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The Condensation Sequences & Planetary Formation

The Condensation Sequences & Planetary Formation
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  • 0:01 Why We All Look Different
  • 0:59 Uncompressed Densities
  • 2:37 The Condensation Sequence
  • 5:49 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
In this lesson, you'll learn why the planets have different densities despite being formed from the same solar nebula. We'll cover condensation, uncompressed densities, the ice line, and the condensation sequence.

Why We All Look Different

The ingredients for human life are all the same regardless of where you are born, what color of skin you have, and how tall or short you are. You are made of the same atoms as anyone else. So, why do we all look so different? 'Genes' is the short and simple answer. We all have different genes that code for how we look. But not all of our appearance is controlled by genes. The environment plays its role, too. For instance, if you are born into poverty and have very little food to eat as a small child, you may not grow to your full potential.

Along the same lines, we can ask: why is it that the planets have unique compositions? A prior lesson pointed out that they, like us, were born from the same stuff, but yet they look totally different! Why? The local environment is once again the answer. This lesson will explain how planetary formation is influenced by temperature and condensation.

Uncompressed Densities

The solar system, including all the different looking planets, was born from the same solar nebula. The solar nebula is a cloud of interstellar gas and dust that condensed to form the entire solar system, including the sun and planets. Knowing this, shouldn't all the planets be gas giants like Jupiter or terrestrial rocks like Mercury if they all formed from the same solar nebula? No. And there's a reason why.

The four inner planets, the terrestrial planets (Mercury, Venus, Earth, and Mars), have the highest densities in our solar system, and the four outer gas giants (Jupiter, Saturn, Uranus, and Neptune) have the lowest densities.

But the uncompressed density of a terrestrial planet, the density of a planet if gravity didn't compress it, isn't the same as the observed density. What I mean is, Earth is more massive than Mercury, and thus its stronger gravity will compact it to a higher density than Mercury.

This is like saying if you place a little ant on top of a clay ball, nothing will happen. But if you, much more massive than the ant, step on the ball, it will become squished or more compact. However, if you were to take the uncompressed density of Mercury, which is like the density of the original material that made Mercury, then it's actually higher than that of Earth. As a general rule, the closer the terrestrial planet is located to the sun, the higher its uncompressed density.

The Condensation Sequence

The reason for this, and one of the reasons for why all the planets look different and have different compositions, has to do with condensation, the process of forming solid particles from the solar nebula. You've surely seen how a morning mist, which is basically really tiny water droplets, condenses into much larger and visible water droplets on your car or window. This happens bit by bit, as little molecules of water coalesce together with time. Except in this lesson, it's little particles of the gas in the nebula that come together as the nebula cools to form solid grains.

Anyways, the solar nebula was basically the same throughout its disk in terms of chemical composition, but the type of matter that could condense in any one region of the solar system all depended on the temperature of the gas in that place.

So, substances with really high melting points, the ones you'd have to get really super hot to change from a solid shape to a liquid one, could condense much closer to the sun as a result. Regions closer to the sun, having a temperature of around 1500 Kelvin, were too hot for much of anything to condense except for metal oxides and metals like iron and nickel. These guys are very dense.

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