<p>To answer this, one really has to understand how the geometric and bond albedos are defined. Let's start with the bond albedo since its simpler.</p>

<h3>Bond Albedo</h3>

<p>The <a href="https://en.wikipedia.org/wiki/Bond_albedo" rel="noreferrer">Bond Albedo</a> is just the fraction of energy hitting a surface that gets reflected. To simplify the process, let's say I shoot 100 photons of all the same energy at Enceladus. Of those 100 photons that hit the surface, 81 of them will be reflected back and 19 of them will be absorbed. That means the bond albedo will be $81/100 = 0.81$.</p>

<p>Simple right? As such, the bond albedo can never be greater than 1 because you obviously can't reflect back more photons than you received. This means that a bond albedo of zero implies no light was reflected and all of it was absorbed, whereas a bond albedo of one implies all light was reflected and no light was absorbed.</p>

<h3>Geometric Albedo</h3>

<p>The definition of the <a href="https://en.wikipedia.org/wiki/Geometric_albedo" rel="noreferrer">geometric albedo</a> is a bit more complicated. The first thing you have to realize though is that it is not a straight zero to one scale. A geometric albedo of one doesn't mean all the light got reflected like it did for the bond albedo. So let's try to define the geometric albedo.</p>

<p>There are two important points about the geometric albedo that make it differ from the bond albedo.</p>

<p>For the bond albedo, we talked solely about the total amount of incident light and the total amount of reflected light. This says nothing about an observer of said light. We just have some magical way of knowing about every photon that hits and is reflected by your surface and that allows us to calculate the albedo according to the bond albedo definition. The geometric albedo on the other hand specifically takes into account your ability to observe the light from a specific vantage point.</p>

<p>The second major point is that the geometric albedo is a measure of how well your particular surface reflected the light back to you <em>compared to a reference surface</em>. Imagine now you have two surfaces, one is the surface of Enceladus, the other is your "reference surface". This reference surface is an "idealized" reflector which means that it reflects every photon that hits it (making it have a bond albedo of 1). The key here though, is that it reflects light isotropically. What I mean by that is that the light doesn't get reflected in any preferred direction, but rather gets reflected in all directions equally. So now you have your 100 photons hitting your reference surface and all 100 photons get reflected, but because they're all reflected in random directions, only 10 photons actually get to your camera/eye/detector. What can happen with Enceladus though, is that the surface has just the right properties such that 100 photons hit the surface, and 14 photons get reflected to your detector because the surface <em>preferentially reflects light in a specific direction</em>. It's sending more photons to your detector than the reference surface. The geometric albedo is the ratio of how much light gets reflected to your detector by Enceladus over how much light gets reflected to your detector by the reference surface. In this case, $14/10 = 1.4$.</p>

<p>One small, additional point, is that this definition relies on your detector being in the same direction as your light source. In other words, the geometric albedo is a measure of how much your surface can <a href="https://en.wikipedia.org/wiki/Retroreflector" rel="noreferrer">retro-reflect</a> (i.e., reflect back to the source of the photons) compared to the reference surface. Technically, the geometric albedo attains a maximum value when your surface is made up of retroreflectors. </p>

CC BY-SA 3.0
2017-04-22T20:45:28.037Z
I like your answer, but maybe next time you could also just write down the definitions as integrals ;) that should make it clear very quick for non-layman users...
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-02-04T05:11:16.333Z
Just fyi I've voted to close my own question [Is albedo ever greater than unity? How is it calculated for various surface diffusivities and specular reflection?](https://astronomy.stackexchange.com/q/29459/7982) as dupe to here. You are welcome to leave a comment there if you think there are any subtleties that I should still be asking about.
uhoh
CC BY-SA 3.0
2017-04-21T16:40:12.230Z
2017-10-10T16:07:34.433Z
2017-10-10T16:07:34.433Z
zephyr
<p>Enceladus, one of Saturn's moons, has a geometric albedo of 1.38 and a bond albedo of 0.81. How can the geometric albedo of Enceladus be greater than one? What are the similarities or differences between a geometric albedo and a bond albedo?</p>

CC BY-SA 3.0
2017-04-21T14:01:10.017Z
@barrycarter that's the answer;  why not post it as such & grab the karma? :-)
Carl Witthoft
CC BY-SA 3.0
2017-04-21T14:01:55.383Z
I'm voting to close this question as off-topic because it takes less time to search wikipedia for the answer than to type it here in the first place.
Carl Witthoft
CC BY-SA 3.0
2017-04-21T14:16:35.270Z
@CarlWitthoft Maybe for you. There's no indication that the OP has the base knowledge to understand the underlying reasons (which are not clearly explained on wikipedia), hence why they're asking here. Wikipedia may describe the answer, but it is not clear for someone who doesn't know the physics/terminology/underlying principles.
zephyr
CC BY-SA 3.0
2017-04-21T19:41:56.683Z
I found the Wikipedia answer quite lacking and I'm happy that @zephyr answered this.    His answer was much easier to follow.
userLTK
<saturn><albedo><enceladus>
Why is Enceladus's albedo greater than 1?
Cyclopropane
# Why is Enceladus's albedo greater than 1?

