Mars has always been a prime target of the space program because of its proximity to Earth and also the suspicion that some form of life might be present on its surface. Table 1 gives an overview of the missions that have been launched to Mars. The few photographs obtained from the early flyby missions gave the impression that Mars was much like the Moon, although there was considerable evidence that the atmosphere of Mars was substantial enough to have caused erosion of some Martian surface features.
The Mariner 9 mission was the first to show us the scope and variety of the Martian surface. Ironically, when it first arrived at the planet in late 1971, it was feared that the mission would be a disaster since the whole planet was engulfed in a huge dust storm that hid all but the very tallest features from the spacecraft's cameras. The dust storm cleared, however, and the Martian atmosphere became transparent. Fantastic images of this foreign world were returned to Earth. In addition to the ancient moon-like cratered terrain seen in the early flybys, Mariner 9 discovered giant canyons, mysterious channels, huge volcanoes and other evidence of geological activity, plus further evidence for aeolian (win and fluvial (water) erosion. The Mars Pathfinder and Global Surveyor missions have completed the transformation of our image of Mars. Once believed to be Moon-like, we now know Mars once had much in common with our own planet.
Table 2 - UNMANNED PROBES LAUNCHED TO MARS
|Spacecraft||Launched by||Arrival Date||Comments|
|Mariner 4||USA||July 1965||flyby|
|Mariner 6||USA||July 1969||flyby|
|Mariner 7||USA||August 1969||flyby|
|Mariner 9||USA||November 1971||orbiter|
|Mars 2||USSR||November 1971||orbiter--lander lost|
|Mars 3||USSR||December 1971||orbiter and lander|
|Mars 4||USSR||February 1974||flew by|
|Mars 5||USSR||February 1974||orbiter|
|Mars 6||USSR||March 1974||orbiter--lander failed|
|Mars 7||USSR||March 1974||orbiter--lander failed|
|Viking 1||USA||June 1976||orbiter and lander|
|Viking 2||USA||August 1976||orbiter and lander|
|Mars Global Surveyor||USA||November 1996||orbiter|
|Mars Pathfinder||USA||December 1996||orbiter and lander|
We have all seen many amazing photographs returned from space and we tend to take them for granted. However, these high quality results are not easy to construct. Each image is divided into a number of lines and each line is divided into a series of points called pixels (picture elements). Each Mariner 6 or 7 image, for instance, consisted of 704 lines with 945 pixels per line for a total of 665,280 pixels per image. The imaging system views the scene to be imaged and assigns a numerical values to each pixel which corresponds to the intensity of light in that portion of the scene. Again, using Mariner 6 and 7 as an example, black was recorded as "1" and white as "255" with the intermediate numbers being assigned to various shades of gray. This string of numbers is then transmitted across millions of kilometers of space where a tracking station receives the signal, amplifies it, and stores the information on magnetic tape. The data is then processed by computer and displayed on a television screen where, finally, a regular photograph is obtained.
As if this weren't already complicated enough, the computer processing can be done in a number of different ways to reveal different features from the same image. In general, two versions of each picture were produced: a photometric version which shows large scale brightness variations (contrast enhancement with the sacrifice of fine detail) and a maximum-definition version which concentrates on the fine detail. This processing is in addition to the processing of the initial signal to remove ghost images and noise introduced by the imaging, transmission, and receiving systems. Hopefully this brief introduction has served to make you more aware and appreciative of the photographs you will be investigating.