Enstatite Chondrites - The Archives of Mercury's Origin
(Originally posted July 11, 2016 on Blogger)
Enstatite chondrite. Perhaps one of the rarest meteorites on Earth. I've always wanted a meteorite (of some sort or another) to add to my collection of rocks and minerals, but an enstatite chondrite... ahh, now that would be the crown jewel of my collection should I ever find one.
Chances of that are slim to none. Only about 2% of chondrites that fall to Earth are of this type. With a surface area of nearly 510 million square kilometers, I doubt I'll ever come across this sort of meteorite here on Earth. But if I were able to visit Mercury... well, according to geologists at MIT, my chances would be nearly 100%.
Before we delve into this incredible revelation, let's first discuss what the heck an enstatite chondrite is... in fact, we should start with what a chondrite is.. no, let's go even further down the ladder and start with what chondrules are.
We all know what meteorites are... they're basically rocks (or iron) that fall to Earth from space that make it to Earth's surface before being completely burned up by the heat caused from friction between molecules in Earth's atmosphere and the rocks'(or irons') surfaces. They are the 'successful' asteroids or meteoroids you could say; asteroids being much larger than meteoroids... primitively speaking... both revolve around our Sun, both can, potentially, wreak havoc upon our planet. Both called "shooting stars".
Not all rocks from space are equal. Some have unique characteristics, and the ones I find most fascinating, are the chondrites. Chondrites get their name from the chondrules that comprise their bodies, save for some carbonaceous chondrites. We'll get to that later. Chondrules look like this:
See all those rocks within a matrix of, well, in this case it appears to be iron? Looks a lot like some sort of conglomerate. And, in a certain sense it is. A conglomerate is a type of sedimentary rock composed of a bunch of smaller rounded rocks "glued" together through lithification.. a cementation and compaction process, the name of which comes from the Greek "lithos" (rock) with a Latin suffix for good measure. Breccias are like conglomerates, with the noted difference being the smaller rocks (clasts) lithified within its matrix are angular. Rounding of rocks comes from them having been tumbled over great distances by flowing water in one form or another.
But chondrites, and their chondrules, are NOT conglomerates. These chondrules were not 'glued' together through lithification. According to a paper that was just published less than a week ago in the journal Science Advances by scientists from Japan and France, chondrules are the probable result of low-velocity collisions between planetesimals and icy bodies some 4.6 billion years before present (ybp). Rather than regurgitate the paper, I invite you to read the details of their research here:
Chondrules are usually made up of olivene and pyroxene, silicate minerals which, are themselves, common in Earth's upper mantle (aka asthenosphere). A quick side note, this is the part of Earth's interior largely responsible for plate tectonics (and associated earthquakes). As it moves, so too do the granitic chunks that float atop the denser basaltic crustal layer which are broken into separate plates... those granitic chunks being our continents.
Though, here's some interesting insight not really explained in textbooks... at least not in any I've ever perused; many are taught that the mantle has convective currents that drive plate tectonics... and many imagine it being similar to lava (the surface version of magma)... molten rock that flows in varying viscosities. But the mantle is not liquid... it's solid. We know this because shear waves (S-waves) generated from earthquakes travel through the mantle. As a transverse wave, these waves simply are unable to propagate through liquid mediums.
So if it's not liquid, then how does the mantle move enough to shift crustal plates? The answer is in the missing atoms of an imperfect crystal matrix throughout the mantle. Over geologic time, missing atoms are replaced by neighboring atoms & the overall effect is movement. The mantle has a viscosity several orders of magnitude greater than that of even glass, yet enough that continents move.
It's hard to imagine, but it's true. Earth's mantle literally moves an atom at a time, and the main reason the crust actually moves faster than the mantle is because as it subducts, it pulls itself ever faster beneath whatever older crust that is overriding it. An effect of gravity in a process called slab pull. If you have a necklace, you can see what I mean by laying it out across a table top, then allowing one end to dangle off the table's edge to the point it begins to slide off.. at first it will slide slowly off the table, and as more of it dangles off the edge of the table, the effect of gravity becomes dominate over friction, and the necklace slides off the table. This is more or less, what's happening within the basaltic crust... the granitic chunks (continents), are fortunately for us, spared this plummet as granite is less dense (specific gravity) than the basalt upon which it "floats".
To see how slow highly viscous materials can move, check out this life-long experiment that is ongoing at the University of Queensland here:
There are different types of chondrites... carbonaceous, rumuruti... but the ultra-rare ones are enstatite chondrites. Enstatite (MgSiO3) is oxygen-poor silicate rock that are among the most driest objects in our solar system, most of which are believed to come to Earth from one of the largest asteroids in our solar system: 16-Psyche... a 200 km diameter behemoth lurking in the Asteroid Belt between Mars and Jupiter.
Today I read that the planet Mercury's origins can be traced to enstatite chondrites. A little over a week ago, researchers at MIT came out with their study which concludes, Mercury is substantially composed of an enstatite chondrite. Rare on Earth, Mercury is, well, partially a huge enstatite chondrite. This is absolutely incredible to conceive.
Data collected from NASA's MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) probe between 2011 and 2015 were used in their study, from which it has been (tentatively) concluded that Mercury's interior cooled unusually quick after the planet's formation (by 240 degrees Celsius over 500 million years... rapid in terms of geologic time). By looking at the crystallization of synthetic rocks from a melt in the lab (at specific temperatures and pressures to mimic Mercury's interior conditions at 'birth'), the closest geologic match was enstatite chondrite.
This if fascinating if Mercury is made up of such a rare material. If true, then Mercury can easily qualify as being the driest planet in our solar system. An irony perhaps, for modern southeast Asian cultures who refer to the planet as the "water star" (水星).
Though this study is not yet confirmed (a sample from the planet would help), it is nonetheless an incredible thing to imagine.
Mercury gets its name (in modern Latin languages) from the Greek god, Hermes. The ancient Greeks called the planet Hermes... in fact, Greeks still refer to the planet as Ermes... the H was lost somewhere between modern and ancient times apparently... perhaps it was taken up at the Council of Nicaea... ok, terrible stretch for a joke! Look up the word "Homoousios" to see how terrible I am at making connections with fewer than 6 degrees of separation! But get it? From one god to another? Meh... At any rate, the Roman god equivalent to Hermes was Mercury. A fitting name indeed. For what better god to name the planet that treks fastest across the night sky than the speedy messenger god, Mercury.