METEORITES, DOCUMENTS OF THE EVOLUTION OF THE SOLAR SYSTEM

The planetary rocks derive from the accumulation of solid particles, generally crystals, produced by the condensation of the protosolar nebula.
The Chondrites testify the earliest stages of the evolution of planetary bodies. The Achondrites are more evolved rocks, that underwent a differentiation by fusion during the magmatic processes. The variety of the meteorites found proves their provenance from different planetary bodies: the belonging of fragments, detached by violent impacts, to large bodies such as Mars or the Moon, has been demonstrated; nevertheless the majority of meteorites derives from minor objects of the Solar System, such as asteroids.

FROM THE PROTOSOLAR NEBULA TO METEORITES 
Evolution scheme of the Solar System

The matter produced by the explosion of supernova in the ring of the Swan: example of the possible scenario in which the Sun and its planetary system originated. (JPEG, 398 K)  (NASA-STScI)

Artistic image of a protosolar disc : the material expelled from the supernova is gaining a new shape.  (JPEG, 127 K)  (D.Berry, STScI)

Scheme of the Solar system with the asteroid belt between the orbits of Mars and Jupiter. (JPEG, 337 K)  (Ward's Science Graphics)

Ida, an example of an asteroidal rocky body. (JPEG, 206 K) (NASA-JPL)

MINERALOGY AND PETROGRAPHY OF METEORITES.

Relative abundance of the different types of meteorites found. As you can see, in the majority of cases they are made of undifferentiated original material. (JPEG, 355 K)  (University of Padua, CNR)

Chemical composition of the main minerals found in meteorites. (JPEG, 448 K)  (University of Padua, CNR)

The photographs that follow depict the mineralogical association that characterizes different meteorites. The petrographical study of chondritic and achondritic meteorites is carried out on "thin sections" (0.03 mm), because at such thickness the silicate minerals are transparent to light. On the contrary the meteorite iron is opaque even at an infinitesimal thickness, and their study is carried out gon "lucid sections", exploiting their reflecting properties.

Gibson: Achondritic meteorite - LODRANITE: olivine, pyroxene, Fe-Ni alloys, feldspar. Rocks with a similar mineralogic association and texture can also be found in the upper mantle of the Earth. (JPEG, 422 K)  (University of Tokyo)

 

Y-792510: Achondritic meteorite - EUCRITE: pyroxene, plagioclase. Rocks with a similar mineralogical association and texture can also be found in the earth's crust.  (JPEG, 531 K)  (Catalogue of Yamato Meteorites, Tokyo)

 

Odessa: METEORIC IRON: kamacite and taenite crystals coexisting in the Fe-Ni alloy are the cause of the "Widmanstatten figures". Analogous terrestrial rocks are not known, but are possible.  (JPEG, 283 K)  (Ward's Catalogue, Rochester NY)

 

Brenham : PALLASITE: olivine crystals cemented in a kamacite and taenite matrix. Analogous terrestrial rocks are not known, but are possible.  (JPEG, 458 K)  (Ward's catalogue, Rochester, NY)

 

Fermo: CHONDRITE. Detail of a H3-type: olivine, pyroxene, kamacite, taenite and troilite. Analogous terrestrial rocks are not known nor supposable. Infact, chondrites are primitive undifferentiated material. In particular, carbonaceous chondrites show a relative abundance of chemical elements similar to that of the solar photosphere. (JPEG, 436 K) (University of Padua, CNR)

 

Fermo: detail of a chondrule with a diameter of 1.2 mm approximately. The chondrites are called after the typical grains the dimensions of which are about 1 millimetre. These chondrules are drops of silicate material that underwent a rapid fusion process and partial re-crystallization.  (JPEG, 495 K)  (University of Padua, CNR)

By international convention, the name of the meteorite is determined by the nearest post office. In the case of antarctic samples three letters indicating the "trap" mountain, the year and the subsequent number of the finding are used. For example FRO-93348 is the finding 348 of the year 1993, Frontier Mountain.  

Micrometeorites have been found in the recent sediment of the ocean floors; their size reach 0.1 mm. The sediments of the ocean floors are an ideal place for accumulation of micrometeorites. Anyway, the fact that micrometeorites fall daily on the whole terrestrial surface means that it is possible to find them in the most disparate places. 

PLANETARY COLLISIONS

Possible collisions between asteroids cause the introduction of fragments into orbits intersecting planets. Thus, the event of an impact on planetary bodies becomes possible.

