"Gentlemen, I would rather believe that two Yankee professors would lie|
than believe that stones fall from heaven." -- Thomas Jefferson, 1809
The Effect of Size and Mass on Meteoroids/Meteorites
Meteorites are pieces of extraterrestrial objects -- primarily asteroids, but to a certain extent, also the Moon and Mars -- or pieces left over from the formation of the Solar System, which have run into the Earth, survived the passage through our atmosphere, and reached the surface of the Earth, more or less intact, so that we can pick them up, and examine them. The -ite ending in their name refers to the fact that, by and large, they are rocks (geologists love to name different rocks and even many minerals with an -ite ending), while the meteor- beginning of their name refers to the fact that they fell out of the sky. While still out in space, they are more properly referred to as meteoroids, the -oid ending meaning they have something to do with the start of their name; and in fact, meteors themselves are streaks of light, usually in the nighttime sky, which are caused by meteoroids running into our atmosphere at tens of thousands of miles per hour, heating up themselves and the air they are passing through to thousands or tens of thousands of degrees, and making the air and that part of the meteoroid which is vaporizing glow, bright enough to see, tens of miles below.
When the incoming object is microscopic in size, it is referred to as a micrometeoroid, and is always stopped by friction with the outer atmosphere of the Earth, with sufficiently little force to do very little to either it or the air it is passing through. Such microscopic bits of dust then slowly float and sink toward the surface of the Earth, forming a very tiny fraction of the dust that covers the surface, each year.
When, however, the incoming object is sand-grain or pebble sized or larger, the outer atmosphere is too thin to have much effect on them, and they plunge into the lower thermosphere or upper mesosphere with tremendous velocities, causing the high temperatures previously mentioned, and the emission of large amounts of light, equal to the energy of motion of the meteoroid. Objects of the size under discussion, regardless of their composition, are invariably vaporized before they have much chance to penetrate the atmosphere, or to be slowed by the backward pressure that their friction with the atmosphere exerts on them. But larger meteoroids, if made of tough enough material, can survive their passage through the upper atmosphere more or less intact. Parts of their surfaces vaporize and slough off (this is called ablation, and is one of the ways in which the Space Shuttle gets rid of the heat of its reentry), but the main mass of the meteoroid remains cold and solid, as it enters the mid and lower mesosphere.
The higher density of the air in the middle atmosphere produces not only greater frictional heating, but also powerful pressure forces, which drastically slow the incoming object, unless it is extraordinarily large. In fact, most objects less than fifty feet or so in diameter, even if made of relatively dense material, will be substantially slowed by the atmosphere, to the point where they no longer have enough speed to greatly heat themselves or it, and basically just plop to the ground, the same as if a rock of similar size were dropped from some great height.
This doesn't mean that such objects can't do considerable damage, when they run into things, as the images below attest; but it does mean that they lack the energy of motion required to bury themselves in the Earth, vaporize and melt themselves and a good part of the surrounding countryside, and create a 'meteor crater'. Instead, they and their surroundings, although perhaps broken, bumped or badly bruised by their fall, remain more or less intact, allowing us to examine them in detail.
A Brief Catalogue of Meteorite Types
Historically, the vast majority of known meteorites were 'irons' -- more or less pure mixtures of metallic iron and nickel, usually exhibiting characteristic interleaved structures called Widmanstatten patterns, which indicated slow cooling of an originally molten material, presumably in the core of an extraterrestrial object. Irons, even in left out in the weather for some time, remain extraordinarily dense compared to ordinary rocks (which are only a quarter to a third as dense as typical iron meteorites), so they are easy to recognize, for some time after they fall to Earth.
This long-lived recognizability of irons does not, however, extend to 'stones'. Those meteorites were traditionally recognized mostly when they were actually observed falling from the sky, as after a few years, weatherin and erosion makes them look much the same as any ordinary rock, at least from the outside. An expert (a meteoriticist) can tell a weathered stone meteorite from an ordinary rock, if he carefully examines its interior structure and composition; but even experts might fail to recognize a meteorite, if it were not something they were used to seeing. As an example, one of the world's foremost experts on lunar rocks and meteorites, when first shown a meteorite which later turned out to be a piece of the Moon, not only failed to recognize it as a lunar meteorite, but declared it wasn't even a meteorite, at all.
