Chapter 6 - At the Earth's Core - From Blue Giant to Blue Marble

I'm glad that I am a geologist. There are so many things involved in understanding how the Earth works, that you really need to be an expert to apply them all . . . 

Figure 17 Cross section of the Earth (
http://en.wikipedia.org/wiki/File:Earth-crust-cutaway-english.svg)

When you look at a cross-section of the Earth, you will notice that there are 5 different regions. The top-most region is the crust and is characterized by (relatively) lightweight and rigid rocks. As you move down toward the Earth's center, the next regions are the upper and lower mantle. Both regions have similar rock composition - but the upper mantle tends to be (relatively) brittle, while the (much warmer) lower mantle tends to be more "play doh"-like (no brittle cracking). The last 2 layers consist of a liquid (outer) core and a solid (inner) core.

We know this information from the use of seismographs - which measure seismic wave motion (from earthquakes). Certain seismic waves can propagate through liquid and others cannot - which is how we discovered that the outer core is liquid. The liquid core had been theorized by the fact that the Earth has a magnetic field (dynamo), but seismology proved it as fact.

It was also noticed that seismic waves moved at a different velocity through the (solid) inner core compared with other layers. This meant that the inner core was made of a different material than the crust and mantle (taking into effect the pressure and temperature data also). So what is it made out of?

There are ways to determine the average density of planets. It's a simple matter of mathematics to determine the overall density of the Earth, and to separate out the parts that we know. We know crustal rock average density, and have good data on mantle rock density. We know the volumes involved and we do due diligence by throwing in pressure and temperature data. When all is said and done, CSB states that the density of the inner core is close to iron.

So maybe our cannon ball idea was correct! But let's get some more data to be sure.

Calculating pressures at different depths is pretty straightforward. You use the mass and volume of the material that is above (that depth) and factor in gravity. We have calculated what the pressure is at the center of the Earth (lots!). Now let's take a look at temperature.

You could specialize in Geothermal studies (studying the how the Earth's temperature varies with depth - called a gradient), so I couldn't do justice to it in such a small book. Here is a simplified version. 

 

There are 2 main sources of heat from the inside of the Earth. The first one is radioactive heating - where radioactive material (located in the mantle),heats up rock through radiation. The Oklo natural (nuclear) reactor (in Gabon Africa) is a neat example of how radioactive materials can generate tremendous amounts of heat inside the earth. 

 

The second source of heat is through a chemical reaction that occurs when a molecule of liquid iron "freezes" into a solid one (this is an exothermic or heat-producing reaction). This happens at the (physical) margin between the inner and outer cores.

 

According to Wikipedia (and many others), these 2 reactions cause all of the heat that we measure coming out of the Earth (the core area is estimated to be about 5700 K degrees - or about as hot as the surface of the Sun). According to my professor (Dr. Henry Pollack) at the University of Michigan, he told me that after adding up all of those heat sources the observed heat is an order of magnitude higher than the "theory" can explain. Let's take my professor's position as I trust him.

 

 

Conundrum 12: Earth's observed geothermal gradient

is an order of magnitude higher than the theory predicts. So how the heck do you explain 10 times the heat coming from the Earth (as expected)?

Chapter 7 - The Last Supernova - From Blue Giant to Blue Marble

Figure 18 The Crab Nebula created from the 1054 AD supernova (http://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Crab_Nebula.jpg/768px-Crab_Nebula.jpg)

In following my Principle 1, I need to address all of the observable features of the Solar System – so here is another important one. Scientists doing isotopic analysis of meteorites (outer space rocks that have fallen to Earth) found some interesting “stuff”. What they found was a certain radioactive isotope of the element aluminum that is commonly found only from the aftermath of a supernova. By using isotopic analysis of ratios of radioactive aluminum isotopes to non radioactive ones, scientists can determine the length of time since all of the aluminum was the radioactive kind. This is exactly the same process that carbon dating uses (but using aluminum instead of carbon). This information has led some scientists to believe that there was a supernova that occurred a bit after the time the Earth was formed (4.5 Gya).

