The Lunar Reconnaissance Orbiter, from which the data for MoonMappers comes, celebrated 1000 days in orbit on March 19th. NASA has released a ~3-minute “Evolution of the Moon” video that I think is pretty neat and shows how much it’s been influenced by cratering: Watch me!
The movie is awesome and gives a good overview of how we think the Moon has changed through its history. I’m going to give a summary of the movie in this blog post with some of my thoughts thrown in.
The movie starts showing a cooling Moon moving through space, in orbit around Earth. As it is cooling (around 25 seconds into the movie), you see relatively evenly spaced islands of material with glowing magma between them. This is probably reasonably accurate, as we think there was a “magma ocean” soon after the Moon formed and the lunar highland material (bright regions, heavily cratered today) almost literally floated to the surface like foam on a sea.
About 33 seconds into the movie, a bright impact forms that’s labeled the “South Pole-Aitken Basin” at about 4.3 billion years ago. The “SPA” for short is the oldest known basin on the moon and is gigantic — somewhere around 3000 km in diameter. My only tiny quibble with the display is the literally perfectly spherically symmetric ejecta that’s thrown out from it as seen around 44 seconds in. Ejecta is messy and is very rarely perfectly emplaced in a perfectly radial manner. But, having dabbled in 3D modeling myself for a bit, I know that this can be a real pain to model. It’s still pretty cool.
About 50 seconds in, we start to see more impactors, depicted as a swarm hitting with bright explosions. It’s labeled the “Basin Formation / Heavy Bombardment” from around 4.1-3.8 billion years ago.
The Late Heavy Bombardment (LHB) is an intriguing – still hypothetical – period of time that may or may not have happened and may or may not have been short lived. I was at a conference specifically on the LHB back in February. Half of us there thought it happened, half didn’t. And of the half that thought it happened, half of those thought it was a short spike around 3.9 billion years ago (“spike” lasting around 100 million years) while the other half thought it was more drawn out, lasting between about 4.2 and 3.8 billion years ago.
So, the jury is still out. I think the evidence from the cratering record is mixed, and I’m actually submitting a paper this month that argues that if it happened at Mars, it must have happened before 4 billion years ago. What we really need is to go back to the moon, sample rocks that melted in each of the major basins when those basins formed (called “melt sheets”), and bring them back to Earth and age-date them. Then we’ll know if all of these basins formed in a relatively short period of time.
Moving forward, you see at about 1 min 5 sec that where these giant impact basins formed, they’re staying molten. Unfortunately, this isn’t quite the right series of events. The basins cooled, but they were so thin that magma did creep up and ponded, making the dark lunar maria we see today. We also know that this is a process that took over 100 million years between the formation of the basins and when magma ponded.
This may just be a time-resolution issue when trying to make a 2.5-minute animation of 4.5 billion years of history. As to whether the cells of lava were all the same size, I don’t think that’s quite correct, and it wasn’t like these were just lava melt plains like you’d have to jump over in a video game, but more just giant regions of flood volcanism like you get in Hawai’i. It’s labeled in the movie as “Mare Volcanism” between about 1.0 and 3.8 billion years ago.
Meanwhile, you see them cooling to form the nice dark maria we know and love today, along with Mare Orientale at the 4:30 position at 1 minute 36 seconds into the movie. Mare Orientale, a roughly 1000-km-diameter basin, is the last of the large basins to form, and it formed at roughly 3.85 billion years ago.
Starting at 1 minute 43 seconds, “Intermediate Cratering” starts to take place. The time span is listed as 1.0 to 3.8 billion years, meaning that it “should” be taking place throughout the entire rest of the movie and been taking place over the last minute of the movie. It’s slightly misleading to have it labeled as spanning the entire time but only showing it briefly. But for clarity purposes, I understand.
What I think is really cool is that throughout this time of cratering that it shows, the formerly clean dark maria regions slowly start to show craters on them. It’s a nice effect, and it’s really how it happened.
Starting about 2 minutes in, “Formation of Ray Craters” from today to about 1.0 billion years ago is listed.
This is an interesting transition. When craters form, they shoot out ejecta that was excavated during the formation process. This material is usually bright. But over time, as it’s exposed to micrometeorite bombardment, bombardment by the solar wind and cosmic rays, and other things, the rays “mature,” and eventually they disappear. Craters that we see that still have bright rays around them on the Moon are estimated to be less than 1 billion years old. A crater that just has ejecta but not bright rays is older. It’s a very rough rule-of-thumb way to classify some crater ages, but it’s still nice to have in the back of your mind.
And that brings us to “Today’s Moon.”