Examining The Kerbol System as if it were a real solar system

[OhioBob]

Yea, well, I figured its escape velocity is still lower than mars, and it should be warmer...

Yes, Mars has an atmosphere... but not 5 atmospheres of pressure at the surface.

The atmosphere of Mars is barely there.... for most purposes, Mars has lost its atmosphere.

If Mars lost its atmosphere, I can't see Eve having much chance of retaining its thick atmosphere.

Although Mars is 5 times more massive than Eve, because of the unrealistic planet densities in KSP, Eve's escape velocity is just slightly less than Mars (4.8 km/s vs. 5.0 km/s). Performing the same calculation for Mars I come up with a minimum MW of 22 g/mol, which makes sense because Mars' atmosphere contains nitrogen (28 g/mol).

I'm also not suggesting that Eve would have an atmosphere. I'm just saying that mathematically it has enough gravity to hold onto some heavier gases. I believe that what atmosphere Eve could have initially retained would have likely been stripped away by the solar wind, in the same way that Mars lost most of its atmosphere. If the bodies in KSP were real life worlds, the only one that I think would have an appreciable atmosphere is Jool.

[billbobjebkirk]

What if we pretend that the Kerbol System bodies are not 1/10 size?

[OhioBob]

What if we pretend that the Kerbol System bodies are not 1/10 size?

That depends on what other assumptions we make. Do you mean ten times larger diameter while having the same mass? Ten times larger diameter while having the same density? Ten times larger diameter while having the same surface gravity? The latter of these scenarios would be the most lifelike, so let's go with that one. In that case many of the planets/moons would be large enough to retain atmospheres, even Duna. The minimum molecular weights for each body become,

[TABLE=width: 250]

[TR]

[TD]Jool[/TD]

[TD]0.33 g/mol[/TD]

[/TR]

[TR]

[TD]Tylo[/TD]

[TD]3.3[/TD]

[/TR]

[TR]

[TD]Eve[/TD]

[TD]3.5[/TD]

[/TR]

[TR]

[TD]Laythe[/TD]

[TD]3.9[/TD]

[/TR]

[TR]

[TD]Kerbin[/TD]

[TD]5.9[/TD]

[/TR]

[TR]

[TD]Vall[/TD]

[TD]22[/TD]

[/TR]

[TR]

[TD]Duna[/TD]

[TD]30[/TD]

[/TR]

[TR]

[TD]Eeloo[/TD]

[TD]38[/TD]

[/TR]

[/TABLE]

Any body not listed is too small to have any chance of retaining an atmosphere.

(ETA) This assumes that each planet is subjected to the same solar constant. If the sun is also scaled up in size, then I'm assuming the planets' distances from the sun are scaled up to compensate.

Edited May 29, 2015 by OhioBob

[KerikBalm]

"Although Mars is 5 times more massive than Eve, because of the unrealistic planet densities in KSP, Eve's escape velocity is just slightly less than Mars "

I know this, if you read my post, I attributed ability to hold on to an atmosphere to escape velocity, and state that Eve's escape velocity was 4.5 km/s which is less than Mars' 5km. If its 4.8 instead of 4.5.... that doesn't change my point.

Yes, the solar wind also factors into it (increasing the velocity of parts of the atmosphere, allowing them to escape beyond what you'd expect purely from the temperature) - but it factors in within the confines of escape velocity.

Jupiter could have no magnetic field, and it wouldn't be losing its hydrogen any time soon.... Likewise, Venus is still sporting one heck on an atmosphere.

Long term, Eve has problems if Mars had problems.

"Ten times larger diameter while having the same surface gravity? The latter of these scenarios would be the most lifelike, so let's go with that one."

Indeed - but if we assume that, we need to go back to page 1 and revist everything else, particularly all this discussion about the star classification.

"Do you mean ten times larger diameter while having the same mass?"

This would negligibly impact the escape velocity needed from the surface (assuming no rotation), but the surface gravity would be much lower.

I suspect this is what Findthepin1 was missing when he made his comments about "an object six inches across, with an unrealistic density that gives it about 9.81 meters per second squared gravity.... able to hold onto an atmosphere of 1 atm"

If you took such an object, and were ~6,300 km(the diamete of Earth) away from it, the surface gravity would be miniscule.

The acceleration due to gravity even 6 inches away would be massively different (6400 meters vs 6300 doesn't result in a big change, but 12 vs 6 inches sure does)... and this is why the escape velocity would be so much lower despite identical surface gravity, and that is why these small but dense bodies would have a lot of trouble holding on to any atmosphere.

[Findthepin1]

...I suspect this is what Findthepin1 was missing...

Probably. Thanks.

I think Jool should be a "Accidental Venus Analogue". It has 88.4% of Venus' gravity, a very thick atmosphere like Venus' (ignoring color), its diameter is slightly less than Venus', it's ridiculously hot at low altitudes and things are destroyed before hitting solid ground, etc.

Edited May 29, 2015 by Findthepin1

[OhioBob]

(Jool is) ridiculously hot at low altitudes

That has changed since v1.0. It's temperature at the datum level is now a very cold 200 K.

[Findthepin1]

That has changed since v1.0. It's temperature at the datum level is now a very cold 200 K.

Oh. Thanks. Never mind that part then.