The moon maybe palely alone in the night sky today but according to scientists it is possible that the there was a second, smaller moon 4.4 billion years ago.
A paper published in the journal Nature theorized that there was a smaller moon created in the same impact that created the moon. Astronomers, Martin Jutzi and Erik Asphaug of the University of California at Santa Cruz have long wondered why the moon had two incongruous sides, one smooth with flat plains and another side full of rugged mountains and craters.
The astronomers started thinking that the mountainous region had been added to a pre-existing surface. This theory follows the generally held belief that the moon was formed after a Mars-sized planetoid crashed into Earth to blast a cloud of debris into space that would eventually congeal into the moon. Smaller pieces could have formed at a Trojan point or a place in the same orbit as the moon but ahead or behind it.
The mini-Moon would eventually become gravitationally unstable and would fall back to the bigger Moon. Jutzi and Asphaug's computer simulations of the event show that the Moon's smaller twin wouldn't fall back to the moon in a fiery explosion but in gentle descent. Not with a bang but with a splat.
"Being slow, it does not form a crater, but splats material onto one side," Professor Asphaug said. The extra mini-Moon mass would account for the extra material on the far side of the moon.
The scenario also explains other features found in the moon including why the side facing Earth has so much lava. At the time of the impact the extra mass added to the other side of the moon could have moved the magma beneath the crust upwards to the near side of the moon.
The theory remains just a theory until it can be tested. Unfortunately there is no easy way to test the existence of the Moon's twin just yet. Another moon probe is on the way in September to get a detailed look at the moon's geological structure and history and could provide the necessary data to support Jutzi and Asphaug's theory.
Until then we can only imagine what it was like to have two moons shining down on us in the night sky.