Planetary geomorphology methods - Introduction to Planetary Geomorphology - page 24

In simple systems, images are produced in a single
wavelength range as a “ black and white ” (and shades of
gray) picture. The wavelength range can be either narrow
(as in the near-infrared) or “ broadband ” to produce a
panchromatic picture. Color images are produced by
obtaining data for more than one wavelength over the
same scene. For example, three frames could be exposed,
one each through red, blue, and green filters. Each lter is
sensitive to a given wavelength and the data correspond-
ing to that wavelength are recorded. The three frames are
then combined to produce a color image.

For example, such a detector ownonanaircraftoverterrain
at night can pick up the heat generated by vehicle engines or
even body heat from individuals. In the mid 1960s, some
thermal detectors were declassi ed for civilian use, leading
to remote sensing systems for use on Earth and in planetary
exploration. The Thermal Emission Spectrometer (TES),
developed by Phil Christensen of Arizona State University
and ownontheMars Global Surveyor spacecraft, revolu-
tionized our understanding of the surface of Mars. The
energy recorded by a thermal detector, such as the TES, is
a function of many complex variables, including surface
composition and texture, the atmosphere between the surface
and the detector, and the detector sensitivity. Understanding
the physics of the transfer of energy from the surface to the
recorder enables the determinations of factors such as the
mineralogy and grain sizes of surface materials.

2.5.2 Multispectral data

Mapping surface compositions using remote sensing tech-
niques is critical in planetary science. The crystal structure
of minerals behaves in characteristic ways when exposed
to EM energy. In the visible near-infrared ( Fig. 2.14 ), or

VNIR, crystals absorb energy, resulting in absorption
bands that are diagnostic for speci c minerals or groups
of minerals. To take advantage of this phenomenon, mul-
tispectral spectrometers are designed to measure the
reflected energy as a function of wavelength using filters
and detectors that are responsive to the absorption bands
of interest. Generally, the narrower the band widths cover-
ing the VNIR spectrum, the more precise the analysis. For
example, the Near-Infrared Mapping Spectrometer
(NIMS) which flew on the Galileo mission to Jupiter
could develop a 408-wavelength spectrum for each pixel
over the range from 0.7 to 5.2 microns.
Multispectral spectrometers can be either “ point ” sys-
tems or mapping systems. In point systems, a single line is
traced across a surface, measuring the spectra as a com-
positional pro le over terrains. The advantage of this
approach is that the instrument is relatively simple and
the total data volume is small. The disadvantage is that
sampling of the surface is very limited and important areas
might be missed. Mapping spectrometers have two-
dimensional arrays of detectors that collect data over an
area rather than as a line-trace. Mapping spectrometers are
much more useful for geologic studies.

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2.5.4 Radar imaging data

Dense clouds obscure two objects of planetary geologic
interest, Venus and Titan, a moon of Saturn. Because of its
long wavelength ( Fig. 2.14 ), radar can

”
clouds and has been used to map both Venus and Titan
using synthetic aperture radar (SAR) imaging systems.
Radar is an active remote sensing technique in that the
energy is generated arti cially, in effect illuminating the
scene to be imaged. SAR systems are mounted on a mov-
ing platform (an airplane or spacecraft) and send short
pulses of radio energy obliquely toward the side, striking
the surface at an angle. Some of the energy is re ected
back to the spacecraft, where it is received as an echo, by
which time the motion of the spacecraft has carried it to a
new position. Several thousand pulses are sent per second,
at the speed of light, resulting in an enormous data set that
must be highly processed. To construct an image, three
factors must be integrated: (1) the round-trip time from the
instrument to the surface, (2) the Doppler shift due to the
motion of the spacecraft, and (3) the radar reflectivity of
the surface, which is a function of composition, surface
roughness, and other factors.
Radar images ( Fig. 2.5 ) can be confused with images in
the visible part of the EM spectrum, and there are signifi- -
cant differences that must be understood for proper inter-
pretation. First, the brightness and apparent shadows in a
radar image do not result from sunlight, but from “ illumi-
nation ” by the radar beam. Thus, the geometry of the
bright and dark terrains depends on the position of the
spacecraft and characteristics of the surface such as topo-
graphy and composition.

“ see through

2.5.3 Thermal data

Surface materials radiate, or emit, energy ( “ heat ” )inthe
0.5 - 300 micron range of the EM spectrum, which can be
recorded as digital files or transformed into images.
Detectors that can measure this energy (sensitive thermom-
eters known as bolometers) were developed by the military.