In New Zealand we are approaching what the journalists say is “The Silly Season”, the reason being that what with Christmas and New Year, and with it being in the middle of summer, a lot of journalists take holidays, and the media, with a skeleton staff, have to find almost anything to fill in the spaces that the media makes available. So, in the spirit of getting off to an early start, I noticed an image from Mars that looks as if someone left a cannon ball lying around. (The image is easily found on the web, but details are not, so I am not sure where it was found.) So what is it?
Needless to say there were some loopy suggestions from “the fringe”, but while it is easy to scoff, it is not so easy to try to guess what it is. The idea of a cannon ball and nothing else borders on the totally bizarre. So what can we see from the image? The remarkable point about this object is it seems to be lying on the surface, which suggest it did not strike it, as otherwise there would be indentations, or, if it were a meteorite, there would be a crater. There clearly isn’t. Equally, however, it looks smooth, which suggests it has been fused, which means it did not arise there. Some have suggested it is a haematite spherule, but that, to me is not that likely, in part because it is so large (the so-called “blueberries” were quite small) and also because there seems to be only one of it, while what created the “blueberries” created a lot of them. In my opinion, it is probably an iron meteorite, and the reason there is no impact crater is that it landed somewhere else, and rolled to this spot.
So maybe time to get a little more serious, and think about iron meteorites. What can we say about them? The Curiosity rover has also found “Egg rock”, which is an iron meteorite about the size of a golf ball. The Rover found iron, nickel and phosphorus as significant constituents, and the phosphorus is present as iron phosphide. There are two important issues here: how did the iron/nickel ball form separately from everything else, and equally important, how did iron phosphide form? That last question may need explanation, because phosphorus does not normally occur as a phosphide, and phosphides only form under highly reducing conditions. (Reducing conditions are usually in the presence of hydrogen and or an active metal at higher temperatures. The opposite, oxidising conditions, occurs when there is oxygen or water present, but not enough hydrogen or metal to scavenge the oxygen.)
Iron phosphide is known to occur in certain iron meteorites, but such meteorites can always be attributed to having formed at a little more than 1 A.U. from, or closer to the star. Chondrites that formed further out, such as in the asteroid belt, always have their phosphorus in the form of phosphate, which is a very stable, oxidised, phosphorus compound. The point about 1 A.U. (the distance of Earth from the sun) is that was where the temperatures were hot enough to melt iron, and the phosphide would form by the molten iron reacting with phosphate to form the phosphide and iron oxide.
Now for the reason for going on about this. One of the JPL team explained that iron meteorites originated from the cores of asteroids. The premise here is that during initial accretion, the dust assembled into an asteroid-sized object, the object got sufficiently hot and the iron and nickel melted and sunk to the core. Later, there was a massive collision and the asteroid’s core shattered, and the meteorites we see are the fragments from the shattering. (Note, the same people argue planets formed by asteroid sized bodies, and bigger, colliding and everything stick together. Here is having your cake and eating it in action.) The first question is, why did the rock melt? One possibility is radioactive isotopes, so it is possible, nevertheless for the explanation to work the asteroid had to melt hot enough to melt iron, and to hold those temperatures for long enough for the iron to work its way to the centre through the very viscous silicates in a very weak gravitational field. A further problem is that the phosphate would dissolve in the silicates, in which case it would not form iron phosphide because the iron would not get there. Calcium phosphate has a density of about 3, very similar to many of the silicates, so it might be difficult for iron phosphide to form in such an asteroid. Only a very few asteroids, and Vesta is one, have iron cores, and there are some reasons to believe Vesta formed somewhere else and moved.
The reason for my interest is that in my ebook, “Planetary Formation and Biogenesis” I argue that the first way accretion started was for the dust in the accretion disk to get hot enough to get sticky, or to form something that could later act like a cement. When the temperatures got up to about 1550 degrees Centigrade, iron melts and in the disk would form globules that would grow to a certain degree. Many of these would also find molten silicates to coat them, so the separation occurred through the temperature generated by the accreting star. Provided these could separate themselves from the gas flow (and there is at least a plausible mechanism) then these would become the raw materials for rocky planets to form. That is why (at least in my opinion) Earth, Venus and Mercury have large iron cores, but Mars does not.
That, of course, has got a little away from the “Martian cannonball” but part of forming a scientific theory is to let the mind wander, to check that a number of other aspects of the problem are consistent with the propositions. In my view, the presence of iron phosphide in an iron meteorite is most unlikely to have come from the core of an asteroid that got smashed up. I still like my theory, but then again, I suppose I am biased.