Supernovas are one thing that you really don’t want to get too close to. These scientists theorize that this supernova was “close” to the Earth. How close? We don’t know, but close enough to cast these chunks of meteorites (with aluminum) to fall upon the Earth.

Let’s make an assumption here – that the supernova happened right here in our Solar System, and not from a nearby system. This is applying Occam’s Razor – it is simpler to believe that a supernova happened here instead of adding another star system to “the mix” (making the model more complex).


Conundrum 13: Nearby (survivable) supernova
Just how do you have a supernova in our Solar System without destroying everything?



Conundrum 14: Which star blew up?
It obviously wasn’t the Sun – so what, another star?



The Sun is here today – so it never supernova-ed. That means that there must have been another star in addition to the Sun in our Solar System 4.5 Gya (star number 2).

Stars the size of our Sun do not supernova. There are 2 types of supernovas and they start from either giant stars (type II) or white dwarf stars (type I). It seems pretty unlikely that the Sun and Solar System could survive a supernova – but if it could survive it would make more sense if it were a type I supernova. A type I supernova has much less mass to blast out than a giant star, and a type II supernova leaves a neutron star or black hole behind – and we don’t see one around here. The second star was probably a white dwarf (Let’s call this star Rabbit).



Proposal 5

There was a white dwarf star (Rabbit) in the Solar System and it supernovaed (around) 4.5 Gya.

Figure 19 A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.(http://upload.wikimedia.org/wikipedia/commons/5/5d/Size_IK_Peg.png)

Let’s explore the mechanics of a Sun/Rabbit Solar System i.e. before the supernova.

Chapter 8 - Rabbit The White Dwarf - From Blue Giant to Blue Marble

A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron degenerate matter.Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth,it is very dense. Their faint luminosity comes from the emission of stored heat.–Wikipedia

If (the white dwarf star) Rabbit blew up in a supernova 4.5 Gya, and the Sun was formed 4.7 Gya, then Rabbit must have been around a lot longer than (predating) the Sun. White dwarf stars are (only) created when a red giant star expels its outer layers of gas (in a planetary nebula), leaving the metal core behind (which becomes the white dwarf). This means that there was a progenitor star to Rabbit and it was a red giant (call it Queen).

But I’m getting ahead of myself! A white dwarf blows up in a supernova when its mass exceeds 1.4 solar masses (1.4x the mass of the Sun) – which is called the Chandrasekhar limit. Usually this is accomplished when a white dwarf “sucks” matter away from a nearby star. This matter (gas, dust) creates a disk around the white dwarf (called an accretion disk) before falling onto the white dwarf. So Rabbit would have been “sucking” matter from the Sun until it got “fat” enough to explode. The disk must have been “wafer thin” . . .

Figure 20 Artist's conception of a white dwarf star accreting hydrogen from a larger companion
(http://en.wikipedia.org/wiki/File:Making_a_Nova.jpg)

Some scientists believe that the Rabbit supernova is what “sparked” the creation of the Solar System. If the supernova occurred 4.5 Gya, the Moon rocks were already formed by then thus the supernova could not have created the Solar System. Even if the supernova occurred 4.7 Gya, that doesn’t leave much time for the planets to accrete (and the supernova “stuff” to settle out). I don’t believe this is the way it happened. The Rabbit supernova did not create the Solar System – the creation of the Solar System triggered the Rabbit supernova!

That will take some explaining, so lets tell the whole story.


A previous conundrum says that supernovas do a poor job of creating boron-5 and beryllium-4, but the Earth has tremendous amounts of the stuff. So where did it come from?

Supernovas are not the only mechanism for creating “star stuff” - there is another process that can create new elements. This process is called a nova (instead of supernova). A white dwarf star (convenient!) accretes gas/dust/star stuff on its surface from a companion star (or cloud) and this material undergoes fusion – which creates new material that gets thrown into space in a bright “flash” (the nova). Novas can create all sorts of material – each flash could be a different element or compound. Usually only the lightest (least mass) material is created – and beryllium-4 and boron-5 fit that description.

