that they look like they are separated pieces of a jig-saw puzzle. For example, look in the adjacent map at the shape of the east coast of North and South Americal relative to the shape of the west coast of Africa and Europe |
| | Many fossil comparisons along the edges of continents that look like they fit together suggest species similarities that would only make sense if the two continents were joined at some point in the past |
| | There is a large amount of seismic, volcanic, and geothermal activity along the conjectured plate boundaries. This is shown clearly below in the figure labeled "Crustal plate boundaries" where the epicenters of earthquakes above Richter magnitude 5.0 are plotted for a 10-year period. The concentration is striking, and indeed this plot serves to define the plate boundaries extremely well. Here is a clickable map of current volcanic activity on Earth |
| | There are ridges, such as the Mid-Atlantic Ridge (see figures above and below) where plates are separating that are produced by lava welling up from between the plates as they pull apart. Likewise, there are mountain ranges being formed where plates are pushing against each other (e.g., the Himalayas, which are still growing) |
| | êîíåöôîðìûíà÷àëîôîðìûIf the crustal plates are pulling apart at boundaries like the Mid-Atlantic Ridge (see the line of earthquake epicenters down the center of the Atlantic in the preceding figure), the sea floor near these ridges should be very young geologically, since it is formed of material upwelling from the interior. This is indeed the case, as the following figure shows. |
| | This figure displays the estimated age of sea floor crustal plates with red the youngest and blue the oldest. One can see clearly that material near the crustal boundaries is very young geologically. |
| | Consequences of Plate Tectonics |
| | Past and future consequences of plate tectonics for the Earth's surface are enormous. |
| | Some Past and Present Consequences |
| | êîíåöôîðìûíà÷àëîôîðìûPlate tectonics has been responsible for many of the features that we find on the surface of the Earth today. A few examples include |
| | The Appalachian Mountains were formed from wrinkling of the Earth's surface produced by the collision of the North American and African plates |
| | The seismic and volcanic activity of the West Coast of the United States (for example, the San Andreas Fault) is produced by the grinding of the Pacific and North American Plates against each other. Indeed, the entire "ring of fire" around the Pacific, corresponding to regions of high volcanic and seismic activity, is caused primarly by the motion of the Pacific Plate |
| | The Himalayan Mountains were formed (indeed are still growing) as a result of the Indian subplate burrowing under the Eurasian plate and raising its edge |
| | The Dead Sea in Israel is part of a rift system produced by plates that are pulling apart in that region |
| | Some Future Consequences of Plate Tectonics |
| | Plate tectonics is still an active process, and will drastically reshape the face of the Earth over the next 50 million years or so. A few consequences of plate tectonics based on projections of present motion include: |
| | As a consequence of plate tectonics (supplemented by wind and water erosion), we live on the surface of a geologically active planet that has obliterated most of its early geological history. |
| | Portions of California will separate from the rest of North America |
| | The Italian "boot" will disappear |
| | Australia will become linked to Asia |
| | Africa will separate from the Near East |
| | The present atmosphere of the Earth is probably not its original atmosphere. Our current atmosphere is what chemists would call an oxidizing atmosphere, while the original atmosphere was what chemists would call a reducing atmosphere. In particular, it probably did not contain oxygen. |
| | êîíåöôîðìûíà÷àëîôîðìûThe atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates. |
| | The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There is a thin buffer zone between the troposphere and the next layer called the tropopause. |
| | The Mesosphere and Ionosphere |
| | Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible. |
| | The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind), which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization. |
| | The Stratosphere and Ozone Layer |
| | êîíåöôîðìûíà÷àëîôîðìûAbove the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun. The formation of this layer is a delicate matter, since only when oxygen is produced in the atmosphere can an ozone layer form and prevent an intense flux of ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution of life. There is considerable recent concern that manmade flourocarbon compounds may be depleting the ozone layer, with dire future consequences for life on the Earth. |
| | Composition of the Atmosphere |
| | êîíåöôîðìûíà÷àëîôîðìûThe original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials. |
| | The oxygen so characteristic of our atmosphere was almost all produced by plants (cyanobacteria or, more colloquially, blue-green algae). Thus, the present composition of the atmosphere is 79% nitrogen, 20% oxygen, and 1% other gases. |
