Meteorite Monday: Hoba Meteorite

Hoba- Image from Giraud Patrich (1)

The Hoba Meteorite represents the largest single piece of meteorite uncovered on the earth. Weighing in at 60 tons, it makes Oregon’s 15 ton Willamette meteorite seem amateur in comparison. This iron behemoth is classified as an ataxite- an iron meteorite whose nickel content in high enough to prevent development of the widmanstatten pattern. 

This meteorite landed on the earth about 80,000 years ago in what would become Namibia, and, due to it’s sheer weight, was never moved and can still be found where it landed. Unfortunately, this meant it was easy prey for vandals who hacked off pieces of it. The good news is that in 1955 the Namibian Government declared the area a national monument and built a visitors center in order to decrease vandalism (2). Now, in what has to be the coolest hands on science demo, people can touch and get a close up view of the worlds largest meteorite.

1. http://commons.wikimedia.org/wiki/File:Namibie_Hoba_Meteorite_03.JPG

2. http://www.namibian.org/travel/meteorites/hoba-meteorite.html

 

 

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Good-bye to the space shuttle


-The space shuttle Atlantis before its last launch (Image courtesy of NASA/Bill Inglalls)

I’m not sure what saddens me more: that today marks the end of US manned space flight or that we don’t have a damn thing to replace it. Congress and NASA want to point fingers at whose to blame for the, hopefully temporary, demise of the manned space flight program. Was it a lack of funding by Congress or NASA’s mismanagement and bureaucracy? I don’t know and I’m not particularly interested in the politics and squabbling. Either way, both parties have squandered our legacy and potential and should be ashamed of themselves.

Meteorite Monday: Aubrites

Bustee Aubrite- Image courtesy of IMCA

Aubrites belong to a group of meteorites called achondrites. This group of meteorites is different from chondrites in that they don’t contain chondrules, the silica-rich sphere like inclusions for which chondrites are known. Aubrites originated from asteroids and have a brecciated texture. This fragmented texture is the product of a violent collision between the aubrites parent asteroid and another asteroid.

Mineralogically, aubrites are very similar to enstatite chondrites. They are composed primarily of the magnesium rich pyroxene, enstatite. The big difference is that aubrites come from asteroids that heated to the point that they experienced melting and underwent some form of chemical change. In the geological sciences this is called differentiation. Another defining characteristic is the addition of the mineral oldhamite (1). This calcium sulfide, for reasons I’m not sure of, doesn’t form naturally in terrestrial rocks. This quality makes it a useful mineral in determining the celestial origin of aubrites.

1. Smith, Caroline; Russell, Sara; Benedix, Gretchen. Meteorites. Firefly Books. 2009. P. 68

Meteorite Monday: Impact craters and surprises

Manicouagan Crater

Manicouagan Crater

When I picked the topic for this week’s Meteorite Monday, I figured it would go something like this:

Throughout it’s 4.5 billion year old history, the earth has been peppered with meteoroids great and small. The small ones either disintegrate in the atmosphere in a blazing spectacle or make it to the earth’s surface and become meteorites. The much larger ones, such as the one that may or may not have killed off the dinosaurs, leave large craters that scar the earth’s surface and inspire some of the worst sci-fi movies and television around (such as this one, that one, and the worst offender of them all).

The one in the picture above is the Manicougan Crater of Quebec, Canada. At approximately 215 million years old, it’s one of the oldest confirmed impact craters on the earth (1). That puts it towards the end of the Triassic age, and in geologic time, makes it relatively young. However, it had the unfortunate luck to land in an area that would be smoothed over and reworked by the advance and retreat of glaciers over thousands of years. Over time, and with the help of receding glaciers and human tinkering, the crater filled with water and is now partially used to generate electricity for Quebec and parts of New England.

The end. Quick, short and to the point.

