U238>Pb206 Halflife: 4.5 billion years Oldest earth rocks 3.96 billion years Meteors and Moon rocks 4.6 billion years This is the time they solidified The solar system is older than this. Radioactive Dating
When Did the Solar System Form? 4.56 billion years ago How do we know? (evidence for formation) Comet Dust 4.4 to 4.6 BY Lunar samples - 4.5 to 4.6 BY Meteorites - 4.56 BY Including rocks and dirt from Mars Earth 3.9 (or 4.4 BY)
What are the facts about our solar It s at least 4.5 BY old Shaped like a disk system? Orbits in the same direction around the sun Rotate in the same direction (one exception) Moons all orbit in same direction (one exception out of hundreds) Inner planets rocky, outer planets gaseous Outermost objects icy (frozen gasses)
The Solar Nebula Theory The sun and solar system formed from the collapse of a cloud of gas and dust. The cloud was slowly rotating, so centrifugal force made it into a disk (accretion disk) and gravity caused a transfer of matter to the center. Conservation of angular momentum made it rotate more quickly as it contracted Near the sun, the rocky planets formed. Farther away formed the gas giants. Farther still, the icy bodies formed.
Planetoidal disk It s important to note that the planets were forming simultaneously with the proto-sun.
The Solar Nebula Theory Instabilities in the disk may have formed smaller sub-disks where giant planets formed (sometimes there is enough material to form a second companion star). A type of Coriolis Effect caused all of these instabilities to rotate in the same direction as the main disk. The gas giants and their systems of moons can be thought of as Mini-solar systems.
Eventually, the sun became a star (nuclear fusion began). The resulting solar winds swept gasses out of the inner solar system.
So, WHY is the solar system differentiated? In other words, why are the rocky planets close to the sun, the gas giants far away, and the icy bodies way, way out there?
The Solar Nebula Theory Dust, rock and ice condense and stick together to make small bodies called planetesimals. Heat from the newly formed sun only allowed certain elements to condense nearby (rock and metal). Gas could only condense farther out. Ice (frozen gasses) can also form beyond the frost line.
Only rocky and metallic planets could exist close to the sun. Too hot for most gasses to condense Any gasses emitted by volcanic activity is swept away by solar winds Why is Earth allowed to have an atmosphere now?
Beyond the FROST LINE gasses could condense and gather around anything with gravity (a large planetesimal) The Jovian planets Gasses can also freeze and accrete into icy bodies, such as most of the outer moons and the trans-neptunian objects.
Why, do you suppose, are the gas giants gaseous? Why don t their gasses freeze solid into large icy bodies?
So, the solar system is differentiated as a result of temperature, ( and density to a lesser degree).
Planetesimals: trillions of trillions of fragments of dust; rocks; ice chunks; even molecules of gas can be considered a planetesimal. No defined shape. Planetoid: thousands (maybe tens of thousands) of them in the early solar system. A result of millions of years of planetesimal collisions. They are larger and resemble a sphereoid shape. They have enough gravity to shape themselves into spheroids.
A protoplanet is just like the planets we have today except they are un-differentiated, meaning they may still be very hot and do not have an organized internal structure. May be completely molten.
The Solar Nebula Theory Two ways of building planets Larger planetesimals attract smaller ones. They collide and merge to make a bigger planetesimals. These attract more and eventually form planetoids (massive enough to have enough gravity to shape itself into a spheroid), then protoplanets (massive enough to generate internal heat, possibly becoming entirely molten). Near the sun, the nebular hydrogen gas is too hot to condense and be collected by planetoids. Distant planets begin to collect hydrogen atmospheres once they get big enough to gravitationally draw it in. The Terrestrial planets created their own atmospheres. The Jovian planets captured theirs.
The Solar Nebula Theory The young planets start out fairly warm (in a liquid or nearly liquid state). Heavy elements start to sink This concentrates the radioactive elements in the center (and explains why the earth s core is hot). Differentiation
The heat comes from three sources: Gravitational contraction Radioactive elements in the core Constant bombardment (kinetic energy to heat energy)
The Solar Nebula Theory On the terrestrial planets, gasses are released from the hot interior to form atmospheres. Volcanic processes release H 2 S, SO 2, CO 2, H 2 O, NH 3, N 2 Solar UV radiation breaks apart NH 3, hydrogen escapes, leaving N 2 Solar UV radiation also breaks apart H 2 O, hydrogen escapes leaving O, which reacts with rock to form solid oxides. H 2 O combines with H 2 S, and SO 2 to make sulfuric and sulfurous acid. This eats away rock to form solid sulfates. What s left? CO 2 and N 2 in the atmosphere. Oxygen and Sulfur in the rocks. (Why is Earth s atmosphere different today?)
