http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== navigation image map ------------------------------------------------------------------------ /On this page, links to other relevant planetary Websites are emplaced. A pitch is made to the reader to consider taking a look at the extensive review of Astronomy/Cosmology in Section 20 as a background to this Section - optionally switching to it here <../Sect20/A1.html> before moving on to the planets. The definition and nature of a "planet" is then considered. A table lists the major facts and parameters pertaining to the solar planets. Some of the characteristics of the motions and distribution of the planets are described in terms of the historical contributions by Copernicus, Brahe, Kepler, and Newton. Finally, a subsection is presented on how meteorites are used to determine compositions of solid planetary, asteroid, and cometary bodies in the Solar System./ ------------------------------------------------------------------------ INTRODUCTION TO THE PLANETS * Internet Links to Planetary Sites; Book References * Planetary exploration has become one of incredible and vast accomplishments, in which huge amounts of data have now accumulated. Much of it has relied exclusively or largely on remote sensing. This Section is one of the longest in the 27 units of the Tutorial. The intention is to provide a thumbnail view of the major missions to the planets. Despite the importance of learning about planetary atmospheres, we will not say much about the results of remote sensing of these gaseous envelopes nor do we discuss in any detail facts and conjectures about planetary interiors. There are many sources of additional images and descriptive information. Among the best of these currently online is a repeat of Chapter 5: /Planetary Geology/, by James Bell III, Bruce Campbell, and Mark Robinson, in the 3rd Edition of the /Manual of Remote Sensing: Earth Sciences Volume/, 1996, at Marswatch . This lengthy and detailed review focuses on remote sensing approaches to planetary exploration. Its one drawback is a sparsity of images (compared with this Section 19 Overview). An excellent chronological survey of the history of space exploration is found at the Planetscapes website. A complete listing of all planetary missions can be accessed on this NASA Goddard web site. Another site worth visiting is the Home Page of the Jet Propulsion Laboratory (JPL) where you can get addresses to visit other sites dealing with terrestrial and planetary space programs. Another NASA source is the National Space Science Data Center NSSDC . Two other exceptional Home Pages are _The Nine Planets_, by Bill Arnett of the Lunar and Planetary Laboratory, University of Arizona (LPL) and _Views of the Solar System_, by C.J..Hamilton of the Los Alamos National Laboratory (Spaceart ). Dr. J. Schombert of the University of Oregon offers three courses on Planets, Astronomy, and Cosmology that he has put on the Web; the first of these - _The Solar System_ - is accessed at his AST121 site. The NASA Headquarters Space Sciences Directorate maintains an excellent Site that summarizes the major findings in both planetary and cosmological realms during the latest 9 to 12 months that can be accessed at Space Science (see its lists, especially News). Many Solar System missions were managed and conducted by the Jet Propulsion Laboratory; descriptions of Past, Current, and Future missions are obtaining by clicking on any of interest at this Missions site. Books that treat planetary remote sensing as part of a larger review of Planetology include a now out-of-print text by this Tutorial's author (Nicholas M. Short), /Planetary Geology/, 1975, Prentice-Hall Publ., still in libraries, Murray, Malin, and Greeley's /Earthlike Planets/, W.H. Freeman & Co., 1981, and Billy P. Glass's /Introduction to Planetary Geology/, 1982, Cambridge University, Press. More recent are /Planetary Landscapes/ by R. Greeley, 1985, Allen & Unwin, /The Planetary System/, by D. Morrison & T. Owen, Addison-Wesley Publ., 1988, and /Exploring the Planets/ by W.K. Hamblin and E.H. Christiansen, MacMillan, 1990. The writer /strongly/ recommends /*Lonely Planets: The Natural Philosophy of Alien Life*/, by astrobiologist David Grinspoon, Harper Collins Publ. published in March, 2003 that covers much of what we knew by that time about planets in the Solar System. Its main purpose, however, is to review the conditions for and likelihood of life (from very primitive to advanced thinking creatures) in our galaxy and beyond - throughout the Universe. Although a bit wordy and redundant, this book explores almost all facets of whether Earth is unique (he thinks not!!) and under what circumstances planets containing living organisms (or once living, now extinct) can develop. Scientists who have spent at least part of their careers studying the planetary bodies of the Solar System are called /Planetologists/. The majority of these are also /Geologists/, although some are instead /Astronomers/ and /Physicists/. Before we start our tour of the planets, you may wish to review some of the main principles and concepts of Astronomy. If so, please skip to Section 20, which is a comprehensive review of this subject as it is subsumed into the closely related field of Cosmology. *The Nature of a Planet* We concentrate in Section 19 almost entirely on the planetary bodies of the Solar System. Other planetary systems, around different stars, have been discovered discovered in recent years, as described on page 20-11. The central body controlling these planetary bodies is the Sun. (For background information on the Sun, check Sol and/or Sun )). Despite the large size of some of the solar planets relative, say, to Earth, these planetary bodies orbiting the Sun taken together, plus also asteroids and comets, contain only 0.14% of the mass of the Sun (99.86%). They do, however, contain most of the angular momentum of all bodies (including the Sun) in the Solar System. To set a framework for our survey of the Solar System's inhabitants (exclusive of the Sun - that star is treated at the top of page 20-5a <../Sect20/A5a.html>), which we will consider as the "Planetary System" despite the recent discovery of more than 200 extrasolar planets (again, Section 20), look first at the illustration below, which shows the relative sizes of the nine planets, with a small segment of the Sun shown to the far left, so that its size relative to the planets is scaled (the distances between them are not actual - each planet is just placed at the same distance