Hunting 4.5 billion year old Easter eggs

A basketful of tiny, rocky eggs is on display in this slice of Northwest Africa 5205, a chondrite meteorite found in the Sahara Desert in 2006. The individual chondrules are from 1-4 mm across. Photo: Bob King

There’s a picture in the newspaper today of a girl placing an Easter egg she found on a hunt in her basket. Astronomers like to go Easter egg hunting too. They slice through meteorites found in Antarctica and the deserts of Africa and Oman to study the little eggs and spheres inside called chondrules (KON-drools). These crystalline grains, which range in size from less than a millimeter to about a half inch, represent the oldest solid material in the solar system. They’re the chief ingredient in chondrites or stony meteorites, the most common type known.

The semi-precious peridot is a gem variety of the common Earth and meteorite mineral olivine.

Chondrules are composed mostly of the silicate minerals olivine and pyroxene, which in turn are constructed of the elements silicon, magnesium, iron and oxygen.

Silicates are very common on Earth too, making up more than 90 percent of the weight of Earth’s crust. Gem-quality olivine is better known as peridot, the birthstone of August. Maybe you’ve seen this beautiful green stone while shopping for jewelry.

Although the photo above shows them as flat ovals and circles, remember you’re looking at a slice of meteorite and seeing a cross section of what are really spherical and egg-shaped beads.

Where did these come from? Scientists trace their origin back to the solar nebula, a rarefied cloud of gas and dust several light years across that collapsed, heated up and eventually formed the sun at its center. Leftover material orbiting the central sun coalesced into the planets, moons, comets and asteroids.

Artist's view of the solar nebula with the sun forming at center. As it collapsed through gravity, the nebula flattened into a disk. Dust and gases in the disk originate in previous generations of stars which either quietly or explosively slough material into space as they age. Credit: NASA

To build the very, very big, nature often starts with the very, very small. Dust in the cloud at some point was flash-heated to 2700-3500 degrees Fahrenheit, melted and then cooled in the low-gravity environment of space into small droplets. These solidified in a matter of hours into chondrules.

No one knows what caused the heating, but there are lots of ideas. Everything from a nearby supernova blast wave to an outburst of energy from the newly-forming sun to titanic lightning discharges within the disk to the shock wave within the nebula itself have been offered as explanations.

A lovely Easter-egg shaped chondrule in the Saharan meteorite Northwest Africa 869. The grain measures almost 1/4 inch across. Photo: Bob King

The path to planet-making began when the millimeter-sized rocky “eggs” and leftover dust stuck together to make clumps. The clumps clumped into larger clumps up to several hundred feet across. Through direct contact and gravitational attraction, these loosely-compacted bodies grew to become planetesimals  - the seeds of the planets – measuring 5-6 miles in diameter. After many collisions, the planets took shape in the next several hundred million years.

Many of the original chondrules were melted and altered by impacts and the heat of radioactive elements within the growing orbs, but some, like those pictured, are nearly as fresh as they day they formed 4.5 billion years ago.

A chondrite is a wonderful souvenir from a time when Earth was only a possibility and Easter farther off yet. Within each chondrule is the liquified dust of long-ago generations of stars.

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