What I didn’t count on, was reading about this:

In 1998 a group of researchers hypothesized that the Manicouagan Crater formed at the same time as four other craters across the world: Rouchechouart in France, Saint Martin in Canada, Obolon in the Ukraine and Red Wing in North Dakota (2). They then used a computer model to determine the position of the earth’s continents at the time of the impact. They found that all but Obolon and Red Wing occurred at the same lattitude. The latter two “on the other hand, lie on great circles of identical declination with Rochechouart and Saint Martin, respectively” (from the abstract, not my words). This means that the craters possibly formed during an impact event from a single asteroid that broke apart as it entered the earth’s surface. The subsequent collision formed a chain of craters that were separated by the slow, random shuffle of the earth’s tectonic plates.

For some this may be old news, but for me its new and represents one of coolest things about science blogging. When I write a post, I already have an idea of where I want to go with it and I do a little background reading to verify my claims. In the case of this post, I didn’t count on learning about impact crater chains on the earth’s surface. It’s that sort of discovery that really makes all the mental sweating worth it.

1. http://en.wikipedia.org/wiki/Manicouagan_crater

2. John G. Spray, Simon P. Kelley& David B. Rowley. Nature 392, 171-173 (12 March 1998) | doi:10.1038/32397 http://www.nature.com.proxy.lib.pdx.edu/nature/journal/v392/n6672/abs/392171a0.html

A few pictures from my petrology trip to the Blue Mountains of northeastern Oregon

Our stops for both days on the trip

My apologies for the dearth of posts lately- especially the last two missing meteorite Monday posts. I’ve been in a bit of a post-finals, I don’t-wanna-do-anything-but-veg, funk as of late. I’m slowly pulling myself out of it though. I changed my blog theme because I felt the old one didn’t handle my side bar as well. It started to look all cluttered as I added to the blog roll. The header image was taken by a friend during our petrology trip. I love the contrast between the flow banded rhyolite and the ubiquitous rock hammer.

Our fearless petrology professor, Martin J. Streck.

So, for this post I wanted to share a few images from my petrology trip. The Google Earth image above shows all our stops during both days of the trip. The first eight stops are from day one and the next seven or so, are from day two. I tried to keep the image as zoomed out as possible so as to convey exactly where we were at in northeastern Oregon. However, the more I zoomed out, the less clear our stops became. Think of it like playing “Where on Google Earth”, but with a lot more detail to work with!

Oregon’s geology is fairly spectacular, but she really hits her stride in the Blue Mountains. It’s here that you get somewhat of a break from the Columbia River basalts and every other basalt group that dominates Oregon’s landscape. This part of the state is home to accreted terrain that was slowly stitched onto the west coast as the North American plate crawled its way over the Farallon Plate. Some of this stitching is marked by plutons that formed large reservoirs of granite and microgranite between the foreign terrains.

This was clearly illustrated by our instructor as we were leaving our sixth stop. We were in a broad valley that had been carved out by the Grande Ronde River. The rocks in the area where mostly rhyolites and welded tuffs of the John Day Formation (approximately 28 M.Y.A.). He kept stopping every minute or so to whack a rock, look at it with his hand lens, shake his head and throw the piece on the road. It was on the fourth or fifth try that he finally found what he was looking for: granite. He enthusiastically explained that the pluton marked the boundary between 140 M.Y.A terrain and  28 M.Y.A. terrain. Within less than a mile we traversed almost 120 million years of history!

Grande Ronde River. This is where we found the 140 MYA granite right next to the 28 MYA rhyolites

Serpentinite and talc outcrop in the John Day Valley

The second day of the trip was spent mostly in the John Day Valley. It was here that we looked at outcrops of serpentinite, chert, and a few pillow basalts. It was pretty cool to think that we were in a rather dry part of the state, but surrounded by rocks that only form on the ocean floor. In fact, one of those pillow basalts can be seen in the picture with my professor. It’s in the center of picture, next to his right hand.

