Reader Question: Lunar Geology

The incredibly diverse landscape of our Moon. Image Credit: NASA

The incredibly diverse landscape of our Moon. Image Credit: NASA

This reader question comes from Google+

AmyBeth Inverness from amybethinverness.com recently asked two very intriguing questions about Lunar Geology.

Since my field of expertise is with variable stars, I enlisted the help of my friends at the Lunar Reconnaissance Orbiter Camera Center at Arizona State University (http://www.lroc.asu.edu/) to help shed some light on AmyBeth’s questions.

Keep reading to learn more about the incredibly diverse geology of our closest neighbor.

How uniform is the soil on the moon? Is dirt in one area pretty much the same as dirt in another give or take more/less dust/big rocks?

So, this is a really interesting and COMPLICATED question. However, a somewhat simple answer is that vertical mixing is very efficient on the Moon whereas horizontal mixing is less dominant. Think of it this way: when you look up at a full Moon, you are able to visually distinguish the lunar mare (volcanic iron- and magnesium-rich basalts) from the highland material (anorthosite), and although the mare are younger than the highland anorthosites, the youngest mare is thought to be around 2 billion years old (samples cluster around the mid to late 3 billion years old age)…if horizontal mixing were really efficient, we wouldn’t be able to discern the boundaries with the naked eye. So, with that in mind, no, regolith (lunar soil) is not the same in one location compared to another — the regolith composition is directly influenced by the rocks from which it originates…so if the soil sample is from Mare Tranquillitatis, it will be basaltic in nature.

However, because horizontal transport of material does happen (impact events excavate material that is then transported ballistically across the surface — think of the ejecta rays of Copernicus that streak across Mare Imbrium), you will get some mixing so your basalt soil will probably have some “exotic” component (the term used by the original sample handlers to catalog material that was not expected due to the locality). Now, the other part (not composition) is grain size — and the answer to that bit of the question is that the grain sizes do vary across the Moon for the regolith, and the regolith grain size is largely influenced by the age of the surface. That is, micrometeorites and solar wind particles are constantly impacting the lunar surface, and although these tiny particles don’t do as much damage as the impactor that formed Copernicus crater, the particles are responsible for “impact gardening” and formation / evolution / mixing of the regolith. So, two surfaces that are say 3.2 and 3.5 billion years old, you will probably have similar (but not exactly the same) grain size distributions. It’s complicated, like I said!

Does the landscape of the moon include cliffs and such? Or are all those craters bowl-shaped with sloping sides?

The landscape of the Moon is very dramatic – but not always in the sense that we can imagine based on our own Earth-based knowledge. The Moon does have a spectacular landscape – mountains that form crater central peaks (like Tycho — http://lroc.sese.asu.edu/news/?archives/411-Tycho-Central-Peak-Spectacular!.html
) and regions like the Apollo 17 landing site at Taurus Littrow Valley (http://lroc.sese.asu.edu/news/index.php?/archives/639-Taurus-Littrow-Valley,-West-To-East.html#extended) not to mention the Apollo 15 landing site at Hadley Rille.

But these mountains didn’t form the way the mountains on Earth form (plate tectonics). There are lobate scarps (see the Apollo 17 post), sinuous rilles, linear rilles (graben). Impact craters generally are bowl-shaped, but once you get to a certain size (~15 – 20 km diameter), complex craters begin to form – with central peaks and wall terraces formed by fracturing/faulting of the crater walls as they succumb to gravitational forces.

Furthermore, don’t take my word for it – take a look at the Hasselblad photos the astronauts took when on the lunar surface. Here’s a link: http://www.apolloarchive.com/apollo_gallery.html

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