Needless to say, the lack of easy recognizability of stony meteorites, compared to iron meteorites, heavily weighted the number of known meteorites in favor of irons; so that as late as the mid-1960's, although it was known, from studies of meteor trails, than 90% of stony and iron meteorites must be stones, over 90% of known meteorites were irons -- an statistical 'error' of a factor of a hundred.
Aside from a division into stones (which turned out to be common, after all, if one knew where to look for them) and irons, there are two other ways in which we subdivide meteorites, and further subdivisions of those, according to their structure and composition.
First, and most commonly recognized, are differentiated meteorites, which are pieces broken off of larger bodies (primarily asteroids) which once melted, differentiated (heavy stuff falling to the center, and light stuff floating to the top), then slowly cooled and resolidified, before being broken up, or having pieces broken off of them, to produce this group of meteorites. (more to follow within a day or two).
Since the Earth is very large, and not nearly as inhabited, in most places, as those urban areas where most people live, the vast majority of meteorites fall far from any human habitation. But it isn't terribly unusual for meteorites to intrude into everyday life, and on rare occasions people or their possessions have been struck by meteorites, although whether anyone has ever been killed by a meteorite is unknown.
One of the brightest meteors observed in recent years, the Peekskill meteor of 1992 was brighter than the full Moon, and captured by sixteen video cameras and many still cameras, during the forty seconds its fireball streaked across the Virginias and Pennsylvania before one of its fragments struck a car in Peekskill, New York (hence the name given to the meteor and meteorite). The meteorite pictured above weighed nearly thirty pounds, and is made of very dense rock. As large as this might seem, prior to fragmentation, the original meteoroid is thought to have had a mass nearly two thousand times larger than this, and there are probably dozens of similar fragments which fell in woods or fields for miles around, and are still lying there, completely unrecognized.
Although you might assume that the owner of the car would have been unhappy about the damage to the car, she presumably received a considerable sum for the car and meteorite, as the combination has been exhibited all over the world by a company that sells meteorites (but only fragments of its most famous and valuable property). (Left, Pierre Thomas (LST), ENS Lyon, apod021118; Right, the meteorite, with red paint from the car, as displayed on the R.A. Langheinrich Meteorites site)
In March of 2003, a meteorite crashed through Colby Navarro's roof, bounced off a printer and dented a nearby wall, while the homeowner worked at a computer, in Park Forest, Illinois. This particular meteorite, about 4 inches across (shown in right photo), created a similar-sized hole in the ceiling (shown in left photo). Dozens of other pieces fell throughout the area, damaging homes and cars, as the result of a spectacular fireball visible across a wide area. It is estimated that a typical homeowner would have to wait the best part of a hundred million years to file an insurance claim, only to find out that such "acts of God" are not covered by their insurance; but since there are many times a hundred million homes in the world, dozens of such incidents occur somewhere in the world every year. (Ivan and Colby Navarro, apod030506)
Meteorites, particularly stony ones, are often difficult to tell from other rocks, especially after being weathered; so in most places, the vast majority of them are never found (for one thing, 3/4 of the Earth's surface being ocean, most of them sink to the bottom of the sea). For this reason, historically, iron meteorites, whose unusual density and composition makes them relatively easy to recognize, have dominated the "finds" that filled museums and research laboratories. But in 1969, it was discovered that certain areas in Antarctica are perfect places to hunt for meteorites, as (1) they experience relatively little weathering in the extreme cold, and (2) much of the ice sheet's surface is relatively free of rocks, so that rocks lying on the surface often prove be meteorites. In particular, in areas where ice flowing downhill runs into buried hills or mountains, eddies can form which, like eddies in a river, pile up relatively large amounts of flotsam. A number of governments have sponsored expeditions to such areas, resulting in the recovery of thousands of meteorites and meteoric fragments, of which 90% are the stony meteorites so rarely recognized elsewhere.
