Say What?   You jokin’, right?   Stop pullin’ my leg, dawg!

Seriously.

I’ve never figured out why people think this is so weird.  In fact, when I was studying at Stephen F. Austin State University (an EXCELLENT geology program, by the way), two of my professors also held music degrees, so they were not at all surprised to have a geo/music double-major in the mix.

So what exactly does being a planetary geologist entail? Well, you study the geology of other hunks of rock floating around our sun.  Hopefully you will remember from your elementary school days that our largest neighbours are gas and ice giants and not very exciting geologically.  That leaves the other three terrestrial planets as the main bodies of study, but even that was not my area of interest.  I was aiming for our nearest rocky friend, the Moon.  I focused on the lunar maria and classifying mare basalts by their iron content (lunar basalts are usually classified by their titanium content).

Is Planetary Geology a common study? No. It’s one of those “make your own major” kind of things.  I followed the regular course of study for classical geology:  introductory geology courses, historical geology, mineralogy and petrology, structural and stratigraphy, geomorphology and geochemistry, advanced hydrogeology and oceanography, as well as some field trip courses.  I also took Astronomy, Astronomical Observation, Advanced Astrophysical Calculations, and some special topics.

Yes. I. Am. A. Nerd.

Since music has been my full-time job for my entire adult life, I really don’t think about the geology stuff very often.  It usually takes something geologically significant for it to even cross my mind. Like local earthquakes.

We’ve had three minor earthquakes in the East Texas area in the last few weeks.  Our local news station (I won’t name names but it starts with a K and ends with a TRE) did a bang-up job getting people all panicky about what was happening.  Glad they finally took my advice after a couple of pointed FB messages and interviewed an actual geologist to dispel the fears that the Mayans were right and the end of the world is approaching and we are all going to die.

Most people don’t know that we live near the Mt. Enterprise Fault System.  I know because I used to go out there with some grad students and their seismometers and ground penetrating radar and (attempt to) study them. (If that sounds glamorous, I assure you it’s not.  It’s hot, dirty, sweaty, and tiring).  Some people think the sudden activity is due to fracking.  I think it’s just a natural relieving of pressure and they should be glad it’s happening in little bursts instead of one big one. Either way, it was kinda fun to be able to share my geology knowledge for a bit… and surprising folks that I’m a geologist is always good for some entertainment.

Here are a couple of pictorial lessons about geology.  All of these pictures were taken in Central Texas:

This is the same photo (unmarked and marked) and is of a normal fault (actually, this one is part of a horst and graben). The fault is the straight, not quite vertical, line. It is marked by the red line in the second picture. I also marked the geologic layers to make it easier to see how the layers were divided and shifted… the area on the right (horst) is higher than the area on the left (graben).

The next lesson is also the same photo twice, but one has been rotated to facilitate learning without frying your brain:

Now you can say you learned something new today.  You’re welcome!

Finally, here are a few pictures of the geology gals (there were only four of us, so of course we had to stick together) and me.  Enjoy!

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