An invisible world, part 1: The Ancient Moon..

Above: The Moon above Arthurs Seat, Edinburgh. Views like this are pretty much the reason I moved here.....

It's there every evening, it controls the tides, and it's the brightest thing in the sky after the Sun. Yet, because it's always just there, we seem to look through the Moon. Which is a pity - it's a world in it's own right, with a long and complex history. A history that is deeply entwined with our own....

Where did it come from?

Above: To state the blindingly obvious - the Moon, as seen out of my living room window..

After dozens of unmanned and manned landings, we still aren't entirely sure how the Moon came to be. There is a leading theory, called the 'giant impact hypothesis'. According to this the Earth was hit by another planet, about the size of Mars, not long after it formed. Massive amounts of molten matter were blasted into space, and the result was a huge ring of molten rock, around an Earth that was covered in an ocean of lava. Over time the ring of rock clumped together to form the Moon -  or possibly the Moons, but we'll get to the other Moon in a minute....

 
Above: The giant impact hypothesis, which....um... involves giant things impacting each other... Courtesy of NASA

The giant impact hypothesis has problems - there's water in the Moon's rocks, which is hard to explain if it formed all at once from a flying cloud of lava. There's also the problem that the composition of the lunar rocks is very similar to Earth's in some ways, which it shouldn't be if the Moon is partly made from debris of another planet. These aren't quiet big enough problems to throw the theory out - it passes most other tests - but they do show it isn't quite the whole truth.

A fiery Moon...

Above: A - probably - extinct lunar volcanoes. Courtesy of NASA

However it came to be, we know that the young Moon was a very different to the fairly placid

world in our skies today. Volcanoes spotted it, and blasted lava bombs and ash into its sky. Gas rich eruptions sent pyroclastic flows scurrying across the lunar hills and valleys. Moonquakes shook the surface, and massive asteroid strikes battered it...

 
Above: Aside from the blue sky (and the occasional person) this is probably a good way to get an impression of what the ancient Moon was like.

Lunar lavas were made of exotic mixes not seen on Earth for billions of years, and they carved out bizarre landscapes where winding red hot rivers became deltas, and fed into lakes of molten rock. Over time the lowlands filled entirely with cooling lava. Once this had solidified it left the dark lowland plains of today.

A windy Moon...

Lunar winds once shuffled the lunar grit Buzz Aldrin stood on.
Between the volcanoes and the impacts there was enough gas released to sustain a thin but real atmosphere -  how dense it was would have varied wildly, but at times the Moon would have had an atmosphere as thick as that of Mars (Check out this paper on the subject by Alan Stern, former head of NASA) and, as we've seen at comet 67/P, even much thinner atmospheres can have a visible impact....


Above: The wind blown dunes of comet 67P - a place where the 'atmosphere' is thin enough to be high quality vacuum by Earth standards

And if the air was once as thick as that of Mars it isn't much of  jump to speculate that the Moon could have had windstorms and weather in the same way the red planet does....

Above: A dust devil, captured by the Spirit Mars Rover. Courtesy of NASA

Two Moons...

Our Moon might not have originally been alone:  One popular theory has a second smaller Moon orbiting Earth. The two Moons would have made a stunning sight from Earths surface - they would've formed much closer than the Moon is today - and things would have gotten more spectacular when they crashed into each other, which is how the theory goes.

 Above: It went a lil' something like this... courtesy of NASA

That said, you'd probably be more worried about the monstrous tides the young Moon(s) would've raised.....

The Moon, Earth's attic:

Earth wasn't getting it easy while the Moon struggled through its youth: Impossibly giant impacts shattered the Earths crust, flinging terrestrial rocks out into space. The Moon, being the nearest world, would have swept up a good amount of that debris. Plenty of Moon rocks have made the trip in the opposite direction and landed as meteorites. It's been estimated that there's 200 kg of matter from Earths most ancient times per square kilometre of the Moons surface. That rock could provide us clues to the early development of Earth we cannot get here - the cycle of plate tectonics, and countless cataclysmic impacts, have destroyed those most ancient rocks.

It is, in fact, illegal to blog about meteorites without playing this clip at least once.

Today?
It used to be held as obvious that the Moon had cooled fast and quickly lost its atmosphere, leaving behind a dead, airless desert where nothing happened except an occasional meteorite strike.

But recent missions have begun to overturn that idea: Lunar volcanoes have erupted in geologically recent times, water is being generated from the lunar rocks themselves every day, and the lunar dust moves itself around on static charges. It turns out that the Moon today is a bizarre and subtle place.

Next: 'The invisible world part 2: The Moon today'

We are beginning to uncover a subtle but complex modern Moon. A world with mountains of eternal light, valleys of perpetual darkness, gas explosions, buried ice, delicate frosts, unknown mists, mysterious magnetised swirls, tunnels miles wide, and historic monuments....

Elsewhere on the internet:

Newly identified meteorite from Mars

Mining asteroids with microbes

New telescope will study exoplanet climates

Payloads for spacehip2 style space planes developed.

The rise of near space ballooning

...And Philae's wild ride from flightgear flug: