The Good Egg

Cracking raw eggs is really an exercise in plate tectonics.  Subduction, rift and then eruption.  Such structural similarities between our planet and an egg have been observed before.  It's an interesting analogy, but is it just a metaphor or an accurate model?

For starters, neither is a perfect sphere.  That's pretty obvious for the egg but the Earth also deviates in two significant ways.  The first is an equatorial bulge caused by the centrifugal force of the planet's rotation. The resulting shape is almost a perfect ellipse rotating around its minor axis.  When expressed mathematically, this shape known as the reference ellipsoid.

How out of round is it?  The polar diameter is about 0.3% smaller than the equatorial diameter.  This difference is imperceptible in photos taken from space but it's a lot — almost 24 miles.  That's enough to make the summit of Mount Chimborazo in Ecuador the furthest point from the center of the Earth.  It is almost a mile and a half further away than the summit of Mt. Everest. 

The Earth's shape is further distorted by irregularities in its crust and internal density.  The gravitational influence of these anomalies cause mean sea level to deviate from the reference ellipsoid by as much as 100 meters.  This deformity is expressed in another mathematical shape called the geoid which must be used to correct the raw coordinates provided by GPS satellites.  So the Earth, indeed, is wobbling around in space like an egg escaping across the counter top.

Another similarity is the fact that the Earth and the egg both have liquid interiors.  Our planet is actually a giant blob of molten iron.  What the Romans thought of as terra firma is just the hardened slag on its surface.  The Earth's crust varies in thickness from a minimum of about 3 miles beneath the oceans to 18 miles under the most massive mountain ranges.  If we consider 12 miles to be the average thickness and use 4,000 miles as the Earth's radius, we can see that this crust is alarmingly thin.  It's only 0.3% of the radius! (0.003 * 4000 = 12).  The shell of a chicken egg is, on average, about 0.01" thick.  If its diameter averages 2" (which is close), we could think of its shell as a rough approximation of the Earth's crust.  I say "rough" because the eggshell is actually 3 times too thick!

Beneath the Earth's crust, is a thick viscous mantle of hot magma that extends down about 1,800 miles.  That's almost halfway and it is analogous to the albumen in a egg.  The center of the Earth, like the yolk, is far more dense.  It is believed to be liquid, 80% iron and around 6,000ºF at its boundary with the mantle.  At the very center, the extreme pressure prevents liquefaction even though the temperature may reach 10,000ºF.

Surprisingly, the Earth's core is rotating slightly faster than the rest of the planet by about 0.2º in longitude per year.  This creates a dynamo effect that may be generating Earth's magnetic field.  Of course seismic data is the only evidence we have of this core structure and scientists collect, study and debate it regularly.  In that respect, our limited vision into the internal mysteries of our planet is a not unlike candling an egg.  We're pretty sure, but it's still fuzzy.

Closer to where we live, and far more critical to our survival, is the thin envelope of atmosphere around the Earth.  Although some gas molecules persist as far as 300 miles out into space, 80% of it lies within 6 miles of the surface.  This area is called the troposphere and it contains all of the clouds and turbulence that produce our weather.  Above that and out another 26 miles is the stratosphere which contains the ozone layer and all but 1% of the remaining gases.

How do these dimensions compare to our egg?  An freshly laid egg also has an outer protective coating called a cuticle which helps prevent infection and moisture loss through the porous calcium of the shell.  On a chicken egg, the cuticle is about 12 microns thick.  There are 25,400 microns to an inch and if an egg radius of 1 inch represents an earth radius of 4,000 miles, the cuticle would model an "atmosphere" 10,000 feet thick.  Most humans live well below that elevation.  Here's the stunning part — you can't even see the cuticle on an egg.  It's thinner than a human hair.  Unbelievably, the troposphere and stratosphere together, at earth-to-egg scale, are only about as thick as an eyelash!

The humble chicken egg is a remarkable thing.  So is our planet.  Is it metaphysical, fractal or simply ironic that the egg is such a nice model?  Maybe it's all three.  The Earth, like an egg, is clearly a living, dynamic and fragile object.


Posted in Geography on Sep 12, 2016