epepke said:
Now, this is getting a bit silly. You argue about mining on the moon based on the idea that it isn't like the Earth. But now all of a sudden you're talking about the Moon based on what hasn't been done on the Earth?
No, its not even a bit silly. We have to work hypothesis based on our experience. What´s our experience? We know something about the geology of the Moon and a lot of things about Earth´s geology, so we can make some inferences regarding aspects such as type of deposits, their locations and how to find them. We have always done mining here, not there. But still we have to use this experience as a basis for any extrapolation or project.
epepke said:
As you said, there's no plate tectonics on the Moon, so there's nothing to subsume what does fall on the Moon. Most meteoric iron on the Earth has long since dropped down to the core. (I remind you that the core of the Earth is mostly iron.)
Both, Earth and Moon have iron/nickel cores, but the Moon´s is smaller. Also, we have plenty of iron ore deposits on Earth, and they have nothing to do with meteoritic iron. A lot (if not most) of the iron present in our crust was originated in the mantle, the "birth place" of the basaltic magmas.
A 100m-big meteor chunk on the surface (or buried under say, 100m of rubble) will be affected by weathering and erosion. So, it will be oxided and divided in smaller chunks that will be transported, scattered and deposited somewhere else. A lot of it will be incorporated in clay minerals or become iron hydroxides. The remains will be deposited in river channels, deltas, sea shores, etc., toghether vith various other materials from other sources, that will be dominant in volume. In other words- the iron will be incorporated into the crust by surficial processes.
Plate tectonics not just drops material in to the mantle or the core. It recycles crustal material. An iron meteor chunk in a subduction zone would behave in one of the following ways (providing it survived all the above cited processes):
-If its on continental crust, it would not be subducted. It could get buried up to 35-40 km, be deformed and perhaps even partially melted, but that´s all. Eventually it could even be exumed after some hundreds of million years. It could also be, if partially melted, incorporated in to magma.
-If its in oceanic crust, it would be subducted. Depending on factors such as subduction zone angle and subduction rate, the basaltic crust with the meteor chunk can be added to the lower crust, partially melted or a mixture of both. The partially melted part would rise, generating igneous intrusions and volcanoes. the non melted residue is incorporated to the mantle.
What means that meteoritic contribution to the core composition after the differentiation of the core and mantle due to plate tectonics was probably small. The ammount of iron added to the crust of our planet by meteor impacts is probably small. Most of the iron was already here when the planet was aggregated.
epepke said:
As I think I said, it wouldn't be much as a suppliment for Earth mining. But as a source of materials for spacecraft, on a place nearby with so little gravity that you can boost two people into orbit in something only a little larger than a Volkswagen, including fuel, you can't beat it.
I agree with this, but remember that not all asteroids are composed by iron and nickel. Many are composed by silicates, organic compounds, ice, etc. OK, we may have some iron, but a lot of other important materials probably are not avalible on the Moon, since their concentration depends on tectonic activity, that is not present on the Moon. They will have to be mined somewhere else or the colonists will have to develop new methods for extracting them in very small concentrations (ppm or even ppb).