next to its neighbors)(this diagram dates before the 2006 demotion of Pluto within our Solar System). Illustration showing the relative sizes of the nine planets of our Solar System; Pluto (at the far right) is no longer classified as a planet. *19-1: Using their appearance, how many of the above planets can you name? Check the answer to see a diagram that shows the size of each named planet relative to the Sun. ANSWER * In late 2003, announcement was made by a group of astronomers of discovery of what they claim to be a possible Solar System planet, which they named Sedna, that lies astride Neptune's orbital path. This is an isolated small body (about 1250 to 1800 km; 750 - 1100 miles in diameter) that is spherical. Such a shape is suggestive of melting and reorganization into a round mass. According to the American Astronomical Society rules, this size is below the lower limit agreed to be the smallest a body can be to be named a planet. It appears to be a new class - between irregular asteroids, which can be larger, and the solar planet sizes; some rounded "moons" are in this size range which could explain it as an escaped Neptunian satellite but a mechanism to remove it from Neptune's orbital family into its own solar orbit is yet to be proposed. Actually, another planet had been imaged in 2003 but astronomers at Cal Tech did not recognize it as such until 2006 when three images taken 90 minutes apart were inspected and analyzed. This is what they saw: The discovery telescope images showing 2003 UB313. The circled bright object in the left image seems to be one body. But in the second image, two bodies are resolved. The upper body moves again in the third image, taken 90 minutes later. The lower body proved to be a distant star. The upper body was shown to be within our Solar System. It was named 2003 UB313. After its acceptance as a planetary object, it was given the name Eris. At the 2003 sighting time, Eris lay a distance of 97 Astronomical Units. (/An Astronomical Unit [A.U.] is defined as the distance between Earth's center and the Sun's center, approximately 151 million kilometers or 93,000,000 miles./) Continuing observations show it to have a highly elliptical orbit so that its closest approach to the Sun will place it at a distance of 38 A.U. This orbit is inclined 44° from the ecliptic. Eris will take 560 years to complete one revolution around the Sun. Eris is one of several larger spherical objects that are within the Kuiper belt of asteroids. Most of those are irregular in shape. So, is the sphericity of Eris enough to qualify it as a planet, along with Sedna and others. As a momentary digression, this plot is an example of how remote sensing - in the form of gathering reflectances as a function of wavelength - is used in planetary studies. The plot shows the spectral reflectance curve for Pluto and for Eris. Both are nearly identical, suggesting that Eris has a surface composition similar to Pluto. Spectral reflectance curves for Pluto and Eris. The Sedna and Eris discoveries along with detection of other similar bodies have reopened one of the great debates in Solar Planetology: Is Pluto really a planet? Its size is the smallest (2288 km diameter) of the 9 traditional ones stated in textbooks. Purests consider it not worth of planet status; some believe it, like Sedna, is an escaped moon. The controversy was the centerpiece of a debate at the 2006 meeting of the International Astronomical Union (IAU) in Prague. A committee there produced this simplified definition of a planet (applicable to any planetary system, not just that of the Solar System): *A planet is a celestial body that (1) is in orbit around a star but is neither a star nor a satellite of a planet; (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes (usually after melting) a hydrostatic equilibrium (nearly round) shape, and (3) has swept up most of the smaller solid bodies (e.g., asteroidal "chunks") in the spatial region around its orbit. * Item 3 is the new criterion that is to be used in defining a true planet. A key word is "most". Both Earth and Mars have some asteroidal-sized rock chunks within their orbital zones but these bodies are in toto only a very small fraction of the mass of the parent planets. Pluto has much more such bodies still in its orbital zone, so it has failed this third criterion. Item 3 leads to another parameter that clearly separates the Main Planets from smaller bodies. This is done by calculating the ratio of the mass of the parent planet to the mass of all small body materials (excepting moons) in the orbital zone of the planet; this ratio is represented by the Greek letter μ. This plots as follows: Planet/Small Body ratios of large objects in the Solar System. The mass ratios show that the 8 Solar System planets all have μ values greater than about 7000; the three spherical bodies Pluto, Ceres, and Eris all have values less 1. On this basis, the reason the IAU now contends that there are only eight major planets is obvious. This reclassification might seem straightforward, but exceptions may cause confusion, e.g., Pluto's moon Charon. But Charon lies beyond the barycenter for the Pluto-Charon pairing. The barycenter is the center of gravity for a two body system that has one body orbiting the other. The Moon remains a satellite despite its size because of the Earth-Moon barycenter still residing within the larger Earth. In time (billions of years), as the Moon recedes the barycenter will migrate outward beyond the Earth, so that, technically, the Moon would become a planet in the sense of Charon. The large satellites of Jupiter, Saturn, Uranus, and Neptune all associate with barycenters located within their parent planet. So, what did the IAU decide: They first mulled over the "strawman" concept that as of 2006 there might be 12 planets in the Solar System, with the three new ones (2003 UB313 is now named Eris) shown in this diagram. They firmly decided at this meeting against these three as 'full blown' planets, keeping them as large members of the Kuiper Asteroid Belt. The earlier proposed 3 new planets, all much smaller than Earth (right); 2003-313UB is provisionally known as Eris. The latest thinking is that there should be three categories: 1) planets; 2) dwarf planets; 3) solar system objects. Dwarf planets must not be satellites of larger planets. Dwarf planets must be round, whereas solar system objects must be irregular in shape. Dwarf planets that are icy can be as small as 200 km in diameter; if they are rocky, the smallest size that is round is about 