Saturday 19 December 2015
Posidonius et Lacus Somniorum - a volcanic paradise
Finally, a clear night, the Moon is up, and I have time for a sketch! I’ve hit the jackpot!!
The feature I sketched tonight was one of the first features I sketched when I took up the pencil after many years, Posidonius. I was not aware at the time that this was Posidonius – did not matter really as the features on offer were most exciting.
What was interesting for me once I found out the identity of the crater, was how much more I was able to identify and details I was able to pick out that previously made no sense to me.
Posidonius is not one of the oldest features. In fact, it is really just middle aged. For the oldest of features, a crater this size would have been totally flooded with lava, and only a ghost crater left – much smaller impacts than Posidonius can be seen close by which are totally flooded, making those features very, very ancient, much older than Posidonius (such as Le Monnier to the top left of Posidonius). The youngest of features happened once the Moon’s crust was so thick that a crater this size on longer was able to punch through to the molten rock below. Posidonius is one crater where the crust was punctured to allow for partial flooding, but the flooding was not complete, and now long extinct volcanos pot-mark its flooded floor and surrounds.
Posidonius is one extraordinary feature. The partially flooded floor not only has many volcanic vent and domes, but it is also highly fractured, with several massive riles running through it. Its walls are also very interesting. Close examination shows what appears to be a second rim on the inside. This is something I have not seen before. There are a few co-centric craters on the Moon, but these all are on the small side. Posidonius is some 100km in diameter – for a second concentric impact to happen of such a close size to the parent impact is just too much of a long shot. The highly fractured crater floor may hold a clue to this apparent secondary crater – massive upheaval from subterranean magma forces pushing up. Mind you, this is only speculation on my part. There are other examples of magma lifted features on the Moon, which is why I suspect this may be the reason for the double rim.
Posidonius’ floor is not the only location of volcanism in this area. Lacus Somniorum appears featureless, but is actually littered with domes all around Posidonius. One clue to spotting the shield volcanoes is to look for totally isolated ‘mountains’ with no other mountain range within cooee of it, only an empty lava field. This would make the chances of this solitary ‘mountain’ to actually be a shield volcano or Dome.
Object: Posidonius et Lacus Somniorum
Scope: C8, 8” SCT
Gear: 8mm LVW, 250X
Date: 17th Dec. 2015
Location: Sydney, Australia
Media: Soft pastels, charcoal and white in on black paper.