FORMATION OF THE SOLAR SYSTEM
Notes from Chapter 8 in Cosmic Perspective
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.
Jovial 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 Jovial 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 1,000 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 – from orbit of Neptune (30 AU from Sun) up 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) Mercury and Pluto have larger eccentricities and inclinations than other planets
2) Rotational axis of Uranus (and Pluto) substantially tilted
3) Venus rotates backward
4) 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 b.y.a.), 2 percent of the original H and He had been converted to heavier elements.
COLLAPSE OF SOLAR NEBULA
Collapsed piece of interstellar 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) but ice flakes were far more abundant.
Process of growing by colliding and sticking is called ACCRETION. The original “planetesimals” were small, probably odd-shaped like asteroids. As they grew, they became spherical because the force of gravity was strong enough to overcome the strength of rock and pull everything toward the center. As the P grew larger, they gained more area on which objects would collide and stick. Some probably grew to 100s of km in size in only a few million years. Once they became large, further growth became more difficult. Gravitational attraction between P. caused collisions producing fragments. Only the largest P. avoided shattering and grew to full-fledged planets.
In the inner solar system, where only metallic and rocky flakes condensed, planets ended up being composed of rocks and metal. Because the solar nebula had only 0.6% rocky and metallic elements, the P. in the inner solar system could not grow very large. This is why the terrestrial planets are small.
Beyond the frost line, P. were built from ice flakes in addition to flakes of rock and metal. Because there was much more ice available in the outer solar system, these P. could grow to huge sizes. The largest icy P. of the outer solar system became the cores of the jovian planets.
The planets closest to the Sun have the highest densities (4-5 g/cc) and are made of rock and metal. The outer planets have densities ranging from below 1 (Saturn) to 3 g/cc.
Many meteorites contain metallic grains embedded in a variety of rocky minerals. Meteorites containing carbon-rich minerals as well as water have come from greater distances.
NEBULAR CAPTURE – MAKING THE JOVIAN PLANETS
As the icy P. grew to huge sizes, there increased gravitational fields enabled them to capture the abundant hydrogen and helium gases. The same processes that formed the protoplanetary disk (heating, spinning and flattening) formed similar but smaller disks of materials around the jovian planets. Condensation (of metals, rocks and lots of ice) and accretion took place within the jovian nebulae, essentially creating a miniature solar system around each jovian planet. The densities of the jovian moons (1-3 g/cc) reflect the mixture of icy and rocky condensates.
Why the huge space between jovian planet orbits? The rapid accretion in the outer SS may have allowed a few protoplanets to gobble up all their neighbors, leaving the jovian worlds widely spaced.
SOLAR WIND – CLEARING AWAY THE NEBULA
The remaining gas in the solar nebula was blown into interstellar space by the solar wind. The solar wind is fairly weak today, but was much stronger when the Sun was young. If the clearing of the gas did not occur when it did, ices might have condensed in the inner solar system. When the gases were cleared by the solar wind, the early solar system was essentially set.
The young Sun had a much faster rotation. The current Sun takes about 1 month to do a complete rotation. In the 1950s scientists realized that the young Sun’s rapid rotation would have generated a magnetic field far stronger than the Sun’s today. A highly “active” Sun would have more sunspots as well as emit higher energy photons (Ultraviolet and X-rays) This high-energy radiation ionized gas in the solar nebula, creating many charged particles. Charged particles and magnetic fields tend to stick together. As the Sun rotated, its magnetic field dragged the charged particles along. This slowed down the rotation of the Sun. The solar wind then blew the charged particles into space, leaving the Sun with its slow rotation seen today. This process is called “magnetic braking.” We can observe young stars that formed in interstellar clouds (i.e. Great Orion Nebula) and see evidence of rapid rotation, strong magnetic fields, strong stellar winds and ionized gases in the nebula.
LEFTOVER PLANETESIMALS
ORIGIN OF ASTEROIDS AND COMETS
The solar wind was not capable of clearing away excess rocks fragments and ice balls. These leftovers became the comets (icy and in the outer solar system) and asteroids (metallic and rocky and in the inner solar system)
The combined mass of all the asteroids in the asteroid belt is 1/1000 of the earth’s mass. Jupiter’s gravity affects the asteroids, often causing them to change orbit and collide with each other. The debris from these collisions may burn in the earth’s atmosphere (meteors or “shooting stars”) or fall into the earth (meteorites).
The icy P. that cruised the space between Jupiter and Neptune couldn’t grow more than a few kilometers in size before suffering either a collision or a gravitational encounter with one of the jovian planets. Many were flung into distant and random orbits becoming the comets of the spherical Oort cloud.
Kuiper Belt comets follow organized orbits and some are hundreds, even thousands of kilometers in diameter. Pluto is a Kuiper Belt comet. In the 1950s predictions were made that comets inhabited the outer solar system. In the 1990s Kuiper Belt comets were verified.
THE EARLY BOMBARDMENT
The collision of a leftover planetesimal with a planet is called an IMPACT. Impacts leave scars called IMPACT CRATERS. The heavy bombardment of planetary surfaces in the early SS would have resembled a “rain of rock and ice from space.” The Earth’s surface was once scarred like the Moon’s surface, but erosion and other geological processes erased most of the scars. Bombardment on Earth ended about 4 billion years ago, 600 million years after the Earth formed.
CAPTURED MOONS
Mars’s two moons, Phobos and Deimos, are captured asteroids.
GIANT IMPACTS – FORMATION OF OUR MOON
Our Moon formed by a giant impact of the Earth by a Mars-sized object.
The Moon’s composition is similar to that of the earth’s outer layers. The Moon did not form at the same time as the Earth. If it did, it would have formed from the same material thus having a similar density. The Moon’s density is much less than that of the Earth.
Giant impacts were probably responsible for the axis tilts of many planets, tipping Uranus on its side, as well as the backwards rotation of Venus.
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