Day 13: Meteorite Collection
by Lori Dajose
Today, Mike Brown’s affinity for throwing objects at late students inspired an inordinate amount of anxiety—because today, he was holding clumps of dense iron meteorites. (#tenure)
We’ve been learning about small bodies in the solar system, so today we got to handle them in the rocky flesh. Mike began the class by passing around three black rocks: a small rounded one with a thin crust (“The Chunk”), a larger flattened and smoothed piece (“The Covert”), and a sharply angular piece (forgot the name on this one).
“Touch it!” Mike urged as he practically shoved the rocks at us, “It’s a piece of the early solar system!”
Indeed, these meteorites are truly a glimpse back in time to the baby solar system. As the solar nebula’s enormous protoplanetary disk began to cool, the first things to solidify out of the mix were calcium and aluminum. They formed into little white blobs, which are now seen as inclusions embedded into the matrix of dark meteorite. Later, more molten droplets of rocky material solidified. These are called “chondrules,” and the entire rock then is called a “chondrite.”
As we passed the rocks around, I couldn’t help but be surprised at their weight, even though I knew they were primarily composed of dense iron. However, Mike Brown wasn’t too impressed with the density, because as it turns out, he owns something several times heavier: a solid gold medal. While I can’t speak for the other students, this immediately induced in me an image of our dear professor rockin’ some serious solar system bling, conducting a drive-by shooting of Pluto.
But, back on topic. My favorite rock of the day was a pallasite: a silvery, topazy matrix that has never been recreated in a lab, because we can’t reproduce the kind of timescales these were formed over. It was incredibly beautiful, and had complex angular patterns called Widmanstatten patterns. These are an interweaving of two nickel-iron alloys, taenite and kamacite. The difference between these alloys is the ratio of nickel to iron. And now for some geological etymology: At temperatures lower than 850 ºC and low nickel content, the alloy is called kamacite. Above 850 ºC, for any amount of nickel, it’s considered taenite. Any alloy with a substantial amount of nickel that is less than 850 ºC is considered a combination of the two. So now, you know a bit of geology trivia. I’d draw a picture, but my Microsoft Paint skills are not that advanced.
The discussion on meteorite compositions lasted for a good hour, and then we broke off individually to work on some questions regarding asteroids and solar system formation. And luckily, not a single meteorite was thrown at a student in this class period.
Today we looked at meteorites!
1) This is an Allende meteorite. The black exterior is the fusion crust. The interior is made up of a grey matrix with white spots made of aluminum and calcium. There are some chondrites visible upon closer inspection. This is a C-type asteroid likely from the middle of the asteroid belt (lecture 3.12).
2) This is an iron-nickel chunk from Meteor Crater in Arizona. More metal meteorites are found than rocky ones because more metal ones make it through our thick atmosphere (also, they're probably much easier to find!).
3) Here's a really awesome chondrite. You can very clearly see the chondrules in this picture, but there are also reflective metallic bits throughout. It has the density of a typical rock.
4) Weird! Looks like a chunk of metal, right? BUT IT'S NOT! When you lift it, it's very light compared to it's size, with an estimated density of 3-4 g/cc. This is actually a breccia (accreted fragments that smushed together). You can see the compositional variations too. (No photo, sorry!)
5) A slab of an iron-nickel alloy. There are two phases here: kamacite and taenite. Intersecting lamallae of kamacite and taenite create a Widmanstatten pattern. To form kamacite we need VERY slow cooling (not possible to achieve experimentally). To form both kamacite and taenite, we need enough nickel (or else only kamacite is formed), as seen in the phase diagram.
6) AND THE BEST FOR LAST: An incredibly beautiful pallasite. These are olivine crystals (aka peridot for gem-lovers) in an iron-nickel matrix. This is absolutely amazing – it's likely from the core-mantle boundary of a differentiated body that broke up into pieces. It's core stuff mixed with mantle stuff!
by Jiabin Liu
Today we looked at Mike Brown’s collection of meteorites samples in person. They are of different types and have different features that can be examined by just looking and touching. Here are a few that we looked at:
• Canyon Diablo: this is a fragment of the asteroid that hit Arizona and created the Barringer Crater. It is a big chunk of iron meteorite and is relatively heavy and shiny.
• Carbonaceous chondrite (C-type): this is a small piece of rock-lite chondrite and has a density of ~3g/cc. This type includes the most primitive meteorites in the solar system.
• Pallasite: we saw a big slice of pallicite that came from Kansas. This is a stony-iron like piece and shows crystal patterns on the surface.
• Kamacite: a type of meteorite with nickel and iron. It shows patterns interweaving iron and nickel, and cannot be manufactured on Earth, because they had to experience slow cooling over a long period of time in order to form their distinctive patterns.