Annual flux of interplanetary material. (JPEG, 394 K)  (University of Padua, CNR)

 

A crater caused by the impact of a meteorite on the terrestrial surface. It's the Meteor Crater in Arizona (USA), with a diameter of approximately 1.2 km and a depth of 300 m.  (JPEG, 451 K)  (Ward's Catalogue, Rochester NY)

 

The Aristarcus impact crater on the surface of the Moon. The multi-ring morphology is typical of a high energy impact.  (JPEG, 401 K)  (NASA-JPL)

 

Artistic reconstruction of a collision event between asteroids, at relative speeds that can be greater than 5 km per second.  (JPEG, 408 K)

METEORITE SEARCH IN THE ANTARCTIC

Cartographical image of the Antarctic continent. The asterisks indicate the glaciological "trap" zones where meteorites are concentrated. (JPEG, 690 K)  (De Agostini)

Concentration mechanisms of meteorites in the Antarctic ices. The strong catabatic winds remove the superficial part of the ices both by mechanical erosion and by sublimation. The meteorites previously trapped are thus discovered in the zones where the ices stagnate.  (JPEG, 200 K)  (Euromet)

Meteorite search in the "blue ice" zone of Frontier Mountain. IX Italian expedition in the Antarctic, 1993/94.  (JPEG, 299 K)  (National Programme of Research in the Antarctic, PNRA)

The finding of a new, interesting meteorite sample at Frontier Mountain. The three gathering campaigns organized by the National Programme in the Antarctic with the collaboration of "Euromet", provided about 350 new meteorites, so far.  (JPEG, 368 K) (PNRA)

FRO 93348. The larger (1.6 km) meteorite sample found at Frontier Mountain, and exposed there.  (JPEG, 257 K)  (PNRA)

ASTEROIDS AND COMETS, THE ORIGINAL MATERIAL 

The asteroids, or minor planets, are a set of thousands and thousands of small rocky bodies, the majority of which orbits in an area between the orbits of Mars and Jupiter. The first asteroid was discovered by Abbot Piazzi in Palermo on new year's eve 1801. Their size vary from 1,000 km approximately for the largest of them, Ceres, up to ten metres. Not all asteroids are in the zone between Mars and Jupiter. Some are in an orbit that crosses the terrestrial orbit, and could therefore be dangerous to our planet. Infact, the Earth, aswell as other bodies of the Solar System, shows evident traces of collisions with bodies even of kilometric size, but such catastrophical events are fortunately very rare.

The Gaspra asteroid was the first of the Solar System to be photographed from a small distance. It is an irregular body with dimensions 19x12x11 km, with a less gray colour than the Moon, and with a surface full of meteorite impact craters. The picture was taken on October 29th 1991 from the Galileo spacecraft.  (JPEG, 276 K)  (NASA-JPL)

 

Ida, the second asteroid to be photographed by the Galileo spacecraft, surprisingly showed to have a micro-satellite, Dactyl, with a diameter of 1.5 km. Dactyl rotates at 100 km from the centre of Ida, which has a dimension of 50 km approximately.  (JPEG, 146 K)  (NASA-JPL)

 

Dactyl, the first satellite of an asteroid. Even if it is only as small as 1.5 km, it is large enough to cause a planetary catastrophe if it fell on the Earth.  (JPEG, 21 K)  (NASA-JPL)

THE COMETS 

Beyond the orbit of Pluto, in the cold vastness of the external Solar System, there is a rich cloud of small frozen bodies with the dimensions of a few km. Occasionally some of them, when disturbed by the passage of a star or by the mutual collisions, fall towards the Sun and are gradually heated.
Within the orbit of Jupiter the heat of the Sun is sufficient to stimulate the evaporation of the superficial ice. The aqueous vapor thus liberated drags dust that diffuses very effectively the light of the Sun, rendering such objects very brilliant. Thus, a "coma" is formed around the frozen nucleus, which usually is the first sign of the cometary nature of the body. The coma later develops into a tail at the side opposite to the direction of the Sun, because of the pressure of the radiation and of the particles that form the solar wind.
Thanks to its formation at a very low temperature and to the distance from other bodies, the material that forms the nucleus of comets can give us precious clues on the initial chemical composition of the Solar system. Hence, the great interest for missions that could place instruments directly on the nucleus or even bring back samples of cometary material.
The large amount of water contained in the nucleus and the organic substances found in the coma and in the tail, could have in some way influenced the formation of the oceans and of the terrestrial atmosphere, and even the appearance of life. The interest in comets, that have always been fascinating, has thus deep scientific reasons that reach as far as the very origin of our planetary system.

