METEORITES

2000 GSA Southeastern section
Shervais p. A-73. Geochem of the lunar highlands.

        Was the formation of the Solar system triggered by a nearby supernova explosion? New evidence from meteorites suggests this old theory has had its day.
      By studying the way in which our solar system formed, we can speculate how likely it is that habitable planets exist in our stellar neighbourhood. Recent work by geologists, cosmochemists and astronomers have lead to new insights into our origins, according to Dr Sara Russell of the Natural History Museum, talking to the British Association today (Tuesday 12 September).
    “Studying the origin of the solar system can be approached in three ways. We can look at ancient rocks to see under what conditions they formed; we can theoretically model what happens as planets are created, and we can look up to the skies to search for young stars that are in the processes of forming their own planetary systems. By using these three approaches together we can obtain a much clearer picture" says Russell. Over the last five years, astronomical observations have given us the clearest picture yet of the formation of the solar system. Disks have been seen around young stars that have yet to form planets, and extrasolar planets have also been identified. Disk observers have discovered that extremely young stars are more often surrounded by disks than not. This suggests that disk and planet formation may be a very common process. Older stars tend not to have disks; and the proportion of disks goes down with age. Stars that are around 10Ma old rarely have disks - perhaps because the planet forming process has taken place in these systems.
      The number of reports of new exoplanets - planets orbiting other stars - is increasing at a great rate, with ten new planets being reported at the recent IAU (International Astronomy Union) meeting in Manchester. This brings the total number of known planets to over fifty - a sample big enough to make some initial conclusions, says Russell. “Planetary systems are common in the galaxy. And if there is one planet around a star, it seems there will probably be others.”
                                                   Clues from meteorites
      Perhaps the best clues about how terrestrial planets (i.e. Mercury, Venus, Earth and Mars) formed come from meteorites. The vast majority of meteorites come from the asteroid belt, between Mars and Jupiter. In this region, small planetesimals were prevented from accreting into “full-blown” planets by the gravitational perturbations of the giant planet Jupiter. By looking at these samples, we can look back in time to the beginning of the solar system, to the time before the planets existed.
      Meteorites that have never experienced any melting are called chondrites. A close up look at chondrites reveals   that are made up of rounded, mm to centimetre-sized objects called chondrules, plus flecks of metal and white, irregularly shaped inclusions. These objects are glued together by a fine-grained matrix composed of silicate and organic materials. All the components of meteorites are relicts from the solar nebula - the dust cloud that was the planetary ancestor.
      The origin of chondrules is still not well understood. Their shape and texture show that they were once melted droplets that formed during a high-temperature, fast, heating event in the early solar system. This event was probably responsible for creating millimetre to centimetre-sized objects from micron-sized dust - perhaps the first stage in the planet-building process.
      What was the mysterious heating event that made chondrules? Is it an essential stage in the formation of planets?   Russell believes it to have been a ubiquitous process - at least among the material from the asteroid belt - the only early solar system that we can sample. “At the moment, we don't know what the heat source was” says Russell, “although shock-waves passing through the solar disk, or jet outflows from the early sun, are popular theories. We are undertaking laboratory experiments to attempt to replicate the chondrule-forming process. The data may point to the jet model being an important process in the early solar system.”
                                                        Isotopes
     Another mystery thrown up by meteorites is the presence of short lived isotopes when they formed. Some meteoritic constituents once contained highly radioactive components, like aluminium-26. These isotopes have now decayed to distinctive stable daughters. They may also have been an essential part of the planet-building story, because they can provide a large amount of heat - enough to melt and bind a young planet.
      “But to find out how important they are, we need to know how these isotopes formed, and how widely distributed they were in the early solar system. There are currently two main schools of thought about the origin of short-lived isotopes. The traditional view is that the isotopes may have been made by a stellar process- either in a supernova or in a red giant star. The rival theory is that the young Sun itself may have thrown out radiation that produced these isotopes. Recent data suggest that the latter may have been the predominant mechanism for the isotope production.”
       Parts of meteorites have been shown to have contained the isotope beryllium-10 when they formed, a nuclide that must have formed by interaction with the Sun. If this interaction took place, then other isotopes, such as aluminium-26, may also have formed by this process. In this case, they may have only been formed in a very localised region, and their effects on planetary heating are less than scientists previously thought.
    Russell says: “Over the last few years we have learnt a lot about how planets formed. Observational evidence, in particular, has been critical, showing us that planetary systems are common in the galaxy. But we still do not know if the formation of the solar system is a unique event, requiring a triggering from a nearby exploding star. The data presented here suggest that a supernova trigger is not required, in which case the formation of solar systems may be a rather common event. A combination of techniques should give us the chance to learn about these processes in the near future.”
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     'In order for the complicated internal structures to be produced that are observed at   Chicxulub and many extra-terrestrial complex craters, the target material must behave as though it were a fluid.'
     'Of course the collapse process cannot be entirely hydrodynamic, as the end result would inevitably be a flat surface. Evidently, the fluid collapse must be frozen or suspended in some way to produce the observed complex crater morphologies. The mechanism driving this transient weakening, however, still remains a mystery - this phenomenon appears to violate current understanding of rock and debris mechanics.'
      The group at the TH Huxley School and their colleagues at the University of Arizona believe that one potential   material weakening mechanism called Acoustic Fluidisation could come into action as the impact generated shock wave transforms the target into a sea of jostling granular material.
    Gareth Collins said, 'We model the collapse stage of the cratering process, which begins after the initial excavation of the cavity. Our simulations show that temporary weakening of the target by Acoustic Fluidisation allows the formation of internal peak and ring structures similar to those observed in terrestrial and extra-terrestrial craters. Our dynamic simulations of peak-ring formation at Chicxulub are remarkably consistent with observations from the seismic data.'
    His research group hopes to use this model for the generation of the peak-ring at Chicxulub to further their understandings of the geology of other cratered planets and satellites, such as Mercury, Venus and the Moon.
    Dr Jo Morgan, Mr Collins's supervisor in the Geophysics Research Group, TH Huxley School, explained,   'Improved understanding of large-impact crater formation will enable us to assess the environmental effects of such impacts and to determine whether this impact was the dominant force driving the mass extinction at the end of the Cretaceous period.'
    An animation based on Mr Collins' computer simulations is on his group's web site.

