Internal structure and densities of the solid worlds I - chemical evidence
Clouds of interstellar gas and dust from Wikipedia
Origin of planetary material:Solar systems form from clouds of interstellar gas and dust. Where does this material come from?
- Big Bang (10-20 billion years ago (ga)): Hydrogen and helium, (maybe a little bit of lithium)
- Nuclear fusion in earlier stars: More helium, also heavier elements, from lithium to iron. Stars constantly shed a thin "wind" of charged particles during their lives, by which this material can be distributed in interstellar space. In the case of our pet star, we call this the solar wind. Most stars end their lives in paroxysms of mass shedding that yields "planetary nebulae" - (unfortunate term) expanding masses of shed gas.
- Supernovae: Extremely massive stars end their lives in massive explosions during which most of their mass is ejected into space. (E.G. the Crab Nebula, an expanding remnant of a supernova observed in 1054.) During these explosions, exotic processes create every element heavier than iron. Note: A star's life span is inversely proportional to its mass, so large stars burn out quickly (in a few million years) so they constantly recycle these heavy elements into the cosmos.
The Milky Way - all sky composite from Astronomy Picture of the Day.
The result of all this is that huge molecular clouds of interstellar gas and dust are spread throughout the galaxy. Sometimes, that material can be compressed by the stellar winds of neighboring stars to the point that it contracts under its own gravity, leading to....
The Rosette Nebula detail from Astronomy Picture of the Day.
Formation of Solar Systems:
Nebulae. Occasionally, conditions are such that clouds of gas and dust (leftovers of the big bang, material ejected from stars during their lifetimes, and the shrapnel of ancient supernovae) become sufficiently compressed by the stellar winds of adjacent stars or the shock waves of supernovae that they form relatively dense clouds of material called nebulae (sing. nebula) like the Rosette Nebula (detail at right.) Particularly dense regions of nebulae can continue to contract as gravity sucks material to their centers, where it piles up forming a protoplanetary disk. Material concentrated in the center of the disk becomes a protostar while material circling it in the disk may accrete into planets. The Sun and Solar System formed in this way. The protostar becomes hot for at least two reasons.
Stages in origin of Sun:
- Gravitational contraction: Initially, as gas molecules and particles of dust plunged toward the center to form a protostar (E.G.: the "protosun"), these shine brightly from the heat produced by the friction of infalling matter and heat liberated by its compression at the center.
- Nuclear fusion: At a certain point, when pressure at the core of the protosun reached a critical point, hydrogen atoms began to fuse into helium, releasing thermonuclear energy. As less stuff fell into the new star, nuclear fusion replaced gravitational contraction as the primary source of solar heat.
Reconstruction of protosolar nebula University of Arizona - Astronomy 170B1.
Origin of planets:
- Radiation from early hot stages of the forming sun would have vaporized the primordial dust of the solar nebula - the protoplanetary nebula that gave rise to our Solar System. This would have condensed into new dust grains that would eventually form the planets. Note: this material would have been largely homogeneous.
- Accretion: As grains of dust in the planetary nebula encountered one another, they often stuck together due to electrostatic forces. Over 10,000 years these formed particles up to 10 mm in diameter. These, in turn, formed larger and larger aggregates called.....
- Planetesimals: At a certain point, as clumps of materials became larger (0.1 - 10 km across), gravity replaced electrostatic attracton as the dominant force. Larger planetesimals would tend to grow at the expense of smaller ones. The ultimate result:
- Planetary embryos: As the largest planetesimals grew their gravety would sweep up smaller ones in an ongoing series of collisions. Eventually, most material would be consolidated in planet-sized planetary embryos. Vesta is a good example of a planetary embryo that never got consolidated into a larger body. Giant collisions of planetary embryos ultimately yielded the planets we know.
A modern analog to this stage in the Solar System's development may be provided by the star Fomalhaut in whose accretion disk the second extrasolar planet ever directly to be imaged orbits.
Paradox of Earth mass and density:
The fact that the Solar System formed from homogeneous material leads to a paradox. We know:
- Earth's mass (5.9737 x 1024 kg) by applying Newton's laws to the moon's orbit
- Earth's volume 1.097509 x 1021 m3 by direct measurement.
