Early Argentines did not witness a meteorite impact

Ten years ago, planetary scientist Peter Schultz and Argentine pilot Ruben Lianza observed several depressions shaped like tear drops while flying over the Pampa. Because they also found meteorites and tektite glass when they examined the structures on the ground, it seemed certain that the depressions had formed by the impact of bodies travelling almost parallel to the Earth’s surface.  The structures were clearly no more than a few thousand years old, and the discovery encouraged lurid artistic impressions of terrified native South Americans cowering from an extraterrestrial firestorm.  The Rio Cuarto structures were a godsend for those who fear social and economic disaster from Earth-bound NEOs (near-Earth objects), and have been lobbying for a sky watch for impending doom.

In reality, the Pampas of northern Argentina has hundreds of similar structures over an area of more than 50 thousand square kilometres, and their long axes parallel the prevailing wind direction (Bland, P.A. and 10 others 2002.  A possible tektite strewn field in the Argentinian Pampa. Science, v. 296, p, 1109-1111).  They are “blow-outs” developed in the fine loess soils of the Pampa, and much the same structures affect most loess plains.  Being formed of wind-blown silica and clay dust, loess is not well known for its content of objects above a millimetre in size, so any larger objects found on wind-deposited plains stand a high chance of having arrived by some extraterrestrial process.  Meteorites and tektites are rare, but ablation concentrates them in wind-blown depressions as they are too heavy to be blown away.  That is the likely origin of the objects that Schultz and Lianza used in support of their hypothesis of impact devastation wrought on early South Americans.  Phil Bland of the Open University, and his colleagues from Brazil, the USA, Australia, Russia, Argentine and Britain, were able to date organic matter in the Rio Cuarto structures using the C-14 method at 4000 years.  Yet Ar-Ar ages of the meteorites range from 52 to 36 thousand years, so the two are unconnected.  The glassy tektite fragments provided yet another age of 57 thousand years.  Along with similar glasses at a couple of other sites in Argentina, these support melting of the homogeneous loess by an impact around that time, although no crater from which they might have been ejected is known.  The search is on for the source of a hitherto unknown field of strewn tektites, although it seems strange that in the featureless plains of southern South America one hasn’t shown up long before now.

The mantle’s breath and Earth’s early evolution

Many lavas contain bubbles, which form when gases dissolved under pressure in magma froth out at low pressures.  For the most part the gas is water vapour, carbon dioxide and sulphur dioxide.  It comes from mantle peridotite, and represents the volatile fraction of the deep Earth.  But there are traces of other gases, the most revealing of which are the noble gases helium, neon, argon, krypton and xenon, because some of their isotopes originate from radioactive decay of other elements (mainly potassium, uranium and thorium.  Noble gases in basalts offer important insights into how the mantle has evolved since the origin of the Earth.  Chris Ballentine of the University of Manchester, reviews how such trace-gas isotopes in basalts help resolve some otherwise intangible challenges (Ballentine, C.J. 2002.  Tiny tracers tell tall tales.  Science, v. 296, p. 1247-1248).

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