The one molten moon. Forged in the magma debris of the giant impact.
Imagine you’re watching the night sky, but you notice somethings different. You can’t see the stars and the usual blackness is soothed by a luminous moon rising above the horizon. With molten rock streaking across its surface, the moons glow comes from the deep lava ocean that submerges the whole globe.
Although the moon appears dry and grey now, it is widely accepted among the scientific community that the moon had a molten birth. Whether we are better off or not, the moon has long since solidified. Any remnants of the lava ocean under the moon’s surface is still at question.
How do we know the moon had a lava ocean?
Astronauts on the Apollo missions brought home some souvenir rocks for analysis. The moon rocks had very low levels of volatile (easy to evaporate) elements like zinc.
Samples of trinitite, a green coloured glass forged in nuclear bomb testing sites in New Mexico were also collected. Scientists found lower levels of volatile zinc in trinitite closer to the detonation site. The results showed that the heat of the nuclear explosion ripped out the volatile elements, where the moon also exhibited a lack of volatiles.
But how did the moon get the lava ocean in the first place?
The moon’s history is still shrouded in uncertainty, but one theory stands out (although it has many variations). It goes by the name: Impact Theory.
(Artists impression) Observations by NASA’s spitzer space telescope of an impact around the nearby star Vega. Image credit: NASA/JPL-Caltech/T. Pyle (SSC) via wikimedia commons
This impact theory describes when a Mars sized proto-planet slammed into the Earth in the early stages of the solar system. The collision between the two massive planets created a lot of heat, vaporising the planet’s surface, melting rock and flinging molten debris into space. The cores of the two planets merged and the space debris formed a swirling ring around the planet. This ring, over millions of years clumped together and formed the magma moon.
Attempting to explain the moons composition and orbit, impact theory keeps getting tweaked and refined. These variations in the details include if:
– the impact was a direct hit or an off-centre hit, causing a slow graze which grinded the debris into orbit (giving the earth-moon system its similar spin-orbit orientations).
– it was one colossal impact (difficult to get accurate simulation results) or multiple large impacts (meaning the moon was a melting pot of many mini moons, considerably more successful in simulations).
– the energy of the collision being enough to give the Earth a ring system of debris, or so energetic that it would vaporise most of both planets, turning the mantles into vapour (which would cool down and coalesce into the Earth and moon).
What was it like?
Along with its lava oceans, various models predicting the moons history use a metallic atmosphere made up of sodium and silicon oxide. With the post impact Earth, radiating like a small dwarf star (rather dim), the elements would be evaporated from the moons ocean surface. This would maintain the atmosphere mostly on the earth facing side.
Since the moon is tidally locked (meaning the same side of the moon always faces the earth as it orbits), a temperature and pressure gradient from the front and back facing sides of the moon would have caused supersonic winds. If these winds existed then lava waves were rolling along the moon’s surface.
Over the span of millions of years after the collision, the moon cooled down. As it cooled, plagioclase feldspar minerals (made of calcium and sodium) crystallised and floated to the top of the ocean, forming the lunar crust. Without the supply of evaporated elements, the atmosphere quickly disappeared. Each time the moon was bombarded by asteroids that cracked surface revealing the lava underneath, the moon reformed its crust leaving big dark basins.
It may have been cool for astronauts to go surfing on a windy lava moon rather then nine hours of walking around on the grey surface. And it may have been interesting with a lava moon lighting up the night. But the era of the lava the moon is over.