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