On September 3, 1976, NASA's Viking 2 lander touched down on Mars at Utopia Planitia, in the northern plains, and started sending back the first colour panoramas from that part of the planet. Viking 1 had already landed at Chryse Planitia on July 20 of the same year.
The Viking missions carried the first serious attempt to detect life somewhere other than Earth. Each lander had a scoop that put martian soil into three separate experiments designed to look for metabolism, for gases released after a nutrient solution was added, and for organic molecules, and the answers those instruments gave did not agree with one another. One experiment produced a signal that looked biological. The gas chromatograph mass spectrometer found no organic molecules at all.
The answers conflicted. Most researchers eventually settled on chemistry: the martian surface holds perchlorate salts and oxidants made by ultraviolet light, and those can produce the signals the biology experiments saw. Viking's own biology team stayed divided for years. They never closed the argument.
Viking 2 lasted three years. Its seismometer never recorded a definite marsquake, and the planet's interior stayed a puzzle until later missions. What the lander did return was daily weather data, soil chemistry, and photographs of water frost forming on the rocks each winter.