planetary craters what made all those holes pt1
By David Bryant
It is a fact that every solid body within the Solar System is pock-marked by high-speed impact craters: not just the four rocky planets Mercury, Venus, Earth and Mars and their moons, but also the satellites of the four gas giants Jupiter, Saturn Uranus & Neptune.
In addition, all the large asteroids that have been imaged from Earth or from space probes show similar dramatic evidence of past multiple impacts.
Currently, the most commonly-held belief is that the objects that created these structures were either large meteorites or small asteroids (accepting the definition that a meteorite is a chunk of metallic or rocky debris left over from the planetary formation phase or from the disruption of a planetissimal or asteroid).
For some time I have been reflecting upon the fact that very few of the meteorites I sell are associated with a known crater: some of course are:
• Barringer / Canyon Diablo, USA
• Wolf Creek, Australia
• Gebel Kamil, Egypt
But the vast majority of meteorites of all major types are found lying on the Earth’s surface:
there are no major craters associated with the huge Campo del Cielo or Sikhote Alin falls.
Furthermore, although there are around 175 large impact craters on Earth, none of those over 20km in diameter have produced meteorite finds! Strange!
A few, such as Chesapeake Bay, Gilf-el-Kebir, Darwin and Ries, have impact glasses associated with them, but wouldn’t you expect that an object large enough to create a hole 20km across would leave significant solid remains?
It has been estimated that the iron meteorite that created the Barringer Crater in Arizona had a mass of 63,000 tonnes: of this around 30 tonnes have been collected as large fragments (up to 639kg) while another 8,000 tonnes probably remain as fine particles in the crater walls and floor.
Rounding this up, we can suggest that about 12% of the impactor survived. Given that the Barringer Crater is quite small (around 1.2 km wide) then shouldn’t a really huge structure such as the 300 km Vredefort Crater have a vast strewn field of hundreds of thousands of tonnes of material scattered around it?
The usual reason given for the lack of such material is that the vast majority was vapourised on impact and dispersed over a wide area. But even if this were true, there should be some big lumps, albeit some distance from the crater. And have any nickel-iron deposits been found in association with a large astrobleme? (impact crater) Well just possibly yes!
Last year I had published an article in which I suggested that the huge Sudbury Ring structure was created by the impact of an asteroidal core that left behind the enormous reservoir of metallic minerals that is an important source of copper, nickel and iron.
I also gave several lectures on this theme and was astonished by the semi-hostile response I evoked!
To me this seems an eminently likely scenario: it is the only way to explain such huge deposits of dense elements at or near the surface. (You may remember from a previous article that it is axiomatic that, during the differentiation phase of planetary formation, such elements sink inwards to form planetary cores).
Some geologists have suggested that plumes rising from the core could bring heavy elements to the surface, but there is no credible mechanism for this!
Let’s sum up this, the first half of this two-part article:
• The Earth, like all rocky objects in the Solar System, shows numbers of structures produced by massive impacts throughout its history.
• Only a very small number of these craters have meteoritic material associated with them.
• There are just a couple of enormous astroblemes which are surrounded by rich deposits of metallic ores and minerals.
• The vast majority of craters must therefore have been produced by something other than meteoritic or asteroidal impacts I shall reveal my theory of their origin (which caused an Apollo Astronaut to change the content of his lecture!) in the next article!