There is another version of the Anthropic Principle, one that
applies only to the planet Earth. We may be more alone, or unique, in this
universe than the Drake Equation - the calculation of the possibility of other life out there in the universe - may have led us to believe.
The deepest hole ever drilled into the Earth's crust reached down
to about 7.6 miles (12 km) below the Kola peninsula of northern Russia. The
technology available to humankind cannot get below that depth (and that depth
took 24 years of drilling and billions of Rubles to achieve). The rocks are so
hot and plastic with overlying rock pressure at those depths that the hole closes in
on the drill bit - and partially fills the shaft back in from the sides as the
bit is drawn back to the surface to be replaced. So... the maximum depth
achieved by humanity's best effort is less than 1/10,000 the Earth's diameter, or the distance of a short commute on a Monday morning. We actually know more
about galaxies, comets, and the moons of Jupiter and Saturn than we do about
what lies below our feet on our own planet. No matter how you look at it, we
cannot really touch virtually all of the world beneath our feet.
In other words, everything we think we know about the interior of
the Earth is obtained by very indirect means, and a lot of this is from
mathematical modeling.
To see below the depth of the Kola well, we must rely in
electrical geophysical methods like magnetotellurics (which is one of the things that I "do" as a geophysicist; it can detect
resistivity layering down to perhaps 50 km or so), and on earthquakeseismology. For nearly a century seismologists have traced the powerful
vibration signals from very large earthquakes as these signals propagate and
refract through the Earth. By comparing the time of arrivals elsewhere around the planet - and whether just P-waves, or P-waves and S-waves together make
it - they can discern contrasts in density and other physical parameters as
these change with depth. P-waves (or primary waves) are pulses of energy,
momentarily compressing the material they pass through. It's the blast wave
from an explosion expanding outward. S-waves (or secondary waves) are shear
waves, oscillating material back and forth, sideways, as they pass through the
material. Think of how you would move your hands forward and backward to tear a
piece of paper. A key feature of S-waves is that they cannot propagate through
a liquid. Think of trying to use your hands to tear water. By the 1920's
seismologists had used the initial earthquake seismic information and some density calculations to
conclude that there is a solid iron core to the Earth, surrounded by an outer
liquid iron part of the core. The outer liquid core is overlain by a hot and
plastic Mantle, and finally by a relatively thin crust serving as a very
thin solid shell above them both. All living things live on or just beneath the top
of that crust.
The methodical genius who first figured all this out was a quiet
Danish lady named Inge Lehmann, who died in 1993 at 104 years of age.
Seismology and magnetotellurics show us the layering in the Earth
with depth. Indirectly we also know that the center of the earth is very hot.
After all, there are volcanoes and fumaroles, and the deeper you mine in places
like South Africa the hotter it gets. Nearly everywhere scientists have
measured temperature in wells, a thermal gradient exists: deeper means hotter.
But we also know there is a lot of heat below us for several other reasons,
including plate tectonics. SOMETHING has to be powering whole continents to be
able to wander around. And then there's the magnetic field of the earth.
What distinguishes Earth from Mars and the Moon? A magnetic
field, an atmosphere, liquid water - and life. The last requires the first
three in our limited observations so far. Without a magnetic field to deflect
it, Solar radiation would sterilize the
Earth and disrupt any attempt for life to gain a foothold. Solar radiation
would also strip away any atmosphere, which is apparently why Mars doesn't have
much atmosphere left to speak of. Mar's atmosphere is only a few percent of the
density of our own atmosphere - though there is evidence of much more at
one time in the distant past.
What distinguishes Venus from the Earth? Venus has an atmosphere,
but it has fallen under a runaway Greenhouse Effect - too hot for water and in
fact so hot that raw sulfur is a liquid on its surface. The Earth lies in what
is sometimes called the "Goldilocks Zone" where it's not too hot and
not too cold, between roasting Venus and frigid Mars. Water on Earth not only exists, but can
exist in all three states (solid, liquid, and gaseous). This is not so for Mars
or Venus, neither of which has a magnetic field, nor plate tectonics, nor
significant water.
It has been apparent for quite awhile that the Earth's magnetic
field is the reason why life exists on our planet. A magnetic field, however,
requires some sort of dynamo to create and sustain it. How to power this? Well,
if there are enough radioactive elements - or sufficient heat from the collapse
of the proto-planetary disk to form our planet - well then maybe there is enough
energy to drive a dynamo. However, this requires a lot of assumptions that
scientists cannot test - they can't drill deep enough.
There is another problem: hot things tend to cool when surrounded
by colder things... like interplanetary space. A magnetic field driven by an
internal dynamo cannot last forever.
Hot things cool in two ways: by conduction and/or by convection.
