The Lunar Surface

Core Concepts

Maria vs. highlands: two worlds in one

The highlands are the Moon's original crust — light-colored rock rich in anorthosite, a calcium-aluminum silicate that floated to the surface as the early magma ocean cooled. They are saturated with craters because they are ancient, dating to the first few hundred million years of lunar history (~4.4–4.5 billion years). The maria are younger, dark plains of basalt — the same volcanic rock that builds Hawaii — that erupted and flooded giant impact basins between about 3.1 and 3.8 billion years ago. Their younger age is stamped on them twice: fewer craters and darker, iron-rich minerals. The "man in the Moon" is just the pattern of maria on the near side.

Curiously, the maria are almost entirely on the near side. The most widely accepted explanation is that the near-side crust is thinner, so magma could reach the surface more easily there. Why the crust differs is still debated, but it shows how a single surface can encode deep interior history.

Regolith: the Moon's ground-up skin

Every surface on the Moon is covered in regolith — a gray, powdery layer of shattered rock, glass beads, and mineral fragments produced by billions of years of impacts, from giant crater-forming strikes down to micrometeorites no bigger than dust. Regolith depth varies from a few meters on young maria to tens of meters on old highlands. Because there is no wind or water, footprints and rover tracks survive essentially forever — Neil Armstrong's boot prints are still exactly as he left them. Regolith is also the raw material of future lunar bases: it can be used for radiation shielding, and possibly processed for oxygen, metals, and water.

Reading ages: samples calibrate the crater clock

The key breakthrough came when Apollo astronauts and Soviet Luna probes returned rocks of known ages. Count craters on a lunar surface, calibrate against the radiometric ages of the samples, and you have a crater-counting chronometer: more craters per area = older surface. This lets geologists "date" the highlands (ancient), the maria (intermediate), and individual fresh craters like Tycho (young, with bright rays) without ever touching them. The same clock is applied to Mercury, Mars, and even icy moons — which is why the Moon's samples matter far beyond the Moon itself.

The far side, rays, and polar ice

Spacecraft images reveal that the far side is almost entirely highlands, with far fewer maria — consistent with a thicker crust there. Fresh impact craters are decorated with rays: bright streaks of ejecta that radiate outward and gradually fade as space weathering darkens them, making rays a rough indicator of relative youth (Tycho's rays are among the youngest). One of the most important modern discoveries is water: remote-sensing spacecraft have found water ice in permanently shadowed craters near the poles, where temperatures never rise above about −150 °C (a commonly taught reference value). Because sunlight never reaches these floors, ice can survive for billions of years. This discovery has made the polar regions prime targets for future exploration and for the eventual use of lunar water.

A volcanic past, a quiet present

The maria prove the Moon once had vigorous volcanism: they are flood basalts, and features like sinuous rilles (snake-like channels carved by flowing lava) and volcanic domes dot the surface. Some regions even show lava tubes — collapsed and intact — that future habitats might use. But all of it ended long ago: the last major eruptions predate complex life on Earth, and today the Moon is volcanically and tectonically quiet, changing only through the slow rain of impacts.

Common Confusions