The inside of the moon may be wetter than we thought. Some dried lava flows on the moon seem to have far more water than their surrounding areas, and that water probably came from the moon鈥檚 mantle, which was once thought to be mostly dry.
In the past decade, scientists have made several discoveries showing that the lunar interior contains at least a little bit of water, but it isn鈥檛 clear whether that water is spread all across the moon or concentrated only in certain areas. However, lava flows that have brought magma up to the surface can give us a better idea of where the water is inside the moon.
Shuai Li at the University of Hawaii and his colleagues used data from the Moon Mineralogy Mapper, an instrument that orbited the moon aboard India鈥檚 Chandrayaan 1 spacecraft from 2008 to 2009, to examine the light reflected from the moon鈥檚 surface in search of water trapped in the molecular structure of minerals.
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The researchers found that 10 out of the 11 dried lava flows they looked at seemed to have far more water than neighbouring areas. In one lava flow that spans a few hundred kilometres of the lunar surface, the water content is higher than 300 parts per million, whereas the rest of the region has almost no water at all. The other nine areas have similarly high concentrations of water, with little to no water in their surroundings.
They compared the lava flows with their surroundings using various kinds of data from other spacecraft and ground-based telescopes, but found no major differences in the brightness, temperature or topography that could explain why these spots have so much water. Instead, it almost definitely came from inside the moon, said Li, who presented this work at the virtual Lunar and Planetary Science Conference on 16 March.
The solidified lava flows may be similar to a type seen on Earth called a pahoehoe. These tend to cool quickly, so there isn鈥檛 time for all the water to evaporate away. 鈥淚n terrestrial lava flows, we found that more water can be trapped in pahoehoe lava flows,鈥� said Li. This idea is supported by radar measurements indicating that the bottoms of the lava flows on the moon, where the lava meets bedrock, look similar to pahoehoes on Earth.
The fact that 10 out of 11 lava flows examined by the team contained a high concentration of water听may mean that the entire lunar interior is more full of water than we thought. 鈥淭he lava flows in this study indicate a mostly wet interior,鈥� said Li.
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