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Unit 1.3: Our Solar System. I. Formation of the Solar System. Nebular hypothesis : bodies of solar system condensed from enormous cloud of interstellar dust as follows: As nebula begins to contract, spins faster and flattens into a disk shape Evidence: planar orbits

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I formation of the solar system
I. Formation of the Solar System

  • Nebular hypothesis: bodies of solar system condensed from enormous cloud of interstellar dust as follows:

    • As nebula begins to contract, spins faster and flattens into a disk shape

      • Evidence: planar orbits

    • Most material pulled to center  becomes protosun

    • Temperature drops, allows iron/nickel to solidify


  • Accretion of these solid particles  forms “planetesimals”

    • Accretion: clumping of debris to make larger planetary bodies (planetesimals)

    • Gravity of planetesimals allows further accumulation to become protoplanets

  • As protoplanets accumulate debris in solar system, helps to “clear out” solar system

    • Now, solar radiation can heat protoplanets

  • Heat causes gases to vaporize from inner planets, carried away by solar winds


The sun 99 85 of mass of solar system
The Sun: ~99.85% of mass of solar system

  • The Sun’s Interior

    • Core: undergoing nuclear fusion H  He

    • Radiative Zone: surrounds the core

      • Energy “radiates” away from core as electromagnetic waves

    • Convective Zone: convection gives rise to sunspots:

      • Convection is the transfer of energy by moving matter

      • As hot gases rise to surface, they expand and cool (light spots)

      • But, as they cool, they condense and sink back into Sun (dark spots)


The sun s atmosphere
The Sun’s Atmosphere

  • Photosphere – “sphere of light”:

    • Energy is given off in the form of visible light

  • Chromosphere: “sphere of color”

    • Thin layer of gases

    • Appears as thin, red rim around the Sun b/c of hydrogen cooling

    • Solar eruptions:

      • Spicules: small flares

      • Prominences: large flares that erupt due to interaction with magnetic fields

      • Solar flares: largest eruptions of energy


  • Corona – “crown”

    • Outermost portion of solar atmosphere

    • Solar wind: ionized gases (plasma) that escape the Sun’s gravity

      • Mostly lost to space

      • But if it reaches Earth, interacts with our magnetic field to create auroras (Northern and Southern lights)


Iii planet formation
III. Planet Formation

  • As remaining dust condenses to become a planet, chemical differentiation occurs:

    • Denser, heavier elements (nickel and iron) sink towards center of planet

    • Lighter elements (silicon, oxygen, hydrogen) move towards surface of planet

    • Gases escape unless a planet has enough surface gravity to hold them in an atmosphere

      • Escape velocity: speed an object must reach to escape the gravitational pull of a planet

        • 11km/sec for Earth

        • Higher speed for Jovian planets b/c greater surface gravity


B inner planets mercury venus earth mars
B. Inner Planets: Mercury, Venus, Earth, Mars

  • Terrestrial planets: ‘terra-’ = Earth

  • Between Sun and asteroid belt

  • Rocky planets, metallic core

    • Dense b/c gravity pulls heavier elements closer to Sun

  • Thin to no atmosphere

    • Because they are closer to Sun, they are warmer, so fewer gases, ices

  • Smaller in diameter

    • Too warm to retain gases = little to no atmosphere = smaller planets

  • Few to no satellites: 0-2 moons

    • Smaller planets = less gravity = fewer moons


C outer planets jupiter saturn uranus neptune
C. Outer Planets: Jupiter, Saturn, Uranus, Neptune

  • “Jovian” planets: ‘jove-’ = Jupiter-like

  • Beyond asteroid belt

  • Gaseous planets, some metals in core

    • Less dense b/c greater amount of light gases than inner planets

  • Thick atmosphere

    • Farther from Sun, colder, able to retain gases

    • Lower temps = less energy, lighter gases cannot “escape”

