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ScienceWeek
PLANETARY SCIENCE: AGAINST STANDING MARTIAN WATER
The following points are made by T.M. McCollom and B.M. Hynek (Nature 2005 438:1129):
1) The Mars Exploration Rover (MER) Opportunity has returned close-up images of exposed bedrocks from Meridiani Planum, as well as data on their chemical and mineralogical composition[1-5]. The most distinguishing features of the bedrock include: (1) spherical haematite nodules (the so-called "blueberries"), (2) "festoon" bedforms in the rock structure, (3) high concentrations of sulphur, probably in the form of sulphate minerals, (4) anomalously high Br:Cl ratios, and (5) lath-shaped cavities (vugs) suggesting that crystals of a sulphate mineral such as gypsum were formerly present. On the basis of these observations, the MER science team[1,2] has interpreted the bedrock as siliciclastic sediments deposited in a shallow body of briny water, with subsequent evaporation leaving behind sulphate minerals. They inferred that the rocks are composed of roughly 50 wt% siliciclastic material derived from basaltic rocks, 40 wt% evaporite sulphates, and 10 wt% haematite[2].
2) However, the composition of the Meridiani bedrocks indicates that the formation model advocated by the MER team is not plausible. Ratios of cations including Fe, Mg, Ca and Na to (Si + Al) in the rocks are nearly identical to the basaltic martian meteorite Shergotty, and also very similar to unweathered basaltic rocks at Meridiani[4] and Gusev crater. The compositional data strongly suggest that the Meridiani rocks represent typical martian basalt with a sulphur component added. Indeed, if the SO3 abundances of Meridiani bedrocks are reduced to the level of other martian rocks, their composition is nearly identical to that of Shergotty
3) Consequently, any model for the formation of the Meridiani bedrocks must account for enrichment in S but not in any major cations. If the sulphate were attributable to precipitation of salts from an evaporating brine as suggested by the MER team, the rocks would be enriched in a balancing cation (for example, Ca, Mg or Fe) -- but this is not observed. Oxidative weathering of metal sulphide minerals has also been proposed as an analogue to the mineralogy of Meridiani bedrock, but this is similarly implausible because it cannot account for addition of S without a concurrent increase in some cation (Fe is typically the predominant cation in such environments). Furthermore, there is no evidence for the presence of metal sulphide deposits at or near Meridiani.
4) In summary: Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water. On the basis of morphological observations as well as high abundances of haematite and sulphate minerals, the rocks have been interpreted as sediments that were deposited in a shallow body of briny water with subsequent evaporation leaving behind the sulphate minerals. The iron-sulphur mineralization at Meridiani has also been inferred to be analogous to that produced during oxidative weathering of metal sulphide minerals, such as occurs at acid mine drainage sites. Neither of these interpretations, however, is consistent with the chemical composition of the rocks. The authors propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed sulphur dioxide- and water-bearing vapors emitted from fumaroles. This scenario does not require prolonged interaction with a standing body of surface water and may have occurred at high temperatures. Consequently, the model invokes an environment considerably less favorable for biological activity on Mars than previously proposed interpretations.
References (abridged):
1. Squyres, S. W. et al. The Opportunity rover's Athena science investigation at Meridiani Planum, Mars. Science 306, 1698 1703 (2004)
2. Squyres, S. W. et al. In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars. Science 306, 1709 1714 (2004)
3. Herkenhoff, K. E. et al. Evidence from Opportunity's microscopic imager for water on Meridiani Planum. Science 306, 1727 1730 (2004)
4. Rieder, R. et al. Chemistry of rocks and soils at Meridiani Planum from the alpha-particle X-ray spectrometer. Science 306, 1746 1749 (2004)
5. Christensen, P. R. et al. Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity rover. Science 306, 1733 1739 (2004)
Nature http://www.nature.com/nature
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Related Material:
GEOCHEMICAL EVIDENCE FOR WATER WITHIN MARS
Notes by ScienceWeek:
Mars is now a desert world on which liquid water, because of ambient conditions, is not likely to be found at the surface: average temperatures are below 273 degrees kelvin and atmospheric pressures are at or below water's triple-point vapor pressure of 6.1 millibars. However, in 1972 the Mariner 9 orbiter mission photographed evidence -- in the form of apparent giant flood channels and arborized networks of small valleys -- that liquid water might have been stable in the surface environment at some time in the past. Analysis of Mars 4 and Mars 5 data, Viking orbiter images (1976-1980), and observations of flood terrain by Mars Pathfinder in 1997 supported this conclusion.
In general, the term "metamorphism" refers to the process of changing the characteristics of a rock in response to changes in temperature, pressure, or volatile content. Most metamorphic changes of rocks do not include bulk chemical changes, but merely the crystallization of new mineral phases. Examples of the transformation of sediments through metamorphism are sand to sandstone and peat to coal. The term "regional metamorphism" refers to the recrystallization of pre-existing rocks in response to simultaneous changes of ambient conditions that include temperature and the weight of overlying rocks (lithostatic pressure). The term "contact metamorphism" (thermal metamorphism) refers to the recrystallization of rocks surrounding an igneous intrusion in response to the heat supplied by that intrusion.
In general, the term "magma" refers to molten rock generated by partial melting of a planet's crust. Igneous rocks are rocks that have congealed from a molten mass, particularly from magma.
The term "pyroxene" refers to any of a group of rock-forming minerals of variable composition, among which calcium-, magnesium-, and iron-rich varieties predominate. Pyroxenes are found in almost every variety of igneous rock, and they also occur in rocks of widely different compositions formed under conditions of regional and contact metamorphism.
Meteorites are divided roughly into 3 main classes according to their composition. "Iron meteorites" consist of an alloy of iron and nickel; "stony meteorites" consist of silicate minerals; and "iron-stony meteorites" are a mixture of the two previous types. The stony meteorites are further divided into "chondrites" and "achondrites". Chondrites contain small spherules of high-temperature silicates ("chondrules") and constitute more than 85 percent of recovered meteorites. The achondrites (which contain no chondrules) range in composition from rocks made up essentially of single minerals (e.g., olivine) to rocks resembling basaltic lava. Each category is further subdivided on the basis of chemical composition. (The olivines are a group of metal-silicate minerals, with the metal as magnesium, iron, manganese, and calcium, with minor amounts of nickel. The most common form is a solid solution of magnesium silicate and iron silicate.)
Basalt is a dark gray to black igneous rock of volcanic origin that cools rapidly.
In this context, the term "outgassing" refers to the release of gases by volcanic activity.
"Shergottite" is a very rare type of achondrite meteorite, named after the meteorite that fell at Shergotty, India, in 1865, the first known meteorite fall of this type. The Shergotty meteorite consists primarily of pyroxene, and it belongs to the class of meteorites known as SNC meteorites (shergottites, *nakhlites, *chassignites; called "snick" meteorites") which on the basis of their composition are considered to be from Mars. The SNC meteorites are igneous rocks that apparently solidified from a cooling magma near the surface of