Why go to the moon?
To learn about Earth.
The surface of the Earth preserves little or no information about its distant past. Constant tectonic activity has recycled Earth’s crust and shifted landmasses. Rainfall, wind, ice and snow have weathered away surface features over billions of years. Most of the craters formed by the impacts of asteroids and comets have been erased from the geologic record, with just over 100 known craters remaining on the continents.
But there is a place that we can go to learn more about the past of our own planet: the moon. In sharp contrast to Earth’s surface, that of the moon is covered with thousands of craters of all sizes, many of them produced shortly after the moon was born. The moon doesn’t have the winds, rivers or plate tectonics capable of erasing these marks of ancient impacts.
For that reason, the surface of the moon is like a window into the early history of our solar system. By studying the chemical composition of rocks and soil on our natural satellite, we could obtain a glimpse of the Earth’s own geological infancy – including the emergence of life.
The Earth formed 4.54 billion years ago, after ancient asteroids known as planetesimals piled up into a single, planet-sized body as they orbited the sun. Scientists think the moon formed roughly 70 million years later, when a planet about the size of Mars collided with the young Earth. With the aid of sophisticated computer models, experts have shown that this huge collision created a donut-shaped envelope of molten rock and hot gas around the Earth. By calculating how this scorching disk would lose its heat, they’ve deduced that the moon condensed from all this hot material in less than 100 years. Fast forward some 500 million years. Around this time, the giant planets Jupiter, Saturn, Uranus and Neptune likely underwent a rearrangement of their orbits around the sun, as a result of complex gravitational interactions with myriad planetesimals. This rearrangement sent many asteroids on a collision course with Earth. When they crashed into our planet, their impacts launched terrestrial fragments into Earth’s orbit. A very exciting possibility is that some of those Earth rocks might have landed on the moon.
If those pieces of Earth did make it to the moon, they’re probably still lying somewhere on the lunar surface. Some studies predict a large concentration of impacts near the moon’s poles. In some regions, there may be as much as a golf cart’s mass worth of terrestrial material spread over an area equivalent to 140 soccer fields. Whether this mass is in the form of rocks or tiny dust particles depends on, among other things, how hard Earth’s fragments hit the lunar ground.
Regardless of their size, terrestrial remnants could contain invaluable information about our planet’s early years. For example, those terrestrial
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