Enceladus albedo Options

[volcanopele]

I got a question at Wikipedia regarding the geometric albedo of Enceladus. Verbiscer et al. 2005 reported that Enceladus geometric albedo at 555 nm was 1.41, greater than unity. This is much greater than the 0.99 reported in Verbiscer and Veverka 1994. The 0.99 figure was listed in Wikipedia until the release of the Porco et al. paper on Enceladus, which cited the Verbiscer et al. 2005 paper when mentioning the visual geometric albedo (1.4 in the Porco et al. paper).

So someone on Enceladus' wikipedia talk page has asked how can Enceladus have a visual geomtric albedo greater than 1. I thought I would direct the question here.

[JRehling]

I got a question at Wikipedia regarding the geometric albedo of Enceladus. Verbiscer et al. 2005 reported that Enceladus geometric albedo at 555 nm was 1.41, greater than unity. This is much greater than the 0.99 reported in Verbiscer and Veverka 1994. The 0.99 figure was listed in Wikipedia until the release of the Porco et al. paper on Enceladus, which cited the Verbiscer et al. 2005 paper when mentioning the visual geometric albedo (1.4 in the Porco et al. paper).

So someone on Enceladus' wikipedia talk page has asked how can Enceladus have a visual geomtric albedo greater than 1. I thought I would direct the question here.

Opposition surge plus a very high albedo. Unity is the case of a lambertian surface.

[The Messenger]

Opposition surge plus a very high albedo. Unity is the case of a lambertian surface.

Isn't 0.99 a better number then, since the opposition surge is a rather special case?

[volcanopele]

Isn't 0.99 a better number then, since the opposition surge is a rather special case?

Well, visual geometric albedo is supposed to be at a phase angle of 0, so opposition surge is supposed to be included. It is included in the values for every other body, so to remain consistent, it should be included for Enceladus.

[JRehling]

Well, visual geometric albedo is supposed to be at a phase angle of 0, so opposition surge is supposed to be included. It is included in the values for every other body, so to remain consistent, it should be included for Enceladus.

Maybe I said it ambiguously before:

VGA is the luminosity at a phase angle of zero (which includes the opposition surge) divided by the brightness of a Lambertian white body the same size at a phase angle of zero (which would have no opposition surge).

A dark body like Mars still has opposition surge, but is far below the brightness of a Lambertian white. Enceladus is close enough to white that the opposition surge puts it over unity.

[Anne Verbiscer]

VGA is the luminosity at a phase angle of zero (which includes the opposition surge) divided by the brightness of a Lambertian white body the same size at a phase angle of zero (which would have no opposition surge).

Just to clarify, the VGA is the brightness at a phase angle of zero divided by the brightness of a Lambertian disk, not a
sphere, at a phase angle of zero. (So comparisons of limb-darkening on Enceladus to limb darkening on a Lambert sphere
are irrelevant when considering the geometric albedo since a disk does not have a limb.) Enceladus, like many icy satellites
(and unlike terrestrial snow and frost), is extremely back-scattering. Most of it's incident sunlight is reflected right back in
the direction from which it came.

As for the other saturnian satellites observed at "true" opposition in January 2005, Tethys also has a geometric
albedo well in excess of unity: 1.23. The geometric albedos of Mimas, Dione, and Rhea are all about 0.95.

Anne