• Taenite: also an alloy of nickel and iron. It shows a Widmanstatten pattern with fine lines. It is notable that larger patterns mean the cooling was more slowly in its history.
Above is a phase diagram of nickel-iron meteorites. Meteorites are composed of 90% iron and 10% nickel at first, and as they cool, they first form taenites. As they further cool down, kamacites start to form when nickel diffuses. Slow cooling could mean that the parent body has at least a significant size of a few kilometers.
As the second part of the class, we answered a few questions.
1. Events between the start of solar system to the formation of Earth:
• Supernova disturbance
• Collapse of solar nebula
• Protostar formation
• CAI formation
• Chondrules formation
• Planetesimal formation
• Oligarchs formation
• Jupiter formation
• Saturn formation
2. Why are there asteroids in the asteroid belt instead of planets?
• Jupiter scatters these small bodies to large inclinations and eccentricities, so they would shatter when they collide (exactly what we did for homework last week).
• Even before they collide, these small bodies cannot go through runaway growth because their gravitational focusing is already disrupted.
by Junjie Yu
Today, Professor Mike Brown brought his collections of exquisite meteorites. The meteorites can be classified as four categories: chondrites, achondrites, stony-iron meteorites, and iron meteorites. Chondrites constitute 86% of the meteorites now falling to Earth, and represent the overall composition of the solar system. However, an iron meteorite is just a chunk of iron. You may wonder how the iron is isolated out. Actually, the iron is formed by melting with planetesimals and by differentiation between the mantle and core.
Seven percent of meteorites falling to Earth are iron meteorites, composed mostly of iron-nickel alloys. They usually consist of two mineral phases, kamacite with lower Ni-content (5 to 15% Ni) and taenite with high Ni (up to 50%). The internal structure of an iron meteorite would be difficult to tell in the presence of dull patina caused by oxidization. But you can cut off a slice, polish and etch it with nitric acid, because kamacite is less resistant to the acid than taenite. Consequently, it will reveal a spectacular crystalline pattern named Widmanstätten pattern (Figure 1).
Figure 1. Widmanstätten pattern of a Gibeon iron meteorite, found in Great Nama Land, Namibia (Picture by Heritage Auctions).
You may have seen monster crystals of feldspar in the granite which has undergone a long cooling time. The Widmanstätten pattern is formed in a similar way. First meteoric iron is exclusively composed of taenite. When cooling off, it passes a phase boundary where kamacite is exsolved from taenite (Figure 2). The formation of intergrowths of the two mineral phases can only take place during very slow cooling, about 100 to 10,000 °C/Myr, with total cooling times of 10 Myr or less (Goldstein et al., 2009). This explains why this structure cannot be reproduced in the laboratory.
Figure 2. Phase diagram explaining how the Widmanstätten pattern forms (From Wikipedia).
Day 14: Ditch Day
by Valerie Pietrasz
Instead of going to class today, I was rudely awoken at 8 a.m. by the sound of seniors running around, banging pots and pans on everyone's doors and screaming to wake up for ditch day.
At Caltech, ditch day is school-wide event in which all classes are cancelled, homework is postponed, and learning is set aside for the sake of fun. Seniors spend the entire year planning ditch day "stacks", which are sequences of competitions, puzzles, and activities for the underclassmen to participate in instead of studying on ditch day. The idea behind ditch day is that seniors leave campus to go have fun somewhere—but that makes everyone else jealous, so they leave behind these stacks to keep underclassman busy while they're gone.
Each stack is themed: there were Harry Potter, Iron Chef, Despicable Me, Nintendo 64, Spongebob, and Hunger Games stacks, to name a few.
My stack was Game of Thrones themed. My group and I, House Targaryen, ran all around campus, slaying monsters, capturing dragons, and most importantly, defending our house's honor. We participated against other Houses (Stark, Lannister, and Baratheon for those who read the books or watch the show) in competitions of strength, speed and wit—including pool noodle jousting, potato slingshotting, giant jenga, rappelling down a building, and more. For each competition, we had the opportunity to win gold coins to be used at the great House showdown at the end of the day.
By dinnertime, we were exhausted, having collected all the coins we could, but we had one competition left: dodgeball against the other houses. For each coin we had, we got an extra life, giving the team with the most coins a huge advantage. The winner of this competition would claim the throne and the prize at the end. After an epic battle, House Baratheon emerged victorious (and my house emerged dead last). However, for competing so well, we all got a share of the spoils—an extravagant seven-course meal.
Needless to say, we ended the day by filling our empty stomachs with food, sharing war stories and eating as friends rather than foes.