Mini Quiz: Is this moon waxing or waning? How can you tell?

Some questions about the moon

I had hoped to just enjoy a peaceful moment with a quiet mind, but the nagging naturalist in me started running down a growing list of questions about the moon:

  • How far is the moon from earth? And how would a scientist calculate this distance today? 100 years ago? 200? 300? And how far back did people even start measuring this distance?
  • How big are the biggest craters?
  • What’s the youngest visible crater on the moon?
  • How long did it take for the moon to freeze into a solid object? It is a solid object, right?
  • Why doesn’t the moon rotate around it’s own axis?
  • Are there tons of large and small meteors and comets strewn about the surface of the moon because there isn’t an atmosphere to burn them up before impact?
  • Have bacteria been introduced to and survived on the moon?

A quick biographical sketch of the moon

The moon is roughly 4.5 billion years old, a slightly younger sibling to good ol’ dusty bones earth. In the way back when, things were pretty different in our solar system. Earth was in the early stages of stitching itself together from astral schmutz, right alongside the 100 or so other planets circling the sun, each growing larger and larger as their gravitational forces pulled in and collided with all that cosmic gas and rock cluttering up the solar system.

Not all the planets had their ducks in a row and their orbits all mapped out. One of these unruly orbs – Althea as it would come to called – came careening towards earth at a staggering 10 miles per second. Their collision would’ve been epic, and under the strain and friction, both planets essentially liquefied on impact. The impact blasted tons (well trillions of tons) of hot dust up into space. Proto-earth’s gravity kept the dust from spilling aimlessly out into the aether, and it was lassoed into a big ol’ cloudy ring that rotated around our planet. The ring was a turbulent mess of hot, hot dust and rock, which over a thousand years collided and coalesced into a ball that would eventually span 2,000 miles across. Because the moon was formed from the earth (kind of like when Gizmo gave birth to the other Mogwai: video), both celestial bodies have essentially the same chemical composition (this isn’t true for other the planets in our galaxy that have moons).

Images credit: NASA / JPL-Caltech / LRO.

Why the front and back look different

The moon was much closer back then, just 14,000 miles away from Big Blue (though BB wasn’t very blue yet). Since its formation, the moon has progressively untethered itself from earth’s gravitational pull and and is now roughly 238,900 miles away (the moon is currently distancing itself from us by about 1.5 inches per year, though scientists think it will reach an equilibrium in about 50 million years). Back then the earth was also spinning much faster, so a day was only about 6 hours. If there had been water (and continents) the tides would’ve been incredible!

You’ll notice in the image above that the face of the moon we get to gaze up at has lots of dark smooth patches on it while the surface facing away from us is covered tip to toe in craters of all sizes. The darker areas are known as marias (okay, okay I know I said no Latin, but marias does mean sea in Latin), and are formed from lava flows. Today the moon’s surface is solid, so when an asteroid or comet crashes into it, it leaves a mark that will remain forever.

However, when the moon first formed, earth was so hot and the moon was close enough to earth that the heat was intense enough (earth was ~2,700 Kelvin) that it kept the face of the moon facing earth hot enough to keep from cooling solid. The far side of the moon, which faces off into the cold recesses of space cooled much quicker. All of those asteroids and comets pelting the moon that hit the molten side failed to leave lasting impressions in the lava sludge.

As the earth cooled and the moon distanced itself, the marias, or basaltic lava flows, cooled and solidified. These basalts are around 3 to 3.5 billion years old and their iron-rich composition makes them much less reflective than the surrounding craterous mountains (making them appear much darker from our vantage down here on earth). So our side of the moon is smoother and darker than the far side of the moon. There are tons of big marias and these are what give the moon that dolorous “Man on the Moon” vibe. Today, the moon is mostly solid but it does have a band of molten lava beneath the 740km thick solid mantle, and there are some features on the moon that suggest more recent lava flows.


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