Where did the nebula go? Solar wind, heat, and light pressure drove the gas away. What about the left over planetesimals? Most of the rocky ones in the inner solar system eventually collided with planets. (That s why the rate of impacts was high 4 Bya, but is low now.) There s about 20,000 left over mostly between Mars and Jupiter (Asteroids!) Jupiter s gravity prevented a planet from forming there. Encounters with the giant Jovian planets kicked most of the remaining icy ones into the outer solar system or interstellar space These are comets! The encounters would kick them in any direction. (This explains why comets aren t concentrated in the plane of the solar system.)
How Do We Know About the Processes that Occurred in the Formation of our Solar System?
Solar System Samples Meteorites 1) Stony Meteorites Chondrites - 90% - tiny balls (chondrules) of silicate minerals formed by rapid cooling. Oldest rocks of solar system. Carbonaceous Chondrites minor organic compounds (amino acids), water in minerals. Achondrites 8% - similar to terrestrial igneous rocks. Younger pieces of igneous rocks produced on larger asteroids, Mars, Moon
Meteorites 2) Iron Meteorites contain iron, nickel - Large crystals indicating slow crystallization. Cores protoplanets. 3) Stony-irons - mix of iron-nickel and silicate minerals. Transition zone between iron-nickel core and silicate mantle.
That s why NASA is so interested in asteroids! NEAR - Near Earth Asteroid Rendezvous - Landed on Eros (because it is big and close) 2/2001 Dawn Orbit Ceres (primitive and wet) and Vesta (evolved and dry) 6/2006
And NASA s Interested in Comets, Too Earliest history of Solar System - chemical and physical info about formation and building blocks of planets (rest of stuff was pulled into the Sun or other planets. Stardust Passed through Comet Wild 2 Coma 1/2004 Sample Return - 1/15/2006
Comets Comet Tempel 1 Observe how a crater forms and measure it Learn about what s inside Learn about comet outgassing Launch 12/04
impact showed the comet to be more dusty and less icy than expected.
How does this theory fit the characteristics of the Solar System? 1. & 2. Collapse to a disk explains the concentration in the plane of the solar system, and why almost everything moves in the same direction. 3. The giant planets had disks of their own so their moons orbit in their equatorial plane (and their rings, which are essentially trillions of small moons) 4a. Because the inner solar system was hot, only rock and metal could condense which resulted in terrestrial planets 4b. The outer solar system was cold enough for ices to condense and for hydrogen gas to be captured by a massive enough body. This resulted in Jovian planets. 4c. If an object in the outer solar system wasn t massive enough to capture hydrogen gas, it remained as a small icy body. (Pluto, Eris, the outer planet moons, comets, Kuiper Belt)
How does this theory fit the characteristics of the Solar System? 4d. The terrestrial planets released their atmospheres from their interiors. The Jovian planets captured theirs. The icy planets weren t massive enough to capture one, or hot enough to release one. 4e. The inner structure of the planets is explained by differentiation. Heavier elements sink to the core. Lighter ones float to the surface. 5. Asteroids and comets are left over planetesimals. Meteors are bits of dust that have fallen off of comets 6. Everything is the same age because it all formed at about the same time. What about the exceptions?
For every exception there is a rule... Tilted orbits of Mercury and Pluto. Mercury probably suffered a large impact late in its formation Pluto might be a left-over planetesimal. Retrograde rotation of Venus: Probably due to a large impact late in formation. Probability favors, but does not require, rotation in the same direction as the orbit. High axial tilt of Uranus and Pluto: Also likely to be due to a large impact Also, in the outer solar system, computer models suggest the nebula was less concentrated in the plane, which could result in large tilt of sub-disks.
For every exception there is a rule... Retrograde moon of Neptune. Probably a captured planetesimal. Oxygen in the atmosphere of earth. Earth s atmosphere is highly modified by life. Earth s moon orbits in the plane of the solar system. This is likely because the moon was formed from an impact with another body traveling in the plane of the solar system.
Some data to explain: 1. Planets isolated 2. Orbits ~circular / in ~same plane 3. Planets (and moons) travel along orbits in same direction. same direction as Sun rotates (counterclockwise viewed from above)
Some more data to explain: 4. Most planets rotate in this same direction
And some more data to explain: 5. Solar System highly differentiated: Terrestrial Planets (rocky, dense with density ~4-5 g/cm3) Jovian Planets (light, gassy, H, He, density 0.7-2) Icy Planets (Kuiper Belt)