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Age of the Solar System
The oldest rocks found on Earth are about 4.55 billion years old, not native but meteorites which fall from space.
The oldest native Earth rocks are 3.85 billion years old.
Radioactive dating
Unstable parent isotopes decay at a constant rate to stable daughter isotopes. By measuring how much of the parent isotope is still present, and how much of the daughter isotope there is, we can calculate the age of the rock.
Rubidium 187 Strontium 87 48800
Uranium 238 Lead 206 4470
Plutonium 244 Thorium 232 83
Iodine 129 Xenon 129 16
Manganese 53 Chromium 53 3.7
Aluminum 26 Magnesium 26 0.72
parent daughter ½ life (millions of years)
Discussion
The age of a rock given by radiometric dating is the time since the rock last solidified.
What happens to the lighter daughter elements when the rock is in a molten or gaseous phase?
The nebular hypothesis
The Sun and planets formed from the gravitational collapse (possibly triggered by nearby supernova) of a single, spherical, slowly rotating cloud of cold interstellar gas and dust.
Formation of the Solar System
Consequence
Planet formation is a natural outcome of star formation.
Planetary systems should be common.
Dynamics of the Planets
1. The planets revolve counterclockwise around the Sun as viewed from above the Sun’s north pole, the same direction that the Sun rotates on its axis.
Discussion
Why can’t all the planets just orbit in any arbitrary direction? Why should they all go around the Sun in the same way?
2. The major planets have orbital planes that are only slightly inclined with the plane of the equator of the Sun’s rotation, i.e. the orbits are coplanar.
Orbital Inclination
Mercury 7.004Venus 3.394Earth 0Mars 1.85Jupiter 1.308Saturn 2.488Uranus 0.774Neptune 1.774
Discussion
If the solar nebula started as a spherical cloudwhy do all the planets lie in a plane above the Sun’s equator. Shouldn’t they be spherically distributed about the Sun?
Eccentricities
Mercury 0.206Venus .007Earth .017Mars .093Jupiter .048Saturn .056Uranus .046Neptune .010
Discussion
Why are all the orbits nearly circular? What happens to planets that formed with highly eccentric (very elliptical) orbits?
4. The planets rotate counterclockwise as viewed from above the north pole, the same direction as they revolve, except for Venus and Uranus.
Rotation of the Planets
Mercury 58.6 0.0Venus -243.0 177.4Earth 0.997 23.5Mars 1.026 25.2Jupiter 0.41 3.1Saturn 0.43 26.7Uranus -0.72 97.9Neptune 0.67 29.0
Period (days) Axis tilt
Discussion
Why do you think the planets rotate in the same direction and why is this direction in the same sense as the planets orbit the Sun?
Discussion
Unlike all the other planets Venus rotates backward. How would the diurnal and yearly motion of the Sun differ on Venus compared to that of the Earth?
5. The Planets’ orbital distance from the Sun follows a regular
spacing.
Titius-Bode rule
Write down 0, 3, 6, 12, … each number, after the first, being double the previous value. Add 4 to each and divide by 10.
Titius-Bode Rule and Distance
Mercury 0.39 0.4Venus .72 0.7Earth 1.0 1.0Mars 1.52 1.6(Ceres) 2.77 2.8Jupiter 5.2 5.2Saturn 9.54 10.0Uranus 19.18 19.6Neptune 30.06 38.8
Distance AU T-B distance AU
A packed Solar System?
The solar system may be as densely packed as possible. There do not appear to be any orbits stable over the lifetime of the solar system between the current planets.
6. Most satellites revolve in the same direction as their parent
planet’s rotation and lie close to their parent planet’s equatorial
plane
An exception is Neptune’s Triton
Discussion
If the Sun formed from a single spherical rotating cloud, wouldn’t you expect that all the pieces would have the same angular momentum as the original cloud? How must the solar system have changed since the time of its formation that this is no longer the case?
Discussion
Either the Sun’s rotation rate has slowed over time, or the planet’s have been spun up in their orbits. How could we decide between these two possibilities?
The Sun rotates once every 33 days, but should rotate once in about 2 hours if angular momentum were distributed evenly.
This two hour rotation rate is common among other young solar mass stars elsewhere in the galaxy as well as higher mass stars.
Slowing the Sun’s rotation
Magnetic breaking – The Sun’s magnetic field might interact with the early solar nebula to slow the Sun’s rotation.
Strong solar winds early in the history of the Sun might have carried the extra angular momentum away.
8. Long period comets come from all directions and orbital inclinations in contrast to the
coplanar orbits of the planets.
Discussion
If the long period comets can have any inclination, what does this tell you about their distribution around the Sun?
2 types of planets
Terrestrial planets – iron-nickel cores and silicate mantles
Jovian planets – silicate/hydrogen compound (methane, ammonia, water) cores and mostly H and He mantles
Density
Mass of the planet divided by the volume of the planet.
Higher density implies a larger percentage of high density materials, such as iron and nickel, lower density implies more silicates.
Mean density of planets
Earth 5.5 g/cm3
Mercury 5.4 g/cm3
Venus 5.2 g/cm3
Mars 3.9 g/cm3
Moon 3.4 g/cm3
Neptune 1.6 g/cm3
Sun 1.4 g/cm3
Jupiter 1.3 g/cm3
Uranus 1.3 g/cm3
Saturn 0.7 g/cm3
11. All the Jovian planets have a core of icy/rocky material with
between 10-15 times the mass of the Earth
The Planetesimal Hypothesis
Fluffy dust grains condensing out of the solar nebula stick together as a result of low-speed collisions, building up to small bodies called planetesimals.
Protoplanets
As the protoplanets grow by accretion of planetesimals, their gravity increases spurring more accretion.
Solar nebula composition
We expect that the solar nebula from which the Sun formed, had the same composition as the current solar surface.
98% hydrogen and helium1.4% hydrogen compounds – CH4, NH3, H2O0.4% silicate rocks0.2% metals
Discussion
If the planets and Sun all formed from the same nebula, why don’t all the planets and the Sun of the same chemical composition?
The outer planets have about the same composition as the Sun but the terrestrial planets do not. Why?