Alan Stahler: Where did the moon come from?
Planet Earth is covered with landforms: High mountains, deep valleys, flat plains. We know – more-or-less – how these landforms come to be.
The Sierra Nevada, for instance, is a huge block of rock, stretching from the westernmost edge of the Central Valley – over by the Coast Range – to the Owens Valley, over by the Nevada border. The Sierran block has broken loose from the rock around, and floats on the hot rock below. Some millions of years ago, the block began to tilt – down in the west, to form the Central Valley, up in the east, to form the Sierra crest.
We see various types of mountains and valleys and plains forming today, giving us clues as to how they’re made. But how was planet Earth assembled? And Earth’s moon, which is such an oddball? No other rocky planet has a moon anywhere near so large; Earth and moon, together, are actually a double planet. How did our moon come to be?
The origin of the moon perplexed folks in the 1800s and 1900s. One good idea after another got shot down.
When the Apollo astronauts stepped off the ladder of their landing vehicle, onto the lunar surface, the first thing they did was reach down, grab a sample of lunar soil, and stuff it into a pocket of their space suit. If they had to beat a hasty retreat – climb right back into the lander and blast off – they would at least come home with that grab sample.
The Apollo missions returned hundreds of pounds of moon rock to Earth. These rocks hold clues as to how the moon formed … and how it could not have formed. As it’s turned out, they also hold clues as to how planet Earth came to be.
Earth and the other planets formed some four-and-a-half billion years ago. The early Earth was moonless … but plenty of remnants of the planet-forming process still drifted through the solar system.
In the 2000s, the leading hypothesis (“educated guess”) has been that a large chunk of debris – something the size of a small planet – plowed into Earth. The impactor vaporized; the vapor went into orbit around Earth. A large chunk of Earth rock also vaporized, and mixed with the impactor’s vapor. From this orbiting cloud, the moon coalesced.
But … there was … a problem.
The flavor of wine depends on “terroir” – the environment – soil and sunshine, water and wind, hills and valleys – in which the grapes grew. Every body in the solar system is also the product of its birth environment, largely determined by its distance from the warming sun. Close to the sun, rocky planets – Earth and Mars – formed; farther out, gas giants – icy, gaseous bodies formed: Jupiter and Saturn.
The object that slammed into Earth – that would have provided most of the material that would become the moon – must have formed at least somewhat distant form Earth … and should have had a composition at least somewhat different from Earth.
But the moon rocks reveal the moon’s composition to be nearly identical to that of the rocky part of Earth, surrounding the core.
Which has led to a new picture of the moon-forming impact: The planet that slammed into the early Earth didn’t just blow a chunk of Earth-rock into space … it blew off – and vaporized – ALL the rock outside the metallic core. Earth-rock vapor mixed with impactor-vapor, cooled, and condensed into a cloud of liquid rock droplets. Liquid rock – magma – rained down onto Earth’s core, rebuilding the thousands-of-miles-deep rocky layer of Earth that surrounds the core.
In orbit around the Earth, the cloud … coalesced into the moon, bathed, at first, by a magma ocean. (When the magma ocean cooled and solidified, the lightest crystals floated to the top, seen today the bright parts of the man-in-the-moon.).
This Saturday, at 7 p.m., local astronomers will set up scopes at the junction of State Route 49 and the old Downieville Highway, to participate in International Observe the Moon Night. It’s free – bring the kids – and wear warm clothes.
Al Stahler enjoys sharing nature with friends and neighbors. His science and nature programs may be heard on KVMR (89.5 FM), and he may be reached firstname.lastname@example.org.
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