The Moon a Geologists Tale

Posted February 19th, 2009 by admin

This was my first lecture on my course last year – this write up origonally appeared on my personal blog with lots of excess geology ramblings in it!  It starts with me being almost late – ok well I was!

After double checking everything I was almost late as I was drinking tea and reading my book when Carina suddenly announced that it was quarter to six – eek! And I didn’t didn’t know my way out of her building! But as it happened I passed Ian Crawford (the lecture) going the wrong way. He hastily said he’d be back and scuttled off.

I found the room having picked up a scared undergraduate on rout who was looking for vertabrate paleo. I suddenly thought – woaw I’ve not done geology for about three and a half years – eek. I sat down everyone was silent. I managed to have to loudly rummage for my pen and paper. Then Ian arrived and we delved into the world of lunar geology.

To my suprise I guest correctly why the crater near the south pole on the farside is not nicely circular like the other mare (or seas – they are the dark patches you see). i probably didn’t use the right terms but I said it had been erroded by other impactors. Yay! I was right but a bit later I stared at the photos of thin section and could not recall the name of the minerals and lunar minerallogy is simple – really really simple. I was staring a Ca-rich plagioclase and pyroxenes and ilmentite. We even had them in cross polars – mew 🙁 And as for the spherals of orange olivine glass at least I remembered that an amorphous glass would come out black under cross polars (cross polars are like a filter on light transmitting microscopes).

We did a bit on lunar stratigraphy where I actually interupted the class to ask a question – for those of you who know me well you’ll know I don’t do such things lightly as I get shy not to mention the class was over running from what he had origonally said. I asked if the graph was assuming a constant rate of impacts as the number of impact creators is used to date bits of the moons surface – this is obviously relative dating rather than absolute but I was sure that there would have been more impacts early on when there would have been more debris and therefore that would need to be accounted for.

I was pretty sure I had even seen somewhere in the distanct past something to do with a high early impact rate. I got a , ‘good question.’ Which I always consider a good thing to have said. He did however then point out that there where points on the graph showing the bore hole data brought back from the Apollo missions. In other words the graph was constrained by those data points which where radiometricaly dated (they had used isotopes and was therefore about as absolute as dating gets). This ment the graph was indeed showing that the impact rate had not been continious – I thought doh! I should have seen that!

However he did point out that the number of data points was pathetically small. This is also the data they use to date all the other bodies in the solar system but as far as I could tell its all guestamets and is very very relative. Like assume more impacts on mars per time unit becuase its bigger – I mean how do you actually scale something like that? Also I would have thought that proximaty to say an asteroid belt and/or large bodies such as a gas giant would affect the rate of impact on a planetary surface. I found myself hooked – I want to find out more. So much so that I spent the train journey home (which was itself very eventful) reading the papers I’d got about isotopic abundence in the moon.

Even once I got home which was gone 1 o’clock in the morning, I couldn’t stop and carried on reading.

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