After hammering away and gawking at the seafloor rocks, we made our way into the John Day formation with all its rhyolites and ash-flow tuff. On one of the stops we looked at a columnar jointing pattern that formed not from basalt, but from a welded tuff. These were of the Rattlesnake Tuff formation and the topic of my instructors PhD dissertation while attending Oregon State University. What was really neat about these, were the pieces of obsidian and lithic fragments that were trapped in the super heated ash flow from the rhyolitic eruptions.

Rattlesnake Tuff Formation- Geology student for scale

Brecciated dacite and a dyke from the Columbia River Basalts

One of our final stops before making the four hour trip back to Portland, was this brecciated dacite outcrop. The vast majority of these rocks were oxidized, hence the pink color. However, there was a small, rounded part that for some reason, resisted the oxidation process and retained its grey color. In either case, the rocks contained some beautiful phenocrysts of plagioclase and some quartz. And that dark, vertical structure in the middle of the sea of pink? That my friends is one of the many dykes  from which the vaunted Columbia River Basalts spewed forth.

There are some more pictures, but I really wanted to highlight some of the more interesting stops. In fact, my contribution to this month’s Accretionary Wedge will highlight an awesome rock that I got from one of the stops. Maybe a little later in the future the other pictures will crop up in some posts.

Meteorite Monday: The OSIRIS-REx Mission

Today’s Meteorite Monday is going to be a short one. I just returned from my petrology field trip to Eastern Oregon and I haven’t had a whole lot of time to put together an original post. So, I thought I’d share a video about NASA’s latest planetary science mission to asteroid 1999 RQ-36. Asteroids are basically the parent body of most meteorites and can tell us a lot about the formation of the solar system. It’s a very exciting mission that will return actual pieces of asteroid to the earth where we can study it and learn about the origins of the solar system.

Meteorite Monday: Lunar meteorites

The Alan Hills Meteorite- The first identified lunar meteorite

The Alan Hills Meteorite- The first identified lunar meteorite (courtesy of NASA)

Lunar meteorites tend to come in two flavors: either the fine grained basalt of the relatively young lunar basins, or those that are rich in plagioclase that hail from the lunar highlands. To date only about 46 kg of lunar meteorite have been collected from the earth’s surface with the largest sample, Dal al Gaani 400, weighing in at about 1.425 kg (1).

The lunar meteorites that come from the highlands are composed mostly of a calcium rich plagioclase called anorthite. The rocks are then classified based on the amount of anorthite in the sample:

  • >90% anorthite is called anorthosite
  • <90% is classified as either norite if it’s poor in calcium rich pyroxene, or it’s called troctolite if it contains olivine.

The lunar basalts that come from the much younger basins are kind of the opposite of their highland cousins. Instead of being composed primarily of calcium-rich plagioclase, they contain calcium rich pyroxenes. Titanium is also found in these rocks.

Our knowledge of the moon came about primarily from the Apollo missions and the Luna missions. Both returned samples from various locations on the moon and have allowed us to compare those returned samples with the lunar meteorites found on the earth. It is believed that some of the highland meteorites come from the far-side of the moon, which gives us a look at the side we don’t see from our terrestrial vantage.

And now for that awesome picture I found off of NASA’s image of the day site:

Gorgeous picture of the moon taken by the Galileo spacecraft on it's way to Jupiter. (Image courtesy of NASA)

1. Hutchison, Robert. Meteorites: A petrologic, chemical and isotopic synthesis. 2004. Cambridge University Press. P. 296-297

Meteorite Monday: Origins of carbonaceous chondrites

Last week I mentioned this paper that was recently published by Linda T. Elkins-Tanton, Benjamin P. Weiss and Maria T. Zuber of MIT. In it, Elkins-Tanton et al., discuss the possibility that carbonaceous chondrites could have originated from differentiated bodies. For those unfamiliar with this term, it basically refers to any asteroid or planetesimal that grew large enough to develop a distinct core/mantle boundary and a cold crust. If you recall, I wrote a few posts about iron meteorites that originated from such bodies.