| | Consequences of Rotation for Weather |
| | êîíåöôîðìûíà÷àëîôîðìûThe Earth is a spinning globe where a point at the equator is travelling at around 1100 km/hour, but a point at the poles is not moved by the rotation. This fact means that projectiles moving across the Earth's surface are subject to Coriolis forces that cause apparent deflection of the motion. |
| | êîíåöôîðìûíà÷àëîôîðìûThe swirling motions evident in the preceding animations are consequences of frontal systems anchored to high and low pressure systems, which are also called anticyclones and cyclones, respectively. The wind flow around high pressure (anticyclonic) systems is clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere. The corresponding flow around low pressure (cyclonic) systems is counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. This is a consequence of the Coriolis force, as illustrated for the Northern hemisphere in the following figure. |
| | Realistic Weather Patterns |
| | The adjacent animation shows GOES-8 weather satellite images over a 72-hour period from Dec. 29, 1996, through Jan. 1, 1997. This is a geosynchrous satellite, which means that it orbits the Earth with the same period as the Earth's rotation and therefore appears to be essentially motionless over a fixed position on the Earth's surface. For GOES-8 this fixed position looks down on North and South America. |
| | In these composite images red indicates visible light (reflected sunlight), green indicates the 11 micron IR channel (thermal emission), and blue indicates the 3.9 micron channel (thermal + sunlight). At night the images are blue and green. The three periods of daylight in this 72 hour sequence are clearly visible as red-orange regions moving from East to West (right to left). In the IR channels, the natural intensity pattern has been inverted: warmer is darker, so that cool cloudtops stand out brightly. |
| | One can see clearly the pronounced cloud flows associated with the strong westerlies at mid-latitudes in each hemisphere. (This is taken in Northern hemisphere Winter, so the heavier cloud cover in that hemisphere is not surprising.) Less obvious are the easterly trade winds and the polar easterlies, though one can see vestiges of each if one looks carefully. Also apparent are the swirling motions associated with frontal systems. These are particularly pronounced at the boundaries between the mid-latitude westerly and polar wind flows in each hemisphere. |
| | Here is a similar weather animation (1.49 MB animated GIF) using GOES-8/9 IR images for North America over a 2 day period from December 31, 1996 through January 1, 1997. The large weather systems that move ashore from the Pacific in this animation produced catastrophic flooding in California, Oregon, and Washington in early January, 1997. |
| | Solar Heating and Coriolis Forces |
| | êîíåöôîðìûíà÷àëîôîðìûSince winds are just molecules of air, they are also subject to Coriolis forces. Winds are basically driven by Solar heating. As the adjacent (highly idealized) image indicates, Solar heating on the Earth has the effect of producing three major convection zones in each hemisphere. |
| | If solar heating were the only thing influencing the weather, we would then expect the prevailing winds along the Earth's surface to either be from the North or the South, depending on the latitude. However, the Coriolis force deflects these wind flows to the right in the Northern hemisphere and to the left in the Southern hemisphere. This produces the prevailing surface winds illustrated in the adjacent figure. |
| | For example, between 30 degrees and 60 degrees North latitude the solar convection pattern would produce a prevailing surface wind from the South. However, the Coriolis force deflects this flow to the right and the prevailing winds at these latitudes are more from the West and Southwest. They are called the prevailing Westerlies. |
| | The following diagram illustrates the effect of Coriolis forces in the Northern and Southern hemispheres. |
| | êîíåöôîðìûíà÷àëîôîðìûNot only does the Earth have a complex atmosphere, but that atmosphere has complicated motion and nontrivial behavior. We Earthlings call this weather in the short term and climate over the longer term. The following images illustrate some of the complex patterns that develop in Earth's atmosphere. |
| | As we launch our exploration of the rest of the Solar System, it is useful to recall what the Earth looks like using the various imaging techniques that we may wish to apply to distant planets and moons. |
| | The Earth in Visible Light |
| | êîíåöôîðìûíà÷àëîôîðìûWe have seen a number of images of Earth in visible light, but mostly at large scale from a great distance. Here are two images from space of smaller objects on the Earth that we will be interested in looking for on other planets and moons: a canyon system, and an active volcano. |
| | êîíåöôîðìûíà÷àëîôîðìûThere is one aspect of the Earth's appearance that we do not expect to be repeated in the near future for other objects in the Solar System: at night the artificial light associated with human civilization is very visible from space. The following image shows the appearance of the United States at night as observed from a composite of many satellite passes. |
