Friday, 23 May 2014

The Formation of Earth - Some Interesting Questions

This is a mainly a summary of this documentary on the formation of earth on YouTube, with a few modifications.

Why did Earth form ?

It had been well-established that the minerals formed as the sun cooled down floated across space around the sun and gradually bundled together to form planets. But nobody could explain how mineral particles could lump together to form planet-sized bodies, and not break up given their high velocities and turbulence in space. Then, an experiment was carried out in 2003 in outer space. Salt particles in a plastic bags were shaken and left to themselves. The result was the they started lumping together spontaneously to form largely stable structures. The result was attributed to electrostatic charges between the particles of salt, and given that electrostatic exchanges are believed to frequently happen in the early stages of stars, this is the primary reason given for the accretion of planets.

Though the initial lumping of minerals was precipitated by these small electrostatic charges, as the lump gained enough mass, its own gravity started to attract an increasing amount of rocky materials towards it, augmented by frequent collisions with other such bodies floating around the sun.

Why are rocky planets so small as compared to gaseous ones ?

Rocky planets are found near the sun and the initial temperature there was so hot that only elements with very high melting points like iron and nickel could survive evaporation and lump together. Since these heavy elements constitute less than 0.1% of the initial mass of the solar system, they are much smaller as compared to planets that are farther away from the sun, and hence, are composed of the more abundant lighter elements.

Why did Earth get neatly divided into layers ?

When this huge lump of rocks started to melt, the heavy iron and nickel elements sunk to the core due to the massive gravity. This led to the neat division of the earth's crust as we see today. The huge core is made of iron and nickel, with a thick mineral layer above it, followed the extremely thin surface on which we live.

Now, what could have caused the lump of solid rocks to heat up to melting point "in the freezing depths of space" ? The answer is perhaps the highly radioactive rocks that formed its initial core. Their decay (precipitated by their collision on earth) released enough energy to melt the entire planet inside out. Now, since the temperatures in space are nearly -450 degrees, the earth had to ultimately cool, from outside in. This created the thin crust (like the top of a wound) on which we live today.

How was the earth's age established ?

Rocks from earth have been dated to a billion years or more. But with billions of years of erosion, and continental drift, its quite possible that there may yet be rocks much older but we are unlikely to find them. Therefore, scientists decided to test the age of meteorites that fell on earth because when the meteorites formed, that would also be the the time when the earth began to lump together, and since the lumping together happened very quickly we can be pretty sure that the age of the earth will be very close to that of the meteors. The most accurate estimate from this source is 4,567,000,000 years.

How was the moon formed ?

The widely held theory was that the moon formed in the same way as the earth, with layers and a heavy iron core. But then a rock from the moon was brought by Apollo 16 and was found to contain considerably less iron that would be expected. This was a clue that the moon might have been formed entirely from the surface of the earth. It was theorized by a scientist called Hartman that another planet, the size of Mars crashed into the earth, and from the debris, arose the moon. The other planet was completely destroyed on impact. Enjoying the support of several other corroborations, this stands as the most widely accepted theory today.

When did continents and oceans form?

In a section of Canada, recently, some strange kind of rocks were found called amphibolite. These were radiometrically dated to around 4.28 billion years back. These are the oldest rocks from the earth's original crust. Their analysis revealed that they were present at around 12 miles depth (because they have minerals that can form only in conditions of very high pressure and temperature), which implies that the earth's crust was at least 12 miles thick at that time. The continents had already formed.

Moreover, specific chemical signatures in these rocks indicated oceans to be already present at that time. In the same rock sequence, magnetite was also found. Magnetite in banded iron formations is only formed under water, further proving that oceans have existed on earth since 4.28 billion years at the least.

Where did all the water come from ? 

A meteorite fell on earth in 1998, and its analysis showed it to contain grains of table salt with tiny amounts of liquid water trapped inside crystals. It has been dated to 4.5 billion years. This is the first evidence that water existed in space before the earth was formed. It has been theorized that water from these meteorites accumulated over billions of years, providing a very significant portion of water that is present on earth today. Another possible source was comets, but analysis of their water revealed a higher percentage of deuterium (an isotope of hydrogen) than what is found on earth water, and was hence ruled out.

A little about life

Amino acids are the building blocks of proteins, and they are essential to all life on earth. It had been postulated that these amino acids had formed due to chemical reactions in the warm ponds of early earth, or in the atmosphere catalyzed by lightning and UV rays. As it turns out, these building blocks have been discovered on hundreds of meteorites implying that they may have originally formed in space.

Where did all the oxygen come from ?

The initial atmosphere on earth consisted of sulphur, methane, carbon dioxide and sulphuric acid. Life had evolved here in the absence of free oxygen. So, how did we get to a position where nearly every living thing needs oxygen to survive ?

Fossilized remains of coral-like structures called stromatolites have been found in rocks from 3.5 billion years ago. These structures are formed by cyanobacteria, a life form that takes water and sunlight and transforms them into oxygen. Acting over billions of years, they produced all the free oxygen that is available on our planet today. Ironically, this explosion of oxygen caused one of the greatest mass extinction events of history as oxygen was toxic to most lifeforms that had evolved in its absence. Only the few that could tolerate oxygen survived and gave rise to all oxygen-dependent life on earth.

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