Rachel Taylor, age 15, of Santa Cruz, Calif., for her question:
HOW DID WE OBTAIN SO MUCH INFORMATION ABOUT MARS?
Mars is the fourth planet from the sun in our solar system, and the third in order of increasing mass. Scientists have obtained a great deal of information and knowledge about Mars because of six missions carried out by unmanned United States spacecraft between 1964 and 1976.
The first views of Mars were obtained by Mariner 4 in 1964 and further information was gained by the flyby missions of Mariners 6 and 7 in 1969.
The first Mars orbiter Mariner 9 which was launched in 1971 studied the planet for almost a year, giving planetary scientists their first comprehensive global view of the planet and the first detailed images of its two moons.
In 1976 two Viking lander craft touched down successfully on the surface and carried out the first direct investigations of the atmosphere and surface. The second Viking lander ceased operating in April 1980. The first lander worked until November 1982.
The Viking mission also included two orbiters that studied the planet for almost two full Martian years, transmitting to earth tens of thousands of high resolution images, as well as temperature and humidity measurements.
The idea that life can or even does exist on Mars has a long history. For a long time it was believed that Mars had canals and large oases. Spacecraft observations have proven that not only are there no canals, but dark areas once thought to be oases are not green and their spectra contain no evidence of organic materials. The seasonal changes in the appearance of these areas are not due to any vegetative cycle, but to seasonal Martian winds blowing sterile sand and dust.
The strongest evidence against the presence of life, however, is the thinness of the atmosphere and the fact that the surface of the planet is exposed to lethal doses of radiation.
Perhaps the most fundamental and far reaching result obtained by the Viking landers is that the soil contains no organic material (there is no reason to assume that the two landing sites are not representative. of Mars). Although small amounts of organic molecules are continually being supplied to the surface of Mars by carbonaceous meteoroids, apparently this material is destroyed before it has a chance to accumulate.
The results of the soil analysis for organic molecules carried out by the Viking landers provide no evidence for the existence of life. Scientists tell us that it would seem that Mars is a sterile desert.
Due to the thinness of the atmosphere, daily temperature variations on Mars of 180 degrees Fahrenheit are common.
Poleward of about 50 degrees latitude, temperatures remain cold enough (less than minus 189 degrees Fahrenheit) throughout winter for the atmosphere's major constituent, carbon dioxide, to freeze out into the white deposits that make up the polar caps.
For the latest information on the Phoenix spacecraft on Mars go to Science News on Youaskandy main menu and select NASA Jet Propulsion Laboratory