The researchers based their hypothesis on the “metamorphic, magnetic, and exposure age data” for the Vigarano type chondrites, with most of the data being taken from the most famous of this type, Allende. To support this hypothesis they created a computer model of a differentiated body with a cold crust. They then used equations that would allow them to model thermal behavior on such a body and also modeled in a core dynamo as a way to explain the magnetic orientation of Allende’s mineral grains. The model was constrained to bodies 500 km in radius with a crust “no thicker than 2% of the bodies radius”.

This crust basically sits on top of a magma ocean. They further speculate that the planetesimal accretes material as it collides with other pieces of rock in the solar system. This process allows for the crust to thicken and undergo varying levels of metamorphism based on it’s proximity to the heat from the magma.

What the researchers have suggested is a significant departure from that of the current thinking behind planetary formation. As I had mentioned last week, it is commonly thought that carbonaceous chondrites originated from undifferentiated bodies. This paper is suggesting that some of these chondrites originated from bodies experienced at least partial differentiation. It’s a pretty neat concept that I hope will see a lot of debate within the meteoritics community. I plan on keeping my eyes open for any responses or research that builds off this work.

I have only one critique for the paper. It’s pedantic really, but I think it makes all the difference in clarifying the purpose of the research. The title of the paper “Chondrites as samples of differentiated planetismals” makes one believe that the research will apply to more than just two or three classes of meteorite. Throughout most of the paper, the researchers rely primarily on Allende, which is part of one class of carbonaceous chondrite. There’s mention of Orgueil, but beyond that one doesn’t see much in regards to Ivuna or any other species of carbonaceous chondrite.

What I would like to see is how this model applies to the other chondrites. Can it be adapted in such a way as to explain the origins of other carbonaceous chondrites? If not, should there be a new classification for those which come from partially differentiated bodies? Those are a couple of questions I’d like to see answered in any follow up papers.

Elkins-Tanton L.T. et al., Chondrites as samples of differentiated planetesimals, Earth Planet Sci. Let. (2011), doi: 10.1016/j.epsl.2011.03.010

Meteorite Monday: Or not…

I’ll admit, I kinda let the ball drop on this week’s Meteorite Monday. Normally, I try to get this post done a few days ahead of time, but this weekend has been filled with work, chemistry, petrology readings and completing scholarship essays so I can get some research money.

However, I do have a neat post planned that talks about a recent paper that was published by some researchers at MIT. In the paper, “Chondrites as samples of differentiated planetesimals”, Elkins-Tanton et al., postulate that carbonaceous chondrites could have originated from asteroids that experienced differentiation. This challenges the current paradigm that such chondrites actually come from bodies that didn’t grow large enough to develop a core and mantle. Some have speculated that if true, this could change the way we view the formation of the planets. It’s a good read and if you have time, I’d recommend giving it a look over.

The Allende Meteorite- One of the meteorites used in the Elkins-Tanton et al., study.

Meteorite Monday: Stony-Iron Meteorites, or space rock bling

Stony-irons are hands down the prettiest meteorites to look at. They’re somewhat similar to their iron meteorite cousins in that they have an  iron-nickel body, or matrix. However, they are unique in that some are studded with these gorgeous olivine crystals called peridot. The accepted consensus is that these meteorites originated at the boundary between the core and the mantle of asteroids that grew large enough to develop such features (1). This is because peridot, a magnesium rich olivine, crystalizes at very high temperatures only found at the mantle. The stony-irons with these olivine crystals are called pallasites.

Esquel Pallasite from Ohio State University

The other type of stony iron is called a mesosiderite. These guys are really cool because their composition is thought to be the result of a collision between two molten asteroids. Mesosiderites still retain the iron-nickel body of the pallasites, but contain chunks of melted rock (clasts) instead of olivine crystals. The angularity and direction of the clasts is suggestive of a large impact between two asteroids, with material  from the crust mixing into the iron-nickel core (1).

Mesosiderite from Arizona State University

1. Smith, C., Russel, S., Benedix, G., Meteorites. Firefly Books. 2009. P. 67.