Mark Robinson and Brett Denevi presented the results from the Lunar Reconnaissance Orbiter mission at last week's joint meeting of the European Planetary Science Congress and the American Astronomical Society's Division for Planetary Sciences.
"Looking up at the Moon, its surface appears painted with shades of grey - at least to the human eye. But with the right instruments, the Moon can appear colourful," said Robinson, of Arizona State University.
"The maria appear reddish in some places and blue in others. Although subtle, these colour variations tell us important things about the chemistry and evolution of the lunar surface. They indicate the titanium and iron abundance, as well as the maturity of a lunar soil."
The Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) is imaging the surface in seven different wavelengths at a resolution of between 100 and 400 metres per pixel. Specific minerals reflect or absorb strongly certain parts of the electromagnetic spectrum, so the wavelengths detected by LROC WAC help scientists better understand the chemical composition of the lunar surface.
Robinson and his team previously developed a technique using Hubble Space Telescope images to map titanium abundances around a small area centred on the Apollo 17 landing site.
Samples around the site spanned a broad range of titanium levels. By comparing the Apollo data from the ground with the Hubble images, the team found that the titanium levels corresponded to the ratio of ultraviolet to visible light reflected by the lunar soils.
"Our challenge was to find out whether the technique would work across broad areas, or whether there was something special about the Apollo 17 area," said Robinson.
Robinson's team constructed a mosaic from around 4000 LRO WAC images collected over one month. Using the technique they had developed with the Hubble imagery, they used the WAC ratio of the brightness in the ultraviolet to visible light to deduce titanium abundance, backed up by surface samples gathered by Apollo and Luna missions.
The highest titanium abundances on Earth are around xx percent. The new map shows that in the mare titanium abundances range from about one percent to a little more than ten percent.
In the highlands, everywhere TiO2 is less than one percent. The new titanium values match those measured in the ground samples to about one percent.
"We still don't really understand why we find much higher abundances of titanium on the Moon compared to similar types of rocks on Earth. What the lunar titanium-richness does tell us is that the interior of the Moon had less oxygen when it was formed, knowledge that geochemists value for understanding the evolution of the Moon," said Robinson.
Lunar titanium is mostly found in the mineral ilmenite, a compound containing iron, titanium and oxygen. Future miners living and working on the Moon could break down ilmenite to liberate these elements. In addition, Apollo data shows that titanium-rich minerals are more efficient at retaining particles from the solar wind, such as helium and hydrogen. These gases would also provide a vital resource for future human inhabitants of lunar colonies.
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