Friday, January 22, 2010

Meteorite falls in Lorton!


On Monday, a meteorite crashed into a doctor's office in Lorton, Virginia! Smithsonian scientists confirm its identity as space rock. 220 mph at the moment of impact. Super cool.

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Tuesday, December 29, 2009

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?

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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.

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Tuesday, April 14, 2009

Another Namibia shot: The Hoba Meteorite

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


More on this, the largest known intact meteorite on the Earth's surface.

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Tuesday, January 27, 2009

Andesitic meteorites and what they mean

Blogging on Peer-Reviewed ResearchJames Day (of the University of Maryland, College Park) presented last Wednesday at the Geological Society of Washington. He gave a talk entitled "Evidence for evolved crust formation in the early solar system." I would describe this presentation as a "game-changer," and I'll tell you why.

James described the Antarctic discovery* of two pieces of a new kind of meteorite with an andesitic composition. A clear fusion crust indicated it was a meteorite, and not just a rock from the Antarctic crust. (Isotopic evidence corroborates this, as you'll see.) The meteorite was in two pieces, which are respectively referred to as Graves Nunatuk (GRA) 06128 and 06129. Here's a plot from James' (et al.'s) Nature paper a few weeks ago showing the meteorite's composition:

meteor_comp

Black dots are actual measurements, and the gray blob is the calculated composition based on variations in mineralogy and mineral major element compositions. The meteorite has an 207Pb-206Pb age of 4.5 billion years, and oxygen isotopes plot far off the terrestrial fractionation trend:

not_from_earth
Everything from our planet plots on that upper horizontal line (including the Moon). This sample of evolved crust is therefore not from the Earth or the Moon. James also ruled out Mercury and Venus as potential sources, and suggested that it may be a fragment of a parent body in the asteroid belt. As the diagram above shows, the oxygen isotopes suggest an affinity with a group of meteorites called brachinites. (As near as I can tell, brachinites are usually ultramafic. At any rate, there have never been andesitic meteorites of any flavor known prior to GRA 06128/9.)

Highly siderophile element patterns suggest that there was no core formation in the parent body (these elements were still present in the sample; indicating they had not sequestered themselves in a metallic core). James also reported that pyroxene exsolution lamellae work by another group indicates a shallow depth of formation, on the order of 15-20 meters depth. (This, however, is extrapolated from pyroxene exsolution lamellae work on the Skaergaard Intrusion in Greenland; how well the method translates to an asteroid forming at the dawn of our solar system is another question. This generated a lot of questions at the GSW talk.) Large amounts of Na-rich plagioclase in GRA 06128/9 suggest partial melting of 10-30% in regions of the parent body. Assuming a chondritic, oxidized, volatile-rich starting composition, this could generate the large amount of Na-rich plagioclase seen in the samples.

So they're andesitic in composition, but otherwise like brachinites. In an e-mail to me, James noted that, "they have uncannily similar HSE patterns (and key ratios like Pd/Ir etc. are similar), O isotopes are in the right ballpark, they required about 30% partial melting (whether they are residues or cumulates; we haven't quite figured that out yet) and the accessory phases in these meteorites also imply a volatile rich parent body."

So why should you care? Why would I call this a "game changer?" It's because it really stretches our thinking. The nebular hypothesis of the solar system's formation has meteorites' composition as the starting material for the rocky planets. On earth, this meteoritic ("chondritic") composition compacted under the influence of gravity, then differentiated into layers based on density (a process facilitated by higher temperatures due to more radioactive decay early in the planet's history). Dense iron and nickel flowed down to make the core (joined by those HSEs), the medium-weight stuff became the 'silicate Earth' (mantle + crust), and the lightweight stuff formed an early atmosphere, most of which was likely stripped away by the erosive effects of the solar wind. (This is inferred to have taken place before the development of a magnetic field.)

Then, over time, the ultramafic-composition mantle partially melted to form basaltic-composition oceanic crust, which probably at first appeared like the surface of a lava lake (e.g. Kilauea Iki). This basaltic scum participated in a rudimentary form of plate tectonics, which encouraged partial melting via subduction (and the generation of a new atmosphere, but that's another story). The resulting magma would likely have been andesitic. In other words, on Earth, our andesite comes from plate tectonics, and that likely took a while to get going.

The assumption, in other words, was that crustal evolution ("distillation," in my parlance) took some serious time on a serious planet. But if crust evolved to andesitic compositions this early on non-Earth, non-plate-tectonic, non-planetary bodies, it really changes our understanding of early-formed materials in the solar system. I am reminded of the example of the Jack Hills zircons in Australia. Preserved as part of sedimentary rocks, these zircons crystallized about 4.4 billion years ago. Isotopic examination of the Jack Hills zircons suggest that they formed in a granitic rock. And granites are the most evolved of igneous rocks (the highest "proof"). Granites make up continental crust.

So the Jack Hills zircons similarly stretched our conception of when the earliest evolved crust formed on the planet Earth. I mean; Earth had granites 4.4 billion years ago? Prior to their discovery, most geologists would not have predicted so early a date for evolved crust. But the evidence suggests that's indeed how it was. And now, thanks to James Day's study, our imaginations are being similarly stretched regarding the origins of evolved crust on extraterrestrial bodies, too.

What else is there we don't know about our planet, our solar system? Probably a lot.
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Original paper in Nature: James M. D. Day, Richard D. Ash, Yang Liu, Jeremy J. Bellucci, Douglas Rumble III, William F. McDonough, Richard J. Walker & Lawrence A. Taylor. "Early formation of evolved asteroidal crust." Nature 457, 179-182 (8 January 2009). doi:10.1038/nature07651

Nature Podcast discussing (among other things) the meteorites.

Press release from the University of Maryland.
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* By the Antarctic Search for Meteorites program, which has blogged their expeditions in the past, and apparently just concluded the 2008-09 search.

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Monday, October 20, 2008

Meteorites in Ordovician limestones!

Friday, September 26, 2008

Virginia's extraterrestrial impact crater

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).

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Thursday, August 28, 2008

"Target Earth" article in National Geographic

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.

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Friday, February 22, 2008

Meteor in Oregon

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.

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Saturday, December 22, 2007

Mars may get an asteroid impact next month

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.

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