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 earthquake seismology. 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.