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Mars he first high-resolution martian ancient towering volcanoes, extensive the surface of Mars were strongly mag- T images acquired by the Mariner 4 spacecraft in 1965 shattered pop- ular notions of Mars. Far from being polar caps, and immense canyons apparently cut by water. Systematic observations of the surface and atmos- netized early in martian history. Moreover, MGS has seen indications of recent or ongoing climate change, and an oasis, the surface of Mars appeared phere by Viking led to a dramatic has found small gullies with character- instead to be as battered and barren as increase in our knowledge of the istics suggesting that they were recent- the Moon. With its thin atmosphere breadth of martian geological history ly carved by fluid flow. Additionally, and bitterly cold temperatures, Mars and the dynamics of the current cli- fundamental information has been seemed more parched than the driest mate. The recent Mars Global derived from the study of martian places on Earth. The prospect that life Surveyor (MGS) mission has again rev- meteorites. Detailed analysis of these could have evolved there seemed dim. olutionized our understanding of mar- samples has invigorated the debate Each subsequent mission to Mars tian evolution. Far from having no over whether life ever existed on Mars. has changed that impression in sur- magnetic field as previously believed, Despite studies to date, we still do prising ways. Mariner 9 revealed MGS discovered that large portions of not know fully where water exists on Mars today. There are direct observa- tions of four exposed martian water reservoirs, which include water vapor in the atmosphere, water ice in the atmosphere, seasonal water ice deposits at the surface, and perma- nent water ice deposits at the north and south poles. Of the four reser- voirs, the martian polar caps are by far the most massive. Recent MGS data sets indicate that the mass of water ice contained within the mar- tian north and south polar caps is equivalent to a global ocean some 22 to 33 meters deep. Recent observa- tions from the Mars Odyssey space- craft also suggest a patchy reservoir of water ice beneath the martian surface. At increasing depth, where the rock is warmer, liquid water may be present in pore spaces. The Mars Pathfinder landing site (above), like much if not all of the martian surface, is drier than Earthâs driest desert. But this may not always have been the case. Gullies on canyon and crater walls, such as these (top) in Sirenum Terra seen by Mars Global Surveyor, are possible evidence that water has flowed on or near to the martian surface in the geologically recent past. 8 New Frontiers in Solar System Exploration
Mars Sample Return and Precursor Missions On Earth, life is found wherever there is liquid water. On Mars, although the peak daytime surface temperature near the equator can rise above the freezing point of water, the average surface temperature is about â55Â°C. The surface of Mars today is cold, dry, oxidized, and exposed to an intense amount of solar ultraviolet radiation. These factors are likely to limit or even to prohibit life at or near the surface of the martian soil. The surface environment of Mars, however, may not always have been as hostile to life as it is today. The geo- logical evidence, especially in the val- ley networks, indicates that the mar- tian climate could have been apprecia- bly more hospitable to life about 3 bil- lion years agoâthe atmosphere appears to have been warmer and more dense, and liquid water existed on the surface. In such a climate, life could have developed, possibly leav- ing behind fossil evidence in mineral deposits created by surface water. To date, a single set of robotic stud- ies has searched directly for existing life on Mars: the Viking life-detection experiments, which were designed to At 3000 km long and up to 8 km deep, Marsâs Valles Marineris canyon system test for organisms using carbon dioxide dwarfs any such feature on Earth. Mosaic constructed from Viking 1 orbiter images. or organic molecules in a manner anal- ogous with terrestrial organisms. The scheduled to be joined by a Japanese â¢ Results that show an absence of life results of the Viking experiments very orbiter, Nozomi, and Europeâs Mars may not be accepted because the strongly suggest that the materials test- Express orbiter and Beagle 2 lander. experiments yielding them were too ed were devoid of organic compounds Moreover, NASA has well-defined geocentric or otherwise limited; or other signs of life, but this conclu- plans to launch additional Mars mis- â¢ Results consistent with, but not sion has been debated. The lack of sions at each launch opportunity for definitive of, the existence of life (e.g., unanimity in the scientific community the remainder of this decade (see the the detection of organic compounds highlights the difficulties inherent in diagram on page 10). of unknown, either biological or non- the detection of microorganisms by Since Mars exploration activities biological, origin) may be regarded as robotic means. Indeed, even if it were for the rest of this decade are well in incapable of providing a clear-cut generally acknowledged that the Viking hand, the SSE Survey concentrated on answer; and experiments did not show the presence identifying gaps in the existing pro- of life, the experiments could still be gram and laying the groundwork for â¢ Results interpreted as showing the criticized as being overly geocentric in activities in the decade beyond 2013. existence of life will be regarded as showing only a lack of evidence for The SSE Survey concluded that, with necessarily suspect, since they might lifeforms on or near the surface of Mars our present state of knowledge and reflect the presence of terrestrial con- that were similar to life on Earth. technological expertise, it is unlikely taminants instead of true martian life. The pace of Mars exploration is that robotic techniques will be able to Definitive answers about the exis- currently breathtaking! NASA current- conclusively prove whether there is or tence of martian life will require labo- ly has two spacecraftâMars Global has been life on Mars. Results ratory analysis of Mars samples Surveyor and Mars Odysseyâoperat- obtained from life-detection experi- returned to Earth. Samples provide ing in orbit about the Red Planet, and ments carried out by robotic means the ultimate ground truth for the two moreâthe twin Mars Exploration on the martian surface can be wealth of data returned from tele- Rovers, Spirit and Opportunityâare challenged as ambiguous for the scopes, orbiting sensors, and in situ en route. The NASA missions are following reasons: missions thoughout the solar system. 9
Mars NASA, together with other national and international space agencies, has detailed plans for missions (above) to follow on from the current Mars Global Surveyor and Mars Odyssey. These missions include the Mars Exploration Rovers (right), Mars Express, and Nozomi currently en route to Mars. Missions beyond 2009 are being planned. The SSE Survey recommends that NASA should spend the next 10 years preparing for a Mars Sample Return campaign near 2015. Sample return should be conducted to obtain rocks from a variety of geological settings. Moreover, to best assess Marsâs potential for life, robotic techniques should be devised to collect samples from beneath the martian surface where conditions are more hos- pitable for living organisms. (For more discussion of sample-return missions, see the Technology Development section on page 25.) 10 New Frontiers in Solar System Exploration
Mars Sample Return and Precursor Missions Profile Mars Sample Return and Precursor Missions Mission Type: Continuing program of landers and orbiters leading toward sample-return missions Cost Class: Small, Medium, and Large Priority Measurements: â¢ Collect and return selected samples of martian soil and rock to Earth. â¢ Measure the chemical and isotopic composition of the atmosphere at ground level over the course of a martian year. â¢ Map the distribution of water in the crust. â¢ Assess the rate of escape of gases from the middle and upper atmosphere. â¢ Conduct a long-lived survey of seismic activity. â¢ Quantify the heat flow from the martian interior. â¢ Determine the composition and age of martian rocks. â¢ Undertake high-resolution magnetic mapping of the southern highlands. Although technically challenging, collection and return of martian samples to Earth for intensive study in terrestrial laboratories is a key scientific priority for the decade beginning in 2013. Advanced rovers may be used to collect samples. Guiding Themes Addressed Important Planetary Science Questions Addressed Volatiles and What global mechanisms affect the evolution of volatiles on planetary bodies? Organics Where is the water on Mars? The Stuff of Life What planetary processes are responsible for generating and sustaining habitable worlds? The Origin and Evolution Where are the habitable zones in the solar system? of Habitable Does life currently exist on Mars? Worlds Did life ever exist on Mars? Why have the terrestrial planets differed so dramatically in their evolution? How do the processes that shape the contemporary character of planetary bodies operate and interact? Processes How did the atmosphere evolve over long periods of time? How Planetary Systems Work What kinds of rocks are in the martian crust? Why does the martian crust show evidence of a strong magnetic field in the distant past? 11