Friday, March 11, 2011

Our Solar System Part 1: Earth

An Exploration of How Earth Was Formed and How Life First Took Hold

A Violent Beginning

Looking at this photo of Earth taken during the Apollo 17 lunar mission, you might get the impression that Earth has always been this beautiful blue life-filled planet.





















Far from it. Our planet was born of fire and violence, and life, once it took hold, hung on tenaciously in one form or another over several mass extinctions. Many events came together in just the right order to allow us to exist here today.

4.6 billion years ago, Earth and the entire solar system itself, was a cloud of dust and gas composed of hydrogen and helium as well as heavier elements spewed away from supernovas, near the edge of a galaxy called the Milky Way. A nearby supernova may have recently exploded, creating a shock wave that created localized pockets of denser matter, dense enough to trigger this cloud to collapse. It began to rotate, thanks to the angular momentum of its particle atoms and gravity, and inertia slowly flattened it into a disk, called a protoplanetary disk. It might have looked something like this artist's conception.













Material began to concentrate in the center and heat up. Its rotational speed increased much like how a skater rotates faster as she draws her arms in. The force of gravity and angular momentum conspired to increase the overall energy of this central mass and, with no way for the energy to escape into the vacuum of space surrounding it, eventually its temperature and pressure became so great that hydrogen atoms began to fuse, igniting a nuclear fusion reaction. At this point the central mass called a protostar, ignited into a star, our Sun. Internal energy, directed outward, soon balanced the inward force of gravity to create a stable state called hydrostatic equilibrium. Radiation from the blast blew much of the dust and gas outward but not all. Bits of small dust-like grains composed of silicon, magnesium, aluminum and oxygen as well as other trace elements collided with each other and accreted. These chunks are preserved in chondrite meteorites. These bits formed larger and larger chunks of matter as they collided with and captured each other within their orbital neighbourhoods, eventually forming planets, moons and asteroids. The thick dust around the newly formed Sun would have made it appear red and opaque as chunks of planetesimals were colliding with each other to form Earth. The accretion process is illustrated here in this artist's rendering of Earth forming from a disk of accreting chunks of rock.







When Earth was forming from an accreting mass of orbiting matter, it regularly melted from the energetic impacts and this process caused the matter inside Earth to differentiate according to mass. 






A core evolved from the densest elements, mostly iron (89%) but other heavy elements as well, including radioactive elements. A mantle composed of a mixture of oxygen (45%), silicon (22%) and magnesium (23%) as well as many other elements, formed around the core. Finally, a crust composed of lighter elements and compounds, most of which is silicon dioxide (61%) and aluminum oxide (16%), would eventually form as the planet cooled.  This process is called planetary differentiation.

The Moon is Born From Catastrophe

Earth had just formed. It was about 100 million years old and it had just differentiated into core and mantle but no crust existed yet because it was still completely molten, when it collided with a Mars-size planet called Theia, shown here in this artist's depiction. Theia is the name of the mythical Greek titan who gave birth to the moon goddess, Selene.


















Computer simulations suggest that Theia's iron core sank into Earth's core on impact while Theia's mantle and a great deal of Earth's mantle were ejected into orbit around Earth. It probably took less than a thousand years for this material to coalesce into the Moon. Earth gained significant mass and angular momentum from this collision. After impact it was rotating so fast a day was only 5 hours long. And the new Moon loomed very large on the horizon. It was about ten times closer, about 30,000 km away. This is what Earth might have looked like then.















The Moon's orbit is expanding at a rate of 3.8 centimetres per year and as it does so, Earth's rate of rotation slows, gaining 2 milliseconds every one hundred years. The reason for this is that the Moon's gravitational attraction literally squishes Earth into an egg shape (actually the distortion is only on the scale of a few meters) and there is a lag time for Earth to reform a sphere shape so the Earth's and Moon's bumps don't perfectly face each other. This creates a torque on the spin of the Earth and a similar back reaction on the Moon, which slows down Earth's rotation and injects energy into the Moon's orbit, pushing it away. All the heat lost from the rhythmic deformation of Earth also translates into lost rotational energy. The moon used to rotate faster but tidal forces slowed its rotation until it became tidally locked with Earth, happening just 50 million years after its formation. That is why we see only one face of it. The moon will continue to move away from the Earth until it is about 1.6 times the distance it is now but its rate of orbit expansion has slowed so much it will take 15 billion years to do so. By then, the Earth and the moon will have long been engulfed by the Sun when it expands into a red giant and the Sun itself will be nothing but a brown dwarf dying star remnant. 

