Monday, October 11, 2010

Moons in the Solar System (1 of 2)

Moons are interesting celestial bodies.  Until recent years, they are often overlooked in favour of other astronomical objects like stars and planets.  So what really is a moon?  By definition, a moon is a natural satellite that orbits a planet.  

It is important to note that gravity plays a role in shaping a planet's moons. Too much gravity and the moon is destroyed. Too little of it and the moon floats away until is no longer a moon. In this and the next articles, we will focus on the moons in our Solar System. Let's start with the one we know best!

Earth's Moon and the Giant Impact Hypothesis

Several theories have been postulated on the formation of our moon. The Apollo mission was sent to the moon in 1969 during which rock samples were secured. These moon rocks were found to be almost identical to those of Earth's crust.  The catch however, is that the rocks were super heated. How could this be? 

The story goes that Earth was actually hit by a Mars-sized planet named Theia about 4.5 billion years ago. This was back in Earth's formative days and the orbits of the planets in the solar system were still unstable. At one point, Earth's gravity distorted Theia's orbit and it eventually collided with Earth at just under 4m/s. The impact destroyed Theia and Earth barely survived. The debris of the impact were ejected into space and gravity eventually pulled it all into the moon. 

The angle of the impact had significant implications. A head-on collision would have destroyed both planets.  A glancing blow would have only sliced off a piece of our planet.  Laboratory experiments showed that it was a 45-degree angle impact which took out a big chunk of Earth and led to cirumstances we see today.

In its current orbit, the moon is 260 000 miles from Earth.  When it first formed  however, it was only 1 500 miles from Earth.  It was the moon's gravity that slowed down Earth's rotation cycle from 6 hrs to 24 hrs. Thanks to the moon, we now have 4x more time to use in a day! 


-PTS

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