What Is the Moon Rabbit Made Of?
– The Dark Patches Are Plains of Lava That Flooded Out More Than 3 Billion Years Ago
"There's a rabbit on the Moon, pounding rice cakes." Many of us were told this as children. So what are those dark patches really? They aren't shadows, dirt or water. They are lava that welled up from inside the Moon, then cooled and hardened.
It's a November night. You're walking home from cram school with your child and looking up at the full Moon. Inside the bright white disc, you can see faint dark patches.
"Hey, what's that dark stuff?" your child asks. "A rabbit," you say. "No, I mean what is it really?"
Even adults tend to fudge this one with "a shadow, maybe?" or "some dents?" Yet the answer holds four billion years of the Moon's history.
Just two reasons explain the dark patches
The white areas are light, pale rock that hardened soon after the Moon formed. The dark areas are iron-rich dark rock, made when lava welled up later from inside and hardened. The difference in colour is a difference in rock type.
The Moon's outer "skin" is thought to be thin on the Earth-facing side and thick on the far side. Only on the thin side did lava reach the surface. That is why the patches sit almost entirely on the side we can see.
Let's take them one at a time. First, how did the dark areas form?
Lava pooled in giant basins over hundreds of millions of years
When the Moon was newly formed, its surface is thought to have been covered in molten rock. As it cooled, light, pale minerals floated to the top and made a white crust. This is the white part of today's Moon, called the "highlands."
After that, many large objects crashed into the Moon. They are thought to have dug giant basins, hundreds of kilometres across, all over its surface.
Later still, some of the rock inside the Moon melted into lava. The lava rose through cracks and collected in the low basins. Move the slider in Figure 1 to the right to watch it happen.
Most of the lava is thought to have flowed out between about 3.9 and 3.1 billion years ago. It spread the plains bit by bit over hundreds of millions of years. In the 3 billion years since, the Moon's face has hardly changed. The rabbit we see is a "face" from long before the dinosaurs.
The dark colour comes from iron. The rock formed from hardened lava (basalt) contains a lot of it. Such rock absorbs light easily and reflects only about half as much as the white highlands. The plains are flat and have few conspicuous craters, so from far away they look like smooth dark patches.
The dark areas have names like the "Sea of Tranquility" and the "Sea of Rains." Early astronomers believed they really were seas full of water. The names given by an Italian astronomer who mapped the Moon in the 17th century are still in use. In fact there is not a drop of water. They are plains of lava.
Why are the patches only on the side facing Earth?
The Moon always shows the same face to Earth (this is explained in this article). In 1959 a space probe photographed the far side for the first time, and researchers were surprised. The far side had almost no dark "seas."
On the near side, about 30% of the area is sea. On the far side, there is very little. The Moon's two faces are so different that they could belong to different worlds.
A major reason is thought to be the thickness of the Moon's outer "skin," the crust. Look at Figure 2. Precise measurements of the Moon's gravity by space probes have shown that the crust is thin on the near side and thick on the far side.
Molten rock is slightly lighter than the rock around it, so it tends to rise. But if the crust above is too thick, the molten rock cannot reach the surface. It got through on the thin near side and did not on the thick far side. This is the leading explanation for why the patches lean toward Earth.
So why is the crust thin only on the near side? Nobody has the answer yet. Ideas include heat-producing elements gathering on the near side and a large object striking the far side. Research is continuing.
People elsewhere are said to have seen the same patches as a crab, a woman reading a book, a donkey and more. The human eye tends to find faces and animals even in scattered shapes. What you see depends on your culture, but the rock is the same in every country.
Summary
The Moon's dark patches are plains where lava from inside the Moon flowed into basins left by giant impacts and hardened. They hold more iron than the white highlands and reflect less light, so they look dark. Lava could reach the surface only on the near side, where the crust is thin. That is why the patches are found almost only on the side visible from Earth.
The Moon rabbit is a map of lava that hardened over 3 billion years ago.
Only on the side facing Earth did the Moon's insides spill out.
Why the Moon always shows the same face is covered in "Why does the Moon always face us with the same side?" and the Moon's phases in "Why does the shape of the Moon change every day?" To learn why the Moon and Earth are round, try "Why is the Earth round?"
- Around the half Moon, look through binoculars at the border between the lit and dark parts of the Moon.
- On the white highland side, the border is jagged. Many crater shadows line up along it.
- On the dark sea side, the border is smooth, because the ground is flat, hardened lava.
At full Moon there are no shadows, so the darkness of the patches shows most clearly. To see the bumps, the half Moon is better. To protect your eyes, never point binoculars near the Sun in daytime.
For those who want more – terms, formulas and links to textbooksFrom junior high science to university courses, each part is labelled with its level
- Jr. highCovered in junior high school science
- High schoolCovered in high school Earth science
- HS+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity-level planetary science, not taught in high school
- ResearchNot yet settled even at university level; researchers are still investigating
Jr. highTerms: this phenomenon has names
- Lunar sea (mare): A dark plain on the Moon's surface formed from hardened lava. It contains no water.
- Highlands: The white, old, heavily cratered regions. They are made of a pale rock called anorthosite.