400 km. As of 2010 about 50 round dwarfs have been found in the Solar System. A number of small round bodies have now been found in the Kuiper (asteroidal) Belt and possibly the Oort (cometary) Cloud (page 19-22). They are part of a vast collection of large to small objects - most non-spherical - orbiting the Sun. According to the IAU definition, some smaller round ones may be eventually raised to the dwarf planet level. Here are some current candidates: Possible future solar dwarf planets. The IAU voted on August 24, 2006 to further redefine planets by size - these falling into Main (large) and Dwarf (small) categories. This enabled Pluto to retain some planet status by becoming a Dwarf Planet along with Ceres and Xena. Pluto's moon Charon remained just that - a satellite. But as in any democratic procedure, those on the losing side will continue to disagree. This diagram depicts some of the Dwarf planets in terms of their relative sizes: The comparative sizes of the named Dwarf planets. Since the Dwarf planets are small, good photo images of them are not easily acquired. Most are depicted from observations that are translated into "artist's concept", visual representations of the 'best guess' of their appearance. Below are some of these planets - read their captions to determine if each one is an actual image or an artist's concept: Sedna; this may be an actual image Eris; artist's concept. Haumea; artist's concept; it seems to be elliptical. Ceres; Hubble image.. Xena and its moon Gabrielle; artist's concept The question "What is a Planet" has been addressed in the January, 2007 Scientific American article by that name as written by Steven Soter. There seems little doubt that Earth is still the premier planet among those now discovered (within and beyond the Solar System). It is (so far) unique in having life (but not organic molecules) as a main characteristic. In the Solar System, Earth is alone in having 1) widespread water oceans, and 2) abundant vegetation. It is the presence of trees, grasses, and other vegetative cover that distinguishes Earth from the other solar planets; but from the standpoint of remote sensing this absence on the other planets makes it possible to examine them in terms of the geologic processes that led to their development (on Earth, extensive vegetation usually masks the underlying geology, as is typical in the eastern United States). This subsection ends with the illustration below, which shows the full disc appearance of each of the four Inner or Terrestrial Planets scaled to their actual relative sizes, of which Earth is the most important (or you wouldn't be reading this): The four Inner Planets (Mercury; left); Venus (with its atmosphere removed); Earth; Mars. * Some Planetary Parameters * Consult the table below, which summarizes the principal characteristics and properties of the nine planets. We list them from top to bottom in the same sequence as those shown from left to right in the above illustration. To simplify, we do not include the names of the principal satellites orbiting some of these planets, but we cover them in a listing below the table. *PLANETARY BODY* *DISTANCE FROM SUN (AU)* *ORBITAL PERIOD (yrs)* *ROTATIONAL PERIOD (days)* *DIAMETER (km)* *DENSITY (gm/cm)^3 * *NUMBER OF SATELLITES* *Mercury* 0.387 0.24 58.6 4,880 5.44 0 *Venus* 0.723 0.62 243R 12,105 5.25 0 *Earth* 1.000 1.00 1.00 12,757 5.52 1 *Mars* 1.524 1.88 1.03 6,786 3.93 2 *Jupiter* 5.203 11.86 0.41 143,797 1.34 8 R; 55 IR *Saturn* 9.539 29.46 0.43 120,659 0.70 21 R; 26 IR *Uranus* 19.18 84.01 0.72 51,121 1.28 18 R; 9 IR *Neptune* 30.07 164.80 0.73 49,560 1.64 6 R; 7 IR *Pluto** 39.44 247.68 6.4 2,288 2.06 1 R; 2 IR * Included in the Table despite its reclassification as a Dwarf Planet. Note: For the number of satellites; that numeral left of R refers to those satellite that are nearly spherical - that left of IR refers to irregular shaped satellites (see page 19-14). AU = Astronomical Unit, which is the mean distance (approx. 150 million kilometers, or 93 million miles) from the Sun to Earth Names of principal satellites (smaller ones omitted): * Earth: Moon * Mars: Deimos; Phobos * Jupiter: Io; Europa; Ganymede; Callisto * Saturn: Mimas; Enceladus; Tethys; Dione; Rhea; Titan; Hyperion; Iapetus; Phoebe * Uranus: Miranda; Ariel; Umbriel; Titania; Oberon * Neptune: Triton; Nereid; 1889N1 * Pluto: Charon The above table lists the distances of the planets from the Sun. This diagram shows these distances in terms of the orbits of the planets: The relative distances of the planets in terms of their orbits. As will be described later, beyond Neptune and Pluto are Sedna and the Kuiper Belt (asteroidal bodies) and Oort Cloud (mostly comets). This diagram indicates the extreme distances of these latter features in relation to the nine planets: The full extent of the Solar System. (A brief mention is made here of an interesting hypothesis maded by Dr. Harold Levinson of the Southwestern Research Institute: He postulates that in the early Solar System the planets had orbits different from the present. The Giant Planets Jupiter and Saturn had orbits such that on very infrequent occasions they would approach each other so that their massive gravitational interactions caused them to start a process called resonance. This eventually forced them to readjust their positions and also hurled Uranus and Neptune further out. About 3.9 billion years ago, the resonance also perturbed the asteroid belt objects, forcing many of these towards the Inner planets. The Moon, formed by then, underwent a cataclysmic bombardment [see subsection on the Moon] during which most of its craters were formed [extensive cratering also took place on Earth but almost all of these have been obliterated by subsequent erosion]). The densities of the planets is one characteristic parameter. Here is a histogram that compares this property: Relative densities of the Solar System planets. The Inner planets all show rocky materials at their surfaces (all but Mercury have atmospheres). Much of the density components depends on the relative sizes of the core, mantle (if present), and crust. The next two illustrations show the measured (or estimated) size of each component: Graph showing proportions of crust, mantle, and core in each of the Inner or Terrestrial planets. The atmosphere thickness and size of the solid interior of each of the Giant or Gas planets. The next table summarizes orbital parameters and atmospheric characteristics of the solar planets. Orbital Parameters & Atmospheric Characteristics of the