Download formation of the solar system

FORMATION OF THE SOLAR SYSTEM
All bodies in S.S. formed at about the same time from the same cloud of interstellar dust
and gas.
Patterns of motion:
1. all planets orbit Sun counterclockwise
2. all planets orbit on nearly same plane
3. nearly circular orbits – and space betw. Planets increases with distance from Sun
(extra-wide gap betw. Mars and Jupiter populated with asteroids)
4. most planets rotate in same direction in which they orbit (CCW as seen from
above Earth’s North Pole) and with fairly small axis tilts, i.e. less than 25 degrees.
5. Almost all moons orbit planet in same direction as planet’s rotation and near the
planet’s equatorial plane.
6. Sun rotates in same direction in which planets orbit
Categorizing Planets
Terrestrial planets – “Earth-like” planets include Mercury, Venus and Mars. Relatively
small; close to Sun; close together; solid rocky surface and abundance of metals in deep
interior. Have few moons, if any. Our Moon is considered a fifth terrestrial world.
Jovian planets – “Jupiter-like” (Jupiter, Saturn, Uranus and Neptune). Large; far from
Sun; widely spaced from each other. Made of Hydrogen, Helium, Hydrogen compounds
(methane, ammonia and water). Have small amts. of rocky material deep in cores. (cores
are still probably 10 earth masses) No solid surface. If you plunged deep into an
atmosphere, you would sink deeper and deeper until crushed by overwhelming pressure.
Each Jovian planet has rings and an extensive system of moons.
Asteroids and Comets – most numerous objects in S.S.
Asteroids are small, rocky bodies that orbit Sun primarily in asteroid belt. (the Trojan
asteroids share Jupiter’s orbit) Orbits lie close to planetary orbits, although most have a
tilt. Some have elliptical orbits (compared with near circular orbits of planets) More than
10,00 have been identified and catalogued. Probably a huge number of yet unknown
small asteroids. Very large asteroids (few hundred kilometers in radius – much less than
½ Moon’s radius.
Comets are small icy bodies that spend most of their lives beyond the orbit of Pluto.
When one dives into the inner solar system, it grows a spectacular tail. Many billions of
comets are probably orbiting the Sun in two broad regions:
KUIPER BELT – betw. orbit of Neptune (30 AU from Sun) to 100AU from Sun
**AU = Astronomical Unit = average distance between Earth and Sun. Kuiper Belt
comets travel around Sun in same direction as planets.
OORT CLOUD – Huge spherical region centered on Sun and extending perhaps halfway
to nearest stars. The comets have completely random orbits.
EXCEPTIONS TO THE RULES:
1.
2.
3.
4.
Mercury and Pluto have larger eccentricities and inclinations than other planets
Rotational axis of Uranus (and Pluto) substantially tilted
Venus rotates backward
Pluto has a moon almost as big as itself
NEBULAR THEORY OF SOLAR SYSTEM FORMATION
Our Solar System formed from a giant swirling interstellar cloud of dust and gas (called –
a “nebula”) MWG originally contained just Hydrogen and Helium.
The “galactic recycling process” gradually enriches the galaxy with heavier elements so
that later generations of stars are born with a greater proportion of heavier elements than
earlier generations.
By the time our SS formed (4.6 bya), 2 percent of the original H and He had been
converted to heavier elements.
COLLAPSE OF SOLAR NEBULA
Collapsed piece of interstllar cloud formed our Solar Nebula. It collapsed under its own
gravity probably triggered by a shock wave.
Before collapse, low-density gas had diameter of a few light years. It collapsed to 200
AU, about twice the diameter of Pluto’s orbit.
Solar nebula became hottest in its center – much of the mass collected to form a protosun
Protosun became so hot that nuclear fusion ignited in its core. The Sun then became a
full-fledged star!
Solar nebula rotated faster and faster as it shrank in radius. It flattened into a disk
(protoplanetary disk) from which the planets eventually formed. This explains why all
planets and most of their moons orbit in the same plane and in the same direction.
Evidence of Solar Nebula Theory: Collapsing nebulas (where new SS’s forming) emit
strong infrared. We can observe locations where other star systems are forming – i.e.
Great Orion Nebula.
BUILDING THE PLANETS
Initial mix – 98% H & He; 2% heavy elements (incl. rock and metals)
Gravity drew much of the material in the collapsing solar nebula into the protosun.
In the rest of the nebula, material so spread out that gravity could not pull together
material to form planets on its own – it required “seeds” or condensates. The different
kinds of planets formed from the different kinds of condensates present at the different
locations in the SS.
Ingredients of the solar nebula fall into these 4 categories based on condensation
temperatures: Metals, Rocks, Hydrogen compounds and Light gases.
METAL – iron, nickel, aluminum, etc. Less than 0.2% of solar nebula’s mass. Condense
into solids betw. 1000-1600 K
ROCKS – primarily silicon-based minerals. About 0.4% of solar nebula’s mass.
Condense at 500-1300 K.
HYDROGEN COMPOUNDS – molecules of methane, ammonia and water. Make up
1.4% of nebula’s mass. Solidify into ices below 150 K.
LIGHT GASES – hydrogen and helium. Never condense under solar nebula conditions.
Make up remaining 98% of solar nebula’s mass.
Close to protosun – too hot, everything remained a vapor. Further out a bit - metal flakes
condensed; still further out - rocks condensed. Area of asteroid belt: minerals containing
small amts. of water as well as dark carbon-rich minerals condensed. Beyond “frost line,”
it was cold enough for hydrogen compounds to condense into ices. Outer SS contained
condensates of all kinds (rocks, metals and ices) however, ice flakes were far more
abundant.
Process of growing by colliding and sticking is called ACCRETION.
NEBULAR CAPTURE – MAKING THE JOVIAN PLANETS
SOLAR WIND – CLEARING AWAY THE NEBULA
LEFTOVER PLANETESIMALS
ORIGIN OF ASTEROIDS AND COMETS
THE EARLY BOMBARDMENT
CAPTURED MOONS
GIANT IMPACTS – FORMATION OF OUR MOON