Friday, January 23, 2009

MANY things that scientists study are invisible to them

MANY things that scientists study are invisible to them. Announcements about the discovery of planets around other stars do not come because astronomers have seen the planets, although they would certainly love to. The discovery is more likely based on observations of the star's motion that indicate strong nearby gravitational forces. So it is with discoveries about the Earth's core and mantle. Because the deepest well ever drilled extends down just 12 kilometers, not even pricking the mantle, researchers have to employ indirect methods to study the Earth.

Using meteorites and seismological evidence as clues, scientists have known almost since the beginning of the century that the Earth has a solid, mostly iron, inner core and a molten outer core with a mantle and crust of rocky, silicate material. But for just as long they have been puzzled about how the core and mantle separated. The primordial planet Earth grew out of bits of gas and dust, aggregating over time into a larger, more solid body. Was there then some cataclysmic event billions of years ago that melted much of the planet, prompting the metals and silicates to separate as oil and water do? Or was the separation the result of a more gradual process, a trickling down of the denser molten metals between solid silicate mineral grains to the center of the Earth? Recent research at Lawrence Livermore National Laboratory by geochemist William Minarik has helped to dispel the second, "trickle down," theory. Using the larger of Livermore's two multi-anvil presses to mimic the pressures and temperatures that exist deep in the Earth, he has shown that metals like those in the Earth's core could not have trickled down (see figure below).

The materials used in the experiments were olivine, a silicate mineral that makes up much of the Earth's upper mantle, and an iron-nickel-sulfur-oxygen combination to represent the core.

The multi-anvil press is a relatively rare research tool. Livermore's two presses have been used for a variety of material property studies, including diffusion and deformation of ceramics and metals, deep-focus earthquakes, and the high-pressure stability of mineral phases. The larger, 1,200-ton hydraulic press can produce pressures of 25 gigapascals (GPa), which is equivalent to 250,000 times the atmospheric pressure at sea level, or the pressure that occurs 700 kilometers deep in the Earth. In addition to pressing on the sample, the experiment passes an electric current through a furnace within the assembly to generate temperatures up to 2,200°C.

These experiments using the multi-anvil press generated a high pressure of 11 GPa, or 110,000 times the atmospheric pressure at sea level. This corresponds to 380 kilometers deep into the Earth, or pressures at the center of moons or asteroids 2,500 kilometers in radius (about the size of Mercury). The sample was also heated to a temperature of 1,500°C. Under those conditions, the metal melts and the olivine remains solid. The geometry of the press is key to creating these enormous pressures. For the 11-GPa experiment, a ceramic octahedron had a 10.3-millimeter-long hole with a tiny rhenium furnace, a thermocouple to measure temperatures, and a graphite capsule containing the olivine and iron-nickel-sulfur-oxygen sample inserted in it. The octahedron rested in the center of eight 32-millimeter tungsten carbide cubes whose inside corners were truncated to accommodate the sample. (Tungsten carbide is used for the cubes, or anvils, because of its hardness, which is close to that of diamonds but at a much lower cost.) Tiny ceramic gaskets were placed at the edges of the carbide cubes to contain the pressure. This assembly of an inner octahedron and eight carbide anvil cubes was put in the press's split-cone, steel buckets as shown in the figure above. In several stages, the steel buckets pushed on the carbide cubic anvils, which pushed on the octahedral volume inside. The multi-anvil press is not Livermore's only device for studying the behavior of the Earth's innards, but in many ways it is the best for this type of study. The diamond-anvil cell can produce 100-GPa pressures, comparable to the pressures at the center of the Earth (see Science & Technology Review, March 1996). But it can accommodate only a 20-micrometer sample, too small for much post-experiment evaluation. With the piston-cylinder press, the sample volume is about 500 millimeters3, but it has only a 4-GPa pressure capability, which is comparable to a depth of just 120 kilometers. The multi-anvil press is in the middle, providing pressures useful for studies of this type and accommodating a sample large enough for evaluation after the experiment.

In Minarik's experiments, the press took about 4 hours to bring the samples to full pressure, after which the samples were heated for periods ranging from 4 to 24 hours. During this time, the porosity of the sample collapsed, and the stable microstructure developed. Then the unit was cooled down and allowed to decompress for about 12 hours. During this process, the graphite capsule turned to diamond, which must be ground off before the sample could be sliced and polished for evaluation.