Snowmobiling ice-trekkers search for meteorites during the Antarctic summer of 1995/6. (Ralph P. Harvey (CWRU), Antarctic Search for Meteorites Program, NASA, NSF, apod021226)
An octahedrite meteorite, showing the characteristic interleaving of crystalline structures referred to as Widmanstatten patterns (after Austrian Count Alois von Widmannstätten). Octahedrites are iron meteorites which formed by the cooling and crystallization of a mixture of nickel and iron over a very long time (hundreds of thousands or millions of years), in the core of a differentiated asteroid. As the molten metal cooled, it separated into two varieties, taenite, with a higher nickel content (about one atom in five), and kamacite, with a lower nickel content (about one atom in ten), forming interleaved fingerlike structures up to two inches in size. To display the pattern, the meteorite is cut, polished, then etched with nitric acid mixed with ethanol. The kamacite crystals are left bright and shiny, while the taenite is duller and darker. Most iron meteorites are octahedrites, but they do not all have the same composition. Differences in their trace element composition separate them into more than a dozen groups, each of which must have come from a different parent body. (British Natural History Museum, Aram Dulyan, Wikimedia Commons)
Although most meteorites are fairly small, some are immense. The Willamette meteorite (named after the location of its discovery, in Oregon), on display at the American Museum of Natural History in New York, is an iron meteorite about 4 by 6 by 10 feet in size, which weighs nearly 16 tons. Despite its size and weight, it is only the sixth largest such meteorite found, to date. The large cavities are believed to be mostly due to weathering, in which troilite (an iron sulfide commonly found in iron meteorites) was converted to weak sulfuric acid by reaction with rainwater. (American Museum of Natural History, Wikimedia Commons)
The largest meteorite in the world, the Hoba meteorite, in Namibia. Approximately 9 feet square and 3 feet thick, the meteorite is believed to weigh at least 60 tons. Due to its large mass, it has remained where it was found, although about 6 tons have been removed by scientific sampling and theft in the ninety-some years since its discovery. (Paul Venter, Wikimedia Commons)
(John Subasavage, Jr)
The Holsinger meteorite (after Samuel Holsinger, the foreman of Barringer's mining operation), on display at Meteor Crater, is the largest of the Canyon Diablo meteorites, which are the remnants of the 150-foot-wide iron meteoroid which created Meteor Crater. Although less than 3 feet across, the iron meteorite weighs nearly 1500 pounds. It is estimated that tens of thousands of tons of meteoric fragments, representing about a tenth of the original impactor's mass, were scattered across more than twenty-five square miles surrounding the crater (also see Meteor Craters).
HED Meteorites (Howardites, Eucrites and Diogenites)
HED meteorites are achondrites believed to be pieces of the asteroid Vesta. Unique among the asteroids, so far as we can tell, Vesta underwent geological activity, in the form of volcanic eruptions, between 4.43 and 4.55 billion years ago (in other words, at the time that it and the other rocky bodies in the inner solar system were forming). Subsequent cratering of Vesta ejected large amounts of material -- smaller asteroids, and meteroids -- into space, and at a still later time, some of those objects struck the Earth.
Approximately 200 HED meteorites are currently known. Their name, or acronym, represents the three types of meteorites -- howardites, eucrites, and diogenites -- which form the grouping. Eucrites are basaltic mixtures of calcium-poor pyroxene, calcium-rich plagioclase (anorthosite, a common lunar material), and pigeonite. Some are formed from cooled lavas, while others are broken pieces of such material (breccias). Having formed by the cooling and/or breakup of surface melts, they contain very small crystalline structures. Diogenites are plutonic rocks, formed by slow cooling of magma at depth, which allowed much larger crystalline structures to form. Diogenites are primarily made of magnesium-rich orthopyroxene, although plagioclase and olivine are usually present in small amounts. Howardites are mixtures of broken pieces of eucrites and diogenites formed by material ejected from cratering impacts, then buried under other material and lithified (formed into a solid mass) by the weight of the overlying material. They are similar to lunar breccias in texture, but not composition.
Top left and right, images of eucrites and diogenites; bottom, howardites
(all images courtesy of NASA; click on images to reach original source)
The vast majority of meteorites found on the Earth are pieces from the asteroid belt -- either pieces left over from the formation of the asteroids, which were never inside larger objects, or pieces broken off of larger objects, at a later date. But a small minority of meteorites are pieces of other objects -- namely, Mars or the Moon.
