Origin of meteorites

Ernst Chladni in the beginning of the 19th century had convinced scientific circles, that meteorites are of cosmic origin, though until the second half of the last century their real origin remained a riddle. Did the cosmic visitors come from the Moon, from other planets or were they vagabonding matter-rocks who had their origin in areas outside our solar system? Only modern methods of the space-travel age and of atomic physics were in the position to coax by and by from meteorites the secrets of their origin.

For example it was possible, to determine quite exactly the origin of some observed meteorite falls, by routing these meteorites back to their origin by equations and calculations. It was almost always found in the asteroids belt, a wide area between the orbits of the planets Mars and Jupiter in which innumerable bigger and smaller matter-rocks circle the sun on different routes. In this quite dense belt, again and again collisions happen between different asteroids, where pieces are spliten off and they get hurled out of their route. These "cosmic stray bullets" - whether big or small - pendulum themselves on new, often eccentric orbits, sometimes crossing the orbit of our earth. Prospectively meteorites, inevitable sooner or later get caught by the gravity of a planet - the earth, the moon or the mars - and fall on their surface.

There are multitudes of asteroids, of which many thousands got names and numbers, but these asteroids are anything else but uniform. Spectrographical investigations of telescopes and space probes resulted in multifarious types of asteroids, that sometimes considerably differ in material consistence and construct. So for example, there are dark C-asteroids composed out of carbonic minerals, S-asteroids, that consist of lighter, more strongly reflecting rock, or M-asteroids whose surface seems to be metallic. At the same time each asteroid has a certain reflection spectrum, characterising it like a kind of fingerprint. By means of these unique fingerprints, modern meteorite science could identify one or the other asteroid as the mother body of certain meteorite classes and -groups .

A good example are the meteorites of the HED-group, the howardites, eucrites and diogenites. These form a unit not only based on their chemical and mineralogical characteristics, which makes one assume that they all come from the same mother body, but rather they also possess a unique reflection spectrum, for which some years ago one has found an exact counterpart in the asteroids belt: 4 Vesta. Vesta is with more than 530 km diameter not only the fourth largest asteroid in our solar system, but rather also differs from other asteroids in being no irregular fragment, but almost a ball, a real planetoide with large resemblance to the terrestrical planets (Merkur, Venus, Earth, Mars). I say, "almost a ball", because a powerful crater at the southpole with 460 km diameter and a depth of 13 km makes Vesta rather look like an apple. Colliding with another asteroid, out of this crater the meteorites of the HED-group were once thrown out, whereby Howardites represent samples of the surface, Eucrites part of the basaltic coat and Diogenites samples of the plutonic depths rock of Vesta .

Yet not all meteorites come from the asteroids belt. In some members of the group of the carbonaceous Chondrites indications were found that they could be core fragments of burned out comets. Comets come from the external areas of our solar system, the so-called Oorth cloud, that lies far outside the orbit of Pluto, and consists out of thousands of "dirty snowballs" - a mixture of rock and waterice, that remained from the time of origin of the solar system. Now and then, some of these objects, disturbed by the gravity of a passing star, penetrate on their eccentric routes into the interior of our solar system, melt in the vicinity of the sun and freeze again on their way into the external areas. Here they not only show the typical comet tail out of evaporating waters, but also lose more and more matter until after numerous rounds around the sun only a burned out kernel is remaining, of which parts may land as meteorites on earth.

For a few meteorites one could prove in the meantime a still more prominent origin. On 18th january 1982 an american research team found in the mountains of the antarctic Allan hills a little meteorite with a weight of only 31,4g, getting the idendity ALH81005. Later examination of the tiny piece yielded an outspoken resemblance with rock that was brought by the Apollo-flights from the moon. After extensive examinations by NASA and by numerous institutes it was soon clear: ALH81005 actually was a moon meteorite, an inestimable valuable rocky sample that had found its way from our direct neighbour to the earth without human help.

ALH81005 should not remain alone for long. After this scientific Sensation japanese researchers examined their stock of antarctic meteorites and found out that in 1979 they already found three moon meteorites, only after 1982 being recognized as such. In consequence of these finds both the USA and Japan reinforced their antarktic meteorite programs that brought new lunar meteorites to light. And in 1990, Australian Aborigines found the first non-antarktic Moon meteorite, that was mamed after its find place Calcalong Creek, weighting just 19 g.

Altogether to the present day twenty-four Moon meteorites are known, probably coming from only eighteen different falls. Of these eighteen falls, eleven are from the ice of the Antarctica, one from Australia, three come from the deserts of North Africa and three from Oman. Further candidates from the deserts of Oman and out of the Sahara are at the moment in analysis so that their number could soon increase. How rare these meteorites are, one can see from the fact, that not even 0,1% of all well known meteorites are of lunar origin. Until now well known Moon meteorites altogether don't bring even 6 kg on the scales!

Similarly rare is the group of the SNC-meteorites, named after the initials of three historic falls: Shergotty, Nakhla and Chassigny. These meteorites were categorized together with some antarctic finds based on mineralogical and chemical resemblances into one group giving scientists some riddles. So for example all of these meteorites being less than 1,5 billions years old are relatively young compared to the usual age of the meteorites out of the asteroids belt, which is about 4,5 billions years old and hereby indicates back into the time of origin of our solar system. In addition SNC-meteorites obviously are of volcanic nature what made the scientists search for a mother body still geological active in younger time. For this only terrestric planets like Merkur, Venus and Mars are possible candidates.

The solution of the riddle was found when American scientists examined gazbubbles included in an antarktic SNC-meteorite on their molecular composition and compared them with the results of the Viking-probes, which in 1976 had examined among others the exact composition of the Mars atmosphere. The gaz samples included in the meteorite corresponded exactly with the rates measured by Viking, and today there is no scientist who sincerely doubts, that the SNC-meteorites actually come from Mars!

To the present day, twenty-two Mars meteorites are known, which probably come from only seventeen falls. Of these seventeen, six are from the Antarctica, three from the deserts of North Africa, two from Oman, two from USA as well as respectively one from Nigeria, India, France and Brazil. Further candidates from the deserts of Africa are presently in analysis. In contrast to the Moon meteorites, being all finds, four of the Mars meteorites were observed falls, under them the name givers Shergotty (25. August 1865, India), Nakhla (28. June 1911, Egypt) and Chassigny (3. October 1815, France) as well as the latest fall, Zagami (3. October 1962, Nigeria). In scientific circles it is heavily discussed, whether it is a coincidence, that two of this outspoken rare meteorites fell on the 3rd october. This coincidence is maybe a reference that a swarm of age old fragments of the Mars the crosses earth always on the 3rd october -therefore keep your eyes open!

The origin of numerous other rare meteorites remains still unexplained or uncertain. In the section Classification you find as far as known, indications for the origin of the respective meteorite types, -classes and -groups. It can be expected that present investigations identify new mother bodies, and who knows, perhaps we can quite soon report from the first meteorites from Merkur or Venus.