| | Imaging in Ways other than Visible Light |
| | êîíåöôîðìûíà÷àëîôîðìûBecause our eyes are sensitive to visible light, our prejudice is to view things at those wavelengths. However, we now have instruments at our disposal that permit observations in many wasy other than the visible light region of the electromagnetic spectrum. These often offer considerable advantage; for example radar cuts through the ever-present thick cloud cover to give us images the surface of Venus that we could not obtain at visible wavelengths. |
| | Infrared radiation is basically radiant heat. Therefore, IR detected from satellites can be used to determine the temperature of objects. The following image shows a color-coded map constructed from a composite of satellite data and surface observations giving surface temperatures on the Earth |
| | Infrared and more Exotic Imaging |
| | We have seen in the preceding sections examples of imaging the Earth in the infrared, ultraviolet, and X-ray regions of the spectrum. Here we show additional examples of IR images, and a more exotic technique combining magnetic and gravitational data that can even locate objects beneath the surface of the planet. |
| | êîíåöôîðìûíà÷àëîôîðìûSan Francisco Bay imaged in IR from space |
| | This figure shows the San Francisco Bay area imaged from space in the infrared (IR). |
| | êîíåöôîðìûíà÷àëîôîðìûFossil crater imaged with representation of gravity and magnetic field data |
| | The figure shows a composite of local gravity and magnetic field variation data to image a 112-mile wide relic meteor crater in Yucatan that presently lies below several hundred meters of sedimentary rock. This crater, called Chicxulub, is famous because it is the leading candidate for the site of the asteroid impact that is thought to have killed the dinosaurs 65 million years ago in the K-T extinction |
| | GOES-8 IR satellite image of water vapor in Earth's atmosphere |
| | This êîíåöôîðìûíà÷àëîôîðìûimage shows a GOES-8 weather satellite image in the 6.7 micron IR channel that is sensitive to the distribution of water vapor in Earth's atmosphere. The imager on this satellite records radiation emitted by water vapor in the upper troposphere. Regions with high concentrations of water vapor are bright, while dark spots signal lower water vapor concentrations. |
| | Radar Imaging of the Earth's Surface |
| | The adjacent images show a comparison of the Mt. Everest region (border of Nepal and Tibet). The top image was taken through thick cloud cover with synthetic aperture radar on the space shuttle Endeavor; The bottom figure is an optical image of the same region taken from Endeavor. One can see many of the same features in the two photographs (the photographs were taken at different times of the year, so they have different snow covers). |
| | The curving and branching features are glaciers. The radar technique used is sensitive to characteristics of the glacier surfaces such as the ice roughness and water content. Thus the glaciers show a variety of colors in the radar image but are a rather featureless gray or white in the optical photograph. |
| | êîíåöôîðìûíà÷àëîôîðìûRadar (upper) and visual (lower) images of Mount Everest |
| | Here is a link to the Current sea surface temperatures (updated daily). |
| | Sea Surface Temperature Maps |
| | Similar methods as described above may be used to construct color coded maps of surface seawater temperatures. |
| | The Earth's Magnetic Field |
| | The Earth has a substantial magnetic field, a fact of some historical importance because of the role of the magnetic compass in exploration of the planet. |
| | The field lines defining the structure of the magnetic field are similar to those of a simple bar magnet, as illustrated in the following figure. |
| | êîíåöôîðìûíà÷àëîôîðìûIt is well known that the axis of the magnetic field is tipped with respect to the rotation axis of the Earth. Thus, true north (defined by the direction to the north rotational pole) does not coincide with magnetic north (defined by the direction to the north magnetic pole) and compass directions must be corrected by fixed amounts at given points on the surface of the Earth to yield true directions. |
| | Van Allen Radiation Belts |
| | êîíåöôîðìûíà÷àëîôîðìûA fundamental property of magnetic fields is that they exert forces on moving electrical charges. Thus, a magnetic field can trap charged particles such as electrons and protons as they are forced to execute a spiraling motion back and forth along the field lines. |
| | As illustrated in the adjacent figure, the charged particles are reflected at "mirror points" where the field lines come close together and the spirals tighten. One of the first fruits of early space exploration was the discovery in the late 1950s that the Earth is surrounded by two regions of particularly high concentration of charged particles called the Van Allen radiation belts. |