A nearly 4.5 billion years old piece of Mars, found in Antarctica (the ALH at the start of its identification number refers to the Allen Hills, an area in Antarctica which is a particularly good spot to look for meteorites). This particular meteorite was thought to contain evidence of Martian microfossils, but their supposedly organic nature is a matter of considerable controversy. The fact that the meteorite comes from Mars, however, is virtually certain. (JSC, NASA, apod960817)
Meteorites on Mars
Most "Martian" meteorites currently known are pieces found on the Earth, which originated on Mars. But quite remarkably, as the Spirit and Opportunity rovers have explored Mars, they have come across several meteorites, scattered among the more ordinary Martian rocks. And in fact, a large number of those "ordinary" rocks may well be meteorites, as a common problem even on Earth is that meteorites, under normal circumstances, look much the same as any other rock. The meteorites discovered by the Spirit rover are two among hundreds of thousands of rocks scattered across Gusev Crater, and might not have been recognized, if the rover had been merely passing by, instead of seeking a safe spot to spend the winter. The meteorite discovered by the Opportunity rover, on the other hand, is lying on a vast, sand-covered area, in which very few rocks are found on the surface.
A view from the Spirit rover, of the rock strewn landscape of Gusev Crater. Most of the rocks in the area are dark in color, but the two light-colored rocks in this cropped image are probably iron meteorites, as they have thermal emission properties similar to the undeniably meteoritic object found by the Opportunity rover, in 2005 (see below). (Mars Exploration Rover Mission, Cornell, JPL, NASA, apod060721)
Ironically, the first meteorite found on Mars, an iron meteorite that must be part of the now broken-up core of an asteroid, was found only a few feet from the heat shield which protected the Opportunity rover on its way to the surface, the previous year.
Above, Opportunity's heat shield, lying in pieces on Meridiani Planum; and to the left, the basketball-sized iron meteorite shown in more detail, below. Unlike the meteorites discovered by the Spirit rover, which have been studied only by their thermal emissions so far, this iron meteorite was directly examined by the Opportunity rover, verifying its high concentration of iron, and conclusively proving its status as a meteorite.
(Above, Mars Exploration Rover Mission, JPL, NASA, apod050121); Below, Planetary Photojournal)
A "fisheye" view of the "Block Island" meteorite, from the Opportunity rover on Mars
(Mars Exploration Rover Mission, JPL, NASA, apod090813)
Tektites are glassy, often aerodynamically shaped pieces of rock which were formed when a large meteoroid hit the Earth, and melted portions of the impact site were lifted into suborbital paths, then fell back to Earth, while still in a partially molten state. They have a complex chemistry dependent on the geology of the site where they were formed, so that tektites associated with one impact can often be distinguished from tektites associated with another impact. They are typically found in large "fields", hundreds of miles across, which may be strewn across thousands of miles, along an arc extending from the impact site. Some tektites have near-vacuum bubbles, formed by the resolidification of part of the meltrock, while still tens of miles above the surface of the Earth. Tektites are usually named, like other meteorites, after the site where they were found, with the -ite ending always associated with meteorites tacked onto some variation of the site's actual name.
Moldavites are translucent, glassy green tektites found throughout the Moldau Valley in Chezchoslovakia. They are thought to have been formed as a result of the impact which created the Ries Crater, fifteen million years ago. Sizes range from microscopic, to nearly a foot across. Moldavites are so common that carving them has been a folk art in the Moldau Valley for centuries; and their semi-extraterrestrial origin and color make them attractive mineral specimens, which sell for a relatively reasonable price, considering their unusual origin.
Left to Right: Rough (natural) piece, carved owl, and carved pendant (TopGeo)
Although glassy in nature and appearance, moldavites are not ordinary glass, but melted rock, and have a greater hardness (6.5 to 7 on the Mohs scale, compared to 5 for glass) and higher melting temperature than most glasses. Their composition -- silica and various metallic oxides, including aluminum, potassium and iron -- is similar to that of the basement rocks in the Ries basin, showing that they consist of meltrock, rather than meteoric material. It is the small amount of iron present in the moldavites that gives them their attractive color.
Natural (rough) moldavites are usually relatively thin, translucent sheets, have aerodynamic shapes attesting to their origin, and may contain "bubbles" of near-vacuum, indicating formation at altitudes of ten to fifteen miles.