Mariner 6 was launched on Feb 24, 1969 and Mariner 7 was launched on March 27, 1969. The both were on the second in whole series of Mariners. They were not heavy because they were not carrying heavy instrument with them. Both were completing same mission, they reached at Mars with a gap of few days. Both flew to mars with altitudes. Mariner 6 flew with an altitude of 3,431 km and Mariner 7 at an altitude of 3430 km.
They were studying the atmosphere of Mars with the help of sensors which are specially designed to study the characteristics of specific area. Mariner 6 brought about 70 plus photos and Mariner 7 brought about hundreds of photos back to the earth it helped in figuring out the structure, radius and mass of Mars. They were successful in bringing the data as compared to previous Mariners.