| | The inner and outer Van Allen belts are illustrated in the top figure. The primary source of these charged particles is the stream of particles emanating from the Sun that we call the solar wind. As we shall see in a subsequent section, the charged particles trapped in the Earth's magnetic field are responsible for the aurora (Northern and Southern Lights). |
| | Origin of the Magnetic Field |
| | êîíåöôîðìûíà÷àëîôîðìûMagnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth's magnetic field is not completely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect. |
| | Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earth's magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood. |
| | The Earth's Magnetosphere |
| | The solar wind mentioned above is a stream of ionized gases that blows outward from the Sun at about 400 km/second and that varies in intensity with the amount of surface activity on the Sun. The Earth's magnetic field shields it from much of the solar wind. When the solar wind encounters Earth's magnetic field it is deflected like water around the bow of a ship. |
| | The imaginary surface at which the solar wind is first deflected is called the bow shock. The corresponding region of space sitting behind the bow shock and surrounding the Earth is termed the magnetosphere; it represents a region of space dominated by the Earth's magnetic field in the sense that it largely prevents the solar wind from entering. However, some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts. |
| | Auroras: The Northern and Southern Lights |
| | êîíåöôîðìûíà÷àëîôîðìûThe aurora, or northern and southern lights, are often visible from the surface of the Earth at high northern or southern latitudes. Auroras typically appear as luminous bands or streamers that can extend to altitudes of 200 miles (well into the ionosphere). |
| | Auroras at Non-Visible Wavelengths |
| | The collisions of trapped charged particles with atmospheric molecules causes spectacular effects in the visible spectrum, but these excited molecules can also emit radiation in other wavelength bands. The following figures show aurora imaged in the ultraviolet (UV) and X-ray regions of the spectrum. |
| | Auroras are caused by high energy particles from the solar wind that are trapped in the Earth's magnetic field. As these particles spiral back and forth along the magnetic field lines, they come down into the atmosphere near the north and south magnetic poles where the magnetic field lines disappear into the body of the Earth. |
| | The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen molecules in the atmosphere. This excites the molecules, and when they decay from the excited states they emit the light that we see in the aurora. |
| | Northern and Southern Lights |
| | êîíåöôîðìûíà÷àëîôîðìûThe following figures show three examples of the often spectacular visible light display associated with auroras. |
| | êîíåöôîðìûíà÷àëîôîðìûPlate tectonic motion, which may be only centimeters per century, is now being studied by careful laser ranging techniques that are capable of detecting such small motions. |
| | êîíåöôîðìûíà÷àëîôîðìûfalse-color image of water vapor in Earth's |
| | êîíåöôîðìûíà÷àëîôîðìûX-ray emission from northern aurora observed by the Polar Satellite |
| | êîíåöôîðìûíà÷àëîôîðìûThe Grand Canyon from space |
| | êîíåöôîðìûíà÷àëîôîðìûMt. Etna from space |
| | êîíåöôîðìûíà÷àëîôîðìûThe Moon is the nearest body to us in the Solar System, and as a consequence of the Apollo missions is the only extra-terrestrial object that has yet been explored directly by humans. As a consequence of that exploration by both manned and unmanned spacecraft, we now know a great deal about our nearest celestial neighbor. |
| | êîíåöôîðìûíà÷àëîôîðìûAn extremely important question is that of how the Moon was formed and came to have its present orbit around the Earth. |
| | êîíåöôîðìûíà÷àëîôîðìûFive serious theories have been proposed for the formation of the Moon (not counting the one involving green cheese): |
| | The Ejected Ring Theory: A planetesimal the size of Mars struck the earth, ejecting large volumes of matter. A disk of orbiting material was formed, and this matter eventually condensed to form the Moon in orbit around the Earth. |
| | The Colliding Planetesimals Theory: The interaction of earth-orbiting and Sun-orbiting planetesimals (very large chunks of rocks like asteroids) early in the history of the Solar System led to their breakup. The Moon condensed from this debris. |
| | The Condensation Theory: The Moon and the Earth condensed together from the original nebula that formed the Solar System. |
| | The Capture Theory: The Moon was formed somewhere else, and was later captured by the gravitational field of the Earth. |
| | The Fission Theory: The Moon was once part of the Earth and somehow separated from the Earth early in the history of the Solar System. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. |
| | Constraints from Recent Data |
| | êîíåöôîðìûíà÷àëîôîðìûA detailed comparison of the properties of Lunar and Earth rock samples has placed very strong constraints on the possible validity of these hypotheses. For example, if the Moon came from material that once made up the Earth, then Lunar and Terrestrial rocks should be much more similar in composition than if the Moon was formed somewhere else and only later was captured by the Earth. |
