MARS GLOBAL SURVEYOR: THE SECOND MARS REVOLUTION

Data from Mars Global Surveyor (MGS), now in its eighth year in orbit and its fourth Mars year of detailed study of the "Red" planet, have led to a second revolution in thinking about Mars geology. Two decades ago, the results from the Mariner 9 mission first revolutionized our views of Mars by revealing the large volcanoes, global tectonic dichotomy, large outflow channels, valley networks, layered polar deposits, and many other exotic features. The synthesis that came out of the Mariner 9 and the large follow-on Viking missions was of a lunar-like planet with hints of terrestrial similarities. The view from MGS is that Mars is neither the Moon with an atmosphere nor the Earth with craters, but a planet with its own unique and strange history. From a distant past when liquid water pooled in depressions on the surface that acted as basins for the accumulation of kilometer-thick sedimentary deposits to a recent history where the south polar CO₂ cap is evaporating at a prodigious rate in response to what can only be considered a recent global warming, Mars is revealing a rich and complex story. Many of these results resonate with similar paradigm shifts in terrestrial geoscience, creating rich opportunities for comparisons that both illuminate and educate. The disequilibrium of landforms within the present environment (for example, the absence of movement of sand dunes at the sub-meter scale, or the existence of what appear to be fluid-cut gullies) argue that the Mars of the not-so-distant past was different from the place we see today. These relationships have led to arguments for Mars that closely mimic the reasoning and logic used in the mid-19th century to conclude that Earth experienced Ice Ages. This presentation will illustrate the revolution that is on-going and set the stage for evaluating the results of other missions that follow (both to Mars and in this Symposium).