The Hyakutake comet, photographed on April 6th 1996 at the Cmi Vrh Observatory, Slovenia. The comet remained visible to the naked eye for the entire Spring 1996 and was one of the most spectacular in the later years.  (JPEG, 627 K)  (Photo: H. Mikuz)

 

The region of the nucleus of the Hyakutake comet photographed by the Space Telescope on March 25th 1996.  (JPEG, 36 K)  (NASA-STScI)

 

Halley is surely the most famous of comets, photographed here during its last passage in 1986. The comet travels along a very elliptic orbit which takes it near the Sun every 76 years. This orbital period was discovered by the English astronomer Edmund Halley; it stimulated various international missions directed to the study of its characteristics during its latest return in 1985-86. Italy participated to the European mission Giotto, which photographed the nucleus obtaining some of the most exciting pictures of the last ten years.   (JPEG, 144 K)  (ESO)

 

Exceptional image of the nucleus of Halley's comet photographed by the Halley Multicolour Camera of the Giotto probe in 1986. A group of Paduan researchers also participated to the construction of HMC. The surface of the nucleus is covered by a dark layer, probably composed of organic material. The ice does not evaporate evenly, but form spurts of gas and dust in areas where the solar illumination stimulates activity. The nucleus has dimensions of 8x8x16 km, and rotates in about 52 hours.  (JPEG, 290 K)  (ESA)

 

Halley's comet moves away in the background of the Milky Way after its passage in 1986. It will be seen again from the Earth in the year 2061. (JPEG, 531 K)  (ESO)

 

Spectacular image of the West comet photographed at La Silla (Chile) in March 1976. Comets generally develop two tails under the pressure of the solar wind: a rectilinear one made of ionized gas and a larger and more luminous one made of dust, that form an arch along the orbit of the comet. The large tail of dust of West reached 100 million km. This comet will not be seen for a long time: its orbital period has been estimated to be as long as a million years!  (JPEG, 321 K)  (ESO)

 

The comet that promises to be the most spectacular of the year 1997 is Hale-Bopp, here photographed by Hubble Space Telescope in September 1995, when only the coma was visible. The material produced forms a spiral structure. The picture captured the detachment of a fragment of ice crust, probably caused both by the tumultuous evaporation due to the heat of the sun, and by the rotation of the nucleus.  (JPEG, 49 K)  (NASA-STScI)

 

Comets have a small nucleus, made of ice and dust, that can be easily disgregated under the action of solar energy and of the tidal forces of the major planets. In 1992 the Shoemaker-Levy comet, during its route towards the Sun, was captured by Jupiter but it broke into many fragments, like a lace of luminous pearls, that fell on the planet in July 1994.  (JPEG, 102 K)  (NASA-STScI)

 

Picture of Jupiter in ultraviolet light that enhances the impact areas of the fragments of the Shoemaker-Levy comet, identified with letters of the alphabet.  (JPEG, 272 K)  (NASA-STScI)

GIOTTO: A STEP AWAY FROM THE COMET

The ESA Giotto probe got as far as 600 km from the nucleus of Halley's comet and photographed it. For the first time, scientists could see the heart of a comet and deeply analyzed its chemical and physical characteristics. Thanks to Giotto we know today that the nucleus of Halley's comet, not larger than a few kilometres, is made of rock, Carbon compounds and ice, and that the magic of its tail depends on the gas and the dust released in space by the nucleus. In 1990, after a hibernation period, the Giotto probe was awakened to meet another comet in 1992, Grigg-Skjellerup.

The plane mirror and the lens hood of the Halley Multicolour Camera, which photographed the cometary nucleus, were designed in Padua and built at the Galileo workshops of Florence. To reduce the weight as much as possible, the mirror has a series of holes, while the cylindrical screen is made of kevlar with inner aluminium winglets.  (JPEG, 434 K)  (University of Padua)

The probe reached the comet taking photographed up to a distance of 600 km from the nucleus, just before the Halley Multicolour Camera stopped working because of the cometary dust that hit it at the speed of 70 km/sec. The remaining equipment on board continued to send information while the probe crossed the tail.

The Giotto probe during the assembly. The solar panels have been removed to see inside. The silver spheres are two of the four tanks containing hydrazine, the fuel for orbit correction and positioning.  (JPEG, 359 K)  (ESA)

 
 
 

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