| | These analyses indicate that the abundances of elements in Lunar and Terrestrial material are sufficiently different to make it unlikely that the Moon formed directly from the Earth. Generally, work over the last 10 years has essentially ruled out the first two explanations and made the third one rather unlikely. At present the fifth hypothesis, that the Moon was formed from a ring of matter ejected by collision of a large object with the Earth, is the favored hypothesis; however, the question is not completely settled and many details remain to the accounted for. |
| | Intrinsic and Orbital Properties |
| | êîíåöôîðìûíà÷àëîôîðìûThe mass of the Moon is about 1/80 that of the Earth, and its diameter is about 1/4 that of the Earth. The orbit of the Moon is very nearly circular (eccentricity ~ 0.05) with a mean separation from the Earth of about 384,000 km, which is about 60 Earth radii. The plane of the orbit is tilted about 5 degrees with respect to the ecliptic plane. The Apollo missions to the Moon left devices that can reflect laser light sent to the moon from the Earth. By timing the roundtrip of such light, it is possible to determine the distance to the Moon at any particular time with an uncertainty of only a few centimeters (!). |
| | Since the synodic rotational period of the Moon is 29.5 days, Lunar day and Lunar night are each about 15 Earth days long. During the Lunar night the temperature drops to around -113 degrees Celsius, while during the Lunar day the temperature reaches 100 degrees Celsius. The temperature changes are very rapid since there is no atmosphere or surface water to store heat. |
| | Tides and Gravitational Locking |
| | êîíåöôîðìûíà÷àëîôîðìûWe have introduced tides in our earlier discussion of the Moon's observational characteristics through the effect of the Moon on the Earth's oceans, but the effect is much more general, and has a number of important consequences. |
| | Tidal Coupling and Gravitational Locking |
| | Some important consequences of tidal forces in the Solar System include: |
| | Tidal forces will distort any body experiencing differential gravitational forces. This will normally occur for bodies of finite extent in gravitational fields because of the strong distance dependence of the gravitational force. Thus, not only the oceans, but the body of the Earth is distorted by the Lunar gravity. However, because the Earth is rigid compared with the oceans, the "tides" in the body of the Earth are much smaller than in the oceans |
| | There is a limiting radius for the orbit of one body around another, inside of which the tidal forces are so large that no large solid objects can exist that are held together only by gravitational forces. This radius is called the Roche Limit. Thus, solid objects put into orbit inside the Roche limit will be torn apart by tidal forces, and conversely, solid objects cannot grow by accreting into larger objects if they orbit inside the Roche limit. A famous example is the rings of Saturn: because they lie inside the Roche limit for Saturn, they cannot be solid objects held together by gravitation and must be composed of many small particles. Obviously solid objects can exist inside the Roche limit (for example, spacecraft) but they must be held together by forces other than gravity. This is true of a spacecraft, where chemical forces between the atoms and molecules are much larger than the gravitational forces. |
| | The tidal forces are reciprocal. Not only will the Moon induce tides in the body of the Earth and the Earth's oceans, but by the same argument the gravitational field of the Earth will induce differential forces and therefore tides in the body of the Moon. Again, because the body of the Moon is quite rigid these Lunar tides will be very small, but they occur |
| | This reciprocal induction of tides in the body of the Earth and the Moon leads to a complicated coupling of the rotational and orbital motions of the two objects. These tidal forces and associated couplings have the following general effects: |
| | The interior of the Earth and Moon are heated by the tides in their bodies, just as a paper clip is heated by constant bending. This effect is very small for the Earth and Moon, but we shall see that it can be dramatic for other objects that experience much larger differential gravitational forces and therefore much larger tidal forces. For example, we shall see that the tidal forces exerted by Jupiter on its moon Io are so large that the solid surface of Io is raised and lowered by hundreds of meters twice in each rotational period. This motion so heats the interior of Io that it is probably mostly molten; as a consequence, Io is covered with active volcanos and is the geologically most active object in the Solar System |