Planets. The orbital inclination is a measure of the departure of a planet's orbital plane, in degrees, from the orbital plane of Earth around the Sun (Sol) which defines the *ecliptic*, a plane containing both the Sun and the Earth's orbit (see page 20-5, near the top). * History of Planet Studies and Aspects of their Motions * The table above shows that the four planets closest to the Sun are small compared with those beyond Mars. These are the Inner or Terrestrial (like Earth, with rocky material at their surfaces) planets. From Jupiter through Neptune, the planets are much larger (the Outer or Giant group) and have surfaces that are all gas (Pluto, the exception, may be a "maverick", possibly being an escaped satellite). Nearly all planetary satellites are either rocky or a mix of rock and ice (one, Saturn's Titan, has a thin atmosphere). The four inner planets and Jupiter and Saturn were known since ancient times; Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. The Sun-orbiting planets are recognized by astronomical observations because they move relative to the background stars (the ancients called them "wanderers"). Despite the efforts of pre-Renaissance astronomers (e.g., the Greek, Ptolemy, living in 2nd Century Alexandria, and later Arab observers) to develop a legitimate model of the Solar System, the frame of reference put the Earth at the center of the System (geocentric model). The ancients perceived the "Universe" (for them, mostly the known planets, and other points of light called stars) as a set of concentrically nesting spheres that had the Sun on one sphere; all spheres rotated around the Earth at different rates. The "map" below is one version of the geocentric Solar System as envisioned in late Medieval times; note the descriptors are in Latin: Medieval map of a geocentric Solar System. This was replaced in 1543 (date of publication) by the heliocentric model, based on work by the Polish scholar and priest Nicolaus Copernicus, who postulated that the Earth rotates and the planets revolve around the Sun. This Copernican model was largely ignored for decades, mainly from philosophical/theological objections, until observations by Tycho Brahe in the 17th Century supported the Sun-centered scheme (which, unfortunately, he rejected after conducting a flawed experiment). Galileo also made vital observations through one of the first telescopes; his discovery of satellites around Jupiter confirmed the notions of bodies revolving around a central body. General acceptance by the scientists of the times was still slow but the laws of planetary motion enunciated by Johannes Kepler (Tycho's protege) and motion in general by Isaac Newton finally led to such overwhelming evidence that scientists and other thinkers and eventually the Church acceded to this reality. Kepler deduced from the patterns of motion that the planets revolving around the Sun did not follow precise circles but instead followed elliptical paths with the Sun at one of the two foci that characterize an ellipse. The ellipses defined by him and later astronomers were only slight departures from circularity, except for Mercury (strongly elliptical) and Pluto (which periodically crosses the ellipse traced by Neptune). Kepler's second law is derived as follows (see figure below): Diagram illustrating Kepler's Second Law; see text. Start with a line from the Sun to a planet at any locus. e.g., a, along its orbital path. After it had moved some distance a-a' along the path, it will define some given area A for the time in transit, For another segment elsewhere along the orbit, a different pattern - area B - ensues as it traverses the distance b-b'. Now, if the elapsed time between orbital transits from positions a to a' and b to b' are specified to be the same, the areas in the patterns will be equal (A = B). The law can thus be stated: Imaginary lines from the Sun to any planet sweep out equal areas in equal elapsed time intervals during different stages in the planet's revolution. Since the distance a-a' is shorter than b-b', it follows that the velocity (distance/time) of the planet moving through b-b' is greater than the speed through a-a'; in other words, planets move faster when closer to the Sun. Separate arguments based on Newtonian mechanics show that the velocities of the planets decrease progressively outward from the Sun. (As an aside which applies both to the planets and to orbiting satellites [like Landsat], the velocity needed to achieve and maintain orbit is a balance between the forward motion vector of the moving body and the gravity vector pulling it towards its parent body [whose mass is assumed to be at its center]; thus the tendency to move away tangentially is offset by gravitational force such that as the parent, e.g., Earth, rotates such that seemingly its surface falls away from the tangential line, in fact the satellite (or planet) is pulled downward just enough to maintain the same distance to the center of mass, describing a path that produces a circular orbit [or is modified to some degree of ellipticity], even as its momentum [mv; v varies for the elliptical case] keeps it in that orbit. Like the planets, the velocity needed to get and keep an Earth-orbiting satellite in place decreases outward. Landsat moves much faster [~26,600 km/hr], and with a period [time to complete one orbit] of 103 minutes, than does a geostationary satellite. The latter, when placed at 22,300 miles [36,235 km] above ground, moves slower [24 hours to complete an orbit] over a much longer orbital path at an orbital velocity of ~11000 km/hr; when inserted so as to move parallel and over the equator, the geostationary satellite moves forward at the same speed as its nadir point on the equator and thus is stationary [no relative movement] with respect to that point on the Earth's surface.) Kepler discovered a third relationship affecting the paths of the planets. If the orbital period P of a planet (third column in the table above) is plotted on log-log graph paper against its distance R (second column) from the Sun (taken as equal to the semi-major axis of the path ellipse), then the result is as appears below. The mathematical expression for the equation representing the resulting line is P^2 = R^3 , the mathematical statement of Kepler's third law. Kepler's Third Law, as determined by the equation of the line shown in this log-log plot of P vs R. Another, rather curious relationship was put forth by Johann Titius in 1766, with later modification and promotion by Johann Bode. To formulate it, look at this