Accretion, anorthite, and aluminum
One of the interesting things I learned about when reading Marcia Bjornerud's Reading the Rocks was about the putting-together of our solar system. The scientific consensus is that our Sun is a second- or third-generation star, with previous iterations having been destroyed through supernovae. (The energy of the supernova is capable of fusing low-atomic-weight elements together into heavier elements.) Post-supernova, a big dispersed cloud of dust and gas existed: the pre-solar nebula. The next phase of history took the nebula and condensed it into a protoplanetary disc, and then that fried-egg-shaped accumulation self-organized (first via static charges attracting particles together -- the dust bunny effect -- and then via gravity). These simple forces brought many small particles of mass together into a smaller number of larger accumulations of mass. For a modern analogue to this process, consider the asteroid 25143 Itokawa, which looks like this:
It is, essentially, a big three-dimensional pile of space rock. I imagine that if you went and kicked it, some boulders would go flying off in all directions. It's a great example of the sorts of objects that we interpret occupying the early solar system. This process is self-amplifying (a positive feedback loop): the more mass you concentrate in a given area, the more gravity it exerts on surrounding masses, which pull towards one another, resulting in more mass, more gravity, more mass, and so on until you have planets. Eventually, if you get a big enough pile of space rock, gravity can condense it, and through warming (via radioactive decay, and potentially frictional heat from continuing impacts), the component elements could self-sort by density. Those with the highest specific gravity could sink down lower, whereas the scummier varieties would "float" up to a higher level.
Bjornerud astutely mentions that this early solar system would have lots of these little planetismals, kind of like those encountered in Antoine de Saint-Exupery's charming book The Little Prince:
Judging from the steam plume from that knee-high volcano, there's clearly some differentiation taking place down below. Now we get to the interesting part. Some asteroids fall to the planet Earth, whereupon we stop calling them asteroids, and start calling them meteorites. These meteorites are examined in great detail for information about our solar system's pre-pubescent years. One of them, the Allende meteorite, fell in the Chihuahua region of Mexico in early 1969:
image from Wikimedia commons
Geochemical analysis of the Allende meteorite by Lee, et al. (1976) showed something weird: the mineral anorthite, a feldspar, had mostly the same elements that anorthite has on Earth (or the moon): aluminum, calcium, silicon, and oxygen. But it also had a decent amount of magnesium. That's odd, because magnesium doesn't fit into anorthite's crystal structure very well at all. What's more, the magnesium in the Allende anorthite was all magnesium-26, not the "usual" magnesium-24. So... What's up with that?
It turns out that you can produce magnesium-26 as the stable daughter product when you break down radioactive aluminum-26. But aluminum-26 has a really short half-life (geologically speaking): only 730,000 years. As Bjornerud puts it, "The fact that a significant amount of aluminum-26 entered the meteorite's anorthite before decaying to magnesium-26 means that fewer than ten half-lives, and probably just a few million years, had passed between the supernova and the time that the anorthite crystals were being smelted out in the new solar refinery."
So that's stunning: the radioactive aluminum-26 was produced through a supernova explosion, and then, less than 5 million years later, a protoplanetary disc had formed and meteorites like Allende were being formed. Wow -- Until I read this passage, I had no idea that this phase of history went by so quickly! 5 million years is not a lot of time when you're talking about events of this magnitude. I was shocked, and I wanted to share this insight here. Are you shocked too?
______________________________________
References:
Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al, Geophysical Research Letters, 3(1), 41-44.
Zimmer, Ernst (2002), Using Aluminum-26 as a Clock for Early Solar System Events, Planetary Science Research Discoveries (website). Downloaded on December 16, 2009.
It is, essentially, a big three-dimensional pile of space rock. I imagine that if you went and kicked it, some boulders would go flying off in all directions. It's a great example of the sorts of objects that we interpret occupying the early solar system. This process is self-amplifying (a positive feedback loop): the more mass you concentrate in a given area, the more gravity it exerts on surrounding masses, which pull towards one another, resulting in more mass, more gravity, more mass, and so on until you have planets. Eventually, if you get a big enough pile of space rock, gravity can condense it, and through warming (via radioactive decay, and potentially frictional heat from continuing impacts), the component elements could self-sort by density. Those with the highest specific gravity could sink down lower, whereas the scummier varieties would "float" up to a higher level.
Bjornerud astutely mentions that this early solar system would have lots of these little planetismals, kind of like those encountered in Antoine de Saint-Exupery's charming book The Little Prince:
Judging from the steam plume from that knee-high volcano, there's clearly some differentiation taking place down below. Now we get to the interesting part. Some asteroids fall to the planet Earth, whereupon we stop calling them asteroids, and start calling them meteorites. These meteorites are examined in great detail for information about our solar system's pre-pubescent years. One of them, the Allende meteorite, fell in the Chihuahua region of Mexico in early 1969:
image from Wikimedia commons
Geochemical analysis of the Allende meteorite by Lee, et al. (1976) showed something weird: the mineral anorthite, a feldspar, had mostly the same elements that anorthite has on Earth (or the moon): aluminum, calcium, silicon, and oxygen. But it also had a decent amount of magnesium. That's odd, because magnesium doesn't fit into anorthite's crystal structure very well at all. What's more, the magnesium in the Allende anorthite was all magnesium-26, not the "usual" magnesium-24. So... What's up with that?
It turns out that you can produce magnesium-26 as the stable daughter product when you break down radioactive aluminum-26. But aluminum-26 has a really short half-life (geologically speaking): only 730,000 years. As Bjornerud puts it, "The fact that a significant amount of aluminum-26 entered the meteorite's anorthite before decaying to magnesium-26 means that fewer than ten half-lives, and probably just a few million years, had passed between the supernova and the time that the anorthite crystals were being smelted out in the new solar refinery."
So that's stunning: the radioactive aluminum-26 was produced through a supernova explosion, and then, less than 5 million years later, a protoplanetary disc had formed and meteorites like Allende were being formed. Wow -- Until I read this passage, I had no idea that this phase of history went by so quickly! 5 million years is not a lot of time when you're talking about events of this magnitude. I was shocked, and I wanted to share this insight here. Are you shocked too?
______________________________________
References:
Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al, Geophysical Research Letters, 3(1), 41-44.
Zimmer, Ernst (2002), Using Aluminum-26 as a Clock for Early Solar System Events, Planetary Science Research Discoveries (website). Downloaded on December 16, 2009.
Labels: analogies, astronomy, books, isotopes, meteors, minerals, planetary geology