  • Larger in diameter

    • Primarily because of thicker atmospheres

  • Many satellites: 8-21 moons

    • Larger planets = more gravity, can “hold” more satellites



  • Dwarf Planets: (beyond Neptune)

    • Orbit the Sun:

      • If they orbited another planet, called a moon of that planet

    • Spherical in shape:

      • Gravity ‘pulls’ them into “spheres”

      • If irregular in shape, then asteroids, not planets

    • Not large enough to “clear” their orbit

      • Larger planets accrete smaller bodies or fling them out of their orbit - dwarf planets cannot

    • Ex: Pluto, Ceres, Eris,

      Haumea, Makemake


Iv the moon
IV. The Moon

  • Giant-impact hypothesis:

    • During formation of planets, large body impacted Earth, liquefied surface and ejected crustal and mantle rock

    • Debris entered orbit around Earth, coalesced into moon

  • Supporting evidence:

    • Density of moon is similar to density of material in Earth’s mantle, but not Earth’s core

      • Small iron core in Moon


C lunar surface
C. Lunar Surface

  • Craters: produced by impact of meteoroids

    • No atmosphere = no friction to slow down impact

    • Meteoroid hits, compresses rock

    • Compressed rock rebounds, ejecting surface material from crater

    • Ejecta builds rim of crater

    • Heat of impact melts lunar rock:

      • Astronauts brought back glass beads made in this way

    • No change in shape b/c no processes of erosion (wind, water, etc…)


  • Highlands: mountain ranges due to tectonic processes

  • Maria: (L. mare = ‘sea’) “Seas” of basaltic lava that bled out when asteroids punctured surface

  • Regolith: soil-like layer of lunar dust generated from erosion by meteors

    • ~10 ft thick!


V small solar system bodies
V. Small Solar System Bodies

  • Asteroids: small, rocky bodies

    • Microplanets: grains of “sand” up to 1000 km across

    • Most found in asteroid belt, between Mars and Jupiter

      • Others originate in Kuiper Belt just beyond Uranus

    • Fragments of broken planet?

      • Not enough mass

    • Or several larger asteroidal bodies that collided, making smaller asteroids?


  • Comets: “icy dirtballs”

    • Originate from Kuiper Belt or from Oort Cloud

    • Rocky, metallic materials held together by frozen gases: H2O, methane, ammonia, CO2, and CO

    • Parts of a comet:

      • Coma: glowing nucleus, the head of the comet

      • Tail: as it approaches the Sun, gases vaporize, begin trailing the head


  • Orbit of a comet:

    • Tail always points away from Sun b/c:

      • Radiation pressure from Sun pushes dust particles away from coma; forms one tail

      • Solar winds move ionized gases, esp. carbon monoxide; forms a second tail

    • As comet moves away from Sun, gases/dust recondense

    • Every revolution, size decreases b/c loses mass


  • Meteors: “shooting star”

    • In space, it’s called a meteor, but as it burns up in Earth’s atmosphere, it’s called a meteoroid

    • Meteorite: remains of a meteoroid, found on Earth

    • Meteor shower: a large # (60+) of meteoroids traveling in same direction, at nearly same speed of Earth


  • Meteors are classified according to composition:

    • Irons: mostly iron, and 5-20% nickel

    • Stony: silicate materials, other minerals

    • Stony-irons: mixtures

    • Carbonaceous chondrites: contain amino acids, other organic compounds – life from outer space?


  • Impact of meteors on Earth:

    • Moon-like craters

    • Meteor Crater, Arizona:

      • ¾ mi across, 560 ft. deep

    • Extinction events?

      • Iridium – common in meteors – found in a layer of Earth that corresponds to time when dinosaurs are believed to have gone extinct

    • Why are there so few craters on the Earth, and so many on the moon?

      • Erosion, weathering has actively “recycled” craters back into Earth’s crust

  • Age of meteorites confirms age of Earth @ 4.5 billion years old



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