Meteorite Monday: The Chondrule Controversy

The rounded objects in the Grassland chondrite are chondrules (image from Wikipedia).

One of the more heavily  debated topics within the field of meteoritics is the origin of chondrules. These are the small, spherical silicate inclusions from where we derive the technical name for the most common type of meteorites, the chondrites. With few exceptions, chondrules are found in all chondrite groups in varying quantities. Sometimes we’ll see chondrites that are nearly 70% chondrules and in other cases, we’ll see chondrites, such the Ivuna, that contain no chondrules. In the simplest of terms, chondrules are composed of olivine and/or pyroxene, occasionally glass and a smattering of feldspar. In not so simple terms, chondrules are a hot mess of textures and compositions- messy enough that I’m not going to cover it in this post, but I did delve into it a bit in this older Meteorite Monday post about our enigmatic friends.

Meteorite Monday: The Antarctic Search for Meteorites

Today’s Meteorite Monday will be a short one. I’m in the thick of finishing my senior project about the meteorite I’ve been working on for over a year and a half. Today I wanted to touch briefly on the NSF funded Antarctic Search for Meteorites (ANSMET). This is one of the most productive, publicly funded ventures in the hunt for meteorites. In its 37 years of existence, the scientists at ANSMET have recovered nearly 20,000 meteorites from the cold, barren ice-fields of the Antarctic. This continent is perfect for the recovery of meteorites because it’s dry, devoid of vegetation and sees little in the way of sediment build-up. All this leads to conditions where meteorites, black from their fusion crust, can easily be recognized against the white backdrop of the slow-moving ice.

One of the ~400 meteorites recovered from Antarctica during the 2012-2013 season. (Image from Planetary Science Research Division at the University of Hawai’i- Manoa)

Some famous meteorite alumni from past ANSMET expeditions include the first two lunar meteorites Yamato 791197 and Alan Hills 81005. The martian meteorite, Alan Hills 84001, was also recovered from Antarctica. This is the martian meteorite that was thought to have fossilized martian microbes. The first two digits of the number is the year that the meteorite was found, and generally speaking, the name, i.e., Alan Hills, is the region where it was found.

Once these meteorites are found, they’re sent to Johnson Space Center where they’re classified and made available for study by qualified meteoriticists and planetary science researchers.

Here’s a cool YouTube video showing how the work is done:

Meteorite Monday: So you think you have a meteorite, part 2.

Part of my responsibilities at the meteorite lab is to handle the e-mails we receive concerning meteorite inquiries. These come in on a nearly daily basis and I can easily receive dozens of requests in a week. I don’t mind answering the e-mails as I consider it an integral part of the outreach that we do as a lab. And if I can teach people about what to look for in a meteorite (or meteorwrong in most cases), then all the better. Generally speaking people are pretty good at following our guidelines for e-mailing us: namely, send us a small amount of high resolution photos. We prefer quality over quantity. As I ranted earlier on Twitter, I occasionally receive e-mails with 25+ photos and it’s a temptation to just delete them and move on.

The sad truth is that the vast majority of people that contact me don’t have meteorites. I would love nothing more than to say “yes you have a meteorite!” to everyone that contacts me. Unfortunately, reality says that earth rocks are far more common than space rocks, but with the added challenge that they all look nearly the same. Take for example this one:

Possible meteorite?

Either it has a fusion crust or it’s a weathering patina. And those indentions are either reglamglypts or weathering features. This is one of the few e-mails I’ve received where I had to stop and examine the picture more closely. Generally speaking I get these sort of pictures:

Definite “meteorwrong”

This rock has a shape that is indicative of being in a fluvial setting. Meteorites are not round and, as a general rule, do not contain vesicles. More importantly, meteorites don’t survive long in a fluvial setting and erode away faster than your typical river sediment.

Some of the worst offenders are slag. This is the byproduct of metal production and is routinely confused for being an iron meteorite. Sometimes slag isn’t even magnetic which just completely rules it out as a meteorite.

A faux-iron meteorite.

My favorite meteorwrongs are scoria. I don’t even need to look at the picture for long to know that it’s not a meteorite.

No. No. and No.

Could I be wrong in my visual assessments? Of course. Identifying meteorites via photo alone is like trying to distinguish between chocolate chip cookies and raisin cookies at a distance. It all looks the same until you’ve bitten into it and was sorely disappointed by the presence of raisins instead of chocolate. Yes, I have trust issues from such experiences, but that’s besides the point. I may not know with certainty what type of rock you have, but I know just enough to determine if it’s from space or not. And if I happen to be wrong, then all the better!