Add Ingredient For Life #1: A Magnetosphere

A part of the differentiation process, which was well under way when Theia collided with Earth, is what is called the iron catastrophe, in which the spinning molten metal outer core created Earth's magnetosphere, shown here.



Earth's magnetosphere formed when the Earth was about 500 million years old, not long after the Theia impact. Contrary to the name "catastrophe," the creation of the magnetosphere was essential for life to take hold on Earth. Without it, solar radiation would have stripped away the solar radiation-deflecting atmosphere and any surface water would have dispersed into space before life could evolve.  

Add Ingredient for Life #2: An Atmosphere

Planets as large as Earth had enough gravitational pull to hang onto gases that came from outgassing within the mantle and from comet impacts. These captured gases created Earth's first atmosphere, which replaced a thin haze of hydrogen and helium, leftover from Earth's formation, with carbon dioxide, nitrogen and water vapour. Water vapour increased in abundance with each comet impact (and there is new evidence that asteroids may have contributed much of Earth's water in addition to the contributions from icy comets. Earth was heavily bombarded with comets and asteroids during this violent and chaotic phase of the solar system's evolution.

Hydrogen and helium gases, being very light, dissipated into space as they formed and were replaced over time with the heavier gases from mantle outgassing. As the mantle began to settle, volcanic outgassing contributed to the atmosphere tremendous amounts of carbon dioxide (CO2), as well as hydrogen sulfide, sulfur dioxide, methane and some ammonia, and additional water vapour released from mineral hydrates within the mantle.

As the Earth began to cool, the surface solidified into a crust and vast amounts of accumulated water vapour in the primordial atmosphere condensed and rained down to form the first ocean. Surface temperatures then were about 230°C. Liquid water existed only because the CO2-heavy atmosphere was extremely dense, creating enough pressure to prevent water from vapourizing.  Frequent super-cyclonic storms raged across the surface, fueled by the Earth's rapid rotation and energetic atmosphere. The Moon, much closer then, may have created tides of superheated water up to a thousand times higher than those today, perhaps 100 km high, crashing over land every few hours! Frequent cosmic collisions continued up to about 3.8 billion years ago. Their impacts regularly re-vapourized part or even the entire ocean, creating high altitude clouds that completely enveloped the planet. As the bombardment slowed, clouds dissipated as water vapour rained out of the atmosphere into the ocean.

Earth As a Giant Organic Chemistry Lab

Volcanic gases such as carbon dioxide, sulfur dioxide and hydrogen chloride readily dissolved in the ocean into acids that would have been neutralized by various minerals. Eventually the ocean became a reducing soup of organic compounds in an environment energized by intense UV radiation. Even though the young Sun was fainter then, most UV radiation reached the surface because the ozone layer had not built up yet. Volcanic activity and intense radioactivity within the young core also contributed energy to the environment. Intense lightning from volcanic eruptions may have played an important part in producing HCN (hydrogen cyanide) from CH3 (methyl groups) and NH3 (ammonia) or N2 (nitrogen gas). HCN is essential for the synthesis of amino acids and nucleobases, parts of the building blocks of proteins and DNA, respectively. Small chemically reactive intermediate molecules such as formaldehydes, ethylene, cyanoacetate and acetylene, which can recombine into more complex intermediates that can in turn form stable biochemicals, would have formed under these conditions, but we don't know how concentrated they might have been in the primordial ocean because it is very difficult to pinpoint what the temperature and pH of that environment was. However, we do know that amino acids did in fact form because life made its first remarkable appearance about 3.8 billion years ago. Experiments such as the Miller-Urey experiment, which involve simulating the conditions of this primordial environment, have successfully created over 20 different amino acids.

Oxygen and Nitrogen

The earliest atmosphere contained very little oxygen gas. What little oxygen was present came from the dissociation of water vapour in the upper atmosphere by ultraviolet radiation. Much of this oxygen would have been photochemically converted into what was then a very thin ozone layer, further reducing the abundance of it in the atmosphere.

Researchers have puzzled over the abundance of nitrogen in our atmosphere. It was a predominant material during Earth's formation, probably in solid form, much of which may have been released as gas from the mantle through volcanic eruptions. Nitrogen is an inert gas that is quite stable under solar radiation so it may have built up slowly and remained in the atmosphere so that now it comprises about 78% of the atmospheric content today.

So far I have described the Hadean (a name aptly derived from Hades or Hell) era Earth, a period lasting from Earth's formation to about 3.8 billion years ago when life was about to appear for the first time.































