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.
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:
The Renazzo stony meteorite shown below as broken open reveals the typical texture of this type:
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.
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.
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?):
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:
As the percentage of native iron decreases and silicates increase the resulting stony-iron meteorites are called Mesosiderites.
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:
The next two figures depict photomicrographs (with the petrographic microscope's Nicols in the cross-polarization mode) of individual chondrules:
Some chondrules show a characteristic radiating structure assumed by the Pyroxene
Plagioclase can be conspicuous in some chondrites, as shown in this photomicrograph.
The bulk texture typical of an Ordinary Chondrite is exemplified in this slab cut into the Homestead meteorite:
Somewhat larger chondrules are present in this sample from the Brenham meteorite:
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.
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 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
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;
Two brecciated achondrites appear here:
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.
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.
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 I129, derived from Xe129 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 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:
At least one meteorite has been traced to a specific asteroid, Vesta, based on strong similarities in spectral properties:
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, 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, 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 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.