ON THE MOON

THE MOON'S LONG MEMORY

THE CRATERS ON THE MOON ARE A HISTORICAL RECORD OF ASTEROID IMPACTS—ON EARTH AS WELL AS THE MOON.

FOR THE LAST THREE BILLION YEARS, asteroid impacts have been almost the only event to shape the Moon's surface. Unlike Earth, the Moon has no life, and almost no geologic activity or water. It also lacks an atmosphere, so it has no wind or weather.

Because the Moon's surface is so inactive, it has preserved a long history of asteroid impacts. The Moon and Earth are so close together that the two bodies share roughly the same history of asteroid bombardment. But whereas geology and erosion have erased most of Earth's craters, the Moon still shows traces of impacts that are billions of years old.

IMPACTS WAY BACK WHEN

To sketch out a rough sequence of events on the Moon, researchers have studied the geology and varying crater concentrations on its surface. When Apollo astronauts brought back lunar samples, scientists used the ages of those rocks to reconstruct the history of asteroid bombardment on the Moon and Earth.

Around 3.9 billion years ago, when Earth and the Moon were relatively young, they were bombarded by more asteroids than at any time since. This cataclysmic period made craters on Earth hundreds of miles across. Most scientists agree that this deluge of impacts was the tail end of the solar system's collision-filled planet formation period.

Once the barrage let up, Earth and the Moon experienced fewer and fewer impacts over the next three billion years, leading to the relatively peaceful present.

The Apollo 17 mission landed among the Taurus Mountains (light-colored region), which formed 3.9 billion years ago during a very heavy cratering period. The nearby Mare Serenetatis (dark region) is a much younger lava formation, and so has far fewer craters.

The largest impact craters on the Moon dwarf any found on Earth. Mare Imbrium (labeled, left) measures 1,200 kilometers (750 miles) across. The titanic collision that formed it penetrated beneath the Moon's crust, churning rock from the mantle layer to the surface.

THE LITTLEST CRATERS

When an object the size of a sand grain or even smaller hurtles through space and into Earth's atmosphere, friction heats it to more than 1,650°C (3,000°F) and it vaporizes in a flash of light. But the Moon has essentially no atmosphere to protect it from incoming objects. So even a microscopic speck of dust, or micrometeoroid, and forms a tiny crater.

The Moon's surface is covered with microcraters smaller than a human red blood cell. The steady pelting of the Moon helps pulverize the fine-grained layer of powder that covers the Moon's surface. This "tilling" of the lunar soil proceeds so slowly that the footprints of the Apollo astronauts will last for at least a million years.

NASA scientists discovered this microcrater (magnified 800 times) on the surface of a tiny fragment of glass collected during the Apollo 11 mission. The glass fragment itself formed from rock melted during an earlier impact.

A VIOLENT BIRTH

THE MOON PROBABLY FORMED IN THE WAKE OF A TITANIC COLLISION BETWEEN EARTH AND A SMALLER PLANET.

HOW DID THE MOON FORM? The leading theory is that the Moon resulted from a glancing collision between the young Earth and an object the size of Mars.

Several observations about the Moon confounded early attempts to explain its origin. Most significantly, the Moon is much less dense than Earth, because it has a tiny iron core.

The Apollo missions showed that although the Moon has much less iron than Earth, it formed from the same original reservoir of materials. Yet on the Moon these ingredients are mixed in different proportions. In the late 1970s a few scientists came up with a theory that seems to explain the data-the Moon formed from debris spun off by the largest collision in Earth's history.

EARTH'S GREATEST HIT

The story begins 4.45 billion years ago, when a Mars-sized planet rammed into Earth with enough force to fling away chunks of the iron-poor mantle layer. (Earth had only recently developed its iron core.) Over the next 10 to 100 years, this debris coalesced to form the Moon. If most of the other planet's iron core melded together with Earth's core, this would explain how the Moon ended up with such a tiny iron core.

Although the giant impact theory was greeted with skepticism at first, successful mathematical simulations of the collision and further chemical evidence helped make it the leading theory. Researchers are still adjusting the variables in the model, such as the size of the impacting object, so that the simulations accurately predict known details about Earth and the Moon.

REACHING BEYOND EARTH

These three moon rocks, unlike all the other samples in this hall, were collected from beyond Earth by human hands—during the Apollo lunar missions.

This moon rock is a breccia, meaning it is made from different types of rock that were broken up and then fused together by the pressure and heat of an asteroid impact. Most rocks collected during the Apollo 16 mission in April 1972 were breccias.

This sample is mainly anorthosite, a rock type rich in the mineral feldspar that makes up most of the Moon's ancient highlands. On the early Moon, anorthosite accumulated on the surface of a viscous ocean of magma and remained to form parts of the lunar crust.

Collected from the eastern edge of Mare Serenitatis, this moon rock is mare basalt, or dark lava. It originally melted deep below the Moon's surface, erupted and then solidified 3.7 billion years ago. Mare basalt from this location contains relatively high concentrations of both iron and titanium.

Astronauts collected this sample during the Apollo 17 mission, in December 1972.

Another type of mare basalt, this moon rock is of a particular type called KREEP. Such rocks are rich in potassium (represented by K in the periodic table of elements), rare earth elements (REE) and phosphorous (P). As the magma ocean solidified, these KREEP components remained in the molten portion, becoming more concentrated. This heavier KREEP magma sank to the lower crust, where it solidified, until it was melted by rising basalt lava.

Apollo 14 astronauts photographed this rock where they found it in February 1971 on the Moon's surface, in the Fra Mauro highlands.