November General Meeting Report
AThe Cranbourne Meteorite Fall@
Peter Skilton, Astronomical Society Frankston.
Cranbourne is an old settlement located just north of the Mornington Peninsula and Westernport. It is 49 km south-east of Melbourne via the M1 and the South Gippsland Highway and is situated at an elevation of 50 metres above sea-level. The population of Cranbourne in the early 1990 was about 20,000. It is now among the fastest growing suburbs of Melbourne and part of the City of Casey.
In the late 1850 Cranbourne came to the attention of the scientific community as the location of a number of meteorites there became public knowledge. Large pieces of meteoric boulders were being discovered in a wide track between Beaconsfield and Langwarrin. In 1853 shepherds and settlers observed some Aboriginals dancing around a large piece of metallic rock, projecting from the ground on the McKay property. At first it was assumed this was a natural outcrop of iron ore in the area. A block the size of a child's head was cut from a projecting spur and some of it forged into a small horseshoe. It, and >a Specimen of iron from Western Port= was put on display in the Melbourne Exhibition in 1854. The town clerk of Melbourne, F.G. Fitzgibbon, a delegate for the City Council, soon determined that the 'pure iron' masses near Cranbourne had no relation to the seemingly ferriferous rock of the district and that they were undoubtedly meteorites.
The statistics of the Cranbourne Meteorite are mind-boggling. Time of impact is difficult to pinpoint due to the swampy environment, but by the degree of rusting it has been set tentatively at 1700CE. 12 pieces have been found so far and their total mass is around 9 tons. In fact, Cranbourne No. 1 and 2, at 3.5 & 1.5 tons respectively, are ranked in eleventh place on the list of the world's heaviest known meteorites, and in Australia second only to the Mundrabilla fall (on the Nullarbor Plain, W.A.) boasting two large masses at 12 and 5 tons. The world's largest meteorite was found in 1920 at Hoba West in Namibia and weighs about 60 tonnes.
Each fragment of the Cranbourne meteorite has a story to it, some have a serendipitous discovery, with others it is the numbering, the categorising and, for some shipment to Museums, Universities and Survey Departments. These stories, as well as scale models of the fragments, can be found at the Cranbourne Meteorite Park on the South Gippsland Highway. Fragment No. 12 is on display in the foyer of the City of Casey Council Offices, Narre Warren and can be visited any time during office hours.
Peter Skilton passed two small samples of iron meteorite around, one surprisingly polished, the other heavily magnetised. The way he kept his eyes on them as they went from hand to hand through the assembly probably means they were quite valuable. He talked about the Cranbourne Meteorite subject as an on-going project, a project not yet complete. On hand of a PowerPoint presentation he showed the various impact locations where the objects have been found (the photos were identified in a novel way by children holding out the required number of fingers each time to match the site number), and explained how he and his group plotted the line and dispersion of the meteorite fragments. This pattern, he says, seems to indicate a substantial missing mass south of the flight path, which he (with a bit of help) hopes to locate some day.
The Cranbourne Meteorite Fragments map shown was produced by John Cleverdon for Peter Skilton. It is based on digital mapping data taken from Mapinfo (GIS), with extra information added as required. The area of the circles is based upon the relative masses of the meteorites. When referring to the map it is evident that with the exception of the Pakenham No. 6 and Pearcedale No. 11, the remaining meteors were all located in a straight line approx. 21 kilometres apart.
There were many questions for Peter during Supper time, covering the whole field of meteoritics, from meteor to meteoroid and meteorite, to comets and asteroidal dust. It is a field were past history and the future clearly meet and influence each other in the present.