Reflections - October 2014

October 9, 2014  |  By  | 

General Meetings: October 21 st Annual General Meeting November 18 h Speaker: Dr Andrew Sheinis (AAO) - The development of astronomical instrumentation. December 16 th Christmas Party NAG Meetings: Every fourth Tuesday of the month Observing Nights: Consult NSAS’ web site at Deadline: Please send your contributions to the January 2015 issue of Reections in time to reach the editor before December 15 th to Calendar A few Meteorites are from Mars O f over 61,000 meteorites found on Earth, a mere 132 have been identied as Martian in origin [1]. This article outlines the detective story of how that identication was achieved. Stony meteorites account for some 94% of all meteorites found on Earth. Of these, over 90% are chondrites, so called because almost all are composed of small spherical particles (chondrules) that appear to have melted while freely oating in space prior to their incorporation in meteorites. Chondrites are typically about 4.55 billion years old (based on radiometric dating) and are thought to represent material from the asteroid belt. 4.55 billion years has been taken to represent the age of the Solar System and the time of formation of asteroids. The other kind of stony meteorite, achondrites, is thought to have crystallised from a magma in the same way as terrestrial rocks. Among achondrites, a small number of igneous meteorites were identied by the 1980s as being anomalously young and having oxygen isotope compositions that differed from those of other meteorite groups. These anomalous meteorites were collectively called SNC meteorites. This name was based on the meteorites’ classication into 3 types, each named after a type specimen: Shergottites, named after the • Shergotty meteorite that landed in the Indian village of Shergotty in 1865 (96) Nakhlites, named after the shower • of meteorite stones that landed in the Egyptian village of El Nakhla el Baharia in 1911 (13) Chassignites, named after the • shower of meteorite stones that fell on the French village of Chassigny in 1815 (2) [The number of meteorites in each sub-group as at 9 January 2013 is shown in brackets; 3 of the total of 114 SNCs are unclassied.] The nakhlites and chassignites have ages, based on radiometric dating, of around 1,300 Ma. Shergottites are even younger at about 165-200 Ma. As igneous rocks are generally formed by crystallisation of molten rock as it cools, the straight-forward explanation for these young ages is that they represent when the SNC rocks crystallised from a melt. But this rules out SNCs having a source in the asteroid belt since the necessary igneous activity would have occurred long after it had been assumed that the asteroids had cooled and solidied. Suggestions that SNCs might have come from a planetary body like Mars initially faced the objection that no meteorites from the Moon had been discovered and an impact capable of ejecting a fragment of the lunar surface into an Earth-intersecting orbit was surely much more probable than such an event on Mars, given the Moon’s lower escape velocity and close proximity to the Earth. This objection fell away, however, with the unanimous acceptance that meteorite ALH A81005, found in the Allan Hills region of Antarctica in 1982, was of lunar origin. Consensus was readily achieved because ALH A81005 was identical in mineralogy, mineral chemistry and isotopic composition to Apollo and Luna [2] samples brought back from the Moon to Earth. The conventional view that the dynamics of cratering of planetary surfaces by asteroid impact was unfavourable to producing meteorites was thus overturned. Since the crystallisation ages of SNCs lie between 165 and 1,300 Ma, SNCs have to originate from an astronomical body that featured molten rocks as recently as 165 Ma ago. The giant planets can therefore be eliminated because their exterior layers are predominantly gas and/or ice, not rock. The absence of blocks of ejecta when the comet Shoemaker-Levy/9 hit Jupiter reinforces this conclusion. Comets and Kuiper Belt objects can be eliminated because they have never been molten. Thus, the source for SNCs narrows EETA 79001 aka Elephant Moraine 79001 as found in Antartica Picture Credit: NASA EETA 79001 Picture Credit: NASA

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