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.

The Colour of Squiggly Lines

This week, I gave a talk about my lunar research to the Toronto chapter of KEGS, the Canadian Exploration Geophysical Society.  The interesting thing about my research is that whenever I want to go into detail about it, I generally have to do that using a bunch of squiggly lines called reflectance spectra. Since most people, even in the planetary field, are not reflectance spectroscopists (yes, that is what we are called), I usually have to explain what that means. And so I found myself explaining reflectance spectra to a group of geophysicists last Tuesday. That got me thinking that it was time to write a blog post about these squiggly lines.

The electromagnetic spectrum includes many different kinds of radiation, such as gamma rays, X-rays, the visible colours of light, radio waves, and microwaves.  Each radiation type has a defining wavelength, which is the distance between two side-by-side crests in the wave that makes up that radiation.
Image Credit: Irene Antonenko

To understand spectra, you first have to understand light. Most people know that white light is made up of all the colours of the rainbow. Each colour represents a different wavelength of light, which is the distance between two crests of a light wave (since light travels in waves). But visible colours are only a small portion of all the possible wavelengths, which together make up the electromagnetic spectrum. At shorter wavelengths, we have X-rays, gamma rays, and ultraviolet light. At longer wavelength, we have infrared light, microwaves, and all the different frequencies of radio waves.

When light hits an object, not all of the different wavelengths (or colours) that make up that light will be reflected from the surface of the object.  The wavelengths that do get reflected are what gives that object its colour. For example, a green object looks green to us because that is the wavelength of light that it reflects. If no wavelengths are reflected, the object will look black. But if all of the wavelengths are reflected the object will look white.

The colour of an object is determined by which wavelengths of light are reflected from the object. If no light at all is reflected, the object will look black. If all light is reflected, the object will look white. If only green wavelengths of light are reflected, the object will look green.
Image Credit: Irene Antonenko

When the percentage of light reflected from an object is plotted for each wavelength of light, the result is a squiggly line called a reflectance spectrum. The colours below this spectrum remind us of the corresponding colour of each wavelength.
Image Credit: Irene Antonenko

 
 

However, most objects are much more complex than this simple example. They reflect in many wavelengths, and they reflect some percentage at each of these wavelengths (it's not just all or nothing). So, when you plot the percentage of light that is reflected at each wavelength, you get a squiggly line that we spectroscopists call a reflectance spectrum (spectrum is the singular form of spectra). The thing to note is that most of these squiggly spectra have a number of peaks and valleys. There is no one wavelength to establish colour. Instead, the colour is determined by the strongest reflectors.

The chlorophyll found in plants reflects most strongly at green wavelengths, giving them their green colour. Deciduous maple leaves lose their chlorophyll as the weather cools, allowing other compounds to dominate the leave's colour.
Image Credit: Irene Antonenko

The three main minerals that are found on the Moon have spectra that can be distinguished relatively easily from each other. This allows us to study the composition of the lunar surface using remotely sensed spectral data.
Image Credit: Irene Antonenko

A great example to explain this is the leaf spectrum.  All vegetation reflects very strongly at infrared wavelengths (greater than 700 nanometers).  If our eyes could see infrared, all trees, leaves, etc. would look infrared to us. But our eyes don't see those wavelengths, so the next strongest reflected wavelengths in the range we do see (about 400 to 700 nm) produce the leaf's colour. Chlorophyll, which converts sunlight to energy in plants, reflects most strongly at green wavelengths. This is why most plants look green. But, in deciduous trees, such as maples, the leaves stop producing chlorophyll when the weather cools. As the amount of chlorophyll drops, the strong chlorophyll reflectance in green disappears. This allows other compounds, which reflect more strongly at red or yellow wavelengths (but not as strongly as the chlorophyll does in green), to be revealed. 

The important thing about spectra, which the leaf example demonstrates quite nicely, is that these squiggly lines can be used to identify components that make up the object you are observing. In the case of the leaf, its spectrum can tell you (among other things), whether or not chlorophyll is present in the leaves - though our eyes can tell us that almost as readily.

Which brings us to the planetary portion of the story. Reflectance spectra can be used to tell us which minerals make up the rocks on planetary surfaces. Each mineral has a very distinct spectrum, which can be used by experts and highly specialized software to identify it, allowing us to study the composition of planetary surfaces with remotely sensed spectral data.

My next post will build on this concept and talk about how we use spectral data to study the surface of the Moon...