Moon Mineralogy Mapper > INSTRUMENT

The Moon Mineralogy Mapper is an imaging spectrometer with high spectral and spatial resolution, reliably uniform results, and a high signal-to-noise ratio.

	The images produced by M3 can be thought of as a cube of information. The width is 600 pixels (each of which has a spatial resolution of 70 m at the instrument's altitude of 100 km), the length is an infinite strip along the spacecraft's orbital path (or wherever one chooses to "crop" the picture for an individual study), and the depth is 261 spectral channels.

The instrument will detect electromagnetic radiation with wavelengths from 430 to 3000 nanometers (0.43 to 3 microns), which covers visible light and the near-infrared. (Unlike previous lunar spectrometers, M³ will include the range from 2.5 to 3 microns, which is uniquely a marker for small amounts of OH and H₂O.) This spectral region is dominated by solar reflection, as contrasted with the longer wavelengths of the thermal-infrared region, which is influenced primarily by heat radiated by the ground.

M³ will divide the approximately 2600-nm range to which it is sensitive into 261 discrete bands, each of which is only 10 nm wide. This is considered very high spectral resolution, and will enable M³ to detect the fine detail required for mineral identification.

Spatial resolution will be similarly high. From its vantage point 100 km above the lunar surface, M³ will be able to resolve features as small as 70 m in diameter.

Mineral map of Cuprite, Nevada produced from measurements by an imaging spectrometer called AVIRIS. M3 will enable similar maps of the lunar surface. Click the picture for a large version with a color key to the minerals.

Simplicity, reliability, and accuracy

Each picture M³ produces will show mountains, craters, or plains like a regular camera, but in a very narrow range of wavelengths (the 10-nm sliver of the spectrum that constitutes one spectral channel). It's like taking a picture using a filter that allows only one precise color of light through the lens.

But M³ will take 261 such pictures simultaneously, each in its own "color." To identify the spectral fingerprint of a particular portion of the lunar surface, one would plot the light intensity of each of the 261 channels, noting how bright each pixel is at each wavelength. Plotting this on a graph produces a spectrum. Each mineral has its own unique spectrum, identified by taking spectrographic readings in a laboratory. (Actually, since scientists will have an idea of what they are looking for, they will be able to limit their plotting to the relevant portions of the spectrum for a given mineral -- up to a few dozen channels each time.)

M³ will sense the terrain beneath it through a strip of 600 spatial elements (pixels) oriented perpendicular to the direction of its flight around the Moon. As the instrument orbits the Moon, all 600 pixels will photograph the lunar surface simultaneously, producing an image 600 pixels wide and many kilometers long (along the endless path the instrument will fly).

Designed for simplicity, reliability, and accuracy, M³ uses a compact system of optics (the mirrors that collect and direct the light) known as an "Offner" design, which produces little or no distortion, either spatially or spectrally. And instead of mechanically scanning the scene as some systems do, M³ will use the "pushbroom" method in which the instrument passively sweeps the scene below as it flies, recording with the entire row of 600 pixels simultaneously.

Each of those 600 pixels, as mentioned above, will simultaneously record images in each of 261 spectral channels. So the product of M³'s measurements can be described as an "image cube" that is 600 pixels wide, infinitely long over time as the instrument flies (an "instantaneous snapshot" would be one pixel long), and 261 spectral channels deep.

Global and target modes

M³ will employ two modes, global (mapping the entire surface) and target (zeroing in on features of special interest).

In global mode, the instrument will merge groups of pixels to reduce the data rate and accommodate the limited available downlink time. This will result in an effective spatial resolution of 140 m and spectral resolution of 40 nm. Resolution will be at the maximum 70 m spatial and 10 nm spectral while in target mode, but data in this mode will be recorded for only six minutes at a time. Thus, the total amount of data transmitted in global and target modes will be the same.

The field of view (FOV) will be 40 km for both modes to allow contiguous orbit-to-orbit measurements at the equator that will minimize variations in lighting conditions. The instrument will use diffraction grating to separate sunlight into its constituent wavelengths, much as a prism does, and will maintain 12-bit precision and use only lossless compression throughout.

M³ Instrument - Flight Configuration Pictures

This is the rear view of the M3 instrument in Flight Configuration with Optical Board Assembly (OBA) and Instrument Electronics Assembly (IEA) connected. Front, right view. Left, front view. Left side. Left, top view. Rear, left view. Rear, right view. Top view.

Links:

http://aviris.jpl.nasa.gov/
Home page for AVIRIS, an imaging spectrometer similar to M³.

http://speclab.cr.usgs.gov/aboutimsp.html
USGS tutorial on imaging spectroscopy, citing AVIRIS as the premier example currently in use.

http://rst.gsfc.nasa.gov/Sect13/Sect13_9.html
AVIRIS and other Imaging Spectrometers (from The Remote Sensing Tutorial).

Optics for Compact, High-Performance Imaging Spectrometers [ PDF, 191 kB ]
Article on Offner design.

Low-Distortion Imaging Spectrometers [ PDF, 124 kB ]
Article on Offner design combined with "pushbroom" method.