How did the Earth Get Here?

25 September, 2008

How did the Earth Get Here?

Clues to the Formation of the Solar System:

• Inner planets are small and dense.


• Outer planets are large and have low density.


• Satellites of the outer planets are made mostly of ices .


• Cratered surfaces are everywhere in the Solar System.


• Saturn has such a low density that it can't be solid anywhere.

Formation of the Earth by accretion:
Initial solar nebula consists of mixtures of grains (rock) and ices. The initial ratio is about 90% ices and 10% grains
The sun is on so there is a temperature gradient in this mixture:
In the inner part of the solar system, only things which exist as a solid at high temperature are available (so how come there is so much water on the earth? -- answer later)
So in the inner part of the solar system you can only make a rocky planet via acretion of grains.
In the outer part of the solar system, ices can exist so you can make larger planets out of the more abundant ices
Jupiter (mostly H and He) formed in a manner similar to the Sun, that is not by accretion. Note that Jupiter can never become a star. A star is a ball of gas sufficiently hot to excite nuclear reactions .

The minimum mass require for this is about 8% the mass of the Sun. Jupiter's mass is an order of magnitude below this limit. Jupiter will never be a star.

Jupiter has a large mass and perturbs orbits of objects near them. There were lots of these objects scattered between Jupiter and Pluto.
Jupiter redirected some of this cometary material into the inner solar system and most of the earth's water was delivered through comet bombardment (therefore would we be here without Jupiter?)
Steps in the accretion process :
Step 1: accretion of cm sized particles

Step 2: Physical Collision on km scale

Step 3: Gravitational accretion on 10-100 km scale Step 4: Molten protoplanet from the heat of accretion
Final step is differentiation of the earth: Light objects float; heavy objects sink. Iron-Nickel Core (magnetic field) and oxygen-silicon crust
In the outer part of the solar system, the same 4 step process of accretion occurred but it was accretion of ices (cometisemals) instead of grains.


Things to note about the formation of planets via accretion

• There is a lot of heat dissipated in the final accretion process resulting in initially molten objects


• Any molten object of size greater than about 500 km has sufficient gravity to cause gravitational separation of light and heavy elements thus producing a differentiated body


• The accretion process is inefficient, there is lots of left over debris.


• In the inner part of the solar system, the leftover rocky debris cratered the surfaces of the newly formed planets.


• In the outer part of the solar system, much of the leftover rocky debris was ejected from the solar system due to the large masses of the planets which formed there. Some of this material was ejected into a large "Comet Cloud" which has a distance of about 100,000 AU from the Sun and some of the leftover debris ( beyond Pluto) could not be ejected (as it was far away from Uranus and Neptune) and hence remained there. This material is known as the Kuiper Belt and it was recently discovered by the Hubble Space Telescope
  More information on the Kuiper belt and the kinds of objects that are located there can be found here
  The asteroid belt represents a relic of the accretion process. A planet tried to form in that location but the gravitational influence of the large mass planet Jupiter was sufficient to accelerate the material there to high velocity. High velocity collisions between chunks of rocks cause them to be shattered and indeed, over the history of the solar system, the sizes of the largest asteroids are decreasing. The asteroid belt is not the remains of a planet that was blown up by the Death Star.