| | The tidal coupling of the orbital and rotational motion tends to synchronize them. In the simplest instance, the period of rotation for the two bodies and the orbital period eventually become exactly equal because of this tidal coupling (and as a result, the size of the orbit is changed in such a way as to conserve angular momentum for the entire system). This is called gravitational (or tidal) locking, because as the two objects revolve around their common center of mass each keeps the same side turned toward the other |
| | Tidal Coupling in the Earth-Moon System |
| | Thus, the fact that the rotational period of the Moon and the orbital period of the Earth-Moon system are of the same length is not an accident. Presumably this was not always true, but over billions of years the tidal coupling of the Earth and the Moon has led to this synchronization. In the case of the Earth-Moon system the synchronization is not yet complete. The Earth is slowly decreasing its rotational period and eventually the Earth and Moon will have exactly the same rotational period, and these will also exactly equal the orbital period. At the same time, the separation between the Earth and Moon will slowly increase in just such a way as to conserve angular momentum for the entire system. |
| | Thus, billions of years from now the Earth will always keep the same face turned toward the Moon, just as the Moon already always keeps the same face turned toward the Earth. We will encounter other examples of such tidal locking in other pairs of objects in the Solar System. |
| | Surface Properties of the Moon |
| | The surface of the Moon has two hemispheres with rather asymmetric properties; as a consequence the nature of the Lunar surface that we can see from the Earth is substantially different from the surface that is always hidden from the Earth |
| | êîíåöôîðìûíà÷àëîôîðìûThe face of the Moon turned toward us is termed the near side (image at right). It is divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare). The Maria are lower in altitude than the Highlands, but there is no water on the Moon so they are not literally seas from the Clementine spacecraft suggests that there may be some water on the Moon, contrary to previous assumptions). The dark material filling the Maria is actually dark, solidified lava from earlier periods of Lunar volcanism. Both the Maria and the Highlands exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands. |
| | The side of the Moon unseen from the Earth is called the far side. One of the discoveries of the first Lunar orbiters is that the far side has a very different appearance than the near side. In particular, there are almost no Maria on the far side, as illustrated in the image shown to the left of a portion of the far side surface. In this figure a number of meteor impact craters are visible. |
| | The amount of cratering is usually an indication of the age of a geological surface: the more craters, the older the surface, because if the surface is young there hasn't been time for many craters to form. Thus, the Earth has a relatively young surface because it has few craters. This is because the Earth is geologically active, with plate tectonics and erosion having obliterated most craters from an earlier epoch. In contrast the surface of the Moon is much older, with much more cratering. Further, different parts of the surface of the Moon exhibit different amounts of cratering and therefore are of different ages: the maria are younger than the highlands, because they have fewer craters. |
| | The oldest surfaces in the Solar System are characterized by maximal cratering density. This means that one cannot increase the density of craters because there are so many craters that, on average, any new crater that is formed by a meteor impact will obliterate a previous crater, leaving the total number unchanged. Some regions of the moon exhibit near maximal cratering density, indicating that they are very old. |
| | The Lunar Surface Material |
| | The bulk density of the Moon is 3.4 g/cc, which is comparable to that of (volcanic) basaltic lavas on the Earth (however, the bulk density of the Earth is 5.5 g/cc, because of the dense iron/nickel core). The Moon is coverered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith; it is made from debris blasted out of the Lunar craters by the meteor impacts that created them. Each well-preserved Lunar crater is surrounded by a sheet of ejected material called the ejecta blanket. |
| | êîíåöôîðìûíà÷àëîôîðìûThe abundances of radioactive elements in rock samples can be used to tell the age of the rock in a process called Radioactive Dating. When such techniques are applied to the Lunar rock samples, one finds the following: |
| | Thus, the oldest material from the surface of the Moon is almost as old as we believe the Solar System to be. This is more than a billion years older than the oldest Earth rocks that have been found. Thus, the material brought back from the Moon by the Apollo missions gives us a window on the very early history of our Solar System that would be difficult the find on the Earth, which is geologically active and has consequently has obliterated its early geological history. |
| | Samples from Mare Imbrium and the Ocean of Storms brought back by Apollo 11 and Apollo 12 are about 3.5 billion years old, which is comparable to the oldest rocks found on the surface of the Earth. |
