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In Brief October 2010

Nullarbor update


Geoscientist 20.10 October 2010


The Bunburra Rockhole achondrite stony meteorite with black fusion crust, and a small area of interior surface showing grey, right; as found.Fifty years ago, when I was involved with the Meteorite Collections at the Western Australian Museum, we realised that the Nullarbor Plain, an arid limestone desert, was littered with meteorites 1, and that these dark objects could easily be recognised on the light limestone surface. Now more than 300 have been found there. New falls will also show up easily.

Dr Phil Bland of Imperial College has reported previously in Geoscientist on the setting up of the network (NDF). He and others have now published an update of results to date 2. Four fireball photographic observatories were established in 2007 and the first ground search mounted in 2008. On the first say of the ground search, the meteorite Bunburra Rockhole was recovered within 100m of its predicted fall position! It was initially described at the 40th Lunar & Planetary Science Conference in 2009 (see Abstract 1664) and consortium studies are ongoing. It is a rare basaltic achondrite, and had unusual orbital characteristics.

The photographic network has now observed about 550 fireballs and additional falls. It will be expanded from a trial facility to a full network, but funding for the necessary follow-up fieldwork is proving difficult to obtain. The facility is scientifically extremely important, being the only one in the southern hemisphere. The first orbits have been obtained of southern hemisphere fireballs. The facility may also have discovered a new meteor shower. Of multi-station events observed, 11 probably produced falls of meteorites on the ground, while five certainly did so. One meteorite had an initial mass of 20 tonnes, but it was carried out into the southern ocean.

References

  1. McCall, G.J.H. 1967. The progress of meteoritics in Western Australia and its implications. In: Moore, P., ed., 1968 Yearbook of Astronomy. Eyre & Spottiswooode, London, 146-155.
  2. Bland, P.A., Spurný, P., Shrbený, L. et al. 2010. Meteorite falls observed by the desert fireball network: an update
  3. http://www3.imperial.ac.uk/pls/portallive/docs/1/65165700.jpg

Too many holes


Confirmed structures in Finland (from Ferrière et al. 3)In the History of Meteoritics 1 I concluded it was unlikely that the 170 or so terrestrial structures attributed to impact would be found to have other causes. However, in a recent review 2, I wrote that the crowd of such structures recognised in Scandinavia remained unexplained (there are 62 in all, of which a likely 26 are confirmed by shock effects 3). A recent article by Ferrière et al. 4 gives a map of those confirmed by shock effects in Finland alone, and all 11 fall within an area extending for ~500km in the country’s southern half. They are of widely different ages and do not represent a single bombardment.

Only a handful of Quaternary craters of up to 1-2km diameter have a meteoritic material association: Wolfe Creek and Barringer being the most renowned, while there is also the single 25km diameter Pliocene Eltanin structure in the deep southern ocean (with mesosiderite and howardite specks). Shock effects in quartz and shatter cones, believed not to occur in endogenous terrestrial explosive structures, have been entirely relied on to confirm the remainder.

The shocked quartz in the Keurusselkä structure, newly described and finely illustrated 3, do, in their droplet-decorated shock lamellae, resemble the remarkable shock-melting droplets which I illustrated long ago when describing the Mount Padbury mesosiderite’s eucrite inclusions5. However, this extreme crowding of large-scale structures in Finland, makes me re-iterate my remarks 1,2 concerning the very marked geographic imbalance in the distribution of such structures, which is not to be satisfactorily explained in terms of large impact frequency. There remains a niggling doubt in my mind whether the opposing views of Bucher6, Nicolaysen7 and Currie8, that shock effects alternatively derive from some form of cryptoexplosion structures, generated from within our planet, may not yet come back into play.

References


  1. McCall, G.J.H. 2006. Meteorite cratering; Hooke, Gilbert, Barringer and beyond. In: McCall, G.J.H., Bowden, A.J., Howarth, R.J., eds., The History of Meteoritics and key meteorite Collections. Geological Society of London Special Publication 256; 443-469
  2. McCall, G.J.H. 2009. Half a century of progress in research on terrestrial impact structures. Earth Science Reviews 92; 99-116.
  3. McCall, G.J.H. 2008. Paasselkä crater. Geoscientist 18 (3); 10
  4. Ferrière, L., Raiskila, S, Osinki, G.R., Pesonen, L.J., Lehtinen, M. 2010. The Keurusselkä impact structure,Finland. Impact origin confirmed by characterization of planar deformation features in quartz grains. Meteoritics & Planetary Science 45; 434-446.
  5. McCall, G.J.H., Cleverly, W.H. 1968. The petrology of the Mt. Padbury mesosiderite and it achondrite enclaves. Mineralogical Magazine 35; 1029-160.
  6. Bucher, W.H. 1963. Cryptoexplosion structures caused from within or without the Earth (‘Astroblemes’ or Geoblemes’ ?). American Journal of Science 262; 596-649.
  7. Nicolaysen, L.O. 1972. North American cryptoexplosion structures interpreted as diapirs, which obtain relief from strong natural confinement. Geological Society of America Memoir 132; 605-620.
  8. Currie, K.L. 1972. Geology and petrology of the Manicouagan resurgent caldera. Geological Survey of Canada Bulletin vol. 198; 153 pp.

Dwarf status revoked


In 2006, astronomy’s governing body ruled that Pluto, because it did not clear the asteroidal debris in its vicinity by gravitation, was a dwarf planet. Those who issued this dictum were henceforth referred to as the ?bernerds of Prague by their opponents, who include Alan Stern, principal investigator of the ‘New Horizons’ mission now on its way to Pluto. He thought the definition was “sloppy science”.

Pluto was the only planet discovered in the USA, by Clyde Tombaugh of Illinois in 1930, and this change was naturally unpopular throughout the USA, and nowhere more so than in Tombaugh’s home state where March 13, the date of its discovery, is celebrated as ‘Pluto day’. Illinois duly passed a law early last year, re-establishing Pluto as a planet there (though nowhere else, of course). This interesting case of local legislation revoking scientific decisions makes the mind boggle, should it set a precedent; even though I am firmly on the side of the Illinois Angels in the matter!