The Solar System and the Layered Earth

• The Solar System and the Layered Earth
  
  
  

   Origin of the Solar System

• age of universe: 15 billion years (a recent, more precise value is 13.73 +/- 0.12 billion years)
(NB for the interested: we know this from estimates of the lifetime of protons which are considered "stable" particles.)
• age of solar system⁽¹⁾ (and Earth): 4.5 billion years (Moon about 100,000 years younger)
⁽¹⁾estimated from the study of meteorites Currently, no hypothesis can explain all observations adequately but the modified nebular hypothesis, initially by Immanuel Kant (1755), then updated, is the most widely used/accepted. The nebular hypothesis supports the idea that sun and planets formed at the same time.

• initially huge rotating spherical cloud (nebula) of ice, gas, debris
• spinning cloud contracts
• to conserve angular momentum, spinning speeds up (like figure skater pulling arms in)
• flattening to disk
  (NB for the interested: the exact reason for this is poorly understood but one hypothesis is that particles from above and below the disk collided near the disk and propelled each other further out - this is needed to explain why the planets carry most of the angular momentum in the solar system)
• immense density at center leads to gravitational collapse and formation of sun
(NB for the interested: immense pressure and temperature during gravitational collapse started thermo-nuclear processes in the sun)
• in rotating disk, masses collide and stick together (accretion)
(NB: exactly how this was done is still under debate; two possible models are called gravitational instability theory and core-accretion theory)
• accretion first leads to planetesimals (a few km radius), then planets (> 1000km radius)
A publication in "American Scientist" likens the core-accretion process to what happens to dust particles under your bed when you don't vacuum for a while. If you are logged in from a UCSD campus machine, you can access the article.

• Members of the Solar System

   

  

• sun has 99.9% of all the mass in the solar system
• planets have 99% of all the angular momentum in the solar system


• terrestrial planets, inner planets: Mercury, Venus, Earth, Mars
(rocky planets)


• gas giants, outer planets: Jupiter, Saturn, Uranus, Neptune
(Jupiter and Saturn are gas planets with relatively small cores; Uranus and Neptune are icy planets; all four consist of mainly H₂ and He)
• Jupiter has 60% of all the mass of the planets and 70% of all the angular momentum
• all planets orbit the sun in the same direction (counterclockwise when viewed from above)
• all planets have nearly circular orbits
NB: a revolution about the sun can take from 80 days (Mercury) to 165 years (Neptune)
• all planets also revolve about a spin axis
a spin can take from 9h (Jupiter) to 243 days (Venus)
• all but Mercury and Venus have Moons/satellites


• Moon
• Earth's satellite, not a planet
• formed during early formation when Mars size impactor hit Earth (~4.4 billion years ago)
• Moon made mostly of Earth mantle rock
• Pluto
• discovered relatively late: on 13 February 1930 by Clyde W. Tombaugh
• Pluto is relatively small (less than half the diameter of Mercury; only 2/3 diameter of Moon); smaller than other celestial objects found in the solar system
• Pluto's orbit is highly elliptical and inclined with respect to all the other planets' orbits
• Pluto has a very large moon, Charon, whose diameter is about half that of Pluto
• Pluto is one of the trans-Neptunian objects (TNOs), meaning they lie beyond Neptune
• Pluto was found to be too unusual to fit the typical 'planet of the solar system'
• used to be one of the 9 planets but was downgraded to a dwarf planet in August 2006 during a conference of the IAU (International Astronomical Union) in Prague, Czech Republic


• dwarf planets:
Pluto is now a member of the dwarf planets. Other examples are Ceres in the asteroid belt and Eris (formerly known as Xena), a TNO further out than Pluto but larger and whose discovery was ultimately responsible to downgrade Pluto to a dwarf planet. Read about Pluto at wikipedia
• definition of a dwarf planet
• has own orbit around Sun
• has sufficient mass to assume nearly round shape
• is not a satellite of a planet or other nonstellar body
• has not "cleared the neighborhood" around its orbit
The only difference between the definition of a dwarf planet and a real planet is that real planets have "cleared the neighborhood" around its orbit, meaning that they have "scooped" up all the debris along its orbit as a result of its large mass and gravitational force.