| | The ejecta blanket from the Imbrium Basin (which was formed by a gigantic meteor impact) was returned by Apollo 14 and found to be about 3.9 billion years old. |
| | Lunar Highlands rocks returned by Apollo 16 are about 4 billion years old. The oldest Lunar rock found was located by Apollo 17 and appears to be about 4.5 billion years old. |
| | The Lunar rocks may also be examined according to the chemicals that they contain. Such analysis indicates: |
| | The high concentration of rare metals like Titanium, and the availability of abundant amounts of Silicon and Oxygen has led to serious proposals about mining and manufacturing operations in the future for the Moon. |
| | êîíåöôîðìûíà÷àëîôîðìûThere is high abundance of elements like Silicon (Si) and Oxygen (O). |
| | êîíåöôîðìûíà÷àëîôîðìûThey are poor in the light elements such as hydrogen (H). |
| | They are rich in refractory elements, which are elements such as calcium (Ca), Aluminum (Al), and Titanium (Ti) that form compounds having high melting points. |
| | êîíåöôîðìûíà÷àëîôîðìûOne striking difference between the Lunar surface material and that of Earth concerns the most common kinds of rocks. On the Earth, the most common rocks are sedimentary, because of atmospheric and water erosion of the surface. On the Moon there is no atmosphere to speak of and little or no water, and the most common kind of rock is igneous ("fire-formed rocks"). Geologically, the Lunar surface material has the following characteristics: |
| | The Anorthosites that are common in the Lunar Highlands are not common on the surface of the Earth (The Adirondack Mountains and the Canadian Shield are exceptions). They form the ancient cores of continents on the Earth, but these have largely been obliterated by overlying sedimentary deposits and by plate tectonic activity. |
| | The Maria are mostly composed of dark basalts, which form from rapid cooling of molten rock from massive lava flows. |
| | The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so were formed under different conditions. |
| | Breccias, which are fragments of different rocks compacted and welded together by meteor impacts, are found in the Maria and the Highlands, but are more common in the latter. |
| | Lunar Soils contain glassy globules not commonly found on the Earth. These are probably formed from the heat and pressure generated by meteor impacts. |
| | Interior and Geological Activity |
| | êîíåöôîðìûíà÷àëîôîðìûBefore the Apollo missions we knew almost nothing about the interior of the Moon. The Apollo missions left seismometers on the lunar surface that have allowed us to deduce the general features of the Lunar interior by studying the seismic waves generated by "moonquakes" and occasional meteor impacts. |
| | Geological History of the Moon |
| | êîíåöôîðìûíà÷àëîôîðìûThe weight of the evidence is that the Moon was active geologically in its early history, but the general evidence suggests that the Moon has been essentially dead geologically for more than 3 billion years. Based on that evidence, we believe the chronology of Lunar geology was as follows: |
| | êîíåöôîðìûíà÷àëîôîðìûThus, Lunar surface features, particularly in the Highlands, tend to be older than those of the Earth, which remains to this day a geologically active body. |
| | The volcanism stopped about 3.1 billion years ago: the Moon has been largely dead geologically since then except for the occasional meteor impact or small moonquake, and micro-meteorite erosion of the surface. |
| | The lava flows associated with the volcanism filled the low areas and many craters. These flows solidified to become the flat and dark maria, which have little cratering because most of the original craters were covered by lava flows and only a few meteors of significant size have struck the surface since the period of volcanic activity. The regions that were not covered by the lava flows are the present Highlands; thus, they are heavily cratered, and formed from different rocks than the seas. |
| | The fracturing and heating of the surface and subsurface by the meteor bombardment led to a period of intense volcanic activity in the period 3.8-3.1 billion years ago. Meanwhile, the meteor bombardment had tapered off because by this time much of the debris of the early Solar System had already been captured by the planets. |
| | As the intense meteor bombardment associated with debris left over from the formation of the Solar System continued, most of the craters that we now see on the surface of the Moon were formed by meteor impact. |
| | By 4.2 billion years ago the surface was solid again. |
| | By about 4.4 billion years ago the top 100 km was molten, from original heat of formation and from heat generated by the meteor bombardment. |
| | The Moon was formed about 4.6 billion years ago; maybe hot or maybe cold. The surface was subjected continuously to an intense meteor bombardment associated with debris left over from the formation of the Solar System. |
| | The Structure of the Interior |
