October 10, 2012
The color of the Moon has been studied for years. Lunar color is a subtle, yet fascinating phenomenon. Just when it seemed that we had an explanation, complications would arise. We now think we have a reasonable explanation for it. So, why is the Moon gray? Or to ask the question “scientifically”— What factors account for the range of spectral reflectance seen on the Moon?
Early Apollo astronauts were very impressed with the Moon’s lack of color. During Apollo 8 (first mission to orbit the Moon in 1968) Jim Lovell remarked, “The Moon is basically gray – no color.” The Apollo 10 crew was struck by the numerous brownish hues exhibited by the Moon – from a bright tan to a dark, chocolate brown. When the first astronauts landed and walked on the Moon (Apollo 11), they had an even closer view. Buzz Aldrin mentioned that although the surface color was basically gray, he could see interesting colors within some rocks outside the LM window. During the EVA, Aldrin mentioned to Neil Armstrong that he had seen “some purple rocks.” Purple? — perhaps so.
The Apollo 15 crew was surprised on their 1971 mission to catch a fleeting glimpse of green on the surface (in film shot earlier by crews on the lunar surface, color was too subtle to be seen). When they raised the sun visors of their helmets to again see that the soil was gray, the disappointment in their voices was palpable. But then, at the very next station, they again saw a flash of green and this time, it was still green when the visors were raised. Despite the predictable remarks about “green cheese,” this lunar material – consisting of volcanic glass erupted from deep (> 400 km depth) within the Moon under high pressure – was still green when brought back to Earth.
During their second lunar traverse in 1972, the crew of Apollo 17 found orange soil at Shorty crater. Also volcanic glass, this soil is made up of tiny (~50 micron) beads of orange glass, again erupted from great depth. It is orange (as opposed to the Apollo 15 green glass) because of its relatively high titanium content. It is mixed with black glass beads, of identical composition, but in this case, partly crystallized. Subsequent study of the Apollo samples have found volcanic glass fragments in almost every color in the spectrum, from red to yellow and brown in addition to the two described above.
At this point, it is tempting to ascribe lunar color seen at a distance to the intimate mixing of a variety of colors present at fine scale. But this is not quite correct. Most returned lunar samples are also gray, ranging from a very dark charcoal to a light, almost white-gray shade. Minor variations can be seen as a result of the presence of certain minerals. In particular, the mineral olivine (an Mg- and Fe-rich silicate) is abundant in the lunar crust and is often green or a brownish yellow. Ilmenite (and iron- and titanium oxide) is bluish-black and probably the source of the “purple” Aldrin saw in some rocks during the Apollo 11 EVA. Moreover, the astronauts could sometimes see significant color units from space. After his surface visit, Apollo 17 astronaut Jack Schmitt (in orbit) saw orange material, excavated by small craters on the southwestern rim of the Serenitatis basin. He suggested that this material might be related to the orange soil collected at the landing site a few days earlier.
Interestingly, one can detect subtle color differences on the Moon with telescopes and from spacecraft. Although the Moon appears gray at first glance, one notices different hues of gray in certain places. The dark Mare Tranquillitatis on the eastern near side is a noticeably darker and “bluish-gray” compared to the dark mare plains just to the north in Mare Serenitatis. Part of the reason the Moon looks whitish-gray in the sky can be attributed to the fact that it is the brightest object in the night sky – dazzling the eye when first looked at (either with your naked eye or through a telescope). Spacecraft views also reveal color differences. It is common practice for lunar scientists to work with “false color” composite images, where color variations are “stretched” to extreme degrees to exaggerate differences in order to make them easier to work with. The typical “false color” version of the near side of the Moon shows brilliantly colored “blue” and “red” maria; these color units do not coincide with mare-highland boundaries. The received wisdom is that the different color units in the lunar maria represent lava flows of differing composition. That some lavas are enriched in titanium was a major finding from the Apollo sample studies. Interestingly, these high-titanium lavas come from “blue” regions in the maria. Initially, this was only an empirical correlation but we now know that it is the presence of ilmenite (the iron-, titanium-rich oxide) in these basalts that makes them “blue.”
It should be noted that color differences on the Moon are extremely subtle, requiring intensive image processing to display them clearly. Typically, color differences on the Moon are less than about one percent or so. We are able to see these differences with a careful look, but mapping the detailed boundaries of individual lava flows requires image processing to make the “false color” composites.
The “true” color of the Moon is a brownish (i.e., reddish) gray, but overall, the surface is fairly neutral in tone. If the Earth had no atmosphere, hydrosphere or biosphere, it too would be largely a brownish-gray, as its crust is made up (more or less) of the same silicate and oxide minerals as the Moon (in slightly different proportions). It is the weathering effects of air and water and biological activity at the Earth’s surface that makes it so colorful. The Moon – having none of these processes – displays the “true color” of the rocky planets of the Solar System. The dominant mineral in the lunar crust is plagioclase, a calcium/aluminum-rich silicate mineral. Plagioclase is gray. Thus, the dusty surface of the Moon, derived from plagioclase-rich rocks, is likewise gray. When we talk about “red” and “blue” in lunar terms (as in “blue mare basalts”), we mean bluer, or less reddish, than comparable mare deposits elsewhere on the Moon. So in reality, lunar color differences are really just varying degrees of reddish gray, some more so than others.
And what of the blue Moon? As Conan the Barbarian might say, “But that is another story…..”
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