• Current list of dwarf planets as of fall 2011, the IAU has recognized five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake and Eris. Only Ceres and Pluto have been observed in enough detail to demonstrate that they meet the definition of a dwarf planet. Eris has been accepted because it is more massive (by 27%) than Pluto. The two trans-Neptunian objects Haumea (rather egg-shaped, having 2 moons) and Makemake currently qualify because their brightness of +1 or larger suggests that their diameter is 838 km or larger). However, hundreds more objects are proposed for consideration. Using the brightness argument, four additional trans-Neptunian objects are candidates, as of August 2011: 2007 RQ₁₀, Sedna, Quaoar and Orcus.



• Pluto's Fall Continues: Pluto's struggle for status isn't over. Charon is currently considered Pluto's satellite but it does not really orbit it. Due to the size of Charon relative to Pluto, the center of mass for the Pluto-Charon system (which orbits the Sun) lies outside of Pluto. One could argue that Pluto does not have its own orbit around the Sun (one of the criteria to be a dwarf planet). There is therefore the suggestion that Pluto and Charon should be re-classified together to be a dual, or binary dwarf planet system.

NB: ER, Earth radius = 6371 km; EM, Earth mass = 5.974x10²⁴ kg; AU, astronomical unit = 1.4960x10⁸ km
* distance to Earth: 3.844x10⁵ km
** this is 2.5 times the mass of all the other planets taken together
*** Ganymede, Callisto, Io and Europa are similar to terrestrial planets; Ganymede is larger than Mercury
**** Titan (largely made of ice) is the only satellite with an atmosphere and is larger than Mercury
^oCeres was discovered on 1 January, 1801, 45 years before Netpune. It was considered a planet for half a century before reclassification as an asteroid.
^oo Charon is currently considered Pluto's satellite but Pluto/Charon are really a binary dwarf planet system
-- Eris is larger and more massive than Pluto

• asteroid belt between Mars and Jupiter:
• belt where there should be another planet, according to Bode's Law
• scientists believe that either an impactor destroyed a planet initially formed there or that, due to the tug of Jupiter, a planet could never form
• most objects too small to be rounded
• 3 objects larger than 500 km in diameter (Ceres, Pallas, Vesta)
• some of the larger objects are planetesimals or dwarf planets, with Ceres being the largest at about 1000 km in diameter
• some have Earth-crossing orbit (Apollos) -> past and future impactors on Earth
• some have Mars-crossing orbit (Amors)
• asteroids are low-density, rocky objects
NB: Ceres was actually considered a planet from 1801 until the 1860s.
NB: Neptune was discovered in 1846 and both Neptune and Pluto are not located where Bode's Law would place them. check out wikipedia.


• comets:
  • have rocky core (a few km across) embedded in icy shell ("dirty snowball")
  • upon approach to sun, solar energy causes glow and ejections of icy material
  • left as debris to make for bright meteor showers when Earth approaches debris on its orbit (e.g. Perseid Meteor Showers in August)
  • the core of some comets are large enough to pose a serious impactor risk (e.g. Shoemaker-Levy 9 comet that hit Jupiter in 1994) view wikipedia page
  • In July 2009, another comet impacted on Jupiter see newsclip
  
  
• Kuiper belt:
  • on the plane of the solar system (ecliptic) beyond Neptune
  • extends out to 55 AU (1AU=Earth-Sun distance)
  • source of short-period comets (recurrence < 200 yrs)
  An example:
  • e.g. Halley's comet comes about every 75 yrs though Halley's comet originally may have come from the Oort cloud, last appearance/apparition: 1986)


• Oort cloud:
  • vast, diffuse spherical cloud around the solar system
  • extends out to 50,000 AU (1 light-year)
  • source of long-period comets (recurrence > 200 yrs)
  • period of comets change after encounter with gravitational pull of gas giants
  Some examples:
  • Kohoutek last came 150,000 yrs ago, will come again in 75,000 yrs, last appearance: 1973; its rocky composition suggests that it was originally from the Kuiper belt
  • Hale-Bopp last came 4,200 yrs ago, will come again in 2,533 yrs, last appearance: 1997; orbit almost perpendicular to the ecliptic, so collision unlikely
  • Hyakutake used to come every 17,000 yrs but now comes every 70,000 yrs, last appearance: 1996


