The Viking 1 lander landed on Chryse Planitia of Mars on July 20, 1976.
This site located at 22.5oN and 48.0oW is adjacent to the major channel
systems of Mars and presents a relatively flat geology. The land of this
region is only marked by scattered craters, which are presumed to be Hesperian
Chryse Planitia was chosen as the landing site of the first lander mission
to the Red planet, because of its flat geology. This geology provided mission
planners of the time with a relatively simple landing site for remote control
Viking 2 landed on Utopia Planitia on September 3, 1976. Utopia Planitia
like Chryse Planitia was chosen as a landing site for its relatively flat
geology. The slopes of the fields surrounding this site are less than 1
degree, while the slopes near the horizon have an inclination no greater
than 2 degrees.
Utopia Planitia was an ideal landing site for the second lander of the
Viking Project, because of its characteristic flatness which represented
much of the regions of the northern hemisphere of Mars.
The selected landing site for the touchdown of the Mars Pathfinder mission
on July 4, 1997 is Ares Vallis. This site was chosen for two main reasons.
The first reason is that in ancient times Ares Vallis was a flood plain.
This provides basis for the assumption that soil samples from different
regions of the planet will be concentrated in this area. The rover will
be able to sample many types of soil without having to displace itself
a great distance from the lander unit.
The other reason for the election of this landing site was the power
supply units used by both the rover and the lander. A site with maximum
sun exposure is needed to power the vehicles of the mission. Ares Vallis
combines both maximum sun exposure with a large variety of rock and soil
samples to be analyzed in the scientific aspect of the mission.