How do you tell how old an astronomical object is? I mean, the next time the Moon is in the sky, take a look at it. How would you even begin to answer that question?
I won’t leave you in suspense. Astronomers use a technique called crater counting and it’s pretty much exactly what it sounds like. The idea is that worlds like the Moon, Mercury, and many of the moons of the outer system are not active. They’ve been dead, in almost every sense of the word, for a very long time. And when a comet or asteroid strikes them, the crater they leave behind sticks around. There’s no air to blow it away. No water to wash it down. No plate tectonics to pull it under the surface.
And so craters just pile up, one after another, and often on top of each other. But not all dead worlds are created equal. Some of them were molten in the recent past, and lava is really good at covering up craters. So if you compare two worlds and count their craters, you can get a relative sense of which worlds solidified sooner.
And some had molten parts alongside the solid parts for some time. Planets and moons don’t always cool down all at once. There might be active volcanic regions over here and just plains of solid nothingness over there. So if you scan across the surface of a world, the places with more craters are probably going to be older (in the sense that they solidified in the more distant past) than the places were fewer craters.
For example, on the Moon we have those two broad regions: the dark basins, or mare, and the lighter-colored highlands. Just by looking at the craters you can tell that the maria are younger because they have fewer craters.
But…how old? If I see a world with tons of craters, how old it is? Not in a relative sense, as in this planet is older than this other one. But in an absolute sense. Like billions of years. Give me a number.
The Apollo missions held the key to unlocking crater records not just on the Moon, but across the solar system.
That’s because scientists have been able to apply radiometric dating to the moon rocks returned from the Apollo missions. If you’re not already familiar, radiometric dating is where you look at the abundances of various radioactive elements and compare their proportions to the numbers of elements that they decay into. Since we know the half-life of those elements, we can calculate the absolute age of that sample.
And since the Apollo missions visited many places on the Moon, we’ve been able to build a detailed accounting of which parts cooled and solidified when. For example, the edge of the Sea of Tranquility, the site of the Apollo 11 landing, is just over 3.5 billion years old, while some other regions in the highlands reach right up to 4 billion years old.
By far the youngest features on the Moon are some of the large impact craters. The Copernicus, Tycho, and Cone craters are all less than a billion years old. The youngest crater of all is probably the Giordano Bruno crater, named after the Italian renaissance smart/crazy guy, and is only 4 million years old. These craters are so young because the impacts carry so much energy that they’re able to erase everything in their vicinity, wiping the slate clean and starting the whole process over.
With these absolute numbers in hand, we can now calibrate crater counts across the entire solar system. Now we can look at regions of Mercury or Callisto and know how old they are, even though we’ve never been there, thanks to the Apollo missions and their geologic handiwork.
We also know thanks to the Apollo missions that the Moon is slowly slipping away from us. This was hypothesized all the way back in the early 1800’s when Sir Edmund Halley (of Halley’s comet fame) read through ancient eclipse records and realized we were slowly getting further and further apart. And we had a pretty good explanation for WHY the moon might be going away – the tides raised by the moon get carried in front of it by the spin of the Earth, and that extra gravitational tug pulls the moon into a higher orbit - but we had no good way of putting a precise number on this.
In 1962 Princeton graduate student James Faller proposed placing reflectors on the Moon’s surface. This would allow us to more easily bounce lasers back and forth from the Moon to here, and use that to measure a distance, the same way you can use one of those little laser measurement thingies to…measure distances.
While measurements had been taken before by just reflecting off the lunar soil, with the reflectors the measurements became much more accurate. We now know that the Moon is receding from the Earth at an average rate of 3.8 centimeters per year. Which isn’t very quick, but over the course of, you know, an eon or two, it really adds up.
In fact, in just a few hundred million years the combined effect of the Moon spiraling away from us and the Sun getting brighter and larger (different article) will mean that total solar eclipses will become impossible.
So enjoy them while they last.