Despite being molten and much denser than the olivine, the metallic melt showed no signs of separating and draining to the bottom of the capsule. For the molten metal to drip down along the silicate grain edges, it has to be able to wet the edges. But in none of the experiments did wetting occur.1 Rather, the iron-nickel mixture beaded up at the corners of the silicate grains like water does on a waxed car, as shown in the figure below.

Livermore's findings agree with similar, lower-pressure studies that have melted meteorites and iron-nickel-sulfur- oxygen mixtures and failed to wet the silicate minerals. Together, these experiments indicate that much higher temperatures were required to separate the Earth's core and mantle--temperatures high enough to melt most of the Earth.

All of these data lend credence to the theory that the young, growing Earth was repeatedly bombarded by large planetoids, with some of these collisions generating temperatures high enough to form a magma ocean from which drops of dense molten metal separated. The largest collision may have been when a large celestial body, about the size of Mars, collided with Earth nearly 4.5 billion years ago, melting most of it and causing the core and mantle to separate. The leading theory today for the Moon's formation postulates that some material from that collision was ejected into orbit and condensed into the Earth's Moon. Livermore scientists have long studied material properties and the effects of high temperatures and pressures. Their work has resulted in some mighty big bangs but none as large as the ones Minarik has postulated.

"We plan to look next at the geochemical aspects of this project, the partitioning of trace elements between molten metal and silicates at the same high temperatures and pressures," says Minarik. "There are many scientists in this country and elsewhere studying the formation of the Earth, and all of us are in the same boat. With all of the direct evidence of the Earth's formation buried far beneath our feet, these laboratory experiments are our only way to recreate what might have happened."

Key Words: Earth core formation, multi-anvil press. Reference 1. W. G. Minarik, et al., "Textural Entrapment of Core-Forming Melts," Science 272, 530-533 (April 26, 1996). For further information contact William Minarik (510) 423-4130
) or Frederick Ryerson (510) 422-6170

The theory put forth to explain the origins of the Universe

The theory put forth to explain the origins of the Universe, our solar system, and our planet is called the Big Bang Theory, which says that all matter in the Universe was, at one time, concentrated in a giant mass (a black hole?) that blew apart about 10 to 20 billion years ago (bya) and is still expanding. It is thought that our solar system had its origins when, about 5 bya, triggered by some unknown cause, a cloud of interstellar dust and gases collapsed and condensed.

Some of the matter in the central mass contracted under its own gravity, condensed, and heated until forces were so strong that thermonuclear reactions began, and this was the origin of our star, the Sun. Interestingly, one astronomy book I read pointed out that the size of a star is related to the amount of fuel it has available to burn for energy and how fast it burns up that fuel. A star smaller than our sun would not contain enough fuel to last long enough for evolution to have occurred here on earth.

A larger star would have burned its fuel too fast, and would have burned itself out long ago, also not lasting long enough for life to evolve on Earth. This book pointed out that our sun is just the right size! About 4.6 to 4.5 bya, a disk-shaped cloud of subsidiary smaller lumps, pieces, dust, gases, etc. orbiting the sun subsequently coalesced and condensed to form the planets, satellites, asteroids, comets, etc.

It is thought that Earth began as a cold world, and the very first atmosphere may have been hydrogen gas, but since that is so light weight and very chemically reactive, most of it would have floated off into space or reacted with other substances, thus would have been rapidly dissipated. The first “real” atmosphere is thought to be due to subsequent volcanic activity and other chemical reactions taking place.

It is thought that the inner four, solid planets may have started out with similar atmospheres of H2O, CO2, CO, and N2. According to current thinking, NH3 is now “off the list” because it is so reactive, that scientists believe it would have formed H2, which would have floated off into space, and N2 which would have stayed in the early atmosphere. It is thought that these chemicals made up the atmosphere of our planet for the first 1 billion years, and initially, provided similar atmospheres for the other three solid planets. However, the distance of each of these planets from the sun has influenced what subsequently took place there. Mercury is too close to the sun and too hot.

Any water that might have been there (and any other volatile chemicals) would, long ago, have evaporated into space. Venus also is too close to the sun to have any surface water. The climate there is classified as a “run-away greenhouse effect.” While Venus probably has torrential rains from its heavy cloud cover, the high heat almost immediately evaporates any surface water. Mars, on the other hand, is too far away from the sun, and so is too cold. Any water and carbon dioxide present on the planet are frozen solid in the “ice” cap (a tiny bit of CO2 thaws out and provides a thin atmosphere over portions of the planet during the Martian summer. Also, the planet is too small to hold very much atmosphere, and there is not enough of a greenhouse effect to keep the planet warm. Thus, there is essentially no atmosphere left.

