Seas That Never Held Water
The Moon's naked-eye face is two terrains — dark lava plains called maria and bright ancient highlands — and its detail shows best where shadows fall, along the terminator. · 12 min
Look at a full Moon and you see a pattern — a face, a rabbit, whatever your childhood named it. That pattern is geology, and it is readable with no equipment at all. The dark patches and the bright ground are two different kinds of terrain, formed about a billion years apart. By the end of this folio you will name the largest features from memory and tell, at a glance, which ground is older.
Guess before you learn
Before anything else, commit to a guess: what are the dark patches on the Moon?
They are plains of basalt — dark volcanic rock that flooded out as lava roughly three billion years ago and froze smooth. Early telescope observers guessed water and named them maria, Latin for seas; the names stuck even though the seas never held a drop. If you guessed sea beds, you are in the company of some very good seventeenth-century astronomers.
9–12
3–5
Long ago, giant space rocks slammed into the Moon and left enormous basins. Later, melted rock from deep inside leaked upward and flooded those basins, then cooled into dark, smooth plains — the dark patches you see tonight. The bright areas are the Moon's original crust: older, higher ground, crowded with craters from four billion years of hits.
Here is the rule scientists use: the longer a surface sits exposed, the more craters it collects. Count the craters on two surfaces and you know which one is older.
6–8
The dark patches are maria (MAH-ree-ah): plains of basalt, a dark volcanic rock, which flooded the floors of giant impact basins between about three and three and a half billion years ago. The bright terrain is the highlands: the Moon's original crust, roughly four and a half billion years old and saturated with craters. Nearly all the maria sit on the side that faces Earth.
Crater counting is the clock. The maria carry far fewer craters than the highlands, so they must have been resurfaced — the lava floods wiped the older record clean and the count restarted. Fewer craters, younger surface: a rule that dates terrain across the whole solar system.
9–12
The two terrains differ in rock as well as age. Highland rock is anorthosite, a pale, calcium-rich rock that crystallized and floated to the top of a global magma ocean about 4.4 billion years ago. Mare basalt is iron-rich lava that welled up through basin-fractured crust roughly a billion years later. Maria reflect about 7 percent of incoming sunlight; the highlands roughly twice that — enough contrast for the naked eye.
The lopsidedness is real: maria cover about a third of the near side but only a small fraction of the far side, where the crust is thicker and lava rarely broke through. The face you know exists only from Earth's vantage point.
K–2
The Moon has two kinds of ground. The dark ground is smooth old lava that cooled and turned hard. The bright ground is bumpy and full of round holes made by falling space rocks.
The holes are called craters. The bright bumpy ground is older — it has been hit for longer. Look up tonight: the dark patches you see are the old lava plains.
Undergrad
Radiometric ages from Apollo and Luna samples anchor the crater-count clock: highland breccias run 4.3 to 4.5 billion years, mare basalts mostly 3.1 to 3.8. Calibrated crater size–frequency distributions then date any unsampled surface — the method behind nearly every solid-body age quoted in planetary science.
Fresh impacts expose bright, immature regolith; over roughly a billion years, micrometeorite gardening and solar-wind implantation darken and redden it — space weathering by nanophase iron. That is why Tycho's rays still sweep bright across the disk: at about 100 million years old, Tycho is too young to have faded.
Postgrad
The near/far dichotomy remains an open problem. GRAIL gravity inversions give near-side crust of roughly 30–40 km against 50–60 km far-side; standing hypotheses include accretion of an early companion moon, asymmetric tidal heating, and the Procellarum KREEP Terrane's radiogenic enrichment (K, rare-earth elements, P) sustaining prolonged near-side melting.
The impact-flux calibration itself is contested: the classical late-heavy-bombardment spike near 3.9 billion years, read from Imbrium-linked sample ages, may be an artifact of a single basin's ejecta dominating the collections — a standing caution when the lunar chronology is exported to Mercury or Mars.
maria
Latin for seas (singular mare, MAH-ray): the dark basalt plains left by ancient lava floods. Named by observers who hoped for water; the name outlived the hope.
Now, when to look. The terminator is the line dividing lunar day from lunar night — the sunrise or sunset line. Along it the Sun stands low, and every mountain and crater rim throws a long shadow that makes the relief stand out. Away from it, under a high Sun, the same ground looks flat. This is why the full Moon, for all its brightness, is the worst night for detail: the Sun is overhead everywhere, and there are no shadows left to read. First quarter, with its terminator running down the middle of the disk, beats it easily.
One more secret, for the patient. The Moon keeps the same face toward Earth — its rotation and its orbit are locked to the same period, which is why there is a near side at all. So you might expect that exactly 50 percent of the Moon is visible from Earth, ever. The true figure is about 59 percent. The Moon rocks.
Libration is the Moon's slow apparent rocking, and it has two main causes. The spin is perfectly steady, but the orbit is an ellipse, so the Moon's speed along it rises and falls; when orbital motion runs ahead of the spin we see a few extra degrees around one limb, and when it lags, around the other. The Moon's axis is also tipped slightly against its orbit, so through a month we look a little over its north pole, then a little under its south. Neither rock is large, but together, over months, they let a patient observer map about 59 percent of the surface — no spacecraft required.
Why is this true?
Why does the same face of the Moon always point at Earth in the first place?
Tidal locking: over billions of years, Earth's gravity raised bulges on the Moon that acted as brakes, slowing its spin until one rotation exactly matched one orbit. The Moon does rotate — once per month, in step with its lap around us.
The Moon is the only world whose geology you can read unaided: lava plains, ancient crust, one young crater's rays, and a slow rocking that shows you more than half of everything. Next folio, we set the Moon, Earth, and Sun in a single straight line — and watch what the shadows do.
Practice — new ink and old, interleaved
1.In one sentence: why does Polaris, alone among the bright stars, appear to stand still?
2.The first-quarter Moon is highest in the sky at about what clock hour? Answer with an hour from 0 to 24.
3.From latitude 50°N, which of these stars never sets?
4.Match each term to what it names.
5.Put the steps of the Dipper-to-Polaris hop in order.
- Find the Big Dipper's bowl
- Take the line from Merak to Dubhe
- Extend it about five pointer gaps
- Land on the lone modest star — Polaris
6.The Moon rises at sunset tonight, so it is full. Roughly how many days until third quarter?
7.Tycho's rays are still brilliantly bright. What does that tell you about the crater?
8.Without looking back: name the two lunar terrains and state which is older.
The dark maria — lava plains — and the bright highlands, the original crust; the highlands are older by about a billion years.
How close were you? Grade yourself honestly — it sets your review date.
9.In one sentence: why does first quarter beat full Moon for viewing lunar detail?