The Lake County Astronomical Society

Albedo Effects

Jack Kramer

So what's with albedo ... and what does it mean to me? (Hint: it helps explain the brightness of objects.)

This odd-sounding term refers to the percentage of sunlight that is reflected by a Solar System body. There are two types: normal albedo and bond albedo. The former, also called "normal reflectance", is a measure of the relative brightness of a surface and is used to determine the surface composition of objects. The brightness of an object as seen from Earth is actually a combination of its albedo, diameter, and distance. Bond albedo also is the fraction of the total solar radiation reflected by a planet back to space, but is dependent on the spectrum of the incident radiation because it's defined over the entire range of wavelengths. Bond albedo is used to measure a planet's energy balance. (It's named for the American astronomer George Bond, who in 1861 published a comparison of the brightness of the sun, moon, and Jupiter.) Earth-orbiting satellites have measured the Earth's bond albedo - the most recent values average approximately 0.33. The Moon has an albedo of 0.12. By contrast, the bond albedo of Venus is 0.76. The different types of albedo yield relatively similar values for each body, so most references don't make a fine distinction between the two, and albedo is generally averaged for the entire surface of the body.

At the other end of the scale, the planet Venus blazes forth brightly because it reflects such a large portion of the sunlight it receives. Only Pluto among the planets has a comparably high albedo from its brightest hemisphere. When we look at Venus through a telescope, it's usually pulsing and throbbing in the atmosphere, and sometimes it's even difficult to discern its exact phase. This turmoil is caused largely by the fact that Venus never rises very high, so is greatly affected by any atmospheric instability. Therefore I often use a neutral density filter on Venus, rather than for lunar observation. As far as I'm concerned, moon filters should be renamed "Venus filters"! Sometimes when Venus is particularly bright I stack a colored filter along with the neutral density filter in order to further darken it.

Back to the darker objects, Mars is the third least reflective object in the Solar System, with an albedo of only .16 (again, not counting asteroids). Part of the reason why it appears so bright in a telescope is that when it's around opposition we're seeing it just about full-face with the sun shining directly on it. (If Venus and Mars were to switch places, Venus would appear even brighter than it does now.) Plus, it's relatively near us at those times when it appears largest - about 34 million miles at its closest approach. Mostly what makes details more difficult to see is that Mars is fairly small, and the low contrast surface features are easily smeared by our atmospheric instability. Mars is the only planet, other than Venus, on which I've found colored filters to sometimes be useful. A blue (#80A) filter helps see thin clouds in the upper atmosphere of Mars. Among my images from the close Martian apparition of 2003, I processed this one to emulate the effect of a blue filter in accentuating limb haze and areas of higher albedo. I prefer either orange (#21) or medium yellow (#12) rather than a red filter for surface features because they pass more light and give an enhanced but less garish view. Note that most images sent back by the Mars landers have been color enhanced toward the red to improve contrast, but in fact Martian surface material tends more toward a uniform rusty brown color.

Here's a representative sample of the average normal albedo of Solar System objects:

(Pluto has a range of albedo because of major reflectance differences over parts of its surface.)