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IN SEARCH
OF THE BIGGEST IMPACT
Earth-Moon
collision takes the prize
by Graeme Addison
If you’re fascinated by
impact craters I have bad news for you: there is no sign of
the biggest-ever impact in the Earth’s history. When our
planet was forming, another half-formed planet came
wandering by - our future Moon. They collided and
virtually remade each other. The giant impact happened very
early in the period between 4.5 million and 4.3 million
years ago when the Earth’s major bulk grew from the
accumulation of planetismals (including meteorites and
asteroids). There is no sign of the impact today because
basically the two planetisimals reformed with new surfaces.
For
more on this amazing story, go to an excellent site (from
which the drawing above is taken)
here.
That’s an impact theory
of note, and the most likely explanation for the Moon’s
existence. But there are others. Did you ever think that
maybe the Moon broke away from the Earth - and could have
done so at Vredefort? It’s a far-fetched theory indeed. Yet
I can I remember British popular astronomer Patrick Moore on
a visit to Vredefort saying in a TV series that it was
possible that the Moon had broken away from the Earth here.
The huge plug of granite forming the Dome, he suggested, was
evidence for the pulling-away of a large space body. That
was in the sixties or seventieis and I have never heard it
mentioned again. But read on…
The planetismal collision
theory is just that, a theory, but of all the explanations
for the existence of the Moon it seems the most convincing
because based on rock-hard evidence. Another theory was that
the Moon was itself a planet that got captured by the Earth
although it never hit. But that would not explain why the
satellite is steadily pulling away from us (which the
collision theory does help to account for). The Moon as a
mini-planet probably hit the Earth a glancing blow and then
spun off into orbit, moving steadily away as it continues to
do today.
There is no sign of an
Earth-Moon impact either on Earth or on the Moon. But
actually we do have evidence of what could have happened, in
the form of the Moon rock which was brought back by the
Apollo astronauts in the 1970s. Much Moon rock is the same
as the lighter rocks on Earth (ie our continents) and from
this we can infer that as the re-formed planetisimals broke
apart, the smaller, lighter Moon drifted away carrying large
amounts of Earth’s surface and leaving the heavier bits -
including the Earth’s iron core - behind. The heavier
elements of the Earth’s makeup sank towards the middle,
becoming molten due to the heat from all the impacts, and
friction caused by gravity.
The theory that the Moon
was expelled from Earth by a blast or centrifugal forces -
the original Moore supposition – may not be so far off the
mark after all. The Vredefort scar is certainly a big one,
some 230km across at its widest today (though it has been
eroded down and might have been about 360km originally). It
occurred, however, only 2.023 billion years ago - relatively
late in the day - long after major bombardments had brought
our seas and sculpted our mobile continents. The reason we
can still see the enormous Vredefort crater so well is that
it sits in the middle of a huge, rather inert shield of rock
called the KaapVaal Craton. It has not been subducted under
the continental plates like many other craters.
But its dates just do not
tie up with the dating of the Moon rocks, so there is
absolutely no foundation for a belief that the Moon broke
away here. In any case, the crater (large as it is) is way
too small to have been the womb of the Moon.
Still, the general theory that maybe the Earth expels huge
chunks of itself in gas blasts is still very much around. I
have been watching the work of the Kiel University planetary
and geological scientists led by Prof Jason Phipps Morgan,
who has promised to get in touch about his latest research.
Morgan hit the headlines in 2004 by announcing the theory of
“Verneshots” named after science fiction writer Jules Verne
who had speculated that it might be possible to shoot a
rocket to the Moon out of a sort of cannon.
The Kiel group suggested
that the Earth has been doing this, blasting bits out in
mighty belches of gas. This is a more sophisticated version
of the old “cryptoexplosion” theory that did the rounds to
explain Vredefort for many decades. The cannon principle was
revived by scientists at the Geomar earth science institute
at Kiel to explain mass extinctions. They speculate that a
huge eruption of gas took place from beneath the crust. This
was not a volcanic event but a release of pressure.
The Kiel group did not
mention Vredefort but dealt with the extinction of the
dinosaurs, which has mostly been associated with the
Chixulub impact 65 million years ago. We can speculate that
the hole in the crust that occurred at Vredefort may not
have come from outside but from within the Earth.
Read about the theory of
what may REALLY have killed the dinosaurs here and more
about Jason Phipps Morgan here. For an amusing take on the
Verneshot theory go to the ExitMundi site, here. The picture
above is from ExitMundi. Incidentally, if planets produce
their own meteors that hurtle out into space, how else could
it happen except from megavolcanoes or Verneshots?
Though a Verneshot
creating the Moon at Vredefort is certainly ruled out, the
debates about what made the Moon - and Vredefort Dome -
continue. There is 99% certainty that the Vredefort
structure was caused by an asteroid impact, but still a 1%
shade of doubt that it might have been a cryptoexplosion of
some kind. Though it did not produce the Moon, such an
explosion could have lobbed large chunks of Earth rock into
space and caused mayhem on our planet itself, sending a
shock wave around the Earth and creating vast tsunamis in
the seas. The “fingerprints” of such a blast could well
resemble those of an asteroid impact, particularly if parts
of the rock that were blasted out fell back to Earth forming
a crater.
For a century past, it was thought that the Vredefort Dome
was volcanic in origin, or caused by a blast from within the
Earth. Only in the mid-1990s did geologists more or less
reach consensus about it’s being an impact crater or
astrobleme (lovely word!).
Meantime, the Moon’s
origins are also being probed. It’s amazing how much new
info is appearing about the Moon. Recently I’ve been
subscribing to a Google news alert about the moon and it’s
fascinating how interest in our satellite has revived, with
new research released almost daily. If using a Google
search, make it for “The Moon” (not just “moon” because you
pick up all sorts of junk about pop groups, restaurants and
romantic nothings).
The Moon is back in focus
probably because various Moon missions are being planned -
by the Chinese among others - and there is currently a
debate in the US as to whether to return to the Moon or go
to Mars (or both, using the Moon as a staging base). Space
tourism may well take numerous paying passengers to the Moon
during the 21st century though I wouldn’t bank on it before
2050 at least.
- Unpublished, 2008
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I'm
a writer of popular science and technology books and
articles, a former Professor of Communication whose
hobby is now "backyard astronomy" with history thrown
in! I'm a keen mountain-biker and kayaker, so the
Dome and Vaal River are my playground.
I'm
a former teacher who has worked with Graeme on many
aspects of science & local history research, and I have
got to know the Dome by preparing the maps and data used
in our various presentations. My passions are mountain
running and education. I manage Otters' Haunt.