Albedo

A strange alleyway, just off of memory lane.

Q. Why does wet sand look darker than dry sand?
A. Reflects less light."
---Answer Dept., THE GRAB BAG, San Francisco Chronicle, April 26, 1992.

And aren't we glad that that's been explained?

The word "albedo" refers to the amount of light that a surface reflects. When it reflects different colors by differing amounts, we get various colors.

Fresh snow has an albedo of about 0.7, indicating that it reflects about 70% of the light it receives. Carbon black reflects only about 3% light, and that is close to a lower limit for flat surfaces. Special "black body" measurements are made by looking at the opening of dark cavities, like the mouth of a cave is darker than any surface.

Of the planets, Venus is the clear winner in the albedo sweepstakes, with a 76% reflectivity. Despite what it might seem on a night with a full moon, Luna is quite dark, with an average albedo of only 0.07, close to that of Mercury, which is easily the darkest planet. There are a number of objects even darker still, mostly carbonaceous chondrite asteroids, or planetary moons which may be captured asteroids, like Phobos and Deimos, the moons of Mars. It's not easy to measure the reflectivity of very dark objects, but the carbonaceous bodies have albedos as low as 0.04.

ALBEDO OF VARIOUS BODIES

• MERCURY: 0.06
• VENUS: 0.76
• EARTH: 0.29
• LUNA: 0.07
• MARS: 0.16
• PHOBOS: 0.05
• DEIMOS: 0.05
• ASTEROIDS: (0.04-0.5)
• JUPITER: 0.34
• IO: 0.6
• EUROPA: 0.65
• GANYMEDE: 0.45
• CALLISTO: 0.18
• SATURN: 0.33
• MIMAS : 0.7
• ENCLADUS: >1
• TETHYS: 0.8
• DIONE: 0.5
• RHEA: 0.6
• TITAN: 0.2
• IAPETUS (leading side): 0.05
• (trailing side): 0.5
• URANUS: 0.34-0.5
• MIRANDA: 0.34
• ARIEL: 0.4
• UMBRIEL: 0.19
• TITANIA: 0.28
• OBERON: 0.24
• NEPTUNE: 0.34-0.5
• TRITON: 0.7-0.9
• PLUTO/CHARON: 0.5

Although the 0.76 albedo of Venus is very bright, it's still not the most reflective body in the solar system. That honor belongs to Enceladus, the second large moon of Saturn, which has an "visible geometric albedo of greater than 1 (some observations make it as high as 1.4).

Like another moon of Saturn, Mimas, Enceladus is mostly water, with perhaps as much as 40 percent of the mass being silicate, including a small rocky core. The interior temperature is quite warm for an icy moon, perhaps due to tidal forcing from Dione, which is much denser, and has a resonant orbital period twice that of Enceladus.

It's pretty obvious that the high albedo of Enceladus is due to anisotropic backscattering, light being reflected back towards it origin preferentially. Since the light comes from the Sun, and since, from Enceladus' point of view, the Earth is always near the Sun, the Earth always gets more light reflected from Enceladus. Even so, the ice on Enceladus' surface must be very clean, although there have been some suggestions (based on spectroscopy) that there is some ammonia mixed in with it. But there is active vulcanism seen on Enceladus, which suggests that the surface is undergoing constant refreshing.

The surface of Enceladus shows evidence of an active geological history. There are six types of terrain ranging from heavily cratered plains to craterless grooved terrain that is similar to the sulci of Ganymede. The sulci terrain features are on the trailing hemisphere of Enceladus, leading to the suggestion that they have been protected from bombardment by the bulk of the moon as it encountered debris in its synchronous orbit.

Saturn's E ring shows a brightness peak along the orbit of Enceladus, and may consist mainly of ice crystals which would escape from the weak gravity of Enceladus when meteors impact the surface or when some other factor causes water to outgas from the interior. If this is true, then the E ring is similar in origin to the "plasma torus" of Io, another moon that is warmed by tidal forcing.

Enceladus is the second classical moon of Saturn, named by Sir John Hershel after a giant in Greek mythology who figured in a revolt against the gods. Son of Tartarus and Gaea, Enceladus had a hundred arms and was so strong that Athene was forced to bury him beneath Mt. Aetna, and his occasional movements were held to be the source of the volcanic earth tremors that move Sicily. The surface features of Enceladus are by convention named after characters in Sir Richard Burton's "The Thousand Nights and a Night."