A few rocks surviving from this very early period have been discovered in Greenland, Canada and Australia. The oldest dated rocks, zircons, have been dated at 4.4 billion years old. Life evolved when there was no oxygen and no protection from UV radiation, when volcanoes ravaged the world and the ocean was a roiling soup. The sky was an ominous reddish colour and the ocean was dead grey. It is possible that life arose more than once under these conditions as the primordial ocean repeatedly vapourized and condensed during repeated asteroid impacts. Life may have evolved in shallow clay-rich waters and/or near hydrothermal vents that spew out concentrated ammonia and methane, two molecules that could serve as building blocks for more complex organic compounds. Some theorists also propose that life may have gotten its start when primitive RNA-like sugar-rich molecules hitched rides on the backs of meteorites. It is difficult to model how these sugar molecules, which form the backbone of RNA and are called ribose, could have been created from Earth's primordial ingredients.

A Molecule That Can Copy Itself

In the chaotic and energetic environment, a molecule somehow gained the ability to replicate itself. An early form of abiotic (meaning nonliving or pre-life) evolution has been postulated in which different potential methods of replication were attempted and either improved upon or eliminated based on the natural tendency of any system to move toward the lowest possible energy state. It is possible that within the ocean's organic soup, the energy from lightning and UV (ultraviolet) radiation drove reactions creating more and more complex molecules from simple compounds such as methane and ammonia. Among these molecules were amino acids and nucleobases, the building blocks of life. Reactions occurred randomly, and by chance a replicator molecule was formed, perhaps something much simpler than but superseded by DNA, perhaps with RNA as an intermediate. DNA is now life's universal replicator molecule, except for some viruses and prions.

A Cell Membrane

Some kind of protective envelope to house the replicator molecule would have been needed and this would likely have come from a primitive phospholipid bilayer sphere, which can form spontaneously when phospholipid molecules are placed in water. Eventually a self-contained organism, a single cell prokaryote, evolved, which used DNA as its genetic code, RNA for information transfer and protein synthesis and enzymes to catalyze reactions. There may have been many kinds of protocells and this one line out-survived the others and evolved. Many of these terms may be new to you. Exploringorigins.org provides an excellent and easy to understand tutorial on this entire process, explaining the importance of RNA, DNA and proteins and how they may have come together to create the first living cell under the extreme conditions of early Earth. It comes with many short animations to illustrate the processes involved.

An Energy Source For Life

The first cells to evolve likely used surrounding organic molecules for energy, much like many extremophiles do today. An example of an extremphile, are primitive bacteria-like unicellular organisms called endoliths that survive by feeding on traces of iron, potassium or sulphur. They can survive long ice ages by simply slowing down their cellular processes.

A Much Better Energy Source For Life: Sunlight

At some point, about 3 billion years ago, cells evolved a new strategy for capturing and using the energy in sunlight, and this evolution drastically changed the atmosphere and climate of Earth. Highly successful cyanobacteria (bacteria that obtain their energy from sunlight) utilized a new cellular process called photosynthesis, using CO2 and water as raw materials to create energy-rich sugars, with oxygen as a byproduct. These bacteria evolved in shallow water, trapping sedimentary grains in their bacterial biofilms to create large structures called stromatolites. Stromatolites were the first life to colonize Earth. A few very rare colonies still exist in hypersaline lakes where animals can't graze on them. The oxygen gas released from enormous colonies of stromatolites was captured by organic matter and by dissolved iron in the ocean, but as these minerals became saturated with oxygen, it began to accumulate in the atmosphere. This process is sometimes called the Oxygen Catastrophe because it probably caused the greatest extinction event of all time. Oxygen would have been toxic to most other kinds of bacteria because it destroys organic compounds. As free oxygen combined with atmospheric methane, a very potent greenhouse gas, and as atmospheric carbon dioxide diminished through photosynthesis, eventually a glaciation event was triggered that was so severe that Earth was entirely enveloped in ice, the first of several "Snowball Earth" episodes to come. This ice age, called the Huronian glaciation, began about 2.3 billion years ago and lasted between 300 and 400 million years. The Oxygen Catastrophe also marked the beginning to a new opportunity for life to evolve because until now, life was energetically limited to fermentation reactions. Now cells could take advantage of the much more effective metabolic process of respiration. The geology of Earth changed dramatically as minerals became oxidized. Mitochondria, organelles acting like little cellular power plants, evolved soon afterward, which could turn sunlight into highly concentrated energy storage molecules called ATP. A new kind of cell called a eukaryote, housing all of these enhancements, made its first appearance about 2 billion years ago. Because it had more energy at its disposal, it could grow larger and even more complex. Meanwhile, some of the oxygen was converted into ozone through ultraviolet radiation, creating a UV-protective layer in the upper atmosphere which allowed cells to colonize the surface of the ocean, and later, on land as well. Before this, the DNA in cells would have been vulnerable to high rates of lethal mutations caused by the radiation. At this time, volcanic islands began to coalesce into one large supercontinent called Nuna or Columbia, which existed around 2 billion years ago and began to fragment 1.6 billion years ago, starting a pattern of repeated supercontinent assembly and fragmentation driven by plate tectonics that continues to occur today, ending with the latest supercontinent, Pangaea. This brief but interesting National Geographic video  illustrates Earth's early formation.


