Albedo is a measure of an object’s reflectivity. Geometric albedo in particular is the brightness of an celestial object as a fraction of the maximum possible brightness for a fully reflective, diffusively scattering object of the same cross-section in the same lighting conditions.

The phrase diffusively scattering means reflecting in all directions (with a reflectivity determined by the cosine law). Ordinary plaster, clay … particle board, things like that are basically not shiny and more or less the same consistency in all directions are diffusively scattering. A glass ball is not because it tends to reflect light back in certain directions: same with a mirror ball.

A white ball of plaster would have a geometric albedo close to 1.

There is only one sizable object in the solar system with a geometric albedo > 1, and that is Enceladus, one of Saturn’s larger moons, with a geometric albedo of 1.38.

There are various suggested explanations for its brightness. One commonly given one is that much of the surface may be covered by very smooth water ice, which causes specular (ie mirror-like) reflection.

One result of this reflectivity is that Enceladus is about 15 K colder than other large moons of Saturn.

That doesn’t seem right. On two counts.

mollwollfumble said:

That doesn’t seem right. On two counts.

Okay, what are those counts?

dv said:

mollwollfumble said:

That doesn’t seem right. On two counts.

Okay, what are those counts?

Previous response cut short due to lack of time. Speculative on my part.

I’ll check out the albedo thing in a minute.

My understanding is that Enceladus is hotter than other moons of Saturn because of tidal heating. You know how Io (close to Jupiter) is heated by tidal heating from Jupiter. Europa is, too, to a lesser extent. Callisto and Ganymede, being further out, are not as strongly affected.

Reasoning from that, I thought that Mimas and Enceladus, being the closest to Saturn, would also be heated by tidal heating. But, am I wrong here? Check orbital distances. This is why I thought it would have tidal heating. And because why else would the centre of Enceladus be hot enough to contain liquid water, and why else would the surface be new enough that there are no craters on it, and not even any dust from Saturn’s rings?
Io 421 Megametres
Europa 671 Mm
Mimas 115 Mm
Enceladus 238 Mm

Now for some maths https://en.wikipedia.org/wiki/Tidal_heating

Tidal heating is proportional to (R/P)^5 * e^2
where R = moon radius, P = orbit period, e = eccentricity.

Fifth power ! I wasn’t expecting that. Let’s calculate tidal heating per unit volume, divide by R^3. No – heat loss is through the surface, so i should divide by R^2 to get something that is a direct function of temperature rise.

Plug in data for Enceladus.
R = 252, P = 33, e = 0.0047
R^3 e^2 / P^5 = 9e-6

For Io
R = 1822, P = 42.5, e = 0.0041
R^3 e^2 / P^5 = 9e-6 = 733e-6

For Europa
R = 1561, P = 85, e = 0.009
R^3 e^2 / P^5 = 9e-6 = 68e-6

For Mimas
R = 198, P = 22.6, e = 0.0196
R^3 e^2 / P^5 = 9e-6 = 505e-6

Um, ohh kay. so Enceladus should have very small tidal heating but the tidal heating of Mimas should be enormous.

Was not expecting that.

Speculative on my part.

Icy what you did there.

My understanding is that Enceladus is hotter than other moons of Saturn because of tidal heating.

But it’s not. It’s colder. Here are the average surface temperatures of all the large (d > 500 km) moons of Saturn.

Titan: 94 K
Rhea: 76 K
Iapetus: 110 K
Dione: 87 K
Tehtys: 86 K
Enceladus: 75 K

Rhea is pretty cold too, and also has a high albedo.

mollwollfumble said:

dv said:

Speculative on my part.

Icy what you did there.

My understanding is that Enceladus is hotter than other moons of Saturn because of tidal heating.

But it’s not. It’s colder. Here are the average surface temperatures of all the large (d > 500 km) moons of Saturn.

Titan: 94 K
Rhea: 76 K
Iapetus: 110 K
Dione: 87 K
Tehtys: 86 K
Enceladus: 75 K

Rhea is pretty cold too, and also has a high albedo.

Thanks. A while back I tried to figure out whether Titan’s atmosphere caused it to heat up. It does, but not much, not nearly as much as Earth for instance, despite both having nitrogen-dominated atmospheres. The black dust on half of Iapetus is what keeps it hot.