# If enough heat is applied to an object, will it reach escape speed?

1. Nov 20, 2013

### Remon

This is just something I'm wondering, if we apply enough heat to an object, and since the speed of the molecules inside it is directly measured by temperature, will the molecules inside the object move fast enough to reach the escape speed of the planet that it is on? I know that this is the case for why many planets don't have an atmosphere (because their escape speed is too low to hold an atmosphere due to their weak gravity) and I'd also think that when enough heat is applied to a solid object, it turns into a gas and (if heated enough) will continue rising up above the planet's surface until it "breaks free" of the planets gravity, thus, reaching escape velocity. Although, I don't know if these assumptions are true.

2. Nov 20, 2013

### gabriel.dac

Uhh no. If you apply heat to an object, it will vibrate more. You won't change it's speed.
However, heated gas becomes more dilated, so, depending on the mass of the planet, it can escape - but not because it reached escape velocity

Also, the correct term is escape velocity, not speed.

3. Nov 20, 2013

### ZapperZ

Staff Emeritus
I don't know what you mean by "escape speed" here, but you might want to look up "thermionic emission". That is a common electron source used in old TV and CRT tube and old diodes. The heated filament is the electron source.

However, applying this to celestial bodies is a completely different matter and physics.

Zz.

4. Nov 20, 2013

### Remon

What? I'm talking about the molecules moving faster (in km/s for gasses) inside the object if it is being heated, not the actual object moving as a whole... well, it does move as a whole through states
I learned that from a homework assignment that I had for astronomy - the question talked about the moon Titan and the molecules inside its atmosphere having a speed of about 0.4 km/s (while Titan's escape velocity is 2.6 km/s I think), the question asked me to compare the molecules' speed with Titan's escape velocity and the speed needed for the molecules in its atmosphere to evaporate (or move faster than Titan's escape velocity so that they evaporate/escape Titan)

Last edited: Nov 20, 2013
5. Nov 20, 2013

### Remon

I mean the escape velocity (you guys get too hung up on terminology, I know its important too but try to focus more on the meaning) of a planet, and I also don't see what "thermionic emission" have to do with what I'm asking, it would be helpful if you can elaborate.

Last edited: Nov 20, 2013
6. Nov 20, 2013

### ZapperZ

Staff Emeritus
First of all, I wasn't the one who was "hung up" on the terminology.

Secondly, I was giving an example of how electrons can escape a solid simply via heat, which is analogous to what you were asking.

Thirdly, you might want to consider the fact that "objects" melt or vaporize at some point as you increase the temperature. So consider the average KE at those critical temperatures.

Finally, molecules, atoms, and particles of such size do not have free, unimpeded travel from earth's surface to the upper atmosphere to escape, even if it has enough speed to escape the earth's gravity in the first place. The presence of air means that it will have a mean free path that is sufficiently short, it won't be able to travel far before it encounters collisions with other particles.

Zz.

7. Nov 20, 2013

### gabriel.dac

I see that english is not your native language. It is not my native language either, but anyway, that question does not make any sense. Titan's atmosphere is made out of gas (obviously, duh). So how can it evaporate? And what does speed have to do with that? Only liquids can evaporate

8. Nov 20, 2013

### Remon

So what you mean is that when molecules obtain more speed due to the increased temperature, they will just collide more with each other and not actually change their place as a 'whole'? Because I was thinking that the reason why gasses rise in the air is because their molecules are moving faster than a solid or a liquid object on the ground (which I think leads to lighter or heavier density, but that's another story), which also means that the faster the molecules move (or the higher the temperature), the higher the gas will rise above the surface; although, I could (and probably) be completely wrong, which is why I'm asking you guys. And also, I did "consider the fact that 'objects' melt", I stated in a previous post that "objects move through states".

Last edited: Nov 20, 2013
9. Nov 20, 2013

### Remon

Here is the question exactly: "Saturn’s moon Titan is the only moon in the solar system with an atmosphere, which is 95% nitrogen molecules (N2), similar to Earth’s atmosphere. At Saturn’s distance from the Sun, the temperature of the atmosphere is only 95 K (−180C), and the molecules have an average speed of about 0.4 km/s. How does this speed compare with the speed needed for the molecules to escape from Titan and evaporate into space? Show how you got your answer."

p.s I've been speaking English for over 8 years now, so I'll try not to take that as an insult (I also don't see how my English is inferior in anyway, but that's not the focus here).

10. Nov 20, 2013

### ZapperZ

Staff Emeritus
This appears to be a HW/Coursework-type question. If this is the impetus of your question, then you've posted it in the wrong section. You should have started with this in the very beginning, rather than asking a rather puzzling question, and you should have posted this in the HW/Coursework forum.

Zz.

11. Nov 20, 2013

### Staff: Mentor

Regardless of the framing, the question points to a discussion of how our atmosphere got its current composition:
The molecular weight and temperature of molecules does indeed determine their speed and if the speed is high, they can indeed escape. That's why our atmosphere has few light gases and Mars is almost entirely carbon dioxide.

For large objects, there is no bulk motion, so the idea does not apply.