(Asked by: username_0 on 2017-04-20)

Enceladus, one of Saturn's moons, has a geometric albedo of 1.38 and a bond albedo of 0.81. How can the geometric albedo of Enceladus be greater than one? What are the similarities or differences between a geometric albedo and a bond albedo?

**Tags:** saturn, albedo, enceladus

> **Comments:**
> **username_1**: @barrycarter that's the answer;  why not post it as such & grab the karma? :-)  
> **username_1**: I'm voting to close this question as off-topic because it takes less time to search wikipedia for the answer than to type it here in the first place.  
> **username_2**: @CarlWitthoft Maybe for you. There's no indication that the OP has the base knowledge to understand the underlying reasons (which are not clearly explained on wikipedia), hence why they're asking here. Wikipedia may describe the answer, but it is not clear for someone who doesn't know the physics/terminology/underlying principles.  
> **username_3**: I found the Wikipedia answer quite lacking and I'm happy that @username_2 answered this.    His answer was much easier to follow.  

---

## 1 answers

### Answer by username_2 on 2017-04-21. Score: 17 | Accepted answer

To answer this, one really has to understand how the geometric and bond albedos are defined. Let's start with the bond albedo since its simpler.

### Bond Albedo

The [Bond Albedo](https://en.wikipedia.org/wiki/Bond_albedo) is just the fraction of energy hitting a surface that gets reflected. To simplify the process, let's say I shoot 100 photons of all the same energy at Enceladus. Of those 100 photons that hit the surface, 81 of them will be reflected back and 19 of them will be absorbed. That means the bond albedo will be $81/100 = 0.81$.

Simple right? As such, the bond albedo can never be greater than 1 because you obviously can't reflect back more photons than you received. This means that a bond albedo of zero implies no light was reflected and all of it was absorbed, whereas a bond albedo of one implies all light was reflected and no light was absorbed.

### Geometric Albedo

The definition of the [geometric albedo](https://en.wikipedia.org/wiki/Geometric_albedo) is a bit more complicated. The first thing you have to realize though is that it is not a straight zero to one scale. A geometric albedo of one doesn't mean all the light got reflected like it did for the bond albedo. So let's try to define the geometric albedo.

There are two important points about the geometric albedo that make it differ from the bond albedo.

For the bond albedo, we talked solely about the total amount of incident light and the total amount of reflected light. This says nothing about an observer of said light. We just have some magical way of knowing about every photon that hits and is reflected by your surface and that allows us to calculate the albedo according to the bond albedo definition. The geometric albedo on the other hand specifically takes into account your ability to observe the light from a specific vantage point.

The second major point is that the geometric albedo is a measure of how well your particular surface reflected the light back to you *compared to a reference surface*. Imagine now you have two surfaces, one is the surface of Enceladus, the other is your "reference surface". This reference surface is an "idealized" reflector which means that it reflects every photon that hits it (making it have a bond albedo of 1). The key here though, is that it reflects light isotropically. What I mean by that is that the light doesn't get reflected in any preferred direction, but rather gets reflected in all directions equally. So now you have your 100 photons hitting your reference surface and all 100 photons get reflected, but because they're all reflected in random directions, only 10 photons actually get to your camera/eye/detector. What can happen with Enceladus though, is that the surface has just the right properties such that 100 photons hit the surface, and 14 photons get reflected to your detector because the surface *preferentially reflects light in a specific direction*. It's sending more photons to your detector than the reference surface. The geometric albedo is the ratio of how much light gets reflected to your detector by Enceladus over how much light gets reflected to your detector by the reference surface. In this case, $14/10 = 1.4$.

One small, additional point, is that this definition relies on your detector being in the same direction as your light source. In other words, the geometric albedo is a measure of how much your surface can [retro-reflect](https://en.wikipedia.org/wiki/Retroreflector) (i.e., reflect back to the source of the photons) compared to the reference surface. Technically, the geometric albedo attains a maximum value when your surface is made up of retroreflectors.

> **Comments:**
> **username_4**: I like your answer, but maybe next time you could also just write down the definitions as integrals ;) that should make it clear very quick for non-layman users...  
> **username_5**: Just fyi I've voted to close my own question [Is albedo ever greater than unity? How is it calculated for various surface diffusivities and specular reflection?](https://astronomy.stackexchange.com/q/29459/7982) as dupe to here. You are welcome to leave a comment there if you think there are any subtleties that I should still be asking about.  

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