You and I got here by way of the stars. They fused simple elements like hydrogen and helium in their cores into more complicated ones like carbon, oxygen, nitrogen, phosphorus and iron crucial for life. Exploding violently as supernovae or releasing these materials in more measured ways, stars seeded space with enough raw material to create new generations of suns and planets.
Nearly five billion years ago, a local cloud of these cast-offs began to collapse under its own self-gravity to form the sun and solar system. During that process, bits of iron, rock and ice collided with each other and stuck together to build larger chunks. These aggregated through gravity into asteroid-like bodies called planetesimals and ultimately into the planets.
Meteorites record this early history in their minerals and textures. Eighty-percent of known meteorites are stony rocks called chondrites (KON-drites). The name comes from chondrules (KON-drools), which are small, round, rocky nuggets that bear an uncanny resemblance to those crunchy, candy pebbles in a box of Nerds. Imagine the sweet spheres cemented together into a rock, and you've got a Nerd meteorite.
If you slice open a real chondrite the cut face will reveal a myriad of chondrules — a beautiful sight to see. Specks of pure iron-nickel, chondrule fragments and grains of rocky grains fill the gaps between the little celestial BBs. Chondrules are typically about 1 mm across but range from 0.1 mm up to 10 mm (0.4 inches).
If you slice a meteorite thin enough it becomes translucent to light. Chondrules' spherical shapes and crystallized textures — best seen when examined as "thin sections" under a microscope — indicate they were once completely molten. Something briefly heated mineral-and-metal-laden dust in the vacuum of space to more than 2,200° F (1,200° C), causing it to congeal into minute spheroids of quivering liquid before crystallizing into solid rock. While their origin still remains an enigma, two leading theories offer plausible scenarios.
In the first and more accepted view, clumps of the dust (basically, dust bunnies!) in the solar nebula were flash-heated by erratic outbursts from the early sun. Shock waves spawned during the formation of the giant planets may also have rippled across 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 it in your fingers. Using radioactive isotopes, scientists have dated their origin to 4.56 billion years ago.
Or chondrules may have formed more violently, when molten planetesimals smashed into each other, a common occurrence in the solar system's chaotic youth. Sprays of molten rock from the impacts splashed into space and congealed into tiny, rocky spheres. Later, gravity gathered them into chondrule-rich planetesimals, some of which undoubtedly helped build the planets. Chondrules are composed mostly of the silicate minerals olivine and pyroxene (pie-ROX-een), rocks that are common in the Earth's upper mantle. A gem variety of olivine called peridot is popular in jewelry.
You can contemplate their long and still-mysterious history by turning over a chondrite in your hand. But to see chondrules in their full regalia we'll examine them in thin section under a basic stereo microscope with a pair of polarizing filters, like the ones photographers use to get rid of unwanted reflections in photos.
When light passes through one filter and then through the meteorite slice, the rock's crystal structure splits it into two beams. This "double refraction" is called birefringence. Many minerals are birefringent including familiar ones like calcite. When the beams then pass through a second polarizer, the light waves in each of them "interfere" and either reinforce or cancel each other. This subdues or cancels some colors while making others more intense.
Viewing chondrules through dual polarizers is an aesthetic experience akin to looking at the gorgeous stained glass we see in old church windows. It has a practical side, too. Scientists identify the minerals that comprise chondrules by how they respond to polarized light. For example, pyroxene is typically gray, while olivine crystals glow pink, blue and green. I photographed several of my favorite chondrules to share with you below:
Church windows often depict sacred religious scenes or divine characters of the distant past. I find chondrules sacred in a similar sense because 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 at duluthnewstribune.com/astrobob.