Radar Suggests an Iron- and Titanium-Rich Upper Lunar Crust

Kunowsky crater on the Moon as seen in radar by Mini-RF (left) and optical by LROC (center and right). The Mini-RF scientists picked dozens of crater floors like these—flat, dusty and devoid of boulders at the very center—and discovered that deeper craters have radar properties suggesting them to be more metal-rich than shallower ones. Image credit: Heggy et al., 2020

The wealth of lunar spacecraft data collected over the last several decades allows for global searches for resources on the Moon. A study led by Essam Heggy from the University of Southern California along with team members from the Miniature Radio Frequency (Mini-RF) instrument on the NASA Lunar Reconnaissance Orbiter (LRO) mission find evidence that the lunar subsurface might be richer in bulk iron and titanium oxides than previously thought. Radar scientists found evidence that the fine lunar soils covering crater floors have differing concentrations of iron and titanium oxides as measured from their radar properties. Among simple lunar craters, larger ones are suggested to have higher metallic mineral abundances in the fines covering their central floors than smaller craters less than 5 kilometers in diameter. Since this thin layer of fines comes from the subsurface that was excavated by the original impact, the team used the properties of these fines to conclude that the lunar crust’s iron- and titanium-oxide content is greater at 500 meters depth than at the surface.

In this study, the LRO Mini-RF radar imager was used to compare the radar properties of crater floors from equatorial latitudes up to the north pole to determine whether there were differences with latitude that might suggest the presence of ice closer to the pole. Instead, the team found that latitude did not play a role in the radar properties, but that the crater size did. Craters with diameters of 2 to 5 kilometers were found to have lower dielectric constants than those with diameters between 5 and 20 kilometers. The team concluded that the dielectric properties of lunar fines on the floors of larger craters, which reveal material at greater depths, corresponded to increasing abundances of iron and titanium rather than ice. This would imply that more metallic oxides had been excavated from depths of 500 meters to 2 kilometers than from the uppermost 200 to 500 meters of the lunar subsurface. This observation confirms that the first few hundred meters of the lunar crust are iron-poor, but reveal that deeper down, more iron and titanium oxides could be expected.

This research project was funded through the University of Southern California under NASA award NNX15AV76G and is published in the link https://www.sciencedirect.com/science/article/abs/pii/S0012821X2030217X