The early Earth had 2 moons

--------- #1291 - Two Moons over Miami

- Most astronomers believe that our Moon was created in a collision between Earth and a Mars-size planet. This Big Splash happened 4,500,000,000 years ago. The best evidence is that the Moon seems to be made out of Earth’s crust. The idea is that the collision simply splashed the crust into orbit and gravity eventually made it into a sphere.


- Attachment - Big Splat

- We have since learned that the far side of the Moon is much different than the side we see. The far side is more hilly with deeply pitted craters. The nearside is relatively smooth basalt plains called “ Maria”. Gravity maps and radar have shown that the crust is much thicker on the far side. One explanation for this imbalance is that the Big Splash actually created two moons in orbit. The smaller moon trailing the larger moon by some 60 degrees in synchronous orbit around the Earth. Our Moon was 3 times wider in diameter and 25 times heavier than the smaller moon. Within 100 million years, the gravity of the larger Moon eventually pulled the smaller moon into a collision at 5,000 miles per hour. The smaller moon was 600 miles side and the resulting “Big Splat” spreading the rocks and crust over the surface without creating a giant crater due to the low velocity of the crash.

- Now we only have one Moon. Mars has two moons. Mercury and Venus none, but there are 166 moons in our planetary Solar System. It is doubtful that these other moons were all created in the same way our Moon was created.

- The moons that have highly elliptical orbits are called “irregular satellites”. There are 109 of these moons. Their orbits trace out more of a daisy peddle that a simple single ellipse. And most irregular moons are orbiting in the opposite direction of their planet’s rotation. This is called a “ retrograde orbit”.

- There are 57 “regular satellites “ with more circular orbits and in the same direction as the planet’s rotation. Astronomers believe these regular moons formed at the same time as their planets.

- Here is a tally of moons discovered to date:

--------------------------------------------Regular --------Irregular ------------Total
-------------------- Jupiter -------------- 8 -------------- 55 ------------------- 63
-------------------- Saturn -------------- 24 -------------- 38 ------------------- 62
-------------------- Uranus ------------- 18 -------------- 9 ------------------- 27
-------------------- Neptune ------------- 6 -------------- 7 ------------------- 13
-------------------- Earth ----------------- 1
-------------------- Mars ----------------- 2
-------------------- Venus --------------- 0
-------------------- Mercury --------------0
-------------------- Pluto ---------------- 4

- A moon is captured when the gravitational pull of the planet is greater than that of the Sun. In our early Solar System evolution 4 to 5 billion years ago there were many small icy bodies called planetisimals orbiting the Sun. Occasionally a planet’s gravity could capture one of these planetisimals and they could orbit the Sun together.

- Another theory for moon is called the “3-body interaction”. This is where two planets or two planetisimals create a gravity trap that catches a 3^rd smaller icy body.

- Astronomers believe the giant planets first formed nearer to the Sun the today’s orbits, say 5 to 17 astronomical units, instead of 5 to 40AU. Planetisimals were orbiting the Sun out to about 35 AU. ( An astronomical unit is the today’s distance between the Earth and the Sun, 93 million miles.)

----------------------- Jupiter -------------- 5.2 AU
----------------------- Saturn -------------- 9.5 AU
----------------------- Uranus -------------- 19.2 AU
----------------------- Neptune -------------- 30 AU
----------------------- Pluto------------------ 40 AU
----------------------- Sedna ----------------- 68 AU
----------------------- Kuiper Belt -------- 30 to 70 AU

- As the early planets orbited they naturally sought resonances in their orbits that tweaked their radius. 2:1 resonance meant Jupiter made 2 revolutions for every one that Saturn made. As the larger planets interacted in this way they also tweaked the orbits of smaller objects that were in their path. All orbits were seeking a balance that did not interfere with each other.

- Some of these icy bodies may have come from the Kuiper Belt that lies outside the orbit of Neptune, 30 to 70 AU. These could have been the source of the inner planets receiving heavy bombardment in the early evolution of the Solar System. This could explain how water arrived on Earth. It could explain why the Moon has so many craters.

- One thing for certain our early Solar System was a chaotic place. There were many collisions and close encounters… Orbits were being tossed around like Frisbees. It took billions of years to settle out to what we see today. Today we see some 200 moons among the 8 planets and the 6 Dwarf Planets, including poor Pluto.

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707-536-3272, Thursday, August 18, 2011