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Impact sites in Australia - part 2 of 2

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

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

Boxhole crater

The Boxhole meteor crater is located about 200 km northeast of Alice
Springs, close to the Tropic of Capricorn. It consists of a single large
crater. The route to the site requires a four-wheel drive vehicle and,
at the time of our visit, involved crossing the Plenty River which
certainly seemed to have plenty of water in it for an area which is
famously arid.
The crater lies on private property close to the Dneiper homestead which
is the centre of a pastoral ranch covering almost 600 square kilometres.
The crater is 200 m in diameter and 15 m deep. The object which created
the crater was, like Henbury, an iron- nickel meteorite. In this case,
it retained its structural integrity to create a single impact crater. A
large number of meteoritic fragments have been found at the site. They
exhibit the characteristic dimpled appearance of iron meteorite finds.
Unlike the Henbury site, no nickeliferous silica fragments were in
evidence, perhaps because the surface geology is quite different. In
fact the gravely surface seemed to be strewn with visually attractive
minerals not obviously associated with the meteorite impact but
certainly interesting to a gem hunter.
The composition of the nickel-iron fragments and their radiometric age
closely match those of the Henbury site leading to the suggestion, not
widely accepted, that the Henbury and Boxhole craters may have been
formed by parts of the same original meteoroid.

Gosses Bluff structure

The Gosses Bluff structure is completely different from the other sites
in this area. It is very much larger and very much more ancient. The
present ring-shaped feature is not the original crater. At the time of
the impact, 130 million years ago, the terrain surface was some 2000 m
above the present level. The original crater, thought to have been 20km
across, has been eroded completely away and what remains is a diapiric
structure, a kind of blister, which formed beneath the crater in a hard
geological layer which used to be five kilometres below the ancient
surface. The upturned outline of this ‘blister’ forms the jagged ring,
now four kilometres in diameter, which stands out in today's eroded
surface.
Because it is so conspicuous, in an otherwise flat plain, the Gosses
Bluff feature is important in the culture of the indigenous aboriginal
people of the area. Its huge size actually makes it difficult to
appraise from the ground.
None of the original meteorite has survived 130 million years of surface
erosion but studies of the shatter cones in the underlying geological
formation indicates that the impacting object had a high velocity
(40km/s) and low density (0.0013kg/m^3) suggesting a comet rather than a
meteoroid. The energy released by the impact has been calculated as the
equivalent of around 22,000 megatons of TNT.

Bev and Jan Ewen-Smith run the COAA astronomy holiday centre in the
Algarve region of Portugal.


Best wishes,

Bernd

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