sequence: N = 0.0 0.3 0.6 1.2 2.4 4.8 9.6 19.2 38.4 76.8 Now, add 0.4 to each N term, giving this new sequence: 0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 77.2 The actual positions (in A.U.s) of the planets are: Mercury = 0.39; Venus = 0.72; Earth = 1.0; Mars = 1.5; Asteroid Belt = ~2.8; Jupiter = 5.2; Saturn = 9.5; Uranus = 19.2; Neptune = 30.0; Pluto = 39.5. For each successive planet we doubled the previous N and added 0.4; for Mars this yields 1.6 and for Neptune this results in 38.4 + 0.4 = 38.8. Remarkably, this set of numbers is closely matched by the actual distances as Astronomical Units for all the planets except Neptune which lies at 30.07 A.U. Pluto (no longer a planet), however, lies at 39.4 close to the 38.8 value. The value 77.2 suggests a "missing" planet at that distance. No obvious physical reason has yet been found for the Titius-Bode "rule", nor is the Neptune anomaly readily explanable (it may relate to an interaction with Pluto). But one consequence was a prediction that some planetary body should exist at A.U = 2.8. None was known at that time but the later discovery of the Asteroid Belt at 2.8 fulfilled the prediction. That gap is evident in the figure above, as is the anomalous position of Neptune. The more general subject of celestial motions, or also referred to as celestial mechanics, is rather complex - beyond the scope of the Tutorial here (some aspects are treated near the top of page 20-2 <../Sect20/A5.html>). But a few ideas relating to the Sun's and the planets' motions relative to Earth as the observing platform are introduced here. These motions with respect to the Earth and to the Sun's path across the sky can be plotted on the Celestial Sphere - usually displayed as either the hemisphere north or the hemisphere south of the Equator or one's local horizon, for each case showing the expanse of celestial objects above the horizon in all directions. Thus from any location on one or the other hemispheres, there is a a local observational hemisphere on which the motions of the Sun, the Moon, the planets, and the stars appear to move as the Earth rotates each day, causing each body or point of light to trace an arcuate path across the hemisphere (best seen at night). An axis around which the heavenly bodies seem to move in circular arcs is imagined to pass from Earth to the hemisphere, with the North Star (Polaris) being very close to the point where the celestial axis would penetrate the (hemi-)Sphere; by convention, the Earth's rotational axis is placed to coincide with the celestial axis, and the Earth' North Pole is near the celestial North Pole. The position of the North Star, and hence all other celestial bodies, will vary with latitude and with season of the year. At different times in a year different distinctive groupings of stars - known as constellations (discussed again on page 20-2) appear in the same part of the Sphere at the same time of night, are illustrated here: Major constellations seen in the same direction into the sky, at Summer and Winter. One approach to understanding celestial motions is to trace the pathway of the Sun across the Sphere as it varies seasonally and with location on the Earth's spherical surface. First, we introduce the concept of the solar ecliptic. This is the plane defined by the elliptical trace of the Earth's annual orbital path around the Sun (the orbital traces of the other planets approximately follow the ecliptic as well; Mercury at 7° and Pluto at 17° have the largest departures from the orbital plane as defined by Earth's path). A corollary: the ecliptic is also the Sun's annual path among the stars as viewed from Earth. In this frame of reference, the Earth, and also six other planets rotate in the same direction - counterclockwise (prograde) as viewed from above (as from the celestial North Pole); Venus and Uranus have retrograde (clockwise) rotations. None move in circular orbits; each follows an elliptical path, usually with small ellipticity, such that it will be varying distances to the Sun with points of closest and farthest approaches. While all the planets orbit on or near the ecliptic, their rotational axis is tilted to varying degrees relative to the ecliptic plane (or to a line normal to [90°] the plane). Only Mercury's axis is at 0° with respect to this normal. Venus and Pluto have their north rotational pole tilted more than 90°, hence the top of the axis lies beyond (south of) the ecliptic. The Earth's axis is tilted 23.5° from the a line at right angles to (normal to) the ecliptic. Two diagrams will indicate the seasonal and the latitudinal effects of the Sun's movement across the celestial hemisphere: Seasonal paths of the Sun, projected on to the Celestial (hemi)Sphere. In the above diagram, the (unspecified) latitude of observation, along with Earth's axial tilt, is responsible for the celestial North Pole being about 60° down towards Earth's Equator (in this projection, an 'ellipse' marked by E, W, N, and S). The highest the Sun will reach is at Summer Solstice (~June 21); the plot of a daily path then across the sky at that latitude is a partial circle (arc) shown in red that has the greatest length, thus resulting in the maximum amount of sunshine on that day. That arc begins at sunrise (after dawn) and ends at sunset. The smallest arc is at Winter Solstice (December 21), shown in green; at that time the daylight is the least - several hours less than 12. At the Spring (~March 21) and Fall (~September 21) Equinoxes - blue line - the arc is at a median length, and the sunrise-to-sunset duration is about 12 hours. Paths of the Sun across the Celestial (hemisphere) for observers at latitude 41 degrees north.. The second diagram has a different orientation of the equator, and a set of seasonal Sun paths, for a location at 41° North. Here the Sun becomes almost vertical (would cross the zenith on the Sphere) on June 21. Thus, the Sun path will vary systematically with latitude (the effect of the Earth's axial tilt is built in). This can be plotted on a diagram: Variation of the Sun's path with latitude. Here's how to read this diagram. First consider your location at 0° latitude, i.e., you are on the Equator. This is represented by the bottommost curve. The starting point is September 21: the Sun is directly overhead at Noon. By December 21 (Winter Solstice) the Sun has migrated northward to 23° North latitude, reached when local Noon occurs. By March 21, the Sun has 'retreated' southward to again cross the Equator and be overhead. The Sun then continues southward into the