Information from the planets has greatly expanded our understanding of the Earth. But where does Earth come from?
Matter in the Universe:
- Is not distributed evenly. It is clumped into galaxies -clumps of stars, gas, dust, and other minor components like planets. These, in turn, occur in galaxy clusters. We see the Milky Way, the galaxy where our solar system resides edge on and form the inside at night.
Origin of stars: Stars from from clouds of interstellar gas and dust. Where does this material come from?
- Big Bang (10-20 billion years ago (g.a.)): 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.
- 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.
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 that they form relatively dense clouds of material called nebulae (sing. nebula) like the Rosette Nebula at right. Particularly dense regions of nebulae can continue to contract as gravity sucks material to there centers, where it piles up forming a protostar. The Sun and Solar System formed in this way. The protostar becomes hot for at least two reasons.
- Gravitational contraction: Initially, as gas molecules and particles of dust plunged toward the center to form the protosun (our protostar), it shone brightly from the heat produced by their friction.
- 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 sun, nuclear fusion replaced gravitational contraction as the primary source of solar heat.
The Sun and Planets to scale
Origin of planets:
- Accretion: As particles of dust in the planetary nebula encountered one another, they often stuck together due to electrostatic forces. These formed larger and larger aggregates called.....
- Planetesimals: At a certain point, as clumps of materials became larger, gravity replaced electrostatic attracton as the dominant force. Planetesimals "fell onto" one another forming planet sized bodies. As these grew, some attracted gasses that formed primitive atmospheres. Then an abrupt change took place.....
- Solar wind: As nuclear fusion became the sun's dominant power source, atoms on its surface became energized to the point that they began flying off of the sun's surface as a solar wind. This wind swept the inner solar system clean of gas and dust. The accretion of new planetesimals stopped.
Planetary types: A function of mass and distance from the sun.
- Terrestrial planets: Small (The Earth is the largest), composed of rock and metals, with small atmospheres. These formed in the solar wind-swept inner solar system. E.G.: Mars.
- Jovian planets: (a.k.a. "gas giants") Large (tens to hundreds of times the mass of the Earth), composed of hydrogen and helium, ices (volatile substances such as water, methane, and amonia) rock and metals, with massive atmospheres. These formed far enough from the sun that lighter gasses like hydrogen and helium were not swept away by the solar wind and could be trapped as part of the growing planet's massive atmospheres. Indeed, much of the bulk of these giant planets consists of hydrogen in gaseous or liquid form. E.G.: Neptune. (The largest is Jupiter.)
- Smaller icy bodies: Pluto, Keuper Belt objects, and most moons of the outer planets. Containing high percentages of solid ices (water, ammonia, and in some cases methane) in addition to rock and possibly metals. (A pet peeve. IMHO, the International Astronomical Union's decision to "demote" the nineth planet Pluto to "dwarf planet" status, joining Ceres, Eris (the largest Keuper Belt object), and a list of other small worlds, was a mistake that will eventually become conspicuous. But I digress...).E.G.: Enceladus, a moon of the jovian planet Saturn.
- Myriad Small bodies: Including rocky, metallic, and icy asteroids and comets. E.G: Eros - an asteroid.
- For more GOOD up to date information on planets, visit Bill Arnett's Nine Planets website.
- For info and images of the surface of Mars, visit the Mars Rover mission website.
- For astounding new info and images of Saturn and it's moons, visit the Cassini/Huygens mission website.
- Breaking news as Messenger visits Mercury for the first time in thirty years.
Early history of the Earth: Remember those undifferentiated meteorites? The Earth was originally made of lots of that undifferentiated material. And yet, the oldest known rocks are very different from undifferentiated meteorites and from one another. What happened in between the formation of the Solar System and the formation of the earliest rocks that we can put our hands on to cause this differentiation?
- Period of Gravitational heating:
- Earth's Interior: uniform and undifferentiated
- Earth's Exterior: being heated by impacting planetesimals yielding magma ocean kilometers deep.
- Earth's Interior: uniform and undifferentiated
- Radiogenic heating brought about by the decay of radioactive isotopes inside the Earth. Some, like 26Al, decayed very quickly, very early in Earth's history, releasing radiogenic heat much faster than is released inside the modern Earth.
- Net effect of all heat sources was to melt or soften the Earth's interior enough to allow lighter materials to rise and heavier materials to sink.
- This gravity-induced movement caused more gravitational heating, accelerating the process.
- Result: A layered, differentiated Earth with heavier materials (mostly metals) at the center and lighter ones (mostly rocks) near the surface.
Differentiation yields three basic compositional layers of the Earth. (NOTE THE WORD "COMPOSITIONAL!" There are other ways of breaking down the Earth's layers that we will learn later.)
The origin of the Moon through the Giant Impact of a Mars-size planetesimal.
- All moon rocks are very depleted in volatile materials (i.e. stuff that would quickly evaporate when heated, like water) compared to Earth rocks.
- The oldest moon rocks are a mere 4.53 billion years old (compared with 4.56 for undifferentiated meteorites.) Apparently when the Moon formed, Earth had been around for around 30 million years. (That's roughly half the interval between the extinction of the dinosaurs and now.)
- Recent observations of nearby young stars with powerful telescopes have revealed dense belts of rock and dust that might expect from planetary collisions.
To date, the only hypothesis that hasn't been falsified by good data is that the rocks that formed the moon were blown off the Earth's surface by a giant impact. Here's the sequence:
- Earth is struck by a Mars-sized impactor. By this point, both planets are well-differentiated.
- The impactor vaporizes large parts of Earth's mantle, but does not shatter its core. The impactor's mantle is vaporized, but its core is mostly captured by the Earth.
- All vaporized material goes into orbit around Earth. Non-volatile substances (rock, metals, etc.) quickly condense into an aerosol of small fragments.
- By this time, the sun's nuclear fusion has begun and the solar wind is had started to blow, so that volatiles (i.e stuff like water that doesn't quickly condense and solidify) are swept away into the outer solar system. New Hubble Space Telescope image shows solar wind's effect on the products of a contemporary collision of planetesimals.
- The remaining non-volatile stuff accretes to form the moon. It is poor in volatiles because they were swept away by the solar wind.
Earth's stratification and other terrestrial planets. Like them, Earth has:
- Stratified realm of solid material
- Atmosphere: Gaseous envelope
However it also has these unique realms:
These are the major subsystems of the dynamic Earth System.
- Hydrologic (water) cycle
- Rock cycle
- Tectonic cycles
Merck adds instructive example of carbon cycle.
Compare Earth to Venus
Venus, unlike Earth, has essentially no carbon cycle. On Earth,
Follow this link for a fictional but realistic cinematic imagining of Venus' surface. BBC's Voyage to the Planets - Venus. But on Venus there is no hydrosphere, so CO2 just gathers in the atmosphere. Venus has no proper carbon cycle.
|Digitally reworked Venera images showing Venus surface in normal perspective.|
A major source of Earth's uniqueness is its abundance of interacting cycles.
But note: Up until a year ago, we thought that Earth was the only planet with an active hydrologic cycle. We now know of one other. Problem is, water is not involved.
Key concepts and vocabulary:
- Sources of elements in universe
- Big Bang
- Nuclear fusion
- Stellar/Solar wind
- Planet types
- Gravitational heating
- Radiogenic heating
- Earth's compositional layers
- Giant impact hypothesis of formation of Moon
- Goldilocks Zone
- Earth Cycles
- Hydrologic cycle
- Rock cycle
- Tectonic cycle
- Carbon cycle
- Comparison to Venus