What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?
I will be limiting my discussion only to Mars.
There have been four truly significant missions of science to Mars: Mariner 4, Mariner 9, Viking, and Mars Global Surveyor.
Mariner 4: Mariner 4 is noteworthy for being the first successful mission to Mars. There were two results from this mission that substantially clarified our view of the planet. The first and most important was the discovery that the atmospheric pressure on Mars was very, very low: 4-7 millibars.
This alone pretty much killed off the idea that Mars was Earth-like. However, it had help from the second most important result: the surface of Mars is covered by impact craters.
Mariner 9: Mariner 9 was the first spacecraft to successfully orbit any planet other than the Earth (not counting the moon as a planet), and the first to orbit Mars.
This mission, again, completely changed our view of Mars: This time from a "dead" planet -- like the moon -- to a planet full of surprises.
Mariner 9 discovered the large Tharsis and Elysium volcanoes, the "Grand Canyon" of Mars (Valles Marineris), erosional terrains, catastrophic flood valleys, what appeared to be fluvial valley networks, layered deposits in both the Valles Marineris and in the polar regions, tectonically fractured regions, and sand dunes.
Mariner 9 also measured the atmospheric pressure and surface temperature distribution on Mars.
Viking 1 and 2 Landers: These were the first successful landers on Mars, but their primary importance was the failure to find life on Mars.
Viking carried some pretty sensitive, but rudimentary life detection instruments, which did not detect any consistent pattern that could be associated with the properties of life (despite alternatives offered by some of the investigators).
The most important results from Viking were those from the Gas-Chromatograph and Mass Spectrometers, which showed not only that there wasn't any life, but also that there were no organic molecules at all. Since such materials would be expected on Mars from the influx of meteorites, these results then showed that Mars was "self-sterilizing" and a highly inhospitable place. Not only because it was cold, but because of its surface chemistry and/or radiation environment.
Mars Global Surveyor (MGS): This orbiter provided data that has revolutionized our modern view of Mars once again. The Mars Orbiter Laser Altimeter (MOLA) provided us a global and regional perspective on the topography of Mars. The Thermal Emission Spectrometer (TES) began to illuminate the nature of the composition of the surface rock materials, and the Mars Orbiter Camera (MOC) provided meter-scale observations of the surface that discovered a totally different place than visible in the Mariner 9 and Viking images. (Mariner 9 and the two Viking orbiters took images at a resolution of 20 to 200 times lower than MOC.)
MGS discoveries included: layered sedimentary rock formations, some exposed from at least 10 km depth, evidence for persistent water flow and aqueous clastic sedimentation. MGS found indisputable evidence for Martian lakes (including a delta), ancient stream courses and evidence for rainfall. In addition, MGS found fluvial-looking gullies that appeared to be active, south polar CO2 and climate change, present-day impact cratering rate, and a host of atmospheric observation results that show that Mars has definite, predictable meteorological phenomena.
Last Updated: 12 February 2013