SUPERMAN’S sparkling Fortress of Solitude they’re not, but giant outcrops of crystals, found on the moon by India’s Chandrayaan-1 probe, prove that a roiling ocean of magma once engulfed the rocky body of our satellite.
The moon is thought to have coalesced more than 4 billion years ago from the molten debris of an impact between the Earth and a Mars-sized object. Models suggest that heat from that impact, as well as from material compressing to form the moon, created a sea of magma that lasted for a few hundred million years. Heavy, iron-bearing minerals should have sunk through this magma to form the moon’s mantle, while lighter, iron-poor minerals called plagioclases should have crystallised and floated to the surface.
But it has been difficult to find direct evidence of the moon’s primordial crystalline crust, as it was likely jumbled by meteoroid impacts and paved over by lava flows early in the moon’s history. Until recently, the only evidence came from lunar samples collected at a few sites by the Apollo astronauts.
Last year, however, Japan’s Kaguya probe spotted patches of the stuff inside a number of craters (Nature, DOI: 10.1038/nature08317). Now, it seems Chandrayaan-1, which orbited the moon for almost 10 months until it failed in August, found the mother lode – vast outcrops of plagioclase crystal along a mountain range inside the moon’s 930-kilometre-wide Orientale basin (below). Lava has resurfaced less of Orientale than other craters of its size.
In 1994, the US orbiter Clementine found regions inside Orientale that seemed to be virtually iron-free, hinting at plagioclase. But Chandrayaan-1 was able to detect the light absorbed by the crystal itself. It found that the rock containing the crystal spans at least 40 kilometres and is quite pure – less than 5 per cent of it is composed of iron-rich minerals.
That is purer than a number of Apollo samples, which until now have been the primary source of information on the moon’s ancient crust. “This is a game-changer,” says Paul Warren of the University of California, Los Angeles. “We now have to rethink a lot of lunar science; issues such as the way the crust originally floated over the denser melt of the magma ocean [and] the extent to which the crust was jumbled by large impacts.” …