Question about Albedo?

[John Jaksich]

I realize that the phenomenon of albedo is the measure of the "star's" reflectance from a planet--but how much does the composition of a planet's atmosphere play into the "albedo"? e.g. what if a planet with the composition of Titan were in the place of Mars' orbit? What would we expect to see?

Excuse me if this question is overly simplistic...but is there a generalization where one may find "an equation" for finding the albedo of a planet based on a hypothetical set of conditions?

Cheers---jaksichj

[Messier Tidy Upper]

Well if Titan replaced Mars it would still be reddish I'd expect as the tholin rich atmosphere of Titan is a smoggy orange. Maybe a little more yellow & less vivid than the Martian hue.

It'd also be smaller given Titan isn't quite Mars sized - although Titan is larger than Mercury and Mars is pretty small as rocky planets go too. Actually Titan is next to Mars in world size order. Mars is larger and redder than Titan but I think Titan may be more reflective with a brighter albedo so there may not be all that much difference if those factors cancel each other out some.

Another complicating factor may be how much difference the increased solar radiation and extra warmth make to Titan's surface. Much of the icy tholin "gumbo" slush on Titan's surface may well evapourate creating a thicker atmosphere which might make a difference although I'm not sure which way.

One other thing that may or may not have an effect - Titan reflects sunlight at Saturn's distance - but may reflect *brighter* Sunlight if closer in where the Sunlight is a bit brighter. Not sure, again, how much difference this makes.

Good question! :-)

[Tog]

Albedo doesn't change with distance. If the moon is 7% in Earth orbit, it would be 7% in Neptune orbit. The difference would be that the intensity of the light would be much less by the time it got there, and even lower by the time it got back to us. The specific reflectivity of the surface doesn't care in the least how far it is from the light.

I don't know for sure what the actual equation might be, but I would think that if the magnitude of the light source (the sun) and the target (Titan), could be measured at known distances, then the reflectivity could be figured using the propagation of light and comparing the actual light return to what it would be if it were a 100% reflective surface.

One thing on that I have no idea how to put in is the size of the object. A bathroom mirror is much more reflective than the moon, but the moon will still be brighter (overall) at a given distance, simply because there so much more of it.

As for clouds, Venus has an albedo of around 70% and this is due to the tops of the clouds. The actual surface would be much lower, but light doesn't reach it.

Someone will probably be along to add some actual math, and hopefully correct anything I got wrong.

[Hornblower]

Titan is about 8th magnitude at its mean distance from us. If we substitute it for Mars at a favorable opposition it will be about -3, a little brighter than Mars. Since it is smaller, that means a somewhat higher albedo. That, of course, is assuming that the reflectivity of the atmosphere does not change when it is warmed up. I have no means of predicting what would happen.

To calculate the magnitude change:

1. Calculate the ratio of the distances of Saturn and Mars from the Sun.

2. Raise that to the 4th power. This gives the brightness change as seen from the Sun.

3. Calculate the ratio of the distance of Mars from the Sun to its distance from Earth at opposition.

4. Square result of #3, then multiply by result of #2. This gives the brightness change as seen from Earth.

5. Find log of #4 and multiply by 2.5 to get magnitude change.

[John Jaksich]

Titan is about 8th magnitude at its mean distance from us. If we substitute it for Mars at a favorable opposition it will be about -3, a little brighter than Mars. Since it is smaller, that means a somewhat higher albedo. That, of course, is assuming that the reflectivity of the atmosphere does not change when it is warmed up. I have no means of predicting what would happen.

To calculate the magnitude change:

1. Calculate the ratio of the distances of Saturn and Mars from the Sun.

2. Raise that to the 4th power. This gives the brightness change as seen from the Sun.

3. Calculate the ratio of the distance of Mars from the Sun to its distance from Earth at opposition.

4. Square result of #3, then multiply by result of #2. This gives the brightness change as seen from Earth.

5. Find log of #4 and multiply by 2.5 to get magnitude change.

Thanks a lot and I am very appreciative

jaksichj