*for part 1, click here*

Meteorite Monday: Yes, the many gods of men do indeed come from space

The Hindu God, Kubaras.

I’ve always maintained that if someone wants to know the solar system, study meteorites. They are literally the left over building blocks of our solar system and give us an unparalleled insight into the solar nebula from which our planet and the others formed. We can use mathematical models to hypothesize the accretionary process, but the chemical composition of meteorites and chondrules help us to constrain the results.

Meteorites become even more impressive when we look at how they’ve influenced  the human species over the course of our relatively short history. Meteorites were often considered messengers from whatever gods were worshiped by people in that area. Some stony meteorites were carved into religious figures, while the iron meteorites were turned into jewelry or even the first metal hunting weapons. Regardless of the civilization, from Egyptian to North American Indian tribes, meteorites served to connect people to their deities.

A new paper in the September 2012 Meteoritics and Planetary Science, Buddha from space, further explores this interesting topic by looking at one religious figure in particular, the Hindu god Kubera (or Vaisravana in Buddhism). This god is considered the Lord of wealth and North-direction and can be found on many statues. This article from MAPS looks at one that is possibly carved out of the iron meteorite, Chinga.

A 700 g. chunk of the Chinga meteorite. (Image from Wikipedia)

The Chinga meteorite was discovered around 1913 in the Tana-Tuva region of Mongolia and Siberia. Researchers have used glacial depositional features to estimate that Chinga landed in the region approximately 20,000 years ago. Based on the best known age of the swastika, the statue itself is probably around 3,000 years old.  In 1938 it was recovered by Ernst Shafer while on an expedition for the German National Socialist Party.

This statue is unique for a couple reasons: 1) it’s an ataxite. This type of iron meteorite is composed of at least 16%-30% nickel, and as such, doesn’t display that beautiful cross-hatched widmanstatten pattern for which iron meteorites are known. They also are among the rarest of iron’s and don’t fit easily into a classification scheme. 2) It’s thought by historians that this little figure is what gave rise to the swastika symbol used by the Nazi’s.

Based on their chemical analysis, Buchner et al., were able to conclude that the statue was made from an ataxite. However, the concentrations of certain elements puts it into an ungrouped category (of which Chinga is part of). When classifying irons, meteoriticists look for the weight percent of Fe, Ni, Co, Cr, Ga, and Ge. It’s that quantity that allows for an iron meteorite to be grouped with others, or even allows us to determine if it belongs in its own group. The researchers do report some problems with the Ga and Ge readings, but they attribute this to the very small sample they had to work with from the statue.

 

 

 

Meteorite Monday: The Middle Ordovician Meteor Shower

No, I didn’t make that name up nor is it the name of some 50’s sci-fi show. The name refers to a time period from nearly 460 million years ago when the earth did look quite alien. It was a time when the continents as we know them weren’t in their present positions and life was dominated by trilobites, snails, clams and other seafaring creatures. The Ordovician occurred right on the heels of the Cambrian period when life first started to make it’s appearance in the fossil record.

The position of the continents during the Ordovician.

At around the same time, another important event was occurring in the solar system: the break-up of the L-chondrite parent body. An L-chondrite is a type of stony meteorite that is relatively common and has a low abundance of iron. Most of the L chondrites are heavily shocked and share other chemical commonalities. This has led researchers to hypothesize that L chondrites came from the same source. With this break up of the parent body came an influx of meteorites that would impact the earth; some of which would land in shallow water among the sea creatures of the Ordovician. Those creatures would die off and become the limestone layers in which those meteorites would be encased.

A fossil meteorite and nautiloid encased in limestone. (Image taken from Hawaii Institute of Geophysics and Planetology. Originally found in Schmitz et al., EPSL 194. 2001. P. 4)

This particular image came from a group of researchers that examined a limestone outcrop from a quarry in Kinnekulle, Sweden. They analyzed the distribution and size of meteorites in multiple layers of limestone in order to constrain the influx rate of the meteorites from the parent body break-up. In total, the researchers retrieved 40 meteorites from about 3 meters limestone, or about 10 feet.

