Labels: meteors, virginia

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.

Labels: analogies, astronomy, books, isotopes, meteors, minerals, planetary geology

Following on yesterday's Namibiferific post, I'd also like to share this image:

Labels: africa, meteors, namibia, travel

Labels: antarctica, astronomy, australia, basalt, gsw, hadean, igneous, isotopes, meteors, minerals

My friend Mark Tyra (a College Park geology master, like myself) just alerted me to a cool site which explores the presence of meteorites preserved in Ordovician limestones in Sweden. Check it out!

Labels: limestone, meteors, sweden, websites

The largest meteorite (or maybe comet?... we don't really know which) impact crater in the United States is in Virginia, underneath the lower Chesapeake Bay. In the Eocene, a large bolide (unidentified space chunk) slammed into the Earth. Dating of microfossils found in the same sedimentary layers as impact ejecta have provided a date of ~35.5 Ma for the event. The impactor hit on the continental shelf offshore of Eocene Virginia, carving through the Atlantic-deposited sediments there and gouging into the crystalline bedrock beneath (igneous and metamorphic rocks like the modern Piedmont province, but buried beneath Coastal Plain layers).

The crater was discovered over a ten-year process that began with offshore sampling near Atlantic City, New Jersey in the mid-1980s. Those drill cores came up with a layer of ejecta (including shocked quartz and little beads of glass called tektites) among the late Eocene layers of sediments. Searching around, eventually the crater was seismically imaged by oil exploration in the Chesapeake Bay in the mid-1990s.

Centered on Cape Charles, Virginia, the crater is about 50 miles across, but appears wider as sedimentary layers adjacent to the hold have slumped inward along listric faults. The James, York, and Rappahannock Rivers all trend into this depression, and ultimately the crater is probably responsible for the Susquehanna River taking on its southerly course. When sea level rose and flooded the valley of the Susquehanna, the Chesapeake Bay was formed.

A similar impact structure offshore of New Jersey, the Toms Canyon Impact Crater, may have formed at the same time as the impactor broke into pieces before impacting.

The lead-off image to this post is by the team at the U-Haul trucking company, which performs a terrific public service by finding out interesting things about the different states (and Canadian provinces) and posting them on the sides of their trucks with eye-catching graphics. A great many of the topics they choose are about geology, from minerals to fossils to impact craters to cartography and canyons. A while ago, I wrote an article for Geotimes looking at their program.

More information on the crater:

Wikipedia's entry on the crater.
W&M Geology Department's page about the crater.
USGS team examining the crater.
National Geographic article (2001).

Labels: art, cenozoic, coastal plain, geology, meteors, new jersey, piedmont, rivers, virginia

Just now getting around to reading last month's issue of National Geographic, but it's a good thing I got to it yesterday rather than tomorrow -- because today's the day I talk about comets, asteroids, and meteors in Physical Geology class, and one of the topics that people always love to talk about is what happens when those big dumb space chunks smack into Earth.



The article's a good read, and illustrated with magnificent images, like this classic 1972 image of a fireball over Jackson Lake, Wyoming. That's the Tetons in the background. Check it out.

Labels: meteors, news, wyoming

National Geographic has a news piece about a large fireball seen over Oregon early on Tuesday. A bunch of people witnessed it falling and then explode in the sky. The article suggests that in the days to come, we may find chunks of the exploded space rock in impact sites around the Pacific Northwest (assuming it's not that spy satellite the Navy shot down around the same time...). Check it out.

Labels: meteors

January 30 may be a bad day on Mars. A space rock discovered in November of this year has a 1-in-75 chance of smacking into the red planet on that day. The rock is about the same size as the one inferred to have leveled the forest in a big swath of Siberia in 1908 (in what is called the Tunguska Event.) The amount of energy released then is estimated to be approximately the equivalent of a 15 megaton nuclear bomb. More details here.

Labels: astronomy, mars, meteors