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Thursday, 19 July 2012

Worlds lost in time: The Ancient Moon

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Firstly, my apologies for the patchy blogging this week - that annoying real life stuff has been getting in the way.

Now: This week the 2012 Lunar Science Forum [1]is on - and some one at work has asked me  "So it's all about craters then?"

Looking at the Moon today, you'd be forgiven* for thinking that it had never seen anything more than asteroid strikes.

You'd be wrong - the Moon, for hundreds of millions of years, was world in geological agony.

The Earth and Moon were born from a massive collision [2] between a Mars sized protoplanet, and an Earth-sized world called Theia. The blast briefly outshone the Sun, and may have exceeded ten thousand degrees Celsius. So the Moon formed hot. As it cooled, it began to crystallise - the heavy materials, like pyroxene [3], sank into its core, and the lighter ones floated to the surface and solidified as its crust.

Video above: A potted history of the Moon, as revealed by the new clues from NASA's Lunar Recconasiaence Orbiter space craft. Video courtesy of NASA Goddard Space Flight Center.

That was the just the beginning: Fifty kilometres beneath the surface, the Moon was still molten, and the crust faulted and cracked - under its own stresses, and under the massive asteroid impacts.

The Moon became a world of  many small volcanoes -  lunar lava is very runny and doesn't pile up into huge mountains well.... 

Image above: Twin lunar volcanoes, photographed by the Lunar Reconnaissance Orbiter. They're only about a mile across each. Aren't they  sweet? Awwww........ What? Image courtesy of NASA/JPL

...which meant the surface was covered in rivers of molten rock, hundreds of kilometres long...

Image above: Both volcanism and impacts can melt enough rock to produce rivers of runny lunar lava: This one flowed from the site of the impact that created Tycho crater, and ends in a small lake of once molten rock on the right.. Image courtesy of NASA/JPL. 

....these flooded the low lying areas, creating the dark areas you can see from Earth: 'Maria' or plains flooded with magma.....

 Image above: Mare Crisium. The 'Maria' were mistaken for seas in ancient times. There was, in fact, no mistake: They were formed by seas of lava spilling into giant impact basins... Image courtesy of NASA.

....there were massive eruptions of gas rich lava from pyroclastic vents leading into the lunar mantle....

Image above: The dark stain, around the long cold vent, is believed to be the signature of a pyroclastic eruption, where magma containing dissolved gas flowed to the surface. At depth, the gas was kept dissolved by the pressure. Like the carbon dioxide in a sealed cola bottle. Shake the cola bottle, and then open the lid. Now imagiune the lid coming off a volcanic vent, and the cola being magma.
Image courtesy of NASA/GSFC/Arizona State University.

....not that unlike this one.....

Video above: Infra red and visible images of a pyroclastic flow, showing temperatures of up to four hundred degrees Celsius. In all seriousness, if you ever see one of these coming your way, run across its path not directly away - they can out race anything alive. Video courtesy of montserratvolcanoobs via You-tube.

......and these scattered beads of volcanic glass across the plains.....

Image above: Beads of orange volcanic glass under the microscope, bought back to Earth by the Apollo  mission. These beads only form in pyroclastic eruptions, which means erupting gas under pressure, as well as molten rock. Image courtesy of  NASA.

...and, while all this was going on, the surface was getting pummeled to pieces by massive asteroid impacts, forming really, really huge impact craters.

Image above: The Giordano Bruno crater. Image courtesy of NASA/JPL.

This was one very, very active world. The Moon we see today is only a shadow of its former self, but everything the Moon ever did is recorded on its surface. There are signs that some of its volcanoes may have been active for as long as three billion years [4] - and there may have been tectonic activity as little as fifty million years ago [5]. The latest evidence shows the core is still cooling, and as it cools the whole Moon contracts...

Video above: NASA's Lunar Reconnaisience Orbiter has forund evidence of relatively (Thats a geologists relatively, which is even longer than a paleantologists relatively) recent tectonic activity, forming new valleys on the Moon. Video courtesy of NASA Goddard Space Flight Centre

....and asteroids and comets have slammed into the surface, mixing everything [6] into a gigantic geological puzzle.

Isn't mother nature helpful?

And there is so very much we still don't have a clue about, when it comes to out nearest planetary neighbour:

What are the mascons - the massive anomalies [7] in its gravitational field?

 Image above: A map of the Moons odd mass concentrations - mascons - that make it's gravity so uneven that no spacecraft can stay in orbit for long... Image courtesy of NASA.

Why are some areas so enriched with thorium [8]?

 Image above: The patterning of thorium deposits on the Moon. Map courtesy of Nature magazine.

Why did the lava floods affect the nearside of the Moon more than the far-side?

 Image above: The near and far sides of the Moon, as mapped by the Clementine mission. Even though the far side has basins as deep as those of the near side, they didn't become seas of frozen magma. Image courtesy of SDIO/NASA.

How did all that ice [9] get to its poles?

What else is frozen there [10], besides water ice?

How does the ultras thin layer [11] of water molecules on its surface form?

Video above: Archived news report on the discovery, and confirmation, of water in the upper millimetres of the lunar soil, by the Indian Chandraayan 1 space probe. Video via YouTube by NTDTV.

The list goes on, and on, and on and on....

We'll have to go back to the surface to find out....

* Not sufficiently to avoid being bored to tears by yours truly, explaining that there's more to the Moon than that, though.

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