Rocky rain

Skies are filled with drifting cold snow today in Cambridge. The snowflakes look grey. I spiked my decaf coffee with a splash of regular coffee (egads!) and a bloop of organic milk (bloop!) because surely that’s how Shackleton drank his decaf. 

In Earth time, all is molten. The collision with Theia has tipped Earth 23°, blasted the atmosphere into space, and made a hot mess of this blessed planet. Like Dante, Shackleton’s great-great grandfather* (*unconfirmed, probably not true) we’ll start at the top and spiral our way downwards for a brief foray into this infernal goop. 

Peridotite, a blend of greenish olivine and blackish pyroxene, is possibly the first rock to solidify, hardening into a crust somewhere near the Earth’s molten surface. Hooray for peridotite, first on the scene! We write a quick haiku for peridotite:

Oh peridotite.

You make a young planet proud.

Too bad you're so dense. 


Yes, alas, too dense to float for long atop the basaltic magma, the peridotite sinks down to the mantle some miles below. We’ll follow it down and return to the surface later. 

This “upper mantle” of predominantly peridotite is maybe 250 miles thick. But the pressure is now increasing as we dive. Below this upper mantle, olivine is compressed into different formations like wadlseyite and perovskite. That’s the lower mantle. I’m sure someone has something interesting to say about this, but I don’t. 

We dive. Here we’re diving with the heavy metals that make up the Earth’s core.  (Yes, at its core, the Earth is all about heavy metal.) Mostly iron. Some nickel. The outer core is a swirling mess of molten iron that is assumed to be responsible for our magnetosphere. The inner core is solid. 

Everything down here is quite warm, so especially in these early days there are cycles of convection bringing cooling, denser minerals down and hotter, less dense materials up towards the earth’s surface. Some of these rising minerals have water packed inside; when they near the surface, they explosively degas. (There is also a good bit of farting in Dante’s Inferno, we must note.) 

Back at the surface, black basalt is spreading over the surface, now rid of the lovely greenish peridotite that sank below. This is more or less the Earth’s first crust, pierced by degassing vents that spew water and other hoo-haw out onto the cooling surface — and that’s how we get ourselves an ocean. 

But where did the Earth first get its water? The planetesimals that first formed the Earth may have had some water bound up in their minerals. Asteroids and other nonsense from the young solar system are likely still bringing water down too during this molten phase. Rocky rain. Mineral blizzards. Water is actually pretty darn common in the universe. You can find it all throughout the solar system (usually, annoyingly, as ice), so it’s not a shocker that it would have been present on the early earth — but it’s only somewhat recently that we’ve thought the earth had these oceans. Maybe a mile deep. Salty, slightly acidic. (Like salad dressing?) Again, volcanoes poke out here and here, but mostly, even in these early days, this is a blue world. 

Somewhere in here, zircon crystals are forming. They will one day be found in the Jack Hills of Australia. And some scientists will look at them and say, those formed  4.4 billion years ago — near water. (We’ll return to Zircon later, and not just because it’s the name of my little-known alien probe manufacturing company.)

Meanwhile, above the young, differentiating, degassing Earth, the moon is cobbling together some rocks that will eventually be plucked by a chap named Neil Armstrong.

 Pluck 'em if you got 'em. 

Pluck 'em if you got 'em. 

Souvenirs from the Apollo missions confirmed what a few folks had suggested: that the lighter-colored, pockmarked highlands were older, and the darker lowlands (the maria) were younger, smoothed by melted basalt. The oldest lunar rocks found were around 4.4 billion years old. 

Anyway, it's a great day for the Earth and the Moon, because they're making things that humans will eventually find. Although this particular human can't seem to decide whether to capitalize moon. Moon! Oh, and the other word of the day is siderophile, meaning metal-loving.