Ancient volcanic deposits distributed across the surface of the Moon contain unexpectedly high amounts of water compared with surrounding terrains, according to an analysis of data from the Moon Mineralogy Mapper instrument onboard India’s Chandrayaan-1 lunar orbiter. This finding bolsters the hypothesis that the lunar mantle is surprisingly water-rich.
Map of ESPAT (effective single particle absorption thickness) values and associated water contents derived from Moon Mineralogy Mapper spectra. Numerous locations of increased water content (arrows) are associated with previously recognized pyroclastic deposits ranging in area from thousands of square km to much smaller deposits recently identified in high-resolution Lunar Reconnaissance Orbiter Camera images. A – Aristarchus Plateau, H – Harbinger, H&D – Humorum & Doppelmayer, O – Orientale, RB – Rima Bode, SA – Sinus Aestuum, SG – Sulpicius Gallus, TL – Taurus-Littrow, V – Vaporum. Image credit: Ralph E. Milliken & Shuai Li, doi: 10.1038/ngeo2993.
The volcanic beads don’t contain a lot of water — about 0.05% by weight — but the deposits are large, and the water could potentially be extracted.
The research, published in the journal Nature Geoscience, was led by Brown University associate professor Ralph Milliken.
“Detecting the water content of lunar volcanic deposits using orbital instruments is no easy task,” said Dr. Milliken and his co-author, Dr. Shuai Li from the University of Hawaii.
“Researchers use orbital spectrometers to measure the light that bounces off a planetary surface.”
“By looking at which wavelengths of light are absorbed or reflected by the surface, they can get an idea of which minerals and other compounds are present.”
“The problem is that the lunar surface heats up over the course of a day, especially at the latitudes where these pyroclastic deposits are located. That means that in addition to the light reflected from the surface, the spectrometer also ends up measuring heat.”
“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water. So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component,” Dr. Milliken explained.
To do that, he and Dr. Li used laboratory-based measurements of samples returned from the Apollo missions, combined with a detailed temperature profile of the areas of interest on the Moon’s surface.
Using the new thermal correction, they looked at data from Chandrayaan-1’s Moon Mineralogy Mapper.
They found evidence of water in nearly all of the large pyroclastic deposits that had been previously mapped across the Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.
“The distribution of these water-rich deposits is the key thing,” Dr. Milliken said. “They’re spread across the surface, which tells us that the water found in the Apollo samples isn’t a one-off.”
“Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle.”
The idea that the interior of the Moon is water-rich raises interesting questions about the Moon’s formation.
Planetary researchers think the Moon formed from debris left behind after an object about the size of Mars slammed into the Earth very early in the history of Solar System.
One of the reasons they had assumed the Moon’s interior should be dry is that it seems unlikely that any of the hydrogen needed to form water could have survived the heat of that impact.
“The growing evidence for water inside the Moon suggests that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified,” Dr. Li said.
“The exact origin of water in the lunar interior is still a big question.”
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Ralph E. Milliken & Shuai Li. Remote detection of widespread indigenous water in lunar pyroclastic deposits. Nature Geoscience, published online July 24, 2017; doi: 10.1038/ngeo2993
This article is based on text provided by Brown University.
