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Impact sites in Australia - part 1 of 2

ASTRONOMY NOW / Nov 1997, pp. 48-49:

Focus: Visitors from Space - Impact sites in Australia (by Bev and Jan
Ewen-Smith)

There are many meteorite craters in Australia. On a recent visit, Bev
and Jan Ewen-Smith made a study of some sites near Alice Springs.

It has been estimated that a thousand or so meteorites of a kilogram or
more, reach the Earth's surface every year.
Fortunately for us, meteorites weighing many tonnes arrive rather less
frequently. A one tonne meteorite arrives every decade or so, and at the
1000 tonne level, perhaps every 100,000 years. Meteorites arrive at
random over the surface of the Earth. However, meteorite craters remain
more easily visible in arid areas such as Arizona and central Australia.

Australian craters

The low rainfall, sparse vegetation and lack of soil, animals and
intensive agriculture mean that craters survive for much longer in
central Australia, without losing their identity, than they do in
Europe, for example. Identical craters formed around 10,000 years ago
would have been subjected to the erosive action of rainfall, disturbance
by plant roots, worms and other soil animals as well as Man, in a
European context, and hardly at all in the arid heart of central
Australia.
Australia is a very large place and to visit all of the meteor craters
would involve a great deal of travelling. Within striking distance of
Alice Springs, however, there are three significant crater sites from
which half a dozen or so separate large meteorites have been recovered.

The Henbury craters

The Henbury meteor crater complex is about 100 km southwest of Alice
Springs. Because of its scientific importance, it has been designated a
conservation area. It is open to the public and is readily accessible by
conventional vehicle. It consists of eleven or more individual craters
within a rectangular area of about 1000 m by 500 m.
The walls of the largest three craters are in contact with each other.
None seems to override its neighbour, indicating that the impacts were
essentially simultaneous.
The largest crater is 180 m by 140 m and 15 m deep. Judging by its
non-circular outline, it may have been the result of the impact of two
meteorite fragments. Several smaller craters lie to the south of the
main craters and one of them exhibits a set of dark rays extending to
the north.
The object responsible for the Henbury craters was an iron-type
meteoroid consisting of a nickel-iron alloy which was still travelling
at a great speed when it hit the ground. When it hit, it peeled back a
layer of the existing rock. Many tonnes of nickel-iron 'shrapnel' have
been collected from the site.
In the ejecta area around the craters it was discovered that the surface
is still strewn with thousands of shiny black nickeliferous silica glass
stones which have the appearance of condensed droplets of fused rock.
>From their composition and strong magnetic properties, they appear to
have incorporated some of the meteorite metal alloy.
Radiometric measurements and the state of preservation of the craters
suggests that the impact event occurred no more than 5000 years ago. An
investigator visiting the area in the 1930s reported that the local
indigenous aboriginal people avoided the area and called the locality
"Sun walk fire devil rock" which may indicate a distant memory of human
witnesses to the event. It must be said that the Sun in the Australian
outback shines so fiercely that the rocks qualify for that name without
any help from meteorites.
While it is common for stony meteorites to fragment on arrival, the iron
meteorites, having greater cohesive strength, usually survive intact.
This fact goes some of the way to explaining why it is that only a small
percentage of meteoroids are iron, but that they constitute a large
proportion of meteorite finds. Their greater resistance to weathering
and more marked contrast with terrestrial rocks must also be factors.


Best wishes,

Bernd

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