The Moon

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Lars Lindhard

This is a fine book. I have read it this weekend and enjoyed it. 

 I will not make a review here, but will mention one thing that puzzles me.  The lava flows.

 The book explains the geologic periods with the heavy bombardment, creation of the maria, the Imbrium basin, new craters inside the Imbrium and shows that the lava flows came last. 

 On page 97 an illustration shows the Moon with all maria and Imbrum etc. before and after the lava flows ”filling pre-existing impact basins”.

 It will take a rather large amount of lava to fill the Imbrium basin up an almost bury craters like Archimedes (page 76 and 79)– not to mention all the other places to be filled up on the Moon (at the same time).

 Where did all that lava come from? I reckon that this flowing has taken some time – the early and late Imbrian periods where the lava flowed lasted from 3.84 – 3.2 billion years ago (p.107), that is 600 million years give or take a million or two. 

 The author writes on p. 97: “The lunar maria were formed when molten magma generated by the heat of radioactive decay and the subsequent melting of rock in the Moon’s mantle erupted to the lunar surface over 3 billion years ago, filling the floors of impact basins that had themselves been created some hundred of million years before that.”

 But the Moon had a solid crust – the crust solidified in the pre-nectarian period, it is said on p. 106 – I would think that the Moon was cooled so there was not that much heat left.  

 I mean, first you must have enough molten lava to fill the entire surface of the Moon and then you have to keep it warm and liquid for up to 500 million years.  

 Unless of course the flow-period was much shorter.

 How long would it take to fill the Imbrium?

 Having the recent eruptions on Hawaii in mind, where the lava comes out of every fissure and runs across the island, one might think that the Imbrium job was a matter of months, but to fill the Imbrium basin with a layer of lava more than 2-3 km (?) thick is something else. 

 Did the lava fill Imbrium like water rising in a bowl, or did the first layer solidify so the next flow had to come up through the first etc.? And how thick is the layer of lava?

 Charles A. Wood writes in “The modern Moon” (p. 42) that there were three lava flows in Imbrum and the second “extend 600 km…is only 35 meters thick and has washed down a almost nonexistent slope of 0.01˚. The lava must have been very fluid, more like water and it must have roared across the Moon’s surface at speeds of many kilometers pr. hour.” 

 35 meter is not much, and the layer of lava around Montes Pico and Piton and the other “costal areas” around Imbrium must have been much thicker – several kilometres, I guess. Archimedes is 2 km. high now and must have been much higher (and deeper) before the lava came.

 Why would all this lava suddenly break through and flood the surface? Was there a special reason?