Once again, conditions happen to be just right here on planet Earth. We are just the right distance from the sun! On Earth, the heat and the size are such that the water is neither all frozen nor all vaporized. Because liquid water is present, this has enabled formation of the lakes and oceans needed for life to evolve. Over the next 3.5 billion years, the amount of CO2 in our atmosphere was reduced as it became incorporated into rocks (limestone is CaCO3 and forms when H2O + CO2 H2CO3 and H2CO3 + Ca++ CaCO3 + 2H+).

The liquid oceans formed about 3.8 bya, and life has been present for nearly as long. Evidence would indicate that on early Earth, there was much more volcanic activity, many more electrical storms, and more violent and destructive meteor impacts which could have heated the earth, melting part of the crust (one theory says the moon formed when a big, molten chunk of crust was knocked/blown off from the rest of the planet) and vaporizing part of the atmosphere. Because of this impact heating (and Earth’s internal heat) the Earth subsequently melted, and heavier molten materials sank to the center, forming the core, while lighter ones floated to surface and formed the crust. Thus Earth now has several layers:

A central core composed of very dense metals such as iron and nickle The mantle, which actually consists of several layers of varying composition, and is a molten, medium density, viscous liquid The crust floats on the outside. Because the minerals in the crust are cooler, they have formed strong, rocky layers collectively referred to as the lithosphere. This is not a single shell, but a patchwork of plates that ride on the mantle and move relative to each other. Plate tectonics is the study of the interactions among the plates of the Earth’s crust. Deep in the oceans, are ridges where new lithosphere is formed and the sea floor is spreading and in other locations, there are trenches where old lithosphere is folding in. The continents float/ride on some of the plates, leading to continental drift. The continents are not recycled like ocean floor, but can separate or collide with each other. Wherever two continental plates collide, mountains are pushed up.

It is thought that several major steps had to occur for life to form on early Earth. Alexander Ivanovich Oparin (publ 1936), a Russian scientist, in The Origins of Life, described hypothetical conditions which he felt would have been necessary for life to first come into existence on early Earth. He thought the atmosphere was made largely of water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), methane (CH4), and ammonia (NH3). As the surface of Earth cooled again, torrential rains of this mixture formed the first seas, the “primordial soup.”Lightening, ultraviolet (UV) radiation, and volcanic action all were more intense than they are now. First, organic monomers (simple sugars, amino acids, fatty acids, and nucleotides) would have to be synthesized abiotically from inorganic substances like methane, carbon dioxide, and ammonia.

This hypothesis was later tested by an experiment done by Stanley Miller as a grad student under Harold Urey in 1953. H2O, H2, CH4 and NH3 (at that time, thought to be components of the early atmosphere) were placed in a sterile, closed system. Heat was added to mimic the heat from volcanic activity, and electric sparks were provided to mimic lightening. After one week, the contents of this system had turned from clear to a murky, brown color. A chemical analysis showed a number of organic compounds were present, including several amino acids and simple sugars.

Other researchers have since tried similar experiments with slight variations in the initial mix of chemicals added, and by now, all 20 amino acids, and a number of sugars, lipids, and nucleotides have been obtained in this manner. From this experiment, scientists generalize that if this can happen in a lab, it could have happened in a similar way on early Earth. The second step would be the formation of organic polymers and genetic material from the existing monomers (polysaccharides from simple sugars, proteins from amino acids, and RNA from nucleotides), possibly using hot sand or finely divided clay as a catalyst.

Thirdly, it is thought that non-living aggregates of these polymers formed. These may have exhibited some properties characteristic of living organisms, but were NOT ALIVE, and did not have all the properties of living organisms. In a research laboratory, scientists have seen mixtures of proteins, lipids, and carbohydrates form globules. If the proteins involved happen to be enzymes, these globules can even carry on "metabolic" activity, although they have no means to replicate themselves.

Simultaneous to this, the genetic code would have to have arisen. Several widely-accepted theories as to how this may have happened include the possibly involvement of damp, zinc-containing clay as a catalyst to help the nucleotides polymerize first into RNA, and later into DNA. It is thought, then, that about 4.1 to 3.5 bya, the first prokaryotes, like bacteria, came into existance.

It is difficult to pinpoint a date for this because bacteria don't have skeletons to leave behind. The first “fossils” (remains of colonies/secretions) of prokaryotes seem to be this age. These would have been very simple cells without many of the organelles present in modern cells, especially modern eukaryotes. Once the first cells, the first living organisms, the first prokaryotes came into existance, then the Theory of Evolution takes over to provide an explanation for how (not why) these primitive cells diversified into the five kingdoms of life which we recognize today.

Initially, the energy needed for growth and development was supplied by glycolysis and fermentation. There was no free oxygen in the early atmosphere, and indeed, any organisms living back then would have probably been poisoned/killed by this highly-reactive chemical. Only later, as photosynthetic organisms released increasing amounts of this toxic waste into the atmosphere, did the process of cellular respiration evolve as a means of making use of this oxygen

The structure of the Earth

The structure of the Earth

Imagine a Scotch egg......
1. The outer shell of the Earth is called the CRUST (breadcrumbs)
2. The next layer is called the MANTLE (sausagemeat)
3. The next layer is the liquid OUTER CORE (egg white)
4. The middle bit is called the solid INNER CORE (egg yolk)
The deepest anyone has drilled into the earth is around 12 kilometres, we've only scratched the surface. How do we know what's going on deep underground?

There are lots of clues:
The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock.
Meteorites (created at the same time as the Earth, 4.6 billion years ago) have been analysed. The commonest type is called a chondrite and they contain iron, silicon, magnesium and oxygen (Others contain iron and nickel). A meteorite has roughly the same density as the whole earth. A meteorite minus its iron has a density roughly the same as Mantle rock (e.g. the mineral called olivine).
Iron and Nickel are both dense and magnetic.
Scientists can follow the path of seismic waves from
earthquakes as they travel through the Earth. The inner core of the Earth appears to be solid whilst the outer core is liquid (s waves do not travel through liquids). The mantle is mainly solid as it is under extreme pressure (see below). We know that the mantle rocks are under extreme pressure, diamond is made from carbon deposits and is created in rocks that come from depths of 150-300 kilometres that have been squeezed under massive pressures.

The Earth is sphere (as is the scotch egg!) with a diameter of about 12,700Kilometres. As we go deeper and deeper into the earth the temperature and pressure rises. The core temperature is believed to be an incredible 5000-6000°c.
The crust is very thin (average 20Km). This does not sound very thin but if you were to imagine the Earth as a football, the crust would be about ½millimetre thick. The thinnest parts are under the oceans (OCEANIC CRUST) and go to a depth of roughly 10 kilometres. The thickest parts are the continents (CONTINENTAL CRUST) which extend down to 35 kilometres on average. The continental crust in the Himalayas is some 75 kilometres deep.
The mantle is the layer beneath the crust which extends about half way to the centre. It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky bit... the mantle is a
solid which flows????) The convection of heat from the centre of the Earth is what ultimately drives the movement of the tectonic plates and cause mountains to rise. Click here for more details
The outer core is the layer beneath the mantle. It is made of liquid iron and nickel. Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field.
The inner core is the bit in the middle!. It is made of solid iron and nickel. Temperatures in the core are thought to be in the region of 5000-6000°c and it's solid due to the massive pressure.
(If you haven't seen a solid that flows then go back
here and have a look)

This diagram shows a detailed picture of the Earth's interior. Crust is being created at the mid ocean ridges and being eaten at the subduction zones. The movement processes are driven by the
convection currents created by the heat produced by natural radioactive processes deep within the Earth.

Inner core: depth of 5,150-6,370 kilometresThe inner core is made of solid iron and nickel and is unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids under extreme pressure.

Outer core: depth of 2,890-5,150 kilometresThe outer core is a hot, electrically conducting liquid (mainly Iron and Nickel). This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents creating the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulphur and oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron.

D" layer: depth of 2,700-2,890 kilometresThis layer is 200 to 300 kilometres thick. Although it is often identified as part of the lower mantle, seismic evidence suggests the D" layer might differ chemically from the lower mantle lying above it. Scientists think that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.

Lower mantle: depth of 650-2,890 kilometresThe lower mantle is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminium. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.

Transition region: depth of 400-650 kilometresThe transition region or mesosphere (for middle mantle), sometimes called the fertile layer and is the source of basaltic magmas. It also contains calcium, aluminium, and garnet, which is a complex aluminium-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.

Upper mantle: depth of 10-400 kilometresSolid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been found. These and other minerals are crystalline at high temperatures. Part of the upper mantle called the asthenosphere might be partially molten.

Oceanic crust: depth of 0-10 kilometresThe majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000 kilometre network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with an igneous rock called basalt. Hawaii and Iceland are two examples of the accumulation of basalt islands.

Continental crust: depth of 0-75 kilometresThis is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust is the surface of the Earth. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).Back to the