Life Hangs In There Through Extremes

The three types of unicellular organisms, archaea (a group that contains, for example, extremophiles and methanogens which live in our gut), bacteria, and eukaryotes (at that time confined to living in the water) continued to diversify, possibly during the extreme cold of the ice age as well as after conditions warmed. By about 1 billion years ago, plant, animal and fungi lines had all split and multicellular life was beginning to evolve as cells began to accumulate in colonies and a division of labour began to take place. The first simple multicellular organisms such as green algae and sponges began to evolve.

Multicellular Life Evolves Twice!

Another supercontinent called Rodinia formed about 1.1 billion years ago, and like Nuna, it was entirely barren because no terrestrial life yet existed. This supercontinent was centered on the equator and some theorists believe that because of its location, the rate of chemical weathering of its rock increased, sequestering CO2 and removing its greenhouse gas function from the atmosphere. Two more Snowball Earth episodes followed, around 710 and 640 million years ago. Permafrost decreased chemical weathering and as atmospheric CO2 levels gradually built back up through volcanic activity, each ice age ended. This ushered in a period of intense evolution of multicellular life forms called Ediacara biota. Plants and animals with tissues performing specific functions evolved. Fossil evidence suggests that these early organisms were completely replaced by those of the later Cambrian explosion, because most current body plans of animals appear only in the Cambrian fossil record and not in the older Ediacaran period. You might be wondering how the fossils of these soft-bodied animals could exist today at all. Life on Earth is made up of organic compounds. These molecules are based on carbon to which atoms of oxygen, hydrogen, nitrogen and other elements are attached. When some of these animals died, their soft bodies were quickly buried in sand, preserving them very well. Hydrogen, nitrogen and oxygen are volatile elements and they were driven off of the fossils over time. What remained is carbon and so the overall shape of each organism was preserved in ancient rocks as a carbon layer, an imprint. Canada has an excellent collection of these fossils. I urge you to click on this link and browse the online exhibit.  This website gives you an idea of how these animals might have lived and what they might have looked like. At around 540 million years ago these once quite plentiful older fossils, representing impossible to classify Ediacaran organisms with discs, tubes, mud-filled bags and quilted mattress-like bodies, vanished. No one is sure why but perhaps these organisms represent a failed first attempt at multicellularity. By about 510 million years ago, the Cambrian explosion was underway. Most of the major phyla of organisms made their appearances as the rate of evolution accelerated and the diversity of life exploded and organisms eventually exploited every available ecological niche. This 32-minute online lecture explores Cambrian life in detail. It is still unclear what triggered the Ediacaran die-out and the Cambrian explosion, but perhaps fluctuating atmospheric oxygen levels contributed to them.

Land Plants Appear

About 510 million years ago, at the start of Cambrian explosion, land plants made their first appearance, evolving from branched filamentous algae that lived in shallow waters. This 5-minute video


gives you an idea of just how essential plants are to all life on Earth. There is some evidence that simple photosynthetic algae lived in fresh water depressions and lakes perhaps as early as 1 billion years ago but they did not colonize in high enough numbers to impact atmospheric gases. Several more mass extinctions, triggered by climate changes or catastrophic events or a combination of these, would threaten life but not obliterate it in the eons to come.

Earth Teaches Us How To Look for Life On Other Planets

The atmospheric effects of life on Earth have helped astronomers to define their search for possible markers of life on extrasolar planets. The presence of liquid water and gases such as methane, carbon dioxide and especially oxygen mark the possibility that life which uses biochemical pathways similar to those used on Earth could be supported. An increasing understanding of how life evolved on Earth may help astronomers refine their search toward eventually finding even more specific biomarkers of life.

Next up: Part 2: Mars

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