What’s really interesting is that not all layers yielded an equal amount of meteorites, nor were they of equal size. Some layers only had one, while another had six. One layer, the Arkeologen, put out 26 meteorites! What this tells us is that we had multiple falls from possibly the same event. And keep in mind, these fossil meteorites were found in a relatively small area. Of the known 3300 m^2 of the Arkeologen,only about 2700 m^2 of it has been searched for meteorites.  Stratigraphically, we can assume that more meteorites could be recovered from the formations across a broader area. They were able to use this information to determine that there was an increase in meteorites delivered to the earth at that time.

One of the difficulties with pairing meteorites from this location is the extreme calcification that occurs from being trapped in limestone. Meteorites tend to crack and weather easily on the earth. This can cause internal alterations, such as oxidation, that chemically alter the meteorites. However, Schmitz et al., were able to use relict chromite grains as a way to possibly link them to the same parent body.

 

Sources:

• A rain of chondritic meteorites in the early Ordovician. Schmitz, Birger., Tassinari, Mario., Peucker-Ehrenbrink, Bernhard. Earth and Planetary Science Letters. Vol. 194. Issue 1-2. P. 1-15. DOI-  http://dx.doi.org/10.1016/S0012-821X(01)00559-3
• The Ordovician Period– University of California Museum of Paleontology.

Meteorite Monday: A new edition to my family of space rocks

Today’s Meteorite Monday is a special one to me for two reasons: 1) I get to formally introduce my brand new meteorite and 2) this is the first time I’m updating my blog from WordPress’s mobile app. I love technology!

I posted pictures of my new jewel on Facebook, Twitter and G+ already, but I thought it appropriate to share it on the blog, too. Especially since I haven’t updated in a while.

Here she is, Northwest Africa 7109:

This gorgeous specimen was purchased at our fund raiser on Saturday. It was actually bought by a good friend of mine, Dave, and he gave it to me because I was smitten by it when I first saw it. Not only did I get a new meteorite, but the lab got a portion of the sale to apply to our research. So I wanna give a big thanks to Dave for that! Also, big thanks to those that either donated directly to the lab or purchased meteorites. Your contributions are greatly appreciated and further the exciting science we do in our lab.

The particular meteorite I was gifted is an L5. The L refers to a low metal content and the 5 refers to a relatively high degree of thermal metamorphism. In spite of that, some chondrules are still visible. That nice big broken one in the center is a good example. The piece is pretty well weathered as can be seen from it’s rust colored appearance. I believe the dark colored patches are the result of shock blackening, but that’s just a guess.

I fell in love with this sample because it’s a great teaching resource. It contains all the hallmarks of a meteorite: metal flecks, some shock features and, most importantly, chondrules. It’s the type of meteorite that I’m not afraid to break out of it’s case and let people touch and examine. And that’s incredibly important when getting people excited about meteoritics and science in general.

Meteorite Monday: The Link Fest Edition

I feel kinda cheap for making the first Meteorite Monday in nearly a month a relatively unoriginal list of links. I wanted to pick up where I left off with the last post about carbonaceous chondrites, but my own work in the meteorite lab is in need of attention. So, here’s a few of my favorite stories floating their way around the interwebz:

• Would This Meteor Make a Huge Crater?

Technically this would be a meteorite since it was found on the earth. Wired physics blogger, Rhett Allain, uses physics to look at the validity of a supposed meteorite hitting a car in commercial for a science channel in the UK.

• Novel Mineral Found in 1969 Meteorite “Allende” from Mexico

Yet another new mineral found in meteorite. The first was a titanium sulfide called Wassonite and this one, Panguite, is a titanium oxide. Interestingly enough, both come from relatively rare meteorites: Wassonite was discovered in an enstatite chondrite (highly reduced chemically) and Panguite is from Allende which isn’t nearly as chemically altered.

• World’s Oldest, Largest Impact Crater

This may be one of the more significant finds in planetary sciences. This supposed impact crater is estimated to be about 3 billion years old. That would make it the oldest crater found on the earth thus far. It’s not completely conclusive, but the science looks solid and it’ll be interesting to watch it unfold as other researchers examine the evidence. This is significant because craters of that age are generally only found on the Moon or Mars. Earth’s a bit too hostile towards such things due to erosion and plate tectonics. Here’s a link to the abstract as it appears in Earth and Planetary Science Letters.

