Making mountains out of a moon

摘要

The Moon dramatically exemplifies the role that collisions played in shaping the solid planets, particularly during the earliest history of the Solar System. The Moon's very existence is believed to be a consequence of debris accreting in Earth's orbit after a Mars-sized body collided with Earth shortly after it formed1'2 - a theory known as the giant-impact hypothesis. Lava-filled basins on the lunar nearside, and the massive South Pole-Aitken basin on the farside, mark the locations of major asteroid-sized impacts after the Moon accreted, melted and cooled. And thousands of craters all over the surface represent 'scars' of abundant smaller impacts, mostly accumulated during the waning stage of planet formation. The common attribute of the geological record of impact events is that they produced 'holes' - craters and basins. Now Jutzi and Asphaug (page 69 of this issue3) present intriguing simulations suggesting that a collision by a lunar companion that was created in the Moon-forming impact produced the extensive mountainous terrain on the lunar farside, rather than a crater or basin (Fig. 1). The idea that multiple moons could have formed after the giant impact had been demonstrated by previous two-dimensional simulations4 of the dynamics of the Earth-encircling debris disk that was generated by the impact. Calculations4 showed that perturbations caused by the largest moonlet (our Moon) as it receded from Earth effectively removed material from the disk inside the Moon's orbit; a companion in this region would probably not have survived long.