Remote Sensing Tutorial Page 19-2
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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 Cosmology in Section 21 as a background to this Section - optionally switching to it before moving on to the planets. 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. Last, we present a subsection on how meteorites are used to determine compositions of solid planetary, asteroid, and cometary bodies in the Solar System.


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, but with the restriction that we show almost exclusively just surface images; usually no more than three to ten representatives of each mission. 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 replica 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. 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. JPL has recently selected choice images of the planets from various missions in a special Web site called the Planetary Photojournal, which you can access at USGS. Some images used in this section come from that source. 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 this site (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 the aforementioned one by Billy Glass of the University of Delaware, and a now out-of-print text by this Tutorial's author (Nicholas M. Short, Planetary Geology, 1975, Prentice-Hall Publ.), still in libraries. More recent are Planetary Landscapes by R. Greeley, 1985, Allen & Unwin, and Exploring the Planets by W.K. Hamblin and E.H. Christiansen, MacMillan, 1990.

A book published in March, 2003 covers much of what we know at 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. The writer strongly recommends Lonely Planets: The Natural Philosophy of Alien Life, by astrobiologist David Grinspoon, Harper Collins Publ. 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.

Most 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. In that Section, the point is made that remote sensing is by far the main tool or means by which scientists have learned about stars, galaxies, and intergalactic material in the Universe and in so doing have arrived at an ever more maturing understanding of the origin and development of the entire Cosmos.

Planetary Parameters and Aspects of their Motions

We concentrate in this Section almost entirely on the planetary bodies of the Solar System (for information on the Sun, check Sol and/or Sun) , excluding Earth.

Despite the large size of some of the planets relative, say, to Earth, taken together, plus also asteroids and comets, these planetary bodies orbiting the Sun 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 (treated in Section 20) which we will consider exclusively as the "Planetary System" despite the recent discovery of more than 100 extrasolar planets (again, Section 20), look first at the illustration below, which shows the relative sizes of the nine planets of our solar system (the distances between them are not to scale). From their appearance, how many can you name?

Illustration showing the relative sizes of the nine planets of our solar system.

19-1: Using their appearance, how many of the above planets can you name? ANSWER

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.

DENSITY (gm/cm)3

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; Hyperior; Iapetus; Phoebe
  • Uranus: Miranda; Ariel; Umbriel; Titania; Oberon
  • Neptune: Triton; Nereid; 1889N1
  • Pluto: Charon

In October of 2000, announcement was made by a group of astronomers of discovery of what they claim to be a possible tenth planet. Orbiting the Sun beyond Jupiter but inside Pluto's orbit, this is an isolated small body (about 500 km; 300 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. Much remains unsettled before the limit is revised to include this new body. 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.

History of Planet Studies

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 protoge) 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). His 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 P2 = R3, 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, start with Mercury and assign it the value N = 0 and add 0.4 to it (yielding 0.4). Next assign to Venus N = 0.3 and add 0.4 (giving 0.7). Now double 0.3 (0.6) and add 0.4 (= 1.0). Fo each successive planet double the previous N and add 0.4; for Mars this yields 1.6 and for Neptune this results in 38.4 + 0.4 = 38.8. This set of numbers if closely matched by the actual distances as Astronomical Units for all the planets, except Neptune which lies at 30.07 A.U. Pluto, however, lies at 39.4 close to the 38.8 value. No physical reason has yet been found for the Titius-Bode "rule", nor is the Neptune anomaly explanable. But one consequence is its 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.

Prior to the space program which has led to visits of unmanned probes past or onto planetary bodies and in one fabulous case 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 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 the Sun's planetary and fragmental bodies.

Meteorites as Samples of Planetary Materials

"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 the 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.

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 falls. 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 stone - 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 and etched, usually by sawing to create slab faces, some distinctive textures are often present, such as what is termed "Widmanstatten strucure" caused by unmixing of the two structural phases, 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.

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.

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 finds (those observed as "shooting stars"), which may not be the same as the percentage of falls (those not observed in passage but found later) 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 will 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. This photomicrograph shows a texture characteristic of chondrites, with subspherical chondrules, crystal fragments, a few iron-nickel grains, and a fine-grained matrix. 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.

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% H2O. 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 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.

A lunar meteorite consisting of a monomict breccia representing shock-lithified anorthositic (feldspar-rich) highlands material.

The next figure is of a Nakrite 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 Nakrite 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 presents of I129, derived from Xe129, 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 Ar40 and He gas point to a time when larger parent asteroids may have broken up from collisions. Still younger ages deduced from He3, Ne21, Al26, and A38 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.

We have said nothing on this page, or elsewhere in this Section, about the origin of the planets and the development of a Solar System. This is treated in some detail on page 20-11.. 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 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, which is covered on an inserted page (19-2a).


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Primary Author: Nicholas M. Short, Sr. email: