Astro Bob: Chondrules – solar system window
This is an artist’s impression of the young solar system with the newly formed sun in the center, surrounded by rings of dusty debris that have fused together by gravity to form planets, moons and asteroids. (NASA)
Almost five billion years ago, a local cloud of these releases began to collapse under its own gravity to form the sun and the solar system. During this process, pieces of iron, rock and ice collided with each other and stuck together to form larger pieces. These aggregated by gravity into asteroid-like bodies called planetesimals and eventually into planets.
These are a selection of chondrules from the meteorite NWA 5929. When melted, they formed in one or more flash-warming episodes caused by shock waves, lightning, or planetesimal collisions. They are generally spherical and measure about a millimeter in diameter. The scale cube in the background measures 10 millimeters (just under 1/2 inch) across. (Tomasz Jakubowski)
Meteorites record this ancient story in their minerals and textures. Eighty percent of known meteorites are stony rocks called chondrites (KON-drites). The name comes from the chondres (KON-drools), which are small, round, rocky nuggets that eerily resemble those crunchy pebbles and candy in a box of the nerds. Imagine the soft spheres cemented together in a rock, and you have a Nerd meteorite.
In this slice of meteorite NWA 7572, the small chondrules boldly stand out from the dark matrix of mineral grains. “NWA” stands for North West Africa. Meteorite hunters found the space rock in Northwest Africa in the Sahara Desert in 2012. (Bob King)
If you open up a real chondrite, the cut face will reveal a myriad of chondruses – a beautiful sight to behold. Grains of pure iron-nickel, fragments of chondrules and grains of rock grains fill the interstices between the small celestial BBs. Chondrules are usually around 1mm in diameter but range from 0.1mm to 10mm (0.4 inch).
When a meteorite is sliced to a thickness of only 0.03 millimeters, it becomes translucent to light. The “thin section” is glued to a slide so that it can be studied under a microscope. This slim section of NWA 4560 features a giant measuring 5mm (1/4 inch) in diameter. (King Bob)
If you cut a meteorite thin enough, it becomes translucent in light. The spherical shapes and crystallized textures of the chondrules – best seen when viewed as “thin sections” under a microscope – indicate that they were once completely melted. Something briefly heated the mineral and metal laden dust in the vacuum of space to over 2,200 ° F (1,200 ° C), causing it to congeal into tiny spheroids of simmering liquid before crystallizing into rock solid. If their origin still remains an enigma, two leading theories offer plausible scenarios.
The solar system started out as a cloud of gas and dust called the solar nebula. Something, possibly a nearby supernova explosion, gave it the push to collapse under its own gravity. Most of the cloud’s mass gathered in the center to form the sun, with the rest being in disks that formed the planets. When the cloud collapsed, it also spun faster, causing it to flatten out. (NASA)
In the first and most accepted view, clusters of dust (essentially, dust bunnies!) The shock waves generated during the formation of the giant planets may also have shaken the nebula and compressed and heated the primordial dust. Either way, it would make chondrules the first truly solid matter to form in the solar system – something you could hold between your fingers. Using Radioactive isotopes, scientists have dated their origin to 4.56 billion years ago.
Abu Panu’s chondrite fell in Nigeria on April 19, 2018. It is rich in chondrules of all sizes and has shiny specks of fresh nickel-iron metal on the cut surface. (King Bob)
Or perhaps chondrules formed more violently, when molten planetesimals crashed into each other, a common occurrence in the chaotic youth of the solar system. Jets of molten rock from the impacts spattered through space and froze into tiny rocky spheres. Gravity later gathered them into chondrous-rich planetesimals, some of which no doubt helped build the planets. Chondrules are mainly composed of silicate minerals olivine and pyroxene (pie-ROX-een), rocks that are common in the Earth’s upper mantle. A variety of gemstone olivine called peridot is popular in jewelry.
You can contemplate their long and still mysterious history by flipping a chondrite by hand. But to see the chondrules in their entirety, we’ll look at them in a thin section under a basic stereomicroscope with a pair of polarizing filters, like the ones photographers use to get rid of unwanted reflections in photos.
Left: Like this block of calcite, many minerals in meteorites double refract light, dividing it into two separate beams. Here, the calcite shows a single straight line in duplicate. Right: This is my simple setup for taking photos of thin sections in polarized light. I glue one polarizer to the objective (top) and place the slide on the other polarizer (bottom). (King Bob)
When light passes through a filter and then through the meteorite slice, the crystal structure of the rock splits it into two beams. This “double refraction” is called birefringence. Many minerals are birefringent, including familiar minerals like calcite. When the beams then pass through a second polarizer, the light waves in each of them “interfere” and reinforce or cancel each other out. This attenuates or cancels out some colors while making others more intense.
There are many different types of chondrules, but these are the eight basic types seen under polarized light. (Antonio Ciccolella / CC BY-SA 4.0)
Seeing chondruses through dual polarizers is an aesthetic experience akin to looking at the magnificent stained glass we see in old church windows. It also has a practical side. Scientists identify the minerals that make up chondrules by the way they respond to polarized light. For example, pyroxene is generally gray, while olivine crystals glow in pink, blue, and green. I have photographed several of my favorite chondres to share with you below:
This colored porphyritic olivine chondr has many large olivine crystals. “Porphyry” refers to porphyry, a type of igneous rock with crystals. (King Bob)
The thin, tower-shaped crystals in this radial pyroxene chondring spread out from different sides of the chondrium. (King Bob)
Here is a close-up of the large olivine-pyroxene chondrion from the NWA 4560 thin slide (above). (King Bob)
I call it angry old man’s chondrule. It is also a polysomatic olivine variety with olivine towers separated by dark glass. (King Bob)
Stained glass windows in churches often depict sacred religious scenes or divine figures from the distant past. I find the sacred chondras in a similar sense as they take us back to our origin as stardust.
“Astro” Bob King is a freelance writer for the Duluth News Tribune. Read more of his work on duluthnewstribune.com/astrobob.