The Tsiolkovskiy central peak, seen by the Lunar Reconnaissance Orbiter. Photo by NASA/GSFC/Arizona State University

May 23, 2014 A peak at the far side of the Moon

Sen—I’ve never liked the phrase, “familiarity breeds contempt.” In my experience, familiarity breeds a sort of mental blindness, a time-dulled acceptance of something without much thought.

I sometimes wonder if the Moon falls into that category. After all, we see it all the time… but do we really see it?

In fact, to be pedantic, no: about half of the Moon is forever hidden from Earthly eyes, because the Moon rotates at the same rate it orbits the Earth—that means it always has one side facing us, and the other facing away (more or less). That’s not a coincidence; it’s a natural outcome of the long-term affect of Earth’s gravity on the Moon, and is called tidal locking (every major moon in the solar system exhibits tidal locking with its parent planet).

But it does mean that the far side of the Moon isn’t as familiar to us. That’s too bad, because it’s weird. The most obvious difference is that it doesn’t have the huge maria, the dark basins that blemish the Moon’s near side. These are caused by ancient giant impacts, where the impactor actually punctured right through the Moon’s crust, letting molten rock flood out and fill the impact site. It turns out the crust of the Moon is thinner on the near side, so it was easier for impacts to do that on the near side versus the far side. So, on the far side, dark, filled impact sites are more rare.

But not non-existent! The crater Tsiolkovskiy is an exception. It’s a 180-km-wide 3.5 billion year old impact feature on the far side, and it stands out in photos. That’s because most of that lunar hemisphere is bright, pocked with smaller craters, but Tsiolkovskiy has a dark, smooth floor, providing a stark contrast to the surrounding surface.

All those eons ago, an asteroid or comet slammed into the Moon. The huge energy of impact excavated a vast amount of material. At those energies, rock can flow like a liquid, and it flowed up and out, forming an enormous bowl into the landscape. Some of it flowed back, splashing upwards to form a central peak, mountains of rock that stretch over 3,000 meters above the crater floor. And the floor itself bubbled up with molten material, dark basaltic lava that partially flooded the bottom of the crater. It didn’t quite fill it, leaving an irregular greyish splotch outlining contours in the floor.

The picture above shows the complex central peak of the crater, taken by the Lunar Reconnaissance Orbiter at a dramatic oblique angle. You can really see the contrast between the bright central peak material and the darker floor material below, a clue to their different origins. If you peruse the zoomable image of Tsiolkovskiy taken by LRO (and holy cow, you really have to!) you can see that the floor really does meet the mountain at a sharply-defined line, as you’d expect if the crater was inundated with a fluid material rising up from below.

In the zoomable image you can see so much detail! Look especially for boulders littering the surface, some having rolled down the slope of the central peak, leaving deep grooves behind. It’s fascinating.

If Tsiolkovskiy were on the near side, it would be one of the best-studied and celebrated lunar features. But that hardly means it’s neglected. It was discovered in 1959 in shots taken by the Soviet spacecraft Luna 3, the first to photograph the back side of our satellite (have you ever wondered why so many far side craters have Russian names…?), and subsequently studied by pre-Apollo flights, as well as the crewed missions. Astronaut Harrison Schmitt advocated for Apollo 17 to land in the crater (using relay satellites for communication), but unsurprisingly NASA nixed that idea for safety reasons.

I expect that in the coming decades it will once again become the focus of astronauts’ attention. It’s not that far over the lunar limb from Earth, and it’s a scientific wonderland, full of interesting mixes of geology that rock scientists drool over. I wonder who will be the first to see it again with their own eyes from orbit, and who will be the very first human in history to set foot in it?

And here's a thought for you: that person may very well be alive today.