I have a question on gravity

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.

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.

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.

willstaruss22

My idea for Io is yes it is small but its also very hot and that heat causes molecules to escape the atmosphere easier. In honesty would give maybe 0.1 Earth masses the minimal size to produce a thick atmosphere for an extented amount of time. We cant forget density also if mars were denser than it is now say by Earths density its gravity would be larger and its internal structure hotter. Another posibilty for Io is maybe Jupiter is eating away its atmosphere but thats a theory, its simply to close to that giant beast.

litup

I wonder if a planet like Mercury could have at one time possessed an atmosphere but then was forced too close to the sun it got boiled off? It has a very rocky core but still with a small gravity, it could have perhaps held on to an atmosphere if it was say 4 AU from the sun, getting 1/16th of the energy we get at Earth, where we get 1355 watts per square meter, at 4 AU it would get barely 80 so being cold could help it maintain an atmosphere also.

willstaruss22

Thats what i was thinking maybe even a little closer. I was also wondering what would happen if mars was a mercury type planet huge core much denser. I wish there was a website or something where i could toy with planets masses densities and basically creat a new planet haha.

onomatomanic

A rule-of-thumb lower bound to atmosphere retention (derived here) is given by

rm ~ (3/2) (k/G) (1/m_proton) (R/M) T ~ 2*10^14 (R in m / M in kg) (T in K)

where R, M and T are the planet's radius, mass and surface temperature in SI units. The resultant number is the molecular mass, in atomic mass units, below which the thermal kinetic energy exceeds the gravitational binding energy.

Plugging in values for Earth, that yields rm ~ 0.06. Gases with a molecular mass below this value cannot be retained at all (which is meaningless here, since molecular masses are by definition integers). Gases between this value and ten times this value are lost quickly, i.e. within a year or so (still meaningless here). Gases between ten times and a hundred times this value are lost slowly, i.e. within a billion years or so. That means everything with a molecular mass below 6, which covers diatomic hydrogen and helium. Gases above a hundred times this value are retained almost indefinitely.

To get a long-lived Earth-like atmosphere, the lightest gas that has to be retained is diatomic nitrogen, I think, which means a molecular mass of 28 and thus rm ~ 0.3. In other words, you need a planet for which the quantity (R/M)*T is no more than five times higher than it is for Earth. If a shorter-lived atmosphere is sufficient, the quantity may be ten or twenty times higher (for reference, "twenty times higher" corresponds to the Moon, roughly).

As mentioned initially, this is a lower bound. It doesn't stop the atmosphere from, say, being chemically absorbed into the crust or being stripped away by solar wind or any number of other relevant processes which have already been discussed above.

mfb

If the planet has water, those molecules (atomic weight of 18) should stay in the atmosphere, too.

Some similar relations:
With the average density ρ, M scales with ρR^3, and the ratio becomes [itex]\frac{T}{\rho R^2}[/itex].

The surface gravity scales with g~M/R^2, and an equivalent ratio is T/(gR).

onomatomanic

^ Good point. Evaporating water straight into space doesn't sound very habitable.

And it occurs to me that methane would be again slightly lighter, and should probably be kept too, considering that it's a minor but not insignificant carbon cycle constituent and greenhouse gas.

So, better call it (rm < 2.5 rm_Earth) for a long-lived and (2.5 rm_Earth < rm < 10 rm_Earth) for a short-lived atmosphere. Which means that the Moon isn't even marginally suitable - domed cities it is. :P