# I have a question on gravity

1. Aug 8, 2012

### willstaruss22

Could a planet hold a significant atmopshere of Co2 O2 N and H2O if the gravity was 50% Earths if under the right conditions?

2. Aug 8, 2012

### Nessdude14

Sure, under the right conditions. Only particles that reach escape velocity will leave the atmosphere, and for that to happen, you need heat. The escape velocity on a planet with weaker gravity will be smaller, but if the temperature of the planet is lower, the particles will be slower. Even if the gravity is weaker, there might still be a smaller portion of gas particles that are able to escape the atmosphere.

One thing to consider though is that with a much lower temperature, you may not see the same chemical processes which form those compounds here on earth.

3. Aug 8, 2012

### willstaruss22

So if a planet with this gravity was inbetween earths and mars distance do you think it could hold onto the atmosphere? Id imaine if it were at venuses distance there would be no atmoshpere.

4. Aug 8, 2012

### Nessdude14

The distance from the star doesn't determine the escape velocity, that's only affected by the planet's gravity. The only thing that will change the strength of gravity on the surface of a planet is a change in density of the planet; it's unaffected by the distance from the star.

5. Aug 8, 2012

### willstaruss22

Ok so i made up a hypothetical exoplanet that has a mass of .205 earths with a density of 5.20 g/cm3 and a radius of .60 earths and gravity is 57% earths with an orbit of 402 days around a sun like star. The temperature for the planet is around 275 K Could a planet like this retain a thick enough atmosphere for life? Any possibility for life?

6. Aug 8, 2012

### Nessdude14

The escape velocity of the planet can be found by:
$\Large v_{e}=\sqrt{\frac{2GM}{r}}$

For your planet, it would be about 6.5 km/s, while earth's escape velocity is about 11.2 km/s.

Using the Maxwell-Boltzmann distribution for the speed of gas particles, you could determine the relative amount of gas you'd expect to lose compared to earth. However, just by taking a look at a plot of the Maxwell-Boltzmann distributions for a few gases at 298°K, you can see that even the lighter gases seem to fit well within the range of your escape velocity.

Without actually going into the math for the Maxwell-Boltzmann probability density, this seems to indicate that a planet of those proportions would be able to sustain an atmosphere similar to earth's. The temperature could even be a bit higher.

7. Aug 8, 2012

### willstaruss22

So a planet like this could be habitable. Wow and some people tell me that a planet like that cant have life because of its size haha.

8. Aug 9, 2012

### willstaruss22

What about 290 K or would that be too hot for this planet?

9. Aug 9, 2012

### Nessdude14

I think it would be fine. The Maxwell-Boltzmann distribution I posted is for 298°K.

10. Aug 9, 2012

### Nabeshin

Indeed, it appears this process only really ever matters for the lightest elements, H and He. Everything else is simply too massive.

(I did some digging and got some numbers. For the hypothetical planet in question, you could probably hold onto any substance with a weight over 6amu for 10Gyr+. Which is basically everything not H and He.)

Apparently though, there are other processes at work which can strip a planet of its atmosphere. Things like stellar wind and ionizing radiation come to mind, but I'm not familiar enough with how these work to state numbers.

11. Aug 9, 2012

### willstaruss22

The planet would have a global magnetic field.

12. Aug 10, 2012

### K^2

Temperatures in Thermosphere can reach 1,800 K. That's going to throw all of the above computations way off.

For a hypothetical planet, the Thermosphere temperature will depend on parent star's UV output. So a planet orbiting a blue star will have harder time holding on to its atmosphere than one orbiting a red star, provided equal energy flows from both.

And, of course, magnetic field is vital for holding on to the atmosphere. Just look at Mars.

13. Aug 10, 2012

### willstaruss22

What about a yellow or orange star? Yea i feel bad for mars like if it kept its field we might actually see small patches of liquid water givin the air pressure was enough.

14. Aug 10, 2012

### Staff: Mentor

It is not. Just look at venus. A magnetic field is relevant, but not required for an atmosphere.
We had a similar discussion some days ago in the astronomy forum here, with the same thread starter.

15. Aug 11, 2012

### willstaruss22

I understand atmospheric stripping in the thermosphere because of the much higher temperatures spacing between particals. But doesnt the maxwell boltzmann thing explain gases closer to the ground? Cause that would make sense earths temp is 288k and as stated above the chart describes gases at 298 k and the higher up you go the lower the temp is untill you et to the mesosphere and thermosphere.

16. Aug 12, 2012

### Staff: Mentor

The Maxwell-Bolzmann-distribution is valid for all gases in equilibrium - to a good approximation, this is true for all gases everywhere in the atmosphere. The temperature is not the same everywhere, therefore the distribution depends on the height.

17. Aug 12, 2012

### willstaruss22

makes sense.

18. Aug 13, 2012

### litup

The thing is, how long a time period do you want the atmosphere to hang around? It has been calculated if we were to terraform Mars to equal our atmosphere, it would hang around for some time, about 100,000 years which would certainly give us enough time to compensate in other ways, make up a planetary magnetic shield and so forth. So in the short term, geologically speaking, yes, we can have an atmosphere hang around a long time on a small planet. The smaller the planet, obviously the shorter time the atmosphere hangs around and whether it has a magnetic shield or not.

19. Aug 13, 2012

### willstaruss22

No such thing as an earth like atmosphere else where as they come in various differences. A earth size planet could have a mars like atmosphere and a mars sized planet could have a venus like atmosphere. I think there are a number of processes that determine what type of atmosphere occours on a planet or moon other than size. Look at titan thick atmosphere then Ganymede no atmosphere despite similar sized. We also have to consider the what the planet or moon is made of and a number of other processes. Mars has 38% Earths gravity yes but the volcanoes are barley active. The planet i created was 57% earths gravity and has active volcanoes.

Last edited: Aug 13, 2012
20. Aug 14, 2012

### litup

I guess conditions have to be right for an atmosphere, there are certainly a lot of styles of atmospheres in the solar system. But as for active volcanoes, look at Io. There are active volcanoes going on daily, yet it has an atmosphere of 1 billionth of Earth's. So volcanic activity alone is not the whole answer.

In fiction, however, you can assign what you want as far as planetary attributes go, just be as close to scientifically sound as possible. For instance you wouldn't want to make an atmosphere on an asteroid 100 miles across that you could land on and breathe, that would stretch things a bit.