DOWN TO EARTH

The universe holds millions of galaxies. Our Milky Way Galaxy formed between some 10 billion and 15 billion years ago. Based on the ages of meteorites and lunar rocks, planetary scientists know our solar system is much younger.

It was born 4.6 billion years ago out of an immense cloud of interstellar dust and gas that collapsed under the weight of its own gravity. It pancaked into a flat, spinning disk. Clumps of matter collided and became bigger, like a snowball collecting snowflakes. The objects grew so big their gravity began to pull in nearby material, forming the protoplanets that became our planets today.

Earth is about 4.56 billion years old. Back then, however, it wasn’t Earth as we know it, says Jonathan Berg, chair of NIU’s Department of Geology and Environmental Geosciences. “Even if the Earth wasn’t completely molten, the outer
part would have been like a magma (molten rock) ocean,” Berg says. “The magma would quickly destroy any crust that formed on Earth, while heavy elements, such as iron and nickel, would sink to the center.”

 

Michael Parrish

Michael Parrish, chair of NIU’s Department of Biological Sciences and a noted paleontologist, was among the scientists who announced the fossil discovery
of a mouse-size mammal that lived in Madagascar 165 millionyears ago. The fossil represents the oldest known modern mammal.

After about 100 million years, Earth began to cool and ocean crust formed, followed by buoyant continental crust. “Over history, the continents would break apart and collide back together,” Berg says. “Younger crust tends to grow around older crust. So it makes sense that today geologists generally tend to find the oldest rocks in the center of continents.”

Scientists in the 1950s through the 1970s analyzed rocks determined to be more than 3 billion years old. They thought the early Earth’s magma oceans and high heat would have consumed and destroyed anything much older. In the past decade,
geologists, using more sophisticated dating methods that employ high resolution mass spectrometry, have analyzed layers of grains of zircon in Australia’s ancient sedimentary rocks that date to 4.4 billion years.

“Zircon is one of the most phenomenal minerals on earth because it’s fantastic at surviving and is loaded with radioactive elements, which allow for precise dating,” Berg says. “With new mass spectrometry instrumentation, we can see there were zircons that showed evidence of having gone through several lives, having crystallized, partially melted and crystallized repeatedly.”

Measurements of oxygen isotopes in zircon also suggest there might have been water on Earth more than 4 billion years ago. “If water was around, it’s possible life could have started on earth very, very early,” Berg says. “That’s total speculation at this point, but there could have been early life forms that were repeatedly destroyed.”

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