Conduction is like the metal pot you cook your cream of wheat in. Heat
transfers from a hot source beneath to a cooler part above without any motion of particles
involved. With convection, however - the bubbling cream of wheat - the heat is
transferred by particles moving in three-dimensional loops called hydrothermal
cells. You see them as bubbles driven by steam in the sauce pan. A hotter
particle of the wheat from the bottom, in contact with the metal pan, rises
because it is hotter (and thus less dense) than the particles above it, thus transferring heat from
the bottom to the top of the cream of wheat. If the stuff cannot convect - if it's not
liquid enough - then it will get hotter and hotter until it burns. It not only
tastes terrible, but the sauce pan is a bear to clean up afterwards. In the
same way, the solid iron core can only conduct heat out; like the metal sauce
pan it cannot convect heat. However, the liquid iron outer core and the hot and
plastic mantle above it can convect heat - and these convection cells of
highly conductive material must be the source of the magnetic dynamo. The
convection cells in the mantle are also what's driving whole continents around across the face of the Earth.
Remnant magnetization in rocks 3.5 billion years old, however,
proves that the Earth's magnetic dynamo has existed for at least that long. The
oldest known life is found in stromatolites - clumps of cyanobacteria - just
about that old. This is not a coincidence. If there was no protective magnetic
field, the stromatolites and then algae (and Earth's atmosphere) would not have
survived Solar winds and radiation. But 3.5 billion years is a long time for
something to stay hot enough to drive a magnetic-field-producing dynamo. Older
computer models based on relatively low thermal conduction assumptions for iron seemed to
suggest that it would take awhile for the solid iron core to give up its heat. This could conceivably sustain a dynamo lasting that long. According to these older models, the heat
from the core would take billions of years to conduct out to the outer liquid
core and Mantle where a different form of heat transfer - the much faster convection -
takes place.
In the last several years, however, scientists have been forced to
re-evaluate what they think they know about the center of the earth. Several
years ago, another piece of information became available from some Japanese
extreme-high-pressure experiments. Iron at pressures and temperatures we
calculate must exist in the center of the Earth has a far higher thermal
conductivity than anyone had thought could be possible. According to milecular orbital theory, if you smash material together hard enough, it frees up electrons and changes its conductivity. This means that the Earth's heat-driven dynamo should have burned out billions of years ago. In other words, the Earth's
magnetic field would have then died, and the atmosphere and any nascent life
would have all disappeared before most of the geologic record could even take place. Think of dead Mars.
Speaking of geology, fluid and gas inclusions in ancient rocks
tell us that around 2.5 billion years ago the Earth's primordial atmosphere of
CO2 and nitrogen transitioned to an oxygen-nitrogen atmosphere. The world as we
presently understand it began then. In part we can blame this on the
stromatolites and photosynthesizing plant life that was expanding at that time.
In the 1970's a few scientists offered what seemed like a
ridiculous idea: the Moon formed well after the Earth formed. It formed in its
current size and shape when a large Mars-sized planetoid crashed into the
proto-Earth and splattered material into space around the Earth. That material blasted into multiple orbits then coalesced to form the Moon, leaving a very different - and very hot -
planet Earth behind. Computer models show that this is easily feasible. If so,
then the Earth would have glowed like a small star from the massive infusion of
heat from all the kinetic and potential energy of the collision. This idea is
now taken seriously for several reasons, but mainly because the rocks on the
Moon are sooooo much like the rocks on the Earth, and sooooo different from
rocks on Vesta, Ceres, Mars, and Venus. We can discern these by optical
spectroscopy, coupled with sampling meteors that the spectroscopy says must come
from those places.
Could that ancient impact hold the answer for why we have such a
long-lasting magnetic field around our planet? That seems to be the best
explanation at this time. If so, then life exists on this planet because of
some pretty amazing circumstances:
- it exists in a narrow Goldilocks Zone,
- it was given a huge heat boost by a collision from a large planetoid, and
- its crust was given a lot of water from impacting comets that allowed it to be less solid, more flexible, and have an ocean of liquid water.
- Photosynthesis then started early and gave this planet an oxygen-nitrogen atmosphere, and finally
- The Earth's magnetic field lasted a very, very long time.
Those are a lot of things that had to come together at just the right time for life to form and evolve here.
There are so many coincidences - like the Anthropic Principle that allows molecules - and thus life - to exist. It seems remarkably like our Earth has its
own local version of the Anthropic Principle: just the right features and
additions at just the right times to allow life to form and evolve over an
extended period of time.
Bruce Buffett, a geophysicist at Berkeley puts it this way:
"The more you look at this and think about it, the more you think it can't
be a coincidence. The thought that these things might all be connected is kind
of wondrous." (Discover, July/August 2014, p. 41)
With all the exoplanets being found in solar systems nearby in
the Milky Way Galaxy, what is the likelihood that one of them could have all
these coincidences? Since Galileo, humanity has been humbled to know that it
isn't the center of the universe.
However, it appears that we certainly are unique.
~~~~~
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