southern hemisphere reaching 23° South at the noontime of the Summer Solstice. It then progresses back to the Equator to repeat this cycle. Now, look the 90° curve (right side) which places you at the North Pole: From March 21 to September 21, the Sun will never set - 24 hour daylight; from September 21 to March 21 the Sun does not rise - "perpetual" night. At high latitudes long days or long nights, but some Sun setting, will occur. For a third case, consider the curve for 40° North: The Sun at that latitude is at 40° North on September 21; it migrates to about 62° North by December 21, then moves southward until it reaches its southmost position at ~19° latitude at the Summer Solstice, then reverses trend, going back to 40° North by Fall Equinox. Try your ability to interpret a curve for some other latitude. From day to day during the year, as Earth orbits the Sun in the plane of the ecliptic and maintains a tilted axis of rotation which points in a fixed direction in space, we see four things change: (1) the sunrise direction changes (2) the sunset direction changes (3) the length of daylight changes (4) the height of the Sun at noon above the southern horizon changes Thus, the two Celestial Sphere diagrams show that the Sun follows arcuate patterns (of varying lengths depending on time of year) across the sky as seen at any location during the day. Stars, on the other hand, will follow circular patterns, centered on the Celestial North or South Poles, over much of the Sphere. Planets, being part of the Solar group circuiting the Sun, will also move during a given night, but their paths over months and the seasons will appear as though they "wander" through the background of stars and constellations. Their shifts are not equal per terrestrial time period and are seen from Earth as influenced by their relative distances from the Sun and their locations in their elliptical orbits relative to earth observers. *Meteorites as Samples of Planetary Materials* Prior to the space program which has led to visits of unmanned probes past or onto planetary bodies and over one fabulous decade the landing of humans to explore the Moon's surface, our knowledge of the planets were largely confined to two avenues of investigation: 1) telescope observations and selected properties measurements using accessible parts of the EM spectrum, and 2) samples of one or more planets and smaller solid bodies that fall to Earth as meteorites (or, as discussed in Section 18, as large bolides - megameteorites, asteroids, and comets). The bulk of the rest of this page will be devoted to a review of meteorites, which continue to be a prime source of information about some of the Sun's planetary and fragmental-bodied associates. "Stars" falling from the skies have been known since ancient times; rarely, stones are found that were tied to these "shooting stars". One such rock has been venerated by Islam (in its encased shrine in Mecca) for more than 1300 years. By the 19th Century, meteorites were identified correctly as samples from other parts of the Solar System. They are part of the nearly 500 tons of extraterrestrial rock material that reaches and enters the atmosphere each day. Most of that material is burned up by friction from the high speed of entry but meteoric dust can remain in the air and a very few individual blocks of material survive this passage to fall in the sea or on the ground as meteorites. A general nomenclature has been developed to describe rocks in space that may reach the Earth's surface. If these rocks are relatively small (say about house-size or less), as they exist in space they are referred to as /meteoroids/. If they reach Earth and pass through the atmosphere, creating intense light as their outer skin is melted by friction, they are called /meteors/. If they do not burn up completely in transit, and land on the Earth's surface, they now are designated /meteorites/. The largest meteorite found so far on land is about the size of an automobile; most meteorites are much smaller. Much larger bodies moving in space, such as asteroids and comets (page 19-22), can strike the Earth, either as still intact bodies or broken into fragments; these will nearly always produce impact craters (Section 18) or shock-induced destruction on the ground (such as knocking down trees) if they explode in the atmosphere. Another terminology distinction: Meteorites whose passage through the atmosphere was observed and then someone soon thereafter locates the object are referred to as /Falls/; those whose passage was not observed but were eventually discovered (often by serendipity) are called /Finds/. By the start of space exploration, nearly 1900 meteorites had been collected. That number has jumped notably (over ten thousand) when scientists exploring the Antarctic deduced that a few of the rocks scattered about the ice surface might be meteoritic debris. Patient collection has since verified this, thus providing a very effective way to find new meteorites. However, of any thousand rocks on the Antarctic surface, only about 1 or 2 prove to be meteorites. But each year, a new expedition (on snowmobiles) continues to add to the total. Meteorite hunter looking down to spot meteorites on the Antarctic ice. In searching for meteorites, two clues call attention to certain stones as candidates to be collect and broken into to reveal indications of their nature: 1) rocks that appear to be composed solely or largely of iron metal; and 2) rocks that have a thin dark fusion crust, where friction has melted the exterior. Although the classification of meteorite varieties consists of various categories, most meteorites fall into two types - Iron and Stony - as shown here: A typical iron meteorite (left) and stony meteorite (right); both specimens have a thin, dark fusion crust. The Renazzo stony meteorite shown below as broken open reveals the typical texture of this type: The Renazzo stony meteorite. The mineral composition of meteorites is distinctive. The iron meteorites contain native iron metal alloyed with 5 to 17% nickel. The stony meteorites are composed of minerals that are common in basic igneous rocks: olivine, pyroxene, and plagioclase feldspar. together with a variety of minerals (some found only in meteorites) present usually in small quantities. Various combinations of these and some other constituents, together with distinctive textures, provide the basis for classifying the different meteorites. One general classification appears below. You can examine a more detailed classification by going online