Seeing the Surface
How CRISM Uses Reflected Light to Detect Minerals
CRISM's investigations begin with sunlight reflected off
the Martian surface. CRISM breaks this light into a spectrum, from
which it measures 544 colors. The wide range of colors helps CRISM
determine the mineralogy of the surface. The science of using color to measure composition is called spectroscopy. CRISM uses a special kind of spectroscopy called reflectance spectroscopy. Reflectance spectroscopy focuses on the
radiation reflected by different materials. Someone with a little familiarity with rocks can at a glance
recognize a piece of basalt - black, hardened volcanic lava - or limestone. In this case, color
and texture give the observer immediate
clues to the rock type. At this simple level a basalt is easily recognized
due to its black color (low albedo) and smooth texture (fine-grained).
Limestone is buff-white in color (high albedo) and also smooth or
fine-grained. Albedo is simply a measure of the amount of light that is reflected
from a surface (rock, mineral, soil, etc.). A dark rock
reflects only a small portion of sunlight that hits it, thus it
has a low albedo. You can think of albedo as the percentage
of light that is reflected from a surface versus the amount
that shines on the that surface.
However, some very different rocks may look alike based on color - at least to the human eye. For example, slate - a metamorphic rock derived from heated, altered sediment - is also
black and fine-grained like basalt. Spectroscopy extends mineral
discrimination based on color to wavelengths not visible to the eye.
Electromagnetic radiation consists of energy - the shorter the wavelength, the more energetic
the light is. Electrons in a mineral - a chemical compound that makes up part or all of a rock - can absorb this energy
but only at specific
energy levels characteristic of a specific atom or molecule. Thus,
minerals are more likely to absorb radiation at certain energies, and so
only at certain wavelengths.
In reflectance spectroscopy, the Sun provides light that is reflected
by a mineral, and a spectrometer like CRISM measures the amount of
light reflected at different wavelengths.
Dips in the
reflectance spectrum occur at wavelengths where a mineral absorbs the light. Those dips are called absorptions.
Most of the minerals whose presence and abundance tell us about Mars'
geology have distinctive fingerprints in reflectance spectra. For
example, three of the main minerals making up igneous rocks on Mars are olivine and two kinds of pyroxene.
These minerals have absorptions near 1 micron and 2 microns, respectively, and their
exact wavelengths and relative strengths indicate the mineral
composition of the rock.
One of the main groups of minerals that form by weathering of igneous rocks is oxidized iron minerals. There are several kinds of these, and which mineral formed tells us about the environmental conditions during weathering.
Oxidized iron minerals are most distinguished by the wavelengths and
relative strengths of absorptions at visible wavelengths (0.4-0.7
microns) and in the infrared out to 1 micron.
Two major groups of minerals formed on Mars due to alteration by water. One of these is clays, or phyllosilicates.
These minerals indicate a prolonged, wet environment, and oftentimes a
warm and wet environment. Clay minerals are recognized by absorptions
near 1.4, 1.9 and 2.2-2.4 microns due to water or hydroxyl that is trapped in the minerals' crystal structure. Different clay
minerals can be distinguished by the wavelengths and shapes of these
absorptions, especially the ones at 2.2-2.4 microns.
The other mineral group that formed by alteration due to water is sulfates.
In general, these minerals indicate a relatively acidic environment.
Sulfates are recognized numerous absorptions from 1 to 2.5 microns due to
water in the minerals' crystal structures. There are many varieties of
sulfates that differ in the amount of water they contain, and they can
be distinguished by the shapes and wavelengths of the absorptions.
How CRISM Takes an Image
This graphic shows a single CRISM observation sequence of a rock
sample obtained during ground testing. The target was a breccia, a kind of sedimentary rock. This observation
was made using a command sequence identical to what will be used
inflight to observe Mars.
One two-dimensional frame of CRISM data, outlined in white in the upper
left panel, is a single line of a spatial image. That's right, CRISM takes an image one line at a time. But each pixel along the
line has a spectrum that fills out the second dimension of the frame. A
two-dimensional spatial image
of a target is built up by taking successive data frames as the
spectrometer field of view is swept across a target, either by scanning
CRISM's gimbal or by MRO's along-track motion over the Martian surface.
The stack of resulting data frames (shown in
the upper left panel) is a multiband image, or "image cube." The upper
right panel shows a three-wavelength slice through the multiband
image to create a red-green-blue image as the eye might see it. The
lower right panel shows the spectrum for a single spatial pixel
from the center panel. The absorptions present in the spectrum are
due to water and phyllosilicates, or clay minerals, present in the breccia.
CRISM's Targeted Observations
To take high-resolution observations, CRISM operates in targeted mode.
The instrument's gimbal - basically, a scan platform - is scanned to
compensate for motion of the spacecraft. If the gimbal weren't there,
CRISM's high-resolution images would be hopelessly smeared. On top of that smooth motion, the gimbal superimposes a scan to cover a region
approximately 10 km x 10 km (about 6 miles x 6 miles) at about 18
meters (60 feet) per pixel, in 544 channels covering 0.36-3.92 microns.
Ten additional abbreviated, spatially binned images are taken before
and after the main image, providing measurements to study the
atmosphere and to correct surface spectra for atmospheric effects. This
sequence of multiple measurements of the same target at different
geometries - over a short time while the illumination is constant - is
called an emission phase function.
In time sequence, as a target is approached, five short scans across it
are performed during which hyperspectral data are taken spatially
binned to conserve data volume (purple). Then, centered on the time of
target overflight, a slow scan across the target is performed with
minimal or no spatial binning (green). Finally, five additional short
scans are performed as the target is departed (red). The result is 11
images, the 6th of which is at high spatial resolution.
Projected onto a map, the footprints of the 11 images overlap. The
first and last of the 11 images have the largest footprint because the
spacecraft range to target is largest. The footprints become smaller as
the spacecraft approaches the target. The
central swath has the most pixels, outweighing the small spacecraft
distance, to create a relatively large footprint. The hourglass shape
of the central image is due to the changing range to the
target as the data are acquired.
CRISM's Multispectral Mapping Mode
CRISM can also build
up images using passive, fixed pointing. The instrument points at nadir
and takes 15 or 30 frames per second. Because the spacecraft's velocity
relative to the surface is about 3000 meters per second, this makes the
along-track dimension of a pixel's footprint 200 or 100 meters,
respectively. To "square off" the pixel footprint, data are binned
spatially.
When it operates in this mode, CRISM returns data from only 72 selected wavelengths. CRISM's normal 544 wavelengths provides a hyperspectral image, meaning a very large number of colors. When it saves only the reduced number of wavelengths, CRISM operates as a multispectral imager, meaning many colors. The 72 wavelengths have been selected
carefully so that they cover the absorptions indicative of the mineral
groups that CRISM is looking to find on Mars.
Spectra of key
Martian clay and sulfate minerals are shown here with both
hyperspectral (left) and multispectral (right) wavelength coverage. All
of the key mineral absorptions are still well measured using the
multispectral wavelengths.
CRISM will map most of Mars in its multispectral operating mode. The
data set it collects by doing this is called the multispectral survey.
The word "survey" is used because one reason for taking this data set
is to survey the surface for new sites for targeted observations. The other reason is to
provide a global map. Targeted observations take so much data volume (about
200 megabytes apiece) that only about 1% of Mars can be imaged
hyperspectrally. The multispectral survey fills in the gaps.
The differences in resolving power of OMEGA data and CRISM targeted and
multispectral survey data have been simulated using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. AVIRIS is a CRISM-like spectrometer that observes Earth. For the
simulation, the CRISM team used AVIRIS data covering a hot-spring
deposit on Mauna Kea, Hawaii, the kind of deposit that is a high
priority to find on Mars. The several-fold increase in spatial
resolution of CRISM's multispectral survey over OMEGA allows smaller
regions of interest to be resolved. In CRISM's high-resolution targeted
observations, even small-scale deposits the size of a baseball diamond
can be detected.
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OMEGA (300-1000 m/pixel): discovers large deposits
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CRISM multispectral survey (100-200 m/pixel): discovers small deposits
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CRISM targeted (~18 m/pixel): characterizes deposits
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| Simulation
of OMEGA and CRISM multspectral mapping and targeted observations of
hydrothermal deposits. AVIRIS data covering hot-spring deposits at
Mauna Kea were resampled to the appropriate resolutions and classified
using a mapping algorithm. Colors represent different phyllosilicate
phases. |
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