Solar System Formation. 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.
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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.
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.
½ life (millions of years)
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.
Why does the gas cloud need to be cold?
Planet formation is a natural outcome of
Planetary systems should be common.
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.
What does this mean for the paths of the planets through the sky?
If the solar nebula started as a spherical cloud
why do all the planets lie in a plane above the Sun’s equator. Shouldn’t they be spherically distributed about the Sun?
What two changes take place as the solar nebula collapses due to gravity?
3. rotation. The planets move in elliptical orbits that are very nearly circular.
Why are all the orbits nearly circular? What
happens to planets that formed with
highly eccentric (very elliptical) orbits?
4. rotation. The planets rotate counterclockwise as viewed from above the north pole, the same direction as they revolve, except for Venus and Uranus.
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?
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?
Is the solar day longer or shorter than the sidereal day on Venus?
Which planet will have the most extreme seasons?
Write down 0, 3, 6, 12, … each number, after the first, being double the previous value. Add 4 to each and divide by 10.
T-B distance AU
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.
An exception is Neptune’s Triton
How would you explain this observation with our formation theory of the Solar System?
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?
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.
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.
If the long period comets can have any inclination, what does this tell you about their distribution around the Sun?
Terrestrial planets – iron-nickel cores and silicate mantles
Jovian planets – silicate/hydrogen compound (methane, ammonia, water) cores and mostly H and He mantles
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.
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
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
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.
As the protoplanets grow by accretion of planetesimals, their gravity increases spurring more accretion.
We expect that the solar nebula from which the Sun formed, had the same composition as the current solar surface.
98% hydrogen and helium
1.4% hydrogen compounds – CH4, NH3, H2O
0.4% silicate rocks
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?