Back in March S&T's Kelly Beatty described how researchers found water in tiny green-glass granules created by ancient volcanoes on the Moon — results that banished a long-coveted perception of a perfectly dry satellite. The team’s conclusions, appearing in the July 10th issue of Nature, have caused a bit of a splash in lunar science with their possible challenges to accepted Earth-Moon evolution models.
Contemporary models of lunar formation tell of a cataclysmic birth: a Mars-size object collides with Earth and throws molten material into space, where some of the fragments coalesce into the Moon. Scientists think that the high temperatures involved in this process vaporized all volatiles (lighter elements and molecules like water).
Alberto Saal (Brown University) and his colleagues offer a different lunar vision. Given how much water the glassy beads contain, the magma’s original water content may have resembled that of Earth’s upper mantle, according to team member Erik Hauri (Carnegie Institution of Washington). If this magma came from the Moon’s interior, water must have been deep inside.
The findings don’t throw out the collision model entirely. While scientists often use the lack of lunar water to support the great impact theory, there have been few attempts to link Moon-forming simulations to predictions of where the volatiles end up, explains Robin Canup (Southwest Research Institute), who specializes in planetary and lunar formation models. If a part of the Moon formed quickly after the impact — or (as simulations predict) from parts of the colliding body that didn’t hit Earth directly and therefore weren’t heated enough for the lighter elements to evaporate — it might have retained its water, she suspects.
The water could also come from meteorites or asteroids that struck the Moon. Saal’s team suggests that these collisions must have happened within 200 million years of the event that launched the molten lunar building blocks into Earth orbit.
Canup adds that the these new results may only reflect water abundance in a fraction of the Moon’s interior. Still, she agrees that the findings are important enough to kick-start attempts to pin down just how much of its light elements the Moon lost.