Moon rocks, primarily those collected by the Apollo missions, were formed in an environment vastly different from Earth, lacking air, water, or life. This unique formation gives them a distinct appearance and composition. Their features tell a unique story of their history. Understanding the visual characteristics of these lunar samples requires appreciating the geological diversity captured in each piece, moving beyond the simple idea of a gray rock.
The General Appearance and Texture
Lunar samples exhibit a color palette restricted to shades of gray, black, and sometimes a subtle greenish or brownish tint. This limited range is due to the Moon’s lack of oxygen and water, which prevents the colorful chemical weathering seen on Earth, such as the red rust of iron oxides. Most moon rocks feel rough and angular, a texture resulting from billions of years of bombardment by meteorites and micrometeorites.
Many of these rocks display a porous or “vesicular” texture, pitted with small holes that resemble tiny bubbles. These cavities formed when gases escaped as molten rock cooled rapidly near the Moon’s surface. Despite this porous appearance, lunar rocks are generally denser than many Earth rocks of similar size, depending on the rock’s specific mineral composition.
The Dark and Light Sides: Primary Rock Types
The most significant visual difference among lunar rocks relates to their origin in the two main geological provinces: the dark plains and the brighter highlands. These two areas are made of different rock types that reflect the Moon’s early crustal formation.
Mare basalts, which make up the dark, smooth plains visible from Earth, are dense, fine-grained, and typically black or dark gray. These rocks are rich in iron and magnesium-bearing minerals. Lunar basalts often contain high concentrations of titanium, which contributes to their darker appearance.
The lighter-colored highlands, which cover most of the Moon’s surface, are composed primarily of anorthosite and breccias. Anorthosite is rich in calcium-rich plagioclase feldspar, a mineral that has a naturally light or grayish-white color. Breccias are composed of angular fragments of older rocks cemented together by the force and heat of meteorite impacts, giving them a fractured, composite appearance.
Distinguishing Lunar Surface Features
The Moon’s unique environment has imparted several external modifications that distinguish lunar rocks from Earth-based samples. The lunar surface is covered by regolith, a fine, abrasive, dark gray material that is not soil because it lacks organic matter. Regolith is essentially pulverized rock fragments created by continuous impact bombardment, and this fine dust often clings to the returned samples.
Micrometeorite impacts melt the surface material, creating tiny glass spherules often found mixed within the regolith or fused onto the rock surfaces. These glass beads can appear dark or shiny and result directly from intense, high-velocity impacts in the vacuum of space.
A visual clue is the presence of “zap pits,” which are tiny, microscopic impact craters found on the exposed exterior surfaces of the rocks. These small pits are often surrounded by a rim of melted, glassy material, formed by the bombardment of hypervelocity particles.