A question for anyone who may know (apologize if this has already been discussed before)...

What is (proven or theoretically) the smallest an object can be to maintain an atmosphere? I know Triton and Pluto both have thin ones...Pluto's would be permanent if it weren't for the orbit. I don't consider Enceladus's a permanent one since it probably vanishes regularly, not to mention it's barely existent in the first place. My guess is anything much smaller than Pluto would not have the gravity to hold on to a true atmosphere, though I could be wrong.

Another question on this subject...what's the smallest possible sized-object for humans to walk/bounce on? I would imagine the limit is a little bit smaller than the moon.

Thanks.

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Well... it depends on temperature as well as gravity. Mercury could certainly hold onto an atmosphere if it were as far from the Sun as Pluto -- or even Saturn. But gases can not get much colder than Triton/Pluto, so I would say Triton is about the minimum size.

Very dense gases like xenon or sulfur dioxide freeze at relatively high temeperature, so that does not help. If the planet is warm enough for such gases to remain gaseous, the molecules also move fast enough that the planet must be fairly large.

As for your second question, it really depends on your defintions of "walk" and "bounce". I would say true walking is not possible even on the Moon, whereas bouncing is possible even on a small asteroid -- it would be very long bounces though.

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Make notes for ammonia, Helium, watervapour and What you need to do is plot a graph which summarizes conditions of temperature and escape velocity for which Planetray-like bodies can retain NH3, H2, CO2...in their atomspheres for long periods. Different gas have different molecular mass and average speeds differ at various temp. Atmospheric components can be lost by other methods such as a weak magnetic field will allow a Solar wind to strip a planet while slow rotation may cause one side of a planet to be much hotter than the other, Hydrogen and Helium are lost easily through thermal escape, think about these factors before you decide which can have atmosphere.

Maybe the answer is both Mercury and Pluto Can hold and Can Not hold N2 and CarbonDioxide. A hint for finding out the turth to this - may be to look at a planet's temp during rotations and its orbit, Pluto for example may not orbit like a normal planet and its temperature change by a much as 50 kelvin.

IIRC, Pluto does not have an atmosphere during it's entire orbit for the reason you've given.

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"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Correct me if I'm wrong but I think that pluto takes 250 E-years to orbit the Sun and for a numbers of years the temp stays low enough that it is able to hold on to a thin N2, carbon dioxide, CH4 atmosphere perhaps similar to Triton's, I think Moon Ganymede may also was found to have a very low prerssure atmosphere found from NASA data, about 10^-6 bars much less than Jupiter's Moon Io.

Good points...it makes sense that if Mercury were much further away it would be able to hold onto an atmosphere...being away from the impact of the solar wind. Pluto is the opposite, closer to the Sun would mean the N2/CO4 ice would evaporate.

I know Ilya said Triton is probably a lower limit...but then again let's say we have a planet w/ an atmosphere of gases that freeze at the coolest temps (N2, etc). Assume it's at the largest possible distance from the sun where these elements can remain in a gas state and not freeze up. Could this object be, say, Rhea size? Or would that sort of object not have the sufficient gravity?

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You misunderstand. Pluto has a thin atmosphere when it is "warm" -- that is, closest to Sun. At the farther parts of Pluto's orbit all gases you listed freeze and fall as snow.

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oh yes, thanks Ilya my brain is frozen

need coffee

First, N2 is far from the "freeze at coolest temps" gas -- otherwise you would not see tanker trucks full of it all the time. IIRC, nitrogen liquefies about 90-95 K. The gas which precipitates at lowest temperature is helium (4 K), but light as it is, I am pretty sure Rhea could not hold onto He even if that cold. Keep in mind, velocity of molecules is proportional to square root of absolute temp, so at 5 K (for example) a helium molecule moves only four times slower than at 80 K.

When I have time, I'll find the chart of gas molecule velocities, and will be able yo answer Macro Mouse's original question precisely.

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Thanks for the help Ilya. My lack of chemistry knowledge is showing, but I see where you're coming from and it makes sense Triton is around the limit. But on the bright side I take chem this upcoming year.

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Neutron stars have atmospheres, and diameters down to 23 kilometers or smaller.

Of course, something only a meter deep might not count as a "true" atmosphere. :wink:

According to this pic, in theory our Moon can have an atmosphere of Xenon

http://ircamera.as.arizona.edu/astr_...es/esc_vel.gif

Meter? My understanding is that neutron star atmospheres are only several centimeters deep, and are almost as dense as the liquid form of the gas in question (usually iron vapor).

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Timeframe?

If our 'planet' can 'hold' it for 1 million years? 1 billion? 1 trillion? Plugging in the numbers will give you different answers, depending on what you set as 'sufficiently long' to 'maintain'

Generate?

We've already discussed freezing and thawing; but what about slow outgassing? an equilibrium atmosphere of (say) sodium liberated by a 'solar' wind? an extreme 'day/night' planet (e.g. synchronous rotation with sufficient libration to 'cycle' volatiles in the twilight zone)? How about an icy planet with a 'thinnish' atmosphere and several deep canyons, all nicely arranged so many triple points occur throughout (the planet will be losing its atmosphere, but also constantly replenishing it; the equilibrium may be very stable)?

The Ultimate?

Consider a rogue Jupiter, lost in the deepest reaches of a giant void, as far from any galaxies or stars as it is possible to be. Is it losing its atmosphere, even if only at the rate of an H molecule a century? If so, it's only a matter of time before it loses all of it

Hardly. This "rogue Jupiter" would freeze solid long before it evaporates.

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Interesting ... how fast does a Jupiter cool? How much of a lonely Jupiter's atmosphere does it lose?

Hmm, how about we equip our lonely planet an atmosphere of He then ... it won't freeze (the CMBR will see to that) ....

Maybe the real question is, what is the largest object for which humans can "bounce around" (jump) without exceeding escape velocity?