An artist's concept of a large impact hitting Earth during the period of heavy bombardment
Last week I wrote about the leading theories for how our moon formed. This week, I’d like to write about what’s happened to the Moon since then and what lunar rocks and element isotopes tell us about the Moon’s evolution.
You would expect that once most of the material in Earth orbit was swept up into the new moon (a process that took only about 10-100 years), the debris that remained would have gradually continued to collide, adding to the Moon’s mass, but slowly tapering off. The leftover planetesimals in the solar system would have occasionally collided, but that should taper off as well to a point almost, but not quite, equal to zero today. However, the rocks brought back by Apollo tell a different story.
The Earth rising over the Moon as seen from Apollo 8
The original surface of the Moon was crystallized out of a magma ocean (the formation of the Moon within less than 100 years would have created sufficient heat to melt the crust). We know this from the pieces of anorthosite brought back, especially the famous Genesis rock found by Dave Scott and Jim Irwin on Apollo 15. These rocks date back to 4.5 billion years. Yet by far the most common type of rock brought back from the six landing sites and the several Luna sample return missions and by lunar meteorites found on Earth are lunar brecchias: small, angular pebbles and regolith (lunar soil) fused together from the heat of lunar impacts. And they’re all the same age; a narrow window between 3.85 and 3.95 billion years ago.
Potassium-40 is fairly common in lunar rocks (in the form of feldspar) and once it breaks down to Argon-40, the argon atoms are too big to escape the rock if it has crystallized, so determining the amount of Argon-40 in a rock gives a very accurate age of crystallization. We hardly find any rocks on the Moon (at least we haven’t found many yet) that date to the time between 3.9 and 4.5 billion years. It’s as if some event occurred that reset the isotope clocks at 3.9 billion years in most of the lunar rocks.
When we look at the lunar highlands, which are the oldest surfaces on the Moon, we see only craters. It’s as if the surface of the Moon has been pounded and pounded repeatedly, so that no area is without craters. Craters lie on top of craters, from the very large basins all the way down to the microscopic level. The pounding has thrown up pulverized rock and fragments that has formed a powdery layer the consistency of flour called regolith that is very deep in some places (it can’t properly be called soil because it wasn’t formed by erosion). The large basins themselves are from big impacts that occurred around 3.9 billion years as well, with the Imbrium basin among the most recent (it overlaps the others).
Cross sectional diagram of the Moon
There are other oddities as well. The lunar maria (what ancient people thought were seas) are large areas of basaltic lava that have filled in the huge impact basins, such as Mare Serenitatis and Mare Nectaris. These lava flows, accompanied by rivers of lava, volcanic domes, lava tubes, and other features, occurred between 3.8 and 3.2 billion years ago. Of the 50 some odd basins, by far the majority are on the near side of the Moon (maria basalts cover about 37% of the near side and only 2% of the far side). Data from the Apollo seismic monitors show that the far side of the Moon has a thicker crust and therefore fewer maria; lava had further to go to reach the surface. How could this be?
At the same time period (3.9 billion years ago), Mars and Mercury also show evidence of heavy bombardment. This is called the Noachis Period on Mars. Until recently, we had only seen 1/3 of the surface of Mercury in detail. Now, with the Messenger probe orbiting Mercury, we see craters on top of craters as well. The solar system at that time was a violent, dangerous place as large planetesimals roamed through the inner solar system and pummeled the planets. Earth would have been hit as well, maybe ten times as often as the Moon. It would have been difficult for any life that developed prior to that point to survive, except a few extremophilic bacteria similar to those living in hot springs today. Interestingly, life on Earth seems to date from about 3.8 billion years, just as soon as this heavy bombardment settled down. Perhaps it was already here but all evidence before that was blasted away. Or maybe life gets going quickly where conditions are favorable.
The heavy bombardment could not have been just a regular trail-off of impacts left over from the formation of the solar system. Something extraordinary happened that dramatically increased the numbers of planetesimals reaching the inner solar system. There are several theories for this increase. One is that a large asteroid or small planet was broken up by Jupiter’s gravity. Contrary to what we might like to think, the solar system hasn’t always been a fixed and stable configuration of planets in nice, regular orbits. At present, 4.5 billion years later, it mostly is, but not back then. The regular pattern of planetary orbits first noticed by Kepler (who thought he’d found the music of the spheres) isn’t an accident or coincidence. The masses and orbits of the planets created resonances that pulled and pushed the planets and other objects around as the solar system settled down. These resonances could have broken up a planet trying to form where the asteroid belt is now and sent pieces flying around to smash into the young inner planets.
Another theory is that Jupiter migrated around in an unstable orbit as it grew larger; Saturn also wobbled around, and when these two planets reached a 2:1 resonance, the combined gravity of Saturn and Jupiter sent Uranus and Neptune spiraling outward, which in turn scattered the large number of planestimals and Kuiper Belt Objects. Many objects were spun outward and escaped the solar system. Some were tossed inward. Computer models show this possibility and agree that about 500 million years after the formation of the solar system would have been a likely time for such a resonance to occur. It was like a cosmic shooting gallery. These icy bodies could have provided much of Earth’s water supply, and caused the blasting of medium and large craters seen on the Moon, Mars, and Mercury.
Cut away diagram of the Moon, with known facts
Since the maria basalts stopped erupting about 3.1 billion years ago, the Moon settled down into a basically steady state. Occasional moonquakes occur deep in the mantle near the boundary with the Moon’s asthenosphere. These are weak and long lasting (several minutes) and help reveal the Moon’s interior. Now and then meteorites still hit the Moon, splashing bright rays over the dark maria (such as those of Tycho, Copernicus, and Kepler craters). But that’s about all.
There’s a great activity done in many Earth Science classrooms to demonstrate the sequence of events that shaped the Moon’s surface. Start with a cake pan about ½ full of flour and sit it on a tarp or drop cloth. Take a number of small and medium sized rocks and drop them from various heights and angles into the flour, carefully removing the rocks each time so as not to disturb the craters made in the flour. After a while, the craters start overlapping, with younger craters showing sharp and clean and older craters getting obliterated. This is the lunar highlands. Then drop in larger rocks to make deep basins. Take cocoa powder and sprinkle it carefully in a thin layer in the deepest holes. This is the maria basalts. Then take small rocks and drop them into the maria. The white flour underneath will splash out over the top of the dark maria, making rayed craters. When you’re done, you have a very convincing model of the Moon’s surface.
Moon crater simulation activity
The Elements Unearthed 