The main thesis of this Section on the Exploration of the Solar System is simply that most of it is carried out using the tools and instruments associated with Remote Sensing. The primary sensors have been the camera, scanners, or comparable imaging devices. Also, geophysical instruments were emplaced on the surface by the Apollo astronauts. But much has been learned from the sample returns brought about by the astronauts who went to the Moon and the study of rare extraterrestrial meteorites found on Earth.
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.
Most of the same instruments that survey the electromagnetic spectrum (EM) making up the radiation emanating from the 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: *
|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).
Comparative Planetology, along with Remote Sensing, really began to take off with the space program. Satellites and flyby probes, and then landers, could visit the planets closeup. No longer was it necessary to rely on poorly resolved images made through earthbound telescopes to learn important details about the planets in our Solar System. New subject fields started to emerge in the 1960s. They were given various names, such as Planetology (which was slanted from an astronomical perspective), Astrogeology (which actually has little "astro"), and Planetary Geology (which emphasizes the geological aspects of the "comparative"). To the best of my knowledge, I, the writer (NMS) wrote the first English language textbook on Planetary Geology, which was published in 1975.
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. This website displays many (but not all) relevant missions, including those dealing with earth-observations and with astronomy, by displaying each mission as a panel logo; click on any of these to access a mission description. 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.