The Spectra of a Complex Moon

Earth's Moon consists of bright highland areas and dark mare areas. 
Image Credit: NASA/JPL/USGS

As a planetary geologist, my work revolves around looking at gorgeous pictures of the Moon and then examining squiggly lined spectra at various locations in these pretty pictures. (To learn more about spectra and how they work, please check out my last blog post, The Colour of Squiggly Lines.) For me, the point of this exercise is to figure out what's going on with the surface of the Moon. From this kind of research, I and other researchers are learning that the Moon is far more complex than had originally been suspected.

Very basically, the Moon has two main types of terrains on its surface. There are the bright areas that make up the rugged and heavily cratered highlands and the dark areas (which Galileo named "mare", the Latin word for seas) that make up the flat and relatively smooth lowlands. The highlands represent the ancient crust of the Moon, the part that solidified from a lunar magma ocean about 4.5 billion years ago. The maria (plural form of mare) are massive deposits of solidified lava. Many of the maria are circular because the hot magma flowed into large impact basins, which had been excavated by bombarded during the early Solar System.

The highlands are made up mostly of a rock called anorthosite, whose main mineral is plagioclase. Plagioclase has a very non-descript spectra. The maria, as was already mentioned, are made up of solidified lava, which is to say basalt. The main mineral of lunar basalts is pyroxene, which unlike the highland plagioclase, has a very distinctive spectra.

The lunar highlands and mare can be distinguished using the spectra of their predominant minerals.
Image Credit: NASA (Images), USGS Spectral Library (Spectra), Irene Antonenko (arrangement)

Pyroxene contains two prominent absorption features, places where the spectra dips downwards. One such feature is located at a wavelength of around 1 micron, and another very broad feature is found at around 2 microns. This very pronounced difference between plagioclase and pyroxene spectra makes it possible to use them to distinguish between highland and basalt materials on the Moon. 

Spectra from the Clementine mission, which only have 5 different wavelength bands, can still be used to separate highland from basalt spectra in many cases.
Image Credit: Irene Antonenko

The very first remotely sensed spectral data returned from the Moon was from the Clementine mission in 1994. The sensor on this mission was fairly minimal, providing only 5 bands in the Ultraviolet-Visible range of wavelengths. These 5-band spectra look quite different from laboratory spectra and contain much less information. Still, they have proven to be very effective at distinguishing between mare and highland minerals. And, although new sensors that have been flown since (such as the Moon Mineralogy Mapper on India's Chandrayaan-1 mission from 2009) have many more spectral bands, the Clemetine data set remains the only fully global data set and continues provide very useful information for studying the lunar surface.

My research uses Clementine spectra to map the locations of basalt minerals on the lunar surface. These studies have shown that basalt minerals are sometimes found in places that are very far away from any known maria. This is telling us that something much more complex is going on at these sites.

Locations of basalt spectra are shown as red dots on this map of the western limb of the Moon. The background colouring gives information about the iron content of the surface. In general, mare are iron-rich (green to red), while the highlands are iron-poor (blue).
Image Credit: Irene Antonenko

The current thinking is that such places represent locations where impact craters excavate hidden mare deposits or basalt dikes (intrusions of lava that didn't quite make it to the surface). This means that mapping the surface composition can also tell us about what is happening below the surface, allowing us to study the stratigraphy of the Moon.

In a later post, I will go into more depth on lunar stratigraphy and talk about how I use these techniques to search for hidden lunar mare deposits, called cryptomare.