Meteorite Monday: Carbonaceous Chondrites Revisited

A chunk of the Murchison Meteorite. This carbonaceous chondrite is probably one of the most studied rocks in science. (Image from Northern Arizona University)

A little over a year ago I started the Meteorite Monday series with this post about carbonaceous chondrites. Interest in these primitive space rocks exploded with the fireball that produced the Sutter’s Mill meteorite in California. This meteorite is the newest carbonaceous chondrite to be found and it’s generating a lot of excitement. So, with everything I’ve learned (and the realization of how little I actually know) I’ve decided to revisit the topic and expand on some of what I wrote. However, I’m going to handle this post a little differently. Since there’s a lot to be said on these rocks, I’m going to break this up into at least two separate Meteorite Monday posts. I say two because I’ll probably forget something and want to cover it later. If I try to cover everything in one post, things will get messy and I don’t want that.

To start, let’s get a basic understanding of a carbonaceous chondrite. These are what we call a stony meteorite as opposed to an iron or stony-iron meteorite. They are the meteorites we turn to when we want to learn about the conditions of the solar system at its inception. That’s because if we were to strip the sun down of its atmophile elements, such as nitrogen, helium, and hydrogen, then we’d have a chemical abundance that is also found in some of the carbonaceous chondrites. It’s like being able to study a blank canvas before the paint goes on it. In fact, there is one group of meteorites called the Ivuna-type (or CI) that is used as the “blank canvas” or “standard”. When we want to understand the evolution of a meteorite and it’s parent body, we plot it’s element constituents against that of the CI type meteorites (1). This allows us to look at the concentration of elements and get an idea of its thermal history.

A bulk composition graph showing element abundances at certain temperatures. (Image from David Mittlefehldt at the PSRD- University of Hawaii)

This graph is showing us the bulk abundance of three groups of elements at temperatures present in the solar nebula. The lithophile elements are those that go into silicates, or the rocky parts of the meteorite. The siderophile elements are found in iron metals and the chalcophile are found in sulfide minerals (2). On the Y-axis we see the numbers range from .1 to 10 with the 1 line being our blank slate, so to speak. At 1 is the composition of the Ivuna-type meteorite. Generally, any element that falls below that line is considered depleted, and above that line is enriched. Carbonaceous chondrites plot at either 1 or above that line when looking at the abundance of refractory-lithophile elements. These are the elements that condensed first out of the solar nebula at temperatures around 1500-1800 K. These elements then formed minerals such as corundum (Al2O3), melilite, and perovskite.

These minerals are all condensed in one of the hallmark physical features of most carbonaceous chondrites, calcium-aluminum inclusions (CAI’s).

Large prominent CAI’s in the cut face of the Allende meteorite. (Image from NASA) (3)

Another prominent feature in carbonaceous chondrites are chondrules. Carbonaceous chondrites have seen little in the way of thermal metamorphism and this has left the chondrules with distinct boundaries and rims. Thermal alteration degrades and destroys chondrules at progressively higher temperatures.

A thin section of a carbonaceous chondrite. The spherical inclusions are the chondrules. (Image taken by author)

The chondrules formed after the CAI’s in the solar nebula and have a different chemical composition than the CAI’s. Since this is an overview, I don’t want to delve too much into the chemical properties of these two inclusions. Instead I’ll save those for a later post.

Resources:

1. Weisberg, Michael. McCoy, Timothy. Krot, Alexander. Systematics and Evaluation of Meteorite Classification. 
2. Mittlefehldt, David. Tagish Lake- A Meteorite from the Far Reaches of the Asteroid Belt. December 12, 2002.
3. Taylor, Jeffrey G. Solar System Exploration: Origins of the Earth and Moon.

Meteorite Monday: Comets

Comet Wild 2 from the Stardust Mission (Image from JPL. Taken by Shigemi Numazawa).

I know some of you may be wondering why I’m writing about comets for a Meteorite Monday post. The answer is that comets and meteorites tell us about the formation of the solar system. Meteorites come from asteroids that record the history of the inner solar system and comets tell us about the evolution of the outer solar system. And I know next to nothing about comets, so blogging about it is a great way to diminish some of my ignorance.

What exactly is a comet? A common description is that they are galactic snowballs with rocky material incorporated into the nuclei  (body). Conversely, some comets can be described as mostly rocky with some form of frozen gas and water mixed into the nuclei of the comet. Regardless of the description you can see the common thread: a small solar system body composed of varying amounts of ice, water and rocky material. At one point it was thought that comets and asteroids had unique mineralogies, but samples returned by  NASA’s Stardust mission to Comet Wild 2 tell us that both objects may have more in common than previously thought (1).

