Here's a view of the moon you don't usually see: the far side.
This video, released today by NASA, was captured over about four hours on July 16 by the DSCOVR satellite. DSCOVR orbits the sun at a point directly between it and the Earth, acting as an early warning system for potentially destructive solar storms.
But DSCOVR also uses this vantage point to take beautiful photos of Earth several times a day. In this case, DSCOVR captured a pretty striking image of the far side of the moon as it passed in front of Earth — looking dramatically different from what we're used to seeing.
Why we never see the moon's far side
In roughly 27 days, the moon completes one orbit of Earth — and also rotates on its axis exactly one time. This means that the same side of the moon always faces Earth. (Calling the far side of the moon the "dark side," while a bit more poetic, isn't really accurate because it gets just as much sunlight.)
That these two events occur over the same time period isn't a coincidence. Given enough time, they'll sync up for any smaller object orbiting a bigger one, a phenomenon called tidal locking.
It happens because the larger object (in this case, Earth) exerts gravity on the smaller one (the moon). This gravity subtly changes the shape of the moon as it orbits, creating a pair of slight bulges (in much the same way, the moon's gravity pulls at the Earth's oceans and creates bulges we call tides).
But if the smaller object isn't tidally locked — as the moon wasn't billions of years ago — these bulges won't directly line up with the larger object. They'll either be slightly ahead of it if the object rotates faster than it orbits or behind it if it orbits faster.
In either case, the misaligned bulges will exert a bit of torque on the smaller object, altering its rotation rate until it eventually matches up with the speed of its orbit — leading to tidal locking.
Why the far side of the moon looks so different
The side of the moon we see every night is covered in large dark spots called maria — plains of basalt rock formed after volcanoes erupted billions of years ago.
For centuries, we had no idea what the far side of the moon looked like. But starting in 1959 — when the USSR's Luna 3 probe first photographed it — we learned that it has very few maria and tons of craters.
Scientists still aren't totally certain why this is the case. Some first proposed that Earth shields the close side of the moon, limiting the number of asteroid impacts. But if you were standing on the near side of the moon, scientists have calculated, Earth would only block out about 4 square degrees (out of 41,000 square degrees that make up the whole sky), which isn't nearly enough to account for the discrepancy in craters.
Other theories hold that the moon's far side has a thinner crust, which has meant fewer volcanic eruptions over time. These would have produced fewer maria and also covered up fewer craters.
Still, the moon's south pole has a very thin crust, but few maria, challenging this theory. The truth is that for all the exploring we've done of the moon, we still don't know a ton about its far side and have never sent a rover or lander to its surface.
That may change in 2020, when China tentatively plans to send the first rover to the far side. Doing so could teach us a lot about the moon's geology — and shed light on one of its biggest mysteries.