Moon rocks, formally known as lunar samples, are specimens collected from the Moon’s surface by crewed missions like Apollo and robotic probes like China’s Chang’e series. These geological fragments exhibit a wide spectrum of colors, ranging from near-white to jet-black, rather than a uniform shade of gray. The difference in their appearance is a direct consequence of the Moon’s formation, its geological history, and the harsh, airless environment of space. Understanding the colors of these rocks provides scientists with a detailed record of the Moon’s internal structure and surface processes.
The Observed Colors of Lunar Rocks
Lunar rocks collected from different regions of the Moon reveal a distinct visual dichotomy. The dark plains known as the maria are composed primarily of fine-grained volcanic rock called mare basalt, which appears dark gray to black. This coloration gives the maria their characteristic dark appearance when viewed from Earth, resembling ancient seas.
Conversely, the elevated, heavily cratered regions known as the lunar highlands are dominated by anorthosite. These rocks are visibly much lighter, often appearing white, pale gray, or transparent. This stark contrast defines the overall visual landscape of the Moon.
The loose, fine surface material covering the Moon, called the regolith, is also a notable visual component. The regolith, essentially lunar soil and dust, typically appears a dusty, muted shade of medium to dark gray. This color is generally darker and duller than the fresh, underlying crystalline rocks due to constant exposure to the space environment.
Internal Factors: Mineral Composition and Color
The inherent color of a moon rock is determined by its internal chemical makeup and the minerals that crystallized within it. The dark colors of the mare basalts are primarily caused by a high concentration of iron and titanium oxides. These elements are incorporated into common rock-forming minerals like pyroxene and olivine, which naturally possess a dark hue.
Many basalts also contain significant amounts of the mineral ilmenite, an iron-titanium oxide that is black and highly opaque. Iron is an efficient light absorber, meaning that rocks with a high iron content naturally appear dark. Basalts from the Apollo 11 and Apollo 17 missions, for example, are known for their exceptionally high titanium content, making them some of the darkest collected samples.
In contrast, the light-colored highland rocks are composed almost entirely of plagioclase feldspar, specifically a calcium-rich variety called anorthite. Plagioclase feldspar is naturally white or transparent and contains very little iron or other elements that absorb visible light. This low concentration of iron and titanium means the anorthosite reflects most sunlight, resulting in its bright, pale appearance. This color difference is a direct result of how the early Moon differentiated, with lighter minerals forming the crust and heavier, iron-rich minerals sinking.
External Factors: Space Weathering and Surface Appearance
While mineral composition dictates the rock’s intrinsic color, the lunar surface appearance is significantly modified by space weathering. This external alteration occurs because the Moon lacks a protective atmosphere, leaving its surface constantly exposed to the harsh elements of space. The two primary agents of space weathering are continuous bombardment by micrometeorites and irradiation by the solar wind.
Micrometeorite impacts generate intense heat that melts and vaporizes the rock and soil, creating tiny glass beads and fragments called agglutinates. These impacts also cause the reduction of iron from iron-bearing minerals like pyroxene. This process liberates metallic iron, which forms microscopic particles known as nanophase iron.
These nanophase iron particles are extremely opaque and accumulate on the surface of rock grains and in the agglutinate glass. The accumulation of this dark material creates a thin, dark rind or patina on exposed rocks and is responsible for the overall darkening of the lunar regolith. Solar wind, which consists of high-energy hydrogen and helium ions, also contributes to the weathering by chemically altering the surface layers.
This constant bombardment and alteration lead to the dusty, dull, and darker appearance characteristic of the Moon’s surface. The surface material is not only darkened but also appears slightly redder compared to fresh rock because the nanophase iron preferentially scatters red light. Ultimately, the color of the Moon results from the combined effect of the inherent mineral colors of the rocks and the pervasive, darkening blanket of space-weathered dust.