Astronomers from ancient times until Galileo knew almost nothing scientific about stars and planets except that the latter moved in a regular fashion through the skies. One of the first examples of planetary remote sensing was Galileo’s use of a primitive telescope to discover the Moon’s craters and the moons of Jupiter. In the early 20th century, Hubble learned that most stars were actually galaxies - clusters of billions of stars. But it took the space program, with its probes to and orbiters around the planets, to open up the other planetary bodies in the Solar System to a systematic examination. The wealth of knowledge this has brought, largely through the remote sensing devices carried on the spacecraft, has given astronomers today remarkable insights into the nature and history of the planets. This Section will convincingly underwrite that statement. The first page reviews the different sensors and parts of the spectrum used in these great advances.
Most of the same instruments that survey the electromagnetic
spectrum (EM) around Earth have been the principal tools for exploring our
planetary associates and beyond; searching well into outer space at stars
and other members of the Universe. Here is a list of remote sensing methods
using EM spectral measurements that have provided exceptional information
about planetary surfaces, atmospheres, and, indirectly, interiors: *
Remote sensing by imaging, as applied to Earth, goes back to the middle of
the 19th Century, when balloonists took the first photos. As applied to the
rest of the solar system, we must look to the first observations (documented
by sketches) made by Galileo in 1610, when he turned a telescope to the heavens
and caught a glimpse of the surface complexities on our nearest neighbor,
the Moon. Later, he confirmed the Copernican theories with his discoveries
of moons, or orbiting satellites, around Jupiter. Since then, we have many
observations of our Solar System neighbors, first with telescopes and, after
the opening of the Space Age, with orbiting spacecraft, flyby, probe, and
lander missions. Nowhere else in the diversified and imaginative programs of NASA and other space agencies from different nations has there been such a plethora of observational and scientific triumphs as the exploration of the planets and the Cosmos beyond.
Gamma-Ray Spectroscopy
Gamma spectrum
K, U, Th Abundances
Apollo 15, 16: Venera
X-ray Fluorescence spectrometry
Characteristic Wavelengths
Surface mineral/ chemical comp.
Apollo; Viking Landers
Ultraviolet Spectrometry
Spectrum of Reflected sunlight
Atmospheric Composition: H,He,CO2
Mariner; Pioneer; voyager
Photometry
Albedo
Nature of Surface; Composition
Earth Telescopes; Pioneer
Multispectral Imagers
Spectral and Spatial
Surface Features; Composition
On most missions
Reflectance Spectrometers
Spectral intensities of reflected solar radiation
Surface Chemistry; mineralogy; processes
Telescopes; Apollo
Laser Altimeter
Time delay between emitted and reflected pulses
Surface Relief
Apollo 15,16,17
Polarimeter
Surface Polarization
Surface Texture; Composition
Pioneer; Voyager
Infrared Radiometer (includes scanners)
Thermal radiant intensities
Surface and atmospheric temperatures; compos.
Apollo; Mariner; Viking; Voyager
Microwave Radiometer
Passive microwave emission
Atmosphere/Surface temperatures; structure
Mariner; Pioneer Venus
Bistatic Radar
Surface reflection profiles
Surface Heights; roughness
Apollo 14,15,16; Viking
Imaging Radar
Reflections from swath
Topography and roughness
Magellan; Earth systems
Lunar Sounder
Multifrequency Doppler Shifts
Surface Profiling and imaging; conductivity
Apollo 17
S-Band Transponder
Doppler shift single frequency
Gravity data
Apollo
Radio Occultation
Frequency and intensity change
Atmospheric density and pressure
Flybys and Orbiters
* Adapted from Billy P. Glass, Introduction to Planetary Geology, 1982, Cambridge University, Press
This list is incomplete but is still highly representative. The exploration of the planets, while dominated by remote sensing devices, is also supported by some non-remote sensing methods. Chief among these is landing astronauts on the Moon to observe first hand, deploy instruments, and collect samples. Landers have set down on other planetary bodies as well. So far in the study of stars and galaxies, the methods used have been entirely remote sensing, as will be evident in the Section 20 review of Cosmology.
The Command and Service Module on the Apollo lunar missions
carried a complement of remote sensors and other instruments including alpha-particle spectrometers,
mass spectrometers, magnetometers, far UV spectrometers, scintillometers, and
others designed to measure geochemical and geophysical properties. The astronauts
also deployed, on the surface, instruments for specific studies. Among these
were seismometers, magnetometers, gravimeters, solar wind gauges, cosmic-ray
detectors, heat flow probes, and laser ranging retroreflectors. However, in
retrospect, sensors that produce images, especially photographs and similar
items, have provided the most direct and readily interpretible sets of data,
and will continue to be a mainstay of future missions. While remote sensing, especially in the optical or visible segment of the EM spectrum, is a mainstay in planetary studies, the resulting data still need to be interpreted. The new observations of a planet's or moon's surface tend to reveal exotic features which at first seem alien to those who live on Earth. Yet the very familiarity of the Earth to these observers is often the key element in explaining extra-terrestrial features, since Earth's surface has been well explored and documented visually. The Earth then is the "frame of reference" that commonly provides features resembling those on other planetary bodies; and, much is normally known about the mode of origin and development of these features. This approach has been termed "Comparative Planetology". As an example of how one proceeds in identifying and describing a geological feature on another planet in terms of a terrestrial counterpart, consider these two views of channels on Mars and then a similar set of channels on the Earth (in Africa):
The Chad volcano has been studied in the field, so that the role of running water in carving out the channels shown (they tend to follow fractures) is well documented. Note the similarity in morphology to the two martian sets of channels. This close resemblance illustrates the type of argument planetologists use to explain martian channels: those channels look like terrestrial channels - they probably have similar origins (this still is inference rather than firm proof).
Before proceeding, it may be helpful to you to visit and browse a website that deals with (mostly NASA's) Solar System programs - past, present, and future. Check, too, the Nine Planets and Solar View websites that list most of the spacecraft sent to other planets and solar system objects. To see a large collection of images of the nine planets, go to JPL's Photojournal website, and click on the planet of interest. Then check out one of JPL's movies. Access through the JPL Video Site, then follow the pathway Format-->Video -->Search to bring up the list that includes "Interplanetary Superhighway", July 17, 2002. To start it, once found, click on the blue RealVideo link.
Primary Author: Nicholas M. Short, Sr.