• Meteoroids, Meteorites and Meteors:
  • meteoroid: fragment of asteroid or comet entering Earth's atmosphere
  • meteor: visual phenomenon when meteoroid burns up in Earth's atmosphere
  • meteorite: remaining piece(s) of meteoroid after it impacts on Earth's surface;
    there are two types
    iron/metallic meteorites are thought to resemble Earth's core
    stony/rocky meteorites are thought to resemble Earth's mantle; rarer than iron meteorites

• Earth's History

• ~ 4.56 Ga ago: initially aggregating mass of particles and gas
(NB: Ga comes from Giga for billion and "annum", latin for year)
• random collision of pieces -> planetesimal -> planet
• collision and compaction led to heating
• differentiation: heavier components (e.g. iron-rich) pulled toward center; lighter components toward surface
• formation of Earth's iron core while Earth's outer shells were still soft
• Mars-size impactor hit earth, ejecting parts of mantle into space to form the Moon
• Earth cools and forms lithosphere
• ~3.9 billion years ago: (small) continents, oceans, dense atmosphere (CO2 dominant)
• ~3.5 billion years ago: first life (bacteria)
• ~2.5 billion years ago: large continents
• plate tectonics for at least last 1.5 billion years

• The Layered Earth

• radius: 6371 km; diameter (without atmosphere): 12742 km
• circumference: ~40000 km
• dense, iron-rich core (radius: 3480 km; diameter: 6960 km); composition similar to metallic meteorites
• solid inner core (radius: 1222 km; diameter: 2444 km)
• liquid outer core (mostly iron), movement of electrically charged particles cause magnetic field
• core-mantle boundary: at 2891 km depth; transition from liquid iron to solid rock
• mantle around core; composition similar to chondritic, stony meteorites (peridotite)
• transition zone between upper and lower mantle: 410-660~km depth
• weak, ductile asthenosphere above 350~km
• strong, brittle lithosphere above about 125~km
• crust above 70~km; very brittle; rich in silicates (SiO2-rich rocks)
• the 5 layer model: inner core, outer core, mantle, asthenosphere, lithosphere
• the 3 layer model: core, mantle, crust

• Elastic, Ductile and Brittle Material

Elastic, ductile and brittle describes a material's reaction when a force (or stress) is applied to it.
elastic material
• can be bent without breaking
• returns to original state after bending force is removed
• example: a metal spring
• stores potential energy (e.g. a loaded spring)
ductile material
• also bends without breaking but deformation is plastic, not elastic
• does not return to original state after bending force is removed
• example: putty, play dough
• no storage of potential energy
brittle material
• may be elastic when force is applied slowly
• breaks if force is applied rapidly or if force is greater than the elasticity of the material allows
• may store potential energy (when force is acting slowly)
• potential energy released when material breaks

• The Lithosphere and Asthenosphere; Earth's Layers relevant to Plate Tectonics

Lithosphere
• from Greek "lithos" for rock
• crust and uppermost mantle
• cool and strong but brittle
• zone of earthquakes (EQs)
• broken into 12 major plates and a few others
• oceanic: thin but dense
• continental: thick and less dense

Asthenosphere
• from Greek "asthenes" for weak
• uppermost mantle below lithosphere
• warm, soft and weak
• ductile

• Isostasy

principle of buoyancy
• 2 counteracting forces: gravity and buoyancy
• isostatic equilibrium: forces are balanced and body floats
Relevance to Earth


• lithosphere less dense than asthenosphere
• rigid lithosphere floats on soft asthenosphere like icebergs in water
• floating continents have roots/keels beneath
• asthenosphere reacts to imbalance by flowing (e.g. like viscous honey)
Example 1:
• oceanic plate cools and thickens with age
• gets denser, eventually losing buoyancy
• sinks into Earth's mantle
• drives mantle convection and plate tectonics
• use of potential energy to drive plate tectonics
Example 2 (Load through Ice Sheets during Glaciation):
• ice sheet adds mass added on top of lithosphere
-> asthenosphere beneath flows away
-> lithosphere sinks
• elastic rebound after ice melts (postglacial rebound)

Read more
http://geol.queensu.ca/museum/index.php?option=com_content&view=article&id=50&Itemid=57