to this helpful website . Classification of meteorites; note the percentages of each major type. Iron meteorites (known as Siderites) are uncommon but quite distinctive. (Most believe they are the core material in differentiated (melted) asteroids. They contain one or both of the structural phases of metallic iron: Kamacite and Taenite. The Iron types are classed by the amount of nickel present and the nature of the iron phase(s). When an iron meteorite's interior is exposed, usually by sawing to create slab faces and then etched by nitric acid, some distinctive textures are often present, such as what is termed "Widmanstatten strucure" caused by unmixing of the two structural phases (the broader bands are Kamacite), displayed here at two magnifications. Widmanstatten structure in an iron meteorite, of the Octahedrite class. Magnified close-up of interlocking iron phases in Widmanstatten structure; this eutectic mix of Kamacite and Taenite is called Plessite. Another planar structure in the Iron meteorites is called Neumann Bands, which is a twinning mode induced by shock. Most likely, this shock effect occurred during the breakup of the parent body of which the Iron meteorite was in the interior (a core analogous to the Earth's?): Thin Neumann Band twins in an Iron meteorite. Transitional to the stony types are the stony irons, that include the Pallasites and the Mesosiderites. An example of the first is the Esquel meteorite (generally, a meteorite is named from a geographic location where it fell and was collected: The Esquel Pallasite; the non-metallic phase is olivine (dark or orange); the silvery metallic phase (tinted reddish) is iron. As the percentage of native iron decreases and silicates increase the resulting stony-iron meteorites are called Mesosiderites. The Lowicz Mesosiderite. Most meteorites based on the percentage of Falls (those observed as "shooting stars"), which may not be the same as the percentage of Finds, since some meteorites are more likely to be destroyed by weathering, etc.), are of the type called Chondrites which in turn are grouped into classes depending on mineralogy and texture. Chondrites contain generally small (millimeters up to a centimeter) spherical bodies called /chondrules/, which most meteoriticists believe were once molten silicate droplets produced by melting of interspatial dust by one or more mechanisms such shock waves or heat from the forming Sun. They then cooled and crytallized into Olivine, often accompanied by pyroxene mineral species (Enstatite, Bronzite, Hypersthene) and Plagioclase (calcium-rich). Most chondrites contain small crystal specks of iron-nickel. The chondrules seem to be embedded in other dust and isolated crystals which incorporate the chondrules as the meteoroid or asteroid built up from the remaining materials in the dust clouds surrounding the growing Sun, over the first few million years of the organizing Solar System. This photomicrograph shows a texture characteristic of chondrites, with subspherical chondrules, crystal fragments, a few iron-nickel grains, and a fine-grained matrix: Texture typical of a chondrite. The next two figures depict photomicrographs (with the petrographic microscope's Nicols in the cross-polarization mode) of individual chondrules: A chondrule made up mostly of Olivine, in the Brownsfield chondrite. Two chondrules in the El Hammami chondrite; the one on the left has some plagioclase; the one on the right has parallel crystals of Olivine, producing a 'barred' texture. Some chondrules show a characteristic radiating structure assumed by the Pyroxene Radiating Enstatite crystals in a chondrule; these appear to converge on a common base. Plagioclase can be conspicuous in some chondrites, as shown in this photomicrograph. Plagioclase in a crude chondrule and pyroxene, seen in this thin section in polarized light. The bulk texture typical of an Ordinary Chondrite is exemplified in this slab cut into the Homestead meteorite: Typical chondritic texture; most of the chondrules in this slab from the Homestead meteorite are too small to be visible here. Somewhat larger chondrules are present in this sample from the Brenham meteorite: The Brenham Chondrite. Classification of the Chondrites is determined to some extent by the particular mineral species present. However, the usual hierarchy (Type 1 through Type 6) is determined by the degree of water content and extent to which the chondrule appears to have been reheated and thus recrystallized by thermal metamorphism. Type 1 is most primitive and contains some water; it probably was never reheated after primary crystallization beyond about 300 °C. Type 6 is anhydrous, shows thermal and/or shock textures, but was reheated up to about 800°C. In the next two photomicrographs are shown 1) a ring of iron metal that accumulated when the chondrule was thermally heated to the extent that iron was melted; 2) a chondrule with veins of glass caused by shock heating. A chondrule with an iron ring Veins of blackish glass is a shocked chondrule. We turn now to a special class of Chondrites called Carbonaceous Chondrites. These contain up to 6% carbon, either in elemental form or in the composition of organic (hydrocarbon compounds, including some amino acids, but not biogenic) molecules that occur within them. Low temperature minerals, such as clay minerals and serpentines, attest to the conclusion that the matrix was never subjected to the high temperatures that melted the associated chondrules. This is supported by the variable water content; some of these meteorites contain up to 11% H_2 O. Many meteoriticists consider carbonaceous chondrites to be the most fundamental and primordial representatives of the solid materials available for making up the planetary system. They are thus held to be condensates of melted silicates that mixed with low temperature organic and inorganic phases which grouped into asteroids and comets, or were aggregated into the planetesimals that evolved into the planets. Here is one of the best-studied of this class - the Murchison meteorite that fell on Australia: One of the pieces of the Murchison carbonaceous chondrite. One of the most famous meteorite falls was the Allende carbonaceous chondrite, in which nearly two tons landed in a farmer's field in northern Mexico in 1969. It's quantity has proved to be a bonanza for researchers. Below is one of the pieces and a thin section which shows a carbon-rich matrix around the chondrules A piece of the Allende meteorite A thin section cut from the Allende meteorite; crossed nicols. About 