Confused framing or not (that's why the question is being asked!), this is not a complicated or controversial enough subject to have been worthy of such aggressive replies. Calm down and consider who you are talking to, people.

12. Nov 20, 2013

### Remon

... Ok, here's what happened: I started with this question about a week ago and I needed help with it so I did post it in the homework section to ask for help. But, after I finished the assignment and got all the help I needed (a few days ago), I started thinking more about the question and thought that the concept implied in the question (which is the relation between molecules' speed and escape velocity) could also be applied to all objects. So, to feed my own curiosity, I decided to post the question that I thought of here (in this section), but my question originated from the assignment question that I posted, but it is not directly related to it at all.

13. Nov 20, 2013

### Staff: Mentor

I prefer "escape speed" because "escape velocity" wrongly implies that direction matters.

14. Nov 20, 2013

### Remon

That's exactly what I was saying, that "if the speed is high, they can indeed escape" the celestial body that they are on. Therefore, with high enough temperature (combined with low molecular weight), a gas can reach and go beyond escape velocity, right? But what I'm not sure of is that if escape velocity can be reached by the increased speed of molecules (within the gas), which would lead the gas to escape from the planet.

15. Nov 20, 2013

### Remon

I know that that is not directly aimed at me but I apologize if I was being aggressive in any way (which I'm sure I wasn't), that is not my goal here.

Last edited: Nov 20, 2013
16. Nov 20, 2013

### gabriel.dac

Oh, so basically what you are saying is that hotter gases will get less dense and will move faster towards space, right? If so, Ramon's question makes sense. But if you heat the whole atmosphere equally, nothing will happen but dilation.

17. Nov 20, 2013

### jbriggs444

The point is that lighter gasses (such as hydrogen and helium) at a given temperature will have a larger fraction of their component molecules at a speed greater than escape velocity. So you can heat the atmosphere evenly and while most or all of hydrogen can escape [over geological time periods] the fraction of the carbon dioxide that escapes over the same period will be much less.

18. Nov 20, 2013

### Remon

So I was right all along when I said that the molecules' speed within a gas (doesn't just have to apply to an atmosphere) can reach and will go beyond the escape speed of the celestial body that the gas is on. I know that molecular mass (not 'weight', as terminology is important) also causes changes in the speed of molecules too, but I was trying to focus more on temperature.

19. Nov 20, 2013

### ZapperZ

Staff Emeritus
See, this is where it gets confusing, and why it seems that this topic is being approached differently by different people.

To me, the term "escape speed" means that the minimum speed needed for something to escape the gravitational field starting from the surface of the earth! But from what I'm seeing, it seems that this includes molecules escaping that are already in the upper atmosphere. Is that true? But is this what is defined as "escape speed" here? It certainly isn't how we defined it in standard classical mechanics.

If it is from the surface of the earth, then I will once again bring out the issue of the non-infinite mean-free path for such particles trying to escape.

Zz.

20. Nov 20, 2013

### Staff: Mentor

No, it has nothing to do with density. It is about molecule speed relative to escape speed/velocity.

21. Nov 20, 2013

### Staff: Mentor

That mostly sounds redundant (confirming your understanding), but it sounds like maybe you are unsure if the speed can really be high enough. The velocity distribution is along a bell curve (as jbriggs said), so even if the average speed is too low, a certain fraction will be high enough.

Wiki has a page discussing atmospheric escape:

http://en.m.wikipedia.org/wiki/Atmospheric_escape

22. Nov 20, 2013

### Remon

So the difference between a gas escaping from the surface of the earth and a gas escaping from the atmosphere (≈10 miles above earth's surface) is that the escape velocity/speed is lower in the atmosphere? I know that's not what you said, but I'm assuming that that's main difference between directly "escaping" from the earth's surface than from its atmosphere. And also, why do gas molecules have less "free space" to move on the surface of earth than in its atmosphere? Wouldn't a particular gas (say CO2) have the same amount of "free space" between each of its molecules on the surface of earth as the atmosphere of earth? or does pressure play a role here (since there is less pressure in the atmosphere to "push" the molecules together)? I hope that made sense

23. Nov 20, 2013

### Integral

Staff Emeritus
But it does. IF your velocity is directed toward the surface of the earth, you will not escape the atmosphere.

24. Nov 20, 2013

### Staff: Mentor

Your interpretation is correct, but your definition is not: The surface of the earth is a common reference point, but not the only useful one and it is not part of the definition. You can calculate it from anywhere.

http://en.m.wikipedia.org/wiki/Escape_velocity

Last edited: Nov 20, 2013
25. Nov 20, 2013

### Staff: Mentor

True...

I know that's half in jest, but just to clarify: direction isn't part of the equation and the equation also treats the object like a point mass. We sometimes get questions about how direction affects escape speed, but the reality is that unless pointed at the ground or a building or another obstacle, it doesn't matter. You could, for example, tunnel through the center of the earth and fire a projectile through it and the equation still works....

....but it probably fails if you start below the surface due to not accounting for the non-point size. Not sure though.

Last edited: Nov 20, 2013