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Why not frozen?

Posted by: deltat - Tue Dec 22, 2009 7:15 pm
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Why not frozen? 
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Post Why not frozen?   Posted on: Tue Dec 22, 2009 7:15 pm
Given that all planets further from the Sun than Mars have temperatures way below zero, why aren't they frozen?


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Post Re: Why not frozen?   Posted on: Tue Dec 22, 2009 7:50 pm
Haha good question!

Is it possibly to do with that the water is underground mostly?
Mars has frozen caps doesn't it? and the Moon has ice in it... if you fly a spaceship into it :) Mars did have rivers etc right? Im no expert but i think a lot of it left the planet or went underground, someone correct me if i am wrong :)

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Post Re: Why not frozen?   Posted on: Wed Dec 23, 2009 9:44 am
Extract from Wikipedia entry on gas giants.

Gas giants are commonly described as lacking a solid surface, although a more accurate description is to say that they lack a clearly-defined surface. Although they have rocky or metallic cores - in fact, such a core is thought to be required for a gas giant to form - the majority of the mass of Jupiter and Saturn is hydrogen and helium. In the planet's upper layers, these elements are gaseous, as they are on Earth, but further down in the planet's interior, they become compressed into liquids or solids, which become denser toward the core. Similarly, although the majority of Uranus and Neptune is icy, the extreme heat and pressure of these planets' interiors put the ices into less familiar physical states. Therefore, one cannot "land on" gas giants in a traditional sense, at least not without being crushed by the planet's own gravity.


More information on the page itself.


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Post Re: Why not frozen?   Posted on: Wed Dec 23, 2009 10:50 am
(I'm sorry if I'm telling you things that you already know, but I figured I'd start from the beginning so that I can be sure that everyone here can follow. Also James beat me to an answer while I was still typing. I guess I should learn to type faster, gain more knowledge so that I don't have to keep looking things up, or write shorter posts. Oh well ;-))

deltat wrote:
Given that all planets further from the Sun than Mars have temperatures way below zero, why aren't they frozen?

Well, "frozen" just means "in the solid state". All materials can be in any of a number of states, depending on pressure and temperature. The common phases are solid, liquid and gaseous, and then there is plasma, also known as the "fourth state of matter". See Wikipedia on states of matter for all sorts of additional states and complicated details. To answer your question, we don't need complicated things though.

If you take some liquid water from the tap and put it into your freezer, its temperature will drop, and when it drops below a certain temperature, it will change (freeze) from its liquid state into its solid state (which we call ice). If you take the ice out again, it will warm up beyond this point, and turn back to liquid water (that is, it will melt). Unsurprisingly, the point on the temperature scale where this happens is called the freezing point or melting point (they're the same) of water.

If you next put your water into a kettle and heat it on a fire, it will turn (evaporate) into gaseous water (water vapour) when it reaches a certain temperature. This point is the boiling point of water. The gaseous water will fly out of the kettle, meet with the colder air around it, cool down again to below the boiling point, turn back into liquid water (that's called condensation), and that is the cloud you see coming out of the kettle. Gaseous water (or water vapour) is invisible.

What exactly "Zero" temperature is depends on the scale you are using. Today, mostly the Celsius, Kelvin and Fahrenheit scales are in use, but others have been used throughout history. Unlike different length scales (metres, feet, ells, and so on), different temperature scales typically have not just a different standard distance, but also a different zero point.

The Celsius (or Centigrade) scale that you are using is based on water: the freezing point of water (at normal sea level pressure) is taken as 0°C, and the boiling point of water as 100°C. Since water is a very common substance on Earth, this is a convenient way of working with temperatures.

However, it is also arbitrary: the freezing point and boiling point differ for different materials. For example, oxygen is gaseous at room temperature, and will only condense (turn into liquid) at -183°C, and into solid at -219°C. That is why rockets have insulated liquid oxygen tanks: so that the oxygen will stay cold enough to stay liquid. Basalt, which is a type of rock commonly found in the Earth's crust, is solid at room temperature, and melts at about 1000°C to 1200°C (it consists of different types of materials with varying melting points).

So, now let's look at the planets in our solar system. The inner planets consist of rock, and while they are close to the sun, they don't get warm enough for the rock to melt. So, the rock is "frozen". Other materials on these planets may be in other states: with temperatures on Earth being what they are, oxygen is gaseous, and water may be in any of the three states, depending on the location. On Mars, there is gaseous carbon dioxide in the atmosphere, solid rock, and solid water and carbon dioxide at the poles.

Next, we have the gas giants, which you were actually asking about. Jupiter and Saturn consist mainly of hydrogen and helium, which both have very low boiling and melting points. While the surface (insofar as you can speak of a surface on these planets) temperature at -118°C is below the boiling point and even the melting point of water, hydrogen and helium are still happily gaseous at that point. As you move towards the core, temperature and pressure increases, and the hydrogen turns into metallic hydrogen, which is another, rather exotic, state of matter that doesn't occur naturally on Earth (although we can make it in the lab). Below that are other, heavier elements, but we don't really know yet what the cores of these planets really look like (and in fact, we're not sure that Saturn looks like this at all).

Uranus and Neptune consist mainly of water, ammonia and methane, with hydrogen and helium on top. These planets do in fact consist mainly of frozen water, with an atmosphere consisting of gaseous hydrogen and helium, and are therefore also called "ice giants".

Beyond that, we have the Kuyper belt, which has rocky objects that are in a solid state, because they're colder than the melting point of the rock.

So, there you have your answer. Different materials have different melting and boiling points, and the materials that Jupiter and Saturn are made of have boiling points below their surface temperatures. So, they are gas giants. Uranus and Neptune have an outer layer of the same materials, but below that consist to a large extent of water, which indeed freezes into ice, making them the ice giants. The other plants consist mainly of rock, and are colder than the melting point of rock, so they are solid.

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Post Re: Why not frozen?   Posted on: Wed Dec 23, 2009 10:03 pm
Thanks for reminding me, I forgot how much it took to freeze hydrogen.


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