Oct 2000 issue
    Olivine, a ferromagnesian silicate mineral, is plentiful on Mars according to new mineral maps. This suggests the planet has been cold and dry throughout its geological history. About 3% of the surface mapped so far appears to contains abundant olivine, and another 3% contains coarse-grained haematite – as is only right for the Red Planet.    The olivine occurs in darker, basaltic extrusive (volcanic) rocks that cover a large portion of the planet. The sulphates occur in brighter rocks - probably mechanically weathered material (meteor impacts, wind driven dust/sand erosion) with trace amounts of fine-grained haematite.
   USGS planetary geologists still do not know where the coarse grained haematite comes from. On Earth it is often associated with water, as in a hot spring or lakebed; but such conditions would also produce other minerals, not seen on Mars. The presence of water and chemical weathering would also produce clay minerals, which are also not seen in the latest data. These lines evidence, both positive and negative, point the same way – Mars is, and always was, cold and dry.
   "Not seeing minerals that indicate chemical weathering is also consistent with the abundant olivine and that implies the chemical weathering is very low. Thus, a consistent picture is forming that says Mars' surface has remained cold and dry for a long time," said Hoefen.
Clark agrees that abundant water probably exists below the surface, but only in a frozen state and rarely, if ever, has it existed at the surface in a warm liquid form. The emerging hypothesis has clear implications for the search for Martian life. Article has Mineral spectra images.

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