Comets have three distinct physical characteristics:

1. Nuclei- This is the body of the comet. This is where you find the varying quantities of frozen gases, rock and water. Due to a comets small size, it doesn’t exert enough gravity to take on a spherical size in the same manner as a planet or a star. As a consequence they tend to take on irregular shapes.
2. Coma- The coma is the thin, tenuous atmosphere that develops around a comet as it gets closer to the sun. Bombardment by solar radiation causes the volatile elements within the nuclei to become vaporized and thus released from the body. This is what gives the comet that fuzzy halo look that we see from the earth.
3. Tail- The tail forms as the solar wind pushes against the coma and strips away the volatiles.

Comets are classified according to the eccentricity of their orbit. This simply refers to how elongated their orbit is in relation to the sun. A short-period comet, such as Halley’s Comet, have an orbit that roughly follows the galactic plane and orbits the sun every 200 years or less. Long-period comets have a highly elliptical orbit that all but removes it from the solar system. For example, Comet West, at its furthest from the Sun can apparently get up to 70,000 AU (2). I say apparently because the only good source of information I found on this subject is in Wikipedia. So, take it with of a grain of salt.

Comet McNaught. This is an example of another long-period comet. (Image from Wikipedia. Taken by Fir0002/flagstaffotos)

Research:

• (1) Comparison of Comet 81P/Wild 2 Dust With Interplanetary Dust From Comets. Science 25, 2008. Vol. 319 no. 5862 pp. 447-450 DOI: 10.1126/science.1150683
• (2) Comet entry from Wikipedia

Meteorite Monday: Allende!

A 520 gram (~18 ounce) piece of the Allende meteorite. (Image from Wikipedia)

All right everyone, say it with me: Ayendeh. The name of this beauty is Spanish so those double L’s are pronounced with a “Y” and not a hard “L”. Think of it like the word tortilla.

Now that we have the pronunciation out of the way, let’s get to the good stuff!

Allende is recognized as one of the most well studied meteorites in the field of meteoritics. It’s a stony meteorite of the carbonaceous chondrite family and is full of chondrules, calcium-aluminium inclusions (CAI’s) and even contains pre-solar grains. In short, it’s a wet-dream come true for those that get their kicks out of deciphering the nursery years of our solar system and the conditions that persisted at that time.

The area within the dotted lines represents the Allende strewn field. It's basically the rubble pile left over from Allende breaking apart as it came through the atmosphere. (Image from Encyclopedia of Meteorites)

Allende came crashing to the earth in February of 1969 in the small Mexican village of Pueblito de Allende. Its fall is significant in that 2 metric tons of the stuff have been recovered from the strewn field. Carbonaceous chondrites such as Allende represent about 5% of the current meteorite finds, so finding 2 metric tons gives scientists a relatively large amount of material to play with.. er, study.

The cut side of the Allende meteorite. The white, irregular shaped ("fluffy") inclusions are CAI's. The round inclusions are chondrules. (Image from the Encyclopedia of Meteorites. Taken by Rene Schmit.)

One of the most notable features in Allende are these large, bright inclusions called CAI’s. These are believed to be one of the first products to condense out of the solar nebula and have been dated to form about 1-5 million years after the formation of the solar system. CAI’s are composed of calcium and aluminium rich minerals such as melilite, corundum and perovskite. These elements condensed out of the solar nebula at temperatures around 1600 K

And then there are the presolar grains. As you’ve probably guessed, these grains formed before our solar system and contain isotopic anomalies that attest to their extrasolar origins. Such grains include nanodiamonds, graphite, corondum, silica carbide, and silica nitride. Some of these grains, such as the nanodiamonds only form at high pressures produced by supernovas from red giant stars (1).

 In terms of metamorphism, Allende is relatively unprocessed. It falls under the designation of CV3 which means that it’s chemically related to the Vigarano chondrite (CV) and the 3 means that it’s relatively unaffected by aqueous alteration. Its chondrules still contain glass and water hasn’t moved minerals from the chondrules to the matrix (or body) of the meteorite (Meteoritical Society Database). Don’t stress too much over those details though. That’s something I plan on covering in later posts.

1. Zinner, Ernst. Stellar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites. Annu. Rev. Earth Planet Sci. 1998.