Artem has a doctor of veterinary medicine degree.
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.
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.
Farther out than these inner regions of the nebula, it got a bit cooler, around 1200 K, and here less dense silicates (or rocky stuff) could form in addition to the metal. The four inner planets, the rocky terrestrial worlds, are thus made of a mixture of silicates, metals, and metal oxides.
Just like you know water can freeze when it gets colder, the same thing happens out in space when it gets too cold as we move farther away from the sun! Beyond the ice line, it got cold enough (less than 200 Kelvin) for particles other than rocks and metals to condense. The ice line is a region of space in the solar nebula, between where Mars and Jupiter are now, beyond which water could freeze into ice.
Even further out than this, it got cold enough where methane and ammonia could also condense out of the nebula to form their own kind of ice. These three types of ices are low-density materials and were quite prevalent in the solar system but could only condense out far away from the sun due to their low melting point. This notion helps explain the reason why the outer planets are of such low density compared to the inner planets, even though the gas giants do have small amounts of metals and silicates in them.
And so, the sequence by which different kinds of material condense out of the solar nebula based on the distance from the sun and decreasing temperature is known as the condensation sequence.
The condensation sequence is the sequence by which different kinds of material condense out of the solar nebula based on the distance from the sun and decreasing temperature.
Condensation is the process of forming solid particles from the solar nebula, whereas the solar nebula is a cloud of interstellar gas and dust that condensed to form the entire solar system, including the sun and planets.
This lesson had one major basic point: the closer a planet formed to the sun, the hotter it was there, and thus, only denser particles like that of metals or silicates could condense out of the solar nebula. Thus, the uncompressed density of a planet, the density of a planet if gravity didn't compress it, is generally higher as a planet gets closer to the sun.
Beyond the ice line, it got cold enough for less dense ice particles to condense out of the solar nebula. The ice line is a region of space in the solar nebula, likely between where Mars and Jupiter are now, beyond which water could freeze into ice.
Following this lesson, you should be able to:
- Define solar nebula and condensation
- Summarize the condensation sequence
- Explain why the uncompressed density of a planet is higher the closer it is to the sun
- Describe what the ice line is
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