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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Full of Surprises
Mike Malin
President and Chief Scientist, Malin Space Science Systems
Mineral maps from the Thermal Emission Spectrometer on NASA's Mars Global Surveyor showed the variety of surface rocks on Mars. .

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.

Data from early Mariner 4 and 5 missions to Mars and Venus was printed out on teletype machines.

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 is a reprocessed Mariner 9 image of Olympus Mons. Olympus Mons is a shield volcano 624 km (374 mi) in diameter (approximately the same size as the state of Arizona), 25 km (16 mi) high, and is rimmed by a 6 km (4 mi) high scarp.

This mission, again, completely changed our view of Mars: This time from a "dead" planet -- like the moon -- to a planet full of surprises.

This is a Mariner 9 view of Elysium Mons taken on 16 October 1972, at 11:37 a.m. PST. North is up, illumination is from the right.

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.

This spectacular picture of the Martian landscape by the Viking 1 Lander shows the rock "Big Joe" and a dune field with features remarkably similar to many seen of the deserts on Earth.

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.

This picture is a composite of Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) daily global images acquired at Ls 107 degrees. Ls, solar longitude, is a measure of the time of year on Mars. Mars travels 360 degrees around the sun in one Mars year. The year begins at Ls 0 degrees, the start of northern spring and southern autumn. Image credit: NASA/JPL/Malin Space Science Systems

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.)

This image by Mars Global Surveyor is a mosaic of Mars Orbiter Camera (MOC) high resolution images acquired between August 2000 and September 2003. The area covered is 14 km (8.7 mi) wide and 19.3 km (12 mi) high; north is up and the scene is illuminated by sunlight from the left.

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.

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