Novas can create carbon and oxygen and sulfur (and neon and other things), and these atoms can combine with others to make molecules like water and carbon dioxide. Hmm, Co2 and sulfur describe Venus’s atmosphere while water and oxygen (and nitrogen!) sound a whole lot like the Earth’s atmosphere. So how do you get lighter elements in the inner Solar System (where you would expect heavier ones)? Can you say No-va (just don’t say it in Spanish)?



Proposal 6

The lighter mass elements that are found in the inner Solar System came from Rabbit’s novas.

But a white dwarf that novas on a regular basis gets rid of “extra” mass – which means no build-up of mass to a supernova. Oh crap. But even if it could go supernova, it would probably destroy the whole Solar System. How do you “moderate” a supernova explosion so that most of the Solar System is spared? And how do you trigger one in a star that already novas?

Let’s examine the roots of Rabbit and maybe we can formulate a better hypothesis.

Chapter 9 - Queen The Red Giant - From Blue Giant to Blue Marble

Figure 21 Red giant vs. Sun size comparison
(http://en.wikipedia.org/wiki/File:Sun_red_giant.svg)

A Red Giant is a luminous giant star of low or  intermediate mass (roughly 0.5–10 solar masses) that is in a late phase of stellar evolution.The outer  atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000K and lower.–Wikipedia

All white dwarf stars are created from a red giant “parent” star. The fact that you see a white dwarf, means that it once was a red giant. Our own Sun (in about 4 billion years) will expand into a red giant which will eventually shed its outer layers and turn into a white dwarf.

An “average” red giant star is around 1 AU in diameter – meaning that its surface would come close to the orbit of the Earth (if placed in the center of the Solar System). This means that none of the inner planets existed when Queen was around. It also means that our own Sun did not exist at that time either (its age is only 4.7 Gya). NOTE: We are talking about the time period between 9 Gya to 5 Gya.

There is only one way a red giant can create a white dwarf – and that is to shed its outer shell of gas – exposing its core which becomes the white dwarf. This process creates a ring of gas called a planetary nebula, which continues to expand away from the white dwarf forever (the “average” planetary nebula expands to a light year in diameter!). But we need that nebular material! How else do you create planets? And how did our Sun get created 4.7 Gya?

We need some way to grab that material so it doesn’t disappear forever! The only way to do that is with gravity We would need a large gravitational source to slow down the expansion of the nebula. But where would this gravity come from?

Figure 22 M57 The Ring Nebula
(http://en.wikipedia.org/wiki/File:M57_The_Ring_Nebula.JPG)

A black hole could provide enough gravity – but the Solar System (probably) wouldn’t survive an encounter with it. A huge gas giant planet like Jupiter wouldn’t have enough gravity – and the speeding nebula might just blow it away! A neutron star would make a good candidate since it can be up to 4 solar masses (see Tolman-Oppenheimer-Volkoff limit). Since we know that this planetary nebular material did stick around (we are here), let’s make the assumption that this “extra source of gravity” came from a neutron star (call it Spider) nearby. So where would Spider have been positioned when Queen shed its planetary nebula?

There are 3 possibilities as to where Spider was when this happened:

1. Spider was orbiting Queen.
2. Queen was orbiting Spider.
3. Both stars were orbiting a common barycenter.

My guess is #1, as that gives Spider the best chance to grab the most material. The planetary nebula would have intersected Spider’s orbit, and the neutron star’s enormous gravity would have slowed it down (so it wouldn’t expand out to a light year away).

Since Queen was 1AU in diameter, Spider must have been orbiting somewhere around Mars’ distance. If it was any closer, it would have intersected the outer surface of Queen and friction would have slowed it down and brought it closer and closer to the center of the solar system (where Queen’s core Rabbit was). But after the planetary nebula expansion and the “sweeping” up of material – Spider would have encountered this friction and “spiraled” toward the center.

Note: Neutron stars like Spider are created from a progenitor star through a supernova. The progenitor of Spider was a blue giant star (call him King). Oh great another star . . .