- Basalt: A dark volcanic rock rich in iron and magnesium. The lunar seas and Earth's ocean floors are mostly made of it.
Jr. highHigh schoolChecking with a formula: how big are the dark patches?
We find the Moon's surface area with the sphere formula, then multiply by the fraction covered by seas to get the area of the dark patches.
| In symbols | S_sea ＝ 4 × π × r² × p |
| In words | Sea area ＝ 4 × pi × (Moon's radius)² × fraction covered by sea |
| Where it comes from | The first part, "4 × π × r²", is the formula for the surface area of a sphere. We multiply it by the fraction of the surface covered by sea. |
| S_sea | Area of the seas (dark patches), in square kilometres |
| r | Radius of the Moon, in kilometres |
| p | Fraction of the Moon's whole surface covered by sea (no unit) |
| Radius of the Moon | About 1737 km |
| 4 × pi | About 12.57 |
| Fraction covered by sea, whole Moon | About 16% (0.16) |
| Fraction covered by sea, near side only | About 31% (0.31) |
| Area of Japan | About 378,000 km² |
| Radius squared | 1737 × 1737 ＝ 3017169 |
| Moon's surface area | 3017169 × 12.57 ≒ 37926000 km² |
| Sea area (whole Moon) | 37926000 × 0.16 ≒ 6068000 km² |
| How many Japans | 6068000 ÷ 378000 ≒ 16 |
| Near-side area (half) | 37926000 ÷ 2 ＝ 18963000 km² |
| Sea on the near side | 18963000 × 0.31 ≒ 5879000 km² |
| Share of all sea that is on the near side | 5879000 ÷ 6068000 ≒ 0.97 |
The dark patches cover roughly 6 million km², about 16 times the area of Japan. About 97% of that lies on the near side, facing Earth. The fractions differ a little between sources, but the conclusion that almost all of it is on the near side does not change.
High schoolHS+How does white rock differ from dark rock?
High schoolThe anorthosite of the highlands is made almost entirely of plagioclase, a light, white mineral. The basalt of the seas is rich in dark, iron-bearing minerals such as pyroxene and olivine. In the classification of igneous rocks, this is the difference between pale rock and dark rock.
HS+Between 1969 and 1972, the Apollo programme brought back about 382 kg of Moon rock. Dating showed that the highland rocks formed around 4.4 billion years ago and most of the sea basalts between about 3.9 and 3.1 billion years ago. The method of estimating the age of a surface by counting craters is also calibrated against these ages.
UniversityThe magma ocean and the Moon's two-faced dichotomy
Just after the Moon formed, its outer few hundred kilometres are thought to have melted into a "magma ocean." As it cooled, light plagioclase floated and gathered to form the highland crust. Heavy minerals sank and formed the mantle. Later, part of the mantle melted (partial melting), and the basaltic magma it produced filled the impact basins. These are the lunar seas. That crust thickness and the amount of sea differ so much between the near and far sides is called the "lunar dichotomy." Observations by NASA's gravity-mapping probes show that the crust averages 30 to 40-odd km thick and is thinner on the near side.
📖 Derivations and further reading: Lunar sea (Wikipedia, in Japanese) / NASA's lunar gravity mission (in English)
ResearchWhat is still not well understood
- Why is only the near-side crust thin? Ideas include heat-producing radioactive elements gathering on the near side, another body striking the far side and piling up crust, and the near side cooling slowly because the newly formed, hot Earth shone on it. None has won out.
- How long were the Moon's volcanoes active? Rocks brought back by a Chinese probe in 2020 were reported to be lava about 2 billion years old. Whether any younger lava exists is still being studied.
- Were the impacts that made the basins concentrated in a short period? One theory says impacts surged around 3.9 billion years ago, but others argue this is only an apparent effect of biased samples.
So even this article is "an explanation based on what we know now." Plans to bring back Moon rocks continue, and each new sample slowly rewrites the history of the rabbit's patterns.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Jr. high | Science: "How the Moon and Venus look," "Volcanoes and igneous rocks" | Patterns on the Moon's surface, the rock called basalt |
| High school | Earth science: "Planets and the solar system," "Classifying igneous rocks" | Anorthosite vs. basalt, calculating the area of the seas |
| HS+ | Advanced Earth science: "Radiometric dating" | Dating the Apollo rocks |
| University | Planetary science, lunar and planetary geology | Magma ocean, partial melting, lunar dichotomy |
| Research | Origin and thermal history of the Moon | Why only the near-side crust is thin, the end of volcanism |
| – | Connection to daily life | Moon-viewing, observing the Moon with binoculars |
- Wikipedia, "Lunar sea" (月の海, in Japanese)
- NASA Science, GRAIL (Ebb and Flow)
- Wieczorek, M. A. et al. (2013) The Crust of the Moon as Seen by GRAIL. Science 339, 671–675.
- Hiesinger, H. et al. (2011) Ages and stratigraphy of lunar mare basalts: A synthesis. Geological Society of America Special Paper 477.
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (values such as the Moon's radius)
*This article is a general-audience science explainer. The figures given are rough guides and estimates meant to help you understand how things work. Estimates of the Moon's ages and crust thickness differ between studies.