The moon may be Earth’s closest neighbor, but there are still countless mysteries surrounding the rocky world.
NASA has spent decades researching the moon — even landing 12 of its astronauts there — but some answers still elude its top space scientists.
Here are four things we still don’t understand about the moon.
The origin of the moon
Exactly how the moon came to be has puzzled stargazers for centuries — though it seems likely it was born from an enormous space collision.
Most scientists agree that a Mars-size object crashed into Earth around 4.5 billion years ago, sending rocky debris flying.
The rubble left behind by this blow fell into Earth’s orbit, eventually combining into a single object.
But to throw out enough debris to form the moon, the collision would have to have been big enough to knock Earth out of orbit.
According to Professor Christopher Palma, an astrophysicist at Penn State: “Even where we think we have the very best answers about the moon, we’re still investigating.”
“The collision had to be strong enough to throw a huge amount of material up into orbit around the Earth, but not so strong as to destroy the Earth.”
Precisely what hit Earth, and with what force, remains a mystery.
How did water end up on the moon?
NASA first found water on the moon in 2009 in the form of ice trapped under its surface.
But its scientists still can’t explain exactly how it got there.
Some believe there was a period in which the Earth and moon were bombarded with asteroids and comets.
Some of these objects may have brought water to the surface of the moon and could have spawned Earth’s oceans.
But a recent analysis of moon rocks brought home during the Apollo 15 and 17 mission in the 1970s threw up a new theory.
The chemical makeup of the rock suggests that water buried deep in the moon’s interior was carried to its surface by volcanic eruptions.
This would mean the rocky satellite’s water stores have been there far longer than can be explained by space rock collisions.
Frozen “cold trap” craters
The moon has no atmosphere and so has no buffer against the sun’s punishing rays.
As a result, its surface temperature rockets to 260 degrees Fahrenheit during the day and plummets to -163 degrees Fahrenheit at night.
While anything exposed to the sun will roast in its heat, any spots permanently shadowed from the star might offer a haven for lunar ice.
Many of these so-called “cold traps” are found on the moon’s far side — the part that constantly faces away from Earth, meaning it is hidden from Earth’s telescopes.
What could be hidden in these frozen regions is unknown to scientists, but they could contain frozen water along with a host of preserved evidence of the moon’s early millennia.
China’s Chang’e 4 probe, which landed on the moon’s far side on Jan. 3, will explore the chemical composition of a cold trap known as the Von Kármán impact crater.
Does the moon harbor alien life?
One of the biggest questions facing lunar scientists is whether the moon houses alien life.
Conspiracies about little green men aside, some scientists believe water stored under the satellite’s surface may hold ancient microbes.
“One idea for the origin of life is that its building blocks were delivered by comets,” Palma says.
“There may be remnants of those in the ice on the moon.”
Scientists won’t know for sure until they send a probe — or human — up to the lunar surface armed with a hefty drill.