8% of the silicate (stony) meteorites do not contain chondrules; the group is known as the Achondrites. There are many varieties, as evident in the classification we pointed you to. Most members are thought to have come from the surfaces of asteroids. Some of these are breccias and other unusual textures may be distinctive. Eucrites are a common class and are either similar to terrestrial basalts in texture or are brecciated. The basaltlike Millbillillie exemplifies the first type here; The Millillillie meteorite. Basaltlike texture of the Millillillie meteorite. Two brecciated achondrites appear here: A polymict breccia A polymict breccia. There is a growing realization that many of the Achondrites may be pieces of the Moon or Mars expelled from these bodies by impact. Lunar meteorites may directly strike the Earth after thrown off by a lunar impact or fall after being captured in orbit. Martian meteorites need to be thrown out beyond martian gravity into orbits that may be perturbed or decay to allow eventual Earth-crossing encounters. Below are three meteorite samples of probably lunar origin, as determined by age and composition. Lunar meteorite QUE94281, NWA2362, a possible lunar meteorite traced to some mare site. The Calcalong Creek Lunaite (lunar meteorite) This last meteorite is remarkably like lunar regolith (the loose debris on the surface). If so, it was shock-lithified by the impact that hurled it to Earth; if not, it was probably breccia rock that was part of an ejecta blanket later lithified. After the Apollo moon rock returns, it became much easier to prove certain meteorites to have come from the Moon; both chemical composition and isotope ratios were particularly diagnostic. Here is a plot of the composition of meteorite specimens of certain-to-probable lunar origin: Less than a 100 meteorites are thought to Lunaites (Moon meteorites). Most are fragmental. A neat review of Lunar Meteorites is found at this Washington University web site. At least 35 meteorites have evidence that they came from Mars. The next figure is of a Nakhrite type meteorite of probable martian origin whereas the second illustration shows the texture of the Zagamil meteorite which is considered of martian origin. We will show other examples of these planetary meteorites on pages in this Section that treat the Moon and Mars. A Nakhrite meteorite, believed to come from Mars. Photomicrograph of the texture of the martian Zagamil meteorite. The age(s) of meteorites can be instructive. Elemental isotopes are used to date them. The chondrites give very old ages (clustering around 4.5 - 4.6 billion years), suggesting that these formed near the beginning of the Solar System. These ages are determined by Uranium-Lead and Rubidium-Strontium isotopic analysis; the presence of I^129 , derived from Xe^129 decay, which has a short halflife, confirms that at least some of the constituents were incorporated early in the inception of the Solar System. But there are one ot two younger ages, called exposure ages, which indicate times when the meteorite body separated from a larger host body and began its travel through space. Abnormalities in the amount of Ar^40 and He gas point to a time when larger parent asteroids may have broken up from collisions. Still younger ages deduced from He^3 , Ne^21 , Al^26 , and A^38 contents are associated with times when the meteoroids were traveling in their final sizes and were subject to cosmic ray bombardment. Genetic implications of the different meteorite types are these: For those not of lunar or martian origin, there may have been four stages of organization of meteorite parent bodies (most believed to be from the asteroidal belt between Mars and Jupiter): 1) condensation of high-temperature refractory silicates, oxides, and metals; 2) separation of silicates from metals as granular particulates in the solar nebula; 3) condensationn of lower temperature or volatile phases; and 4) varying degrees of remelting of the earlier condensates. From a different persepective: 1) the Carbonaceous Chondrites are the most primitive; 2) Chondrites formed from aggregation of chondrules (melted by shock, thermal radiation, or other process[es] and dust into bodies that never became large enough to melt; these bodies may, however, have experienced collisional breakup of asteroids (thus, some of the Achondrites might be so derived), 3) the Iron meteorites may (?) be cores of completely melted large asteroids, or less likely, bigger planets that were destroyed by collisional disruption, and 4) Some of the Achondrites were made by fragmentation/reassembly of differentiated planetary or asteroidal surfaces subjected to impact bombardment, or may be shock-lithified surface rubble (such as the regolith deposits on the Moon, as discussed later). Some of the general conditions that lead to different meteorites derived from asteroids are depicted in these diagrams: Schematics of different asteroidal histories that lead to different types of meteorites. At least one meteorite has been traced to a specific asteroid, Vesta, based on strong similarities in spectral properties: The asteroid Vesta, some 370 km in long dimension. The Vesta meteorite, purported to come from the asteroid Vesta. As space exploration goes on, more answers to organizational details are forthcoming, e.g., the similarity of asteroidal material to carbonaceous chondrites has been established by probes that approach or land on the asteroids. The importance of asteroids in the makeup of our Solar System is paramount. But we will defer further discussion of these bodies until after we have examined the major planets. However, for the curious who would like some insight now, go to page 19-22 <../Sect19/Sect19_22.html>. We have said nothing on this page, nor elsewhere in this Section, about the origin of the planets and the development of a Solar System. These topics are treated in some detail on page 20-11 <../Sect20/A11.html>, after astronomical principles are considered. We will start our extraterrestrial planetary tour with the Earth itself and then Earth's sole satellite, the Moon. The geological aspects of Earth were covered on page 2-1a <../Sect2/Sect2_1a.html> and 2-1b, to which the curious user can refer now by clicking on this page number for a refresher review. However, the Earth does deserve a brief overview of its general nature and history as one of the planets within the Solar System. navigation image map ------------------------------------------------------------------------ Primary Author: Nicholas M. Short, Sr.