| | êîíåöôîðìûíà÷àëîôîðìûOur present picture of the Moon's interior is that it has a crust about 65 km thick, a mantle about 1000 km thick, and a core that is about 500 km in radius. A limited amount of seismic data suggests that the outer core may be molten. There does appear to be some amount of differentiation, but not on the scale of that of the Earth. It has no magnetic field to speak of, but magnetization of Lunar rocks suggests that it may have had a larger one earlier in its history. Although there is a small amount of geological activity on the Moon, it is largely dead geologically (the energy associated with the Earth's seismic activity is about 10^14 times larger than that of the Moon). Most Lunar seismic activity appears to be triggered by tidal forces induced in the Moon by the Earth. |
| | êîíåöôîðìûíà÷àëîôîðìû"I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth. No single space program in this period will be more impressive to mankind, or more important in the long-range exploration of space; and none will be so difficult or expensive to accomplish." |
| | - President John F. Kennedy, to a joint session of Congress, May 25, 1961 |
| | êîíåöôîðìûíà÷àëîôîðìû"That's one small step for (a) man. . . one giant leap for mankind." |
| | - êîíåöôîðìûíà÷àëîôîðìûApollo 11 êîíåöôîðìûíà÷àëîôîðìûcommander Neil Armstrong, as he sets foot upon the surface of the Moon; 10:56 PM EDT, July 20, 1969 |
| | Here is an interactive viewer which displays views of the Moon from the Earth, Sun, night side, above named formations on the lunar surface, or as a map showing day and night (Credit: John Walker). |
| | êîíåöôîðìûíà÷àëîôîðìûThe planet Mercury is very difficult to study from the Earth because it is always so close to the Sun. Even at elongation, it is never more than 28 degrees from the Sun in our sky. It is the second smallest planet (it was believed to be the smallest until the discovery that Pluto is actually much smaller than originally thought), and also the fastest in its orbit since it is the innermost planet. In fact, the name Mercury derives from its speed in moving around its orbit. |
| | We began to learn more about Mercury with radar imaging from the Earth in the 1960s, and obtained most of what we know about the planet from the Mariner 10 space probe was placed into a complicated orbit involving Venus and Mercury and which passed close to Mercury and sent back information three times in the period 1974-1976 |
| | The Surface and Interior of Mercury |
| | Speaking loosely, we may characterize Mercury as being much like the Earth on the inside and much like the Moon on the outside. However, the analogies cannot be pushed too far. For example, the detailed early history of the surface was probably different for Mercury than for the Moon. |
| | As noted previously, the density of Mercury and the magnetic field suggest geological differentiation and a large iron core. In the case of the Earth the metallic core occupies about 16% of the interior by volume and in the case of the Moon the core occupies about 4% by volume. In the case of Mercury the core is thought to occupy about 50% of the interior by volume (and a whopping 70% by mass). Thus, Mercury is a planet with a very large iron core and a comparatively thin mantle compared with the Earth. |
| | êîíåöôîðìûíà÷àëîôîðìûThere are three major types of surface features on Mercury: |
| | Smooth plains that resemble Lunar maria. |
| | Intercrater plains, which are pocked with small craters and occupy about 70% of the surface that we have examined. |
| | Rugged highlands that bear some resemblance to the corresponding regions on the Moon. |
| | The adjacent image shows a mosaic of photgraphs taken from Mariner 10 in 1974 that summarizes the character of the surface. |
| | The following images illustrate three features from the surface of Mercury: (1) a large impact basin that is similar to Mare Imbrium on the Moon, (2) the highlands of Mercury, (3) and an example of a large geological fault. |
| | êîíåöôîðìûíà÷àëîôîðìûHills of Mercury |
| | êîíåöôîðìûíà÷àëîôîðìûLarge Faults |
| | After formation the mass of the planet was differentiated into a large iron core and thin mantle. |
| | Lava flowed outward through cracks in the mantle and formed the intercrater plains. The oldest surfaces are about 4.2 billion years old. |
| | Solar tides slowed the rotation and formed large-scale linear features (scarps and lines of cliffs) that we see today. |
| | The intense meteor bombardment characteristic of the early Solar System caused cratering and produced large impact basins like the Caloris Basin. |
| | Starting about 3.8 billion years ago, the smooth plains were created, possibly from volcanic activity triggered by the earlier meteor impacts and erupting from the large impact basins. |
| | After the volcanic activity subsided, the planet has been very quiet geologically except for the occasional meteor (however, it is probably geologically more active than the Moon). |
| | General Features of Mercury |
| | Mercury, the innermost planet, is 0.4 A.U. from the Sun on the average. It revolves about the Sun once every 88 days in an orbit that is the most elliptical of any planet except Pluto. The adjacent image shows the November, 1996, locations of the planets in the inner solar system |
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