We have access to three general classes:
Granite - Joshua Tree National Park___________________Basalt - Sullivan Bay, Santiago, Galápagos Islands___________________Mantle xenolith in basalt from Wikipedia
- Continental crust rocks: The rocks making up the buoyant, topographically upstanding rocks of the continents, including materials like granite (above left) - 2.7 103 kg m-3.
- Oceanic crust rocks: The rocks making up the dense, topographically low-lying rocks of the ocean floor, mostly basalt (above center) - 3.0 103 kg m-3.
- Mantle xenoliths: Rocks brought up from great depths in volcanic conduits including peridotite (above right) - 3.5 103 kg m-3. Note: Mantle rocks contain large quantities of olivine ((Mg, Fe)2SiO4), the most abundant mineral inside planet Earth.
The average density of surface rocks is nothing like the bulk density of planet Earth. One way or another, the low density of surface rocks must be offset by denser material somewhere. This paradox allegedly led Sir Edmund Halley to propose in 1692 that large parts of the interior of Earth were actually hollow. Today, serious possible explanations include:
- Gravitational self-compression by overlying rocks increases rock density at greater depth
- Gravitational self-compression causes chemical change in rocks, increasing density
- Earth's inner layers are made of fundamentally different, denser material.
Lines of evidence:
- Chemical composition of Earth rocks vs. meteorites
- Laboratory studies
- Moment of inertia
- Magnetic fields
- Gravitational anomalies
Meteorites come in several compositional classes:
- Chondrites: Meteorites containing chondrules - small beads of rocky material but including some amount of iron and nickel.
- Achondrites: Stony meteorites without chondrules whose material seems to have crystallized from a molten state.
- Iron meteorites: Meteorites consisting largely of iron with traces of other metals.
- Stony-iron meteorites: (A.K.A. pallasites) meteorites containing distinct metallic and silicate materials.
Earth composition vs. chondrites:
Chondrite normalization: CI carbonaceous chondrites are considered sufficiently reliable reflections of the Solar System's overall composition that it is common to refer to the concentrations of trace substances in rock samples by comparison to their concentrations in chondrites. These are typically displayed in spider diagrams or "spidergrams" (right.) As fig 2.3 0f the text indicates, Earth's crust and upper mantle are significantly enriched in some elements and depleted in others, especially iron, which is known to be common in the sun and in chondrites. The presence of an iron reservoir deep in the Earth might explain Earth's density paradox.
Laboratory simulations of the deep Earth:
What about self-compression or chemical alteration of mantle silicates? We can actually study the behavior of silicate minerals under conditions of temperature and pressure that they might encounter in the very deep Earth using equipment such as:
- Diamond anvil cells: Table-top devices in which a vanishingly small (0.1 mm) sample can be subjected to deep-Earth pressures while being pressed between two cut diamonds, and heated with a laser. These also allow optical access through a window.
- Shock wave guns: Very high pressures (100s of GPa, or several million atm) and simultaneous high temperatures (1000s of K) are obtained using shock wave methods, in which a projectile is accelerated into a target at impact velocities of ~km/s. The high P,T state is obtained for only a few microseconds.
Such tests experimentally verify that:
- Increased pressure results in mineralogical changes as mineral's crystal structures assume more close-packed arrangements. E.G. the common mineral olivine ((Mg, Fe)2SiO4) sequentially transforms to wadsleyite and ringwoodite at increasing pressure.
- Increased pressure drives chemical reactions. For example, (Mg, Fe)2SiO4->(Mg, Fe)2SiO3 + (Mg, Fe)O. (Olivine -> perovskite + magnesium oxide).
Key concepts and vocabulary:
- Big bang
- Nuclear fusion
- Supernova (pl. supernovae)
- Nebula (pl. nebulae)
- Protoplanetary disk
- Solar nebula
- Gravitational contraction
- Planetary embryo
- Earth density paradox:
- Earth rock compositional zones:
- Continental crust
- Oceanic crust
- Mantle xenoliths
- Meteorite types:
- Iron meteorites
- Stony-iron meteorites (pallasites)
- CI carbonaceous chondrites
- Chondrite normalization
- Spider diagram (spidergram)
- Diamond anvil cell
- Shock wave guns