Geol 110 Exploration of the Solar System
Discussion Section 1 Outline
Objectives
1 Understand how scientific investigations are carried out with the scientific
method
2 Learn about the electromagnetic spectrum and how it is used in remote sensing
Introduction
1 Scientific method*
1.1 What is the scientific method?
The scientific method is a sequence of steps for analyzing scientific problems
by which scientists endeavor to construct an accurate representation of the world.
1.2 The scientific method has four steps:
a. Recognizing the problem through observation and description of a
phenomenon or a group of phenomena.
b. Formulation of an hypothesis to explain the phenomena.
c. Use of the hypothesis to predict the existence of other phenomena, or to
predict quantitatively the results of new observations.
d. Performance of experimental tests of the predictions by several independent
experimenters and properly performed experiments. Experimental tests may lead either to
the confirmation of the hypothesis, or to the ruling out of the hypothesis.
1.3 Hypotheses, Models, Theories and Laws
An hypothesis is a limited statement regarding cause and effect in specific
situations; it is no more than an idea that can be either right or wrong.
Model is reserved for situations when it is known that the hypothesis has at
least limited validity.
Theories are scientific ideas supported by an abundance of evidence; they have
passed many tests and failed none. Theories are not necessarily correct, some may
eventually be disproven and replaced by better ones.
Some scientific ideas must be considered absolutely correct, and such ideas are
called scientific laws. For example, the law of gravity and the laws of thermodynamics.
*above material adapted from sources [1] and [2]
2 Electromagnetic spectrum [3]:
The distribution of electromagnetic radiation according to wavelength is
called the electromagnetic spectrum. For example, violet light has a wavelength of 450
nm, red light has longer wavelength of 650 nm, and microwave spans wavelength from
3x106 to 3x109 nm. Note that cm (10-2 m) is used in the figure above, while nm (10-9 m)
and µm (10-6 m) are most commonly used in remote sensing. It is important to recognize
when different units are used and to be comfortable converting between them.
Different regions of the electromagnetic spectrum are used by scientists to
detect surface geologic features or determine the composition or mineralogy. For example,
there are two sensors on-board the 2001 Mars Odyssey spacecraft, which utilize three parts
of the electromagnetic spectrum [4]:
THEMIS: The Thermal Emission Imaging System is actually two cameras that
image Mars in both the visible (425-860 nm) and thermal infrared (10 wavelength bands in
the range 6.78 µm to 14.88 µm – or stated as 6780 nm to 14880 nm) regions of the
spectrum. The visible light images are useful for morphology and identifying gross
differences in color – which relate to mineralogy and/or weathering of the surface. The
infrared images are also useful for morphologic studies but they can also give information
concerning the physical properties of the surface like grain size (loose soil or bedrock),
thermal inertia and mineralogy.
GRS: The Gamma Ray Spectrometer uses the gamma-ray part of the spectrum
to look for the presence of 20 different elements from the periodic table (e.g., carbon,
silicon, iron, magnesium, etc.).
The Mars Reconnaissance Orbiter (MRO) to be launched in 2005 will carry a
visible-infrared hyperspectral imager CRISM (Compact Reconnaisance Imaging
Spectrometer). CRISM will cover wavelengths from 400 to 4050 nm and will be able to
identify a broad range of minerals on the surface of Mars [5] at resolutions of up to 20
m/pixel.
Onboard the Clementine spacecraft that orbited the Moon in 1994, the UV/Vis
multispectral imager covered wavethlengths ranging from 415-1000 nm. This experiment
yielded information on the basic color properties of the surface material on the Moon at a
scale of abput 100 m/pixel, as well as providing images useful for morphologic studies and
cratering statistics [6].
A Few Basic Remote Sensing Terms [7]
Pixel - a contraction of picture element, the smallest subdivision of a digital image that
represents the brightness of the target at that point (see DN).
Digital Number (DN) – numeric value representing relative or absolute brightness of a
pixel (radiance, I/F, thermal inertia, elevation, etc).
Raster data - data structured as an array or grid of cells, i.e. pixels.
Sample - one element in a row or column of a raster array (synonomous with pixel)
Albedo - the ratio of energy reflected by a surface to the amout of energy incident upon it.
CCD – charged couple device, an array of silican detectors that measure the amount of
visible to near-infrared radiation per unit time. CCDs are replacing
conventional film in cameras.
References:
[1] Marshak, S, Earth: Portrait of a planet, Norton & Company, Inc, New York, 2001.
[2] http://teacher.nsrl.rochester.edu/phy_labs/AppendixE/AppendixE.html
[3] http://imagers.gsfc.nasa.gov/ems/waves3.html
[4] http://mars.jpl.nasa.gov/odyssey/technology/index.html
[5] http://crism.jhuapl.edu/instrumentDev.html
[6] http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1994-004A&ex=1
[7] Sabins, F.F., Remote Sensing: Principles and Interpretation, Freeman, SF, 1978.
Subarea of idealized image of uniform filed of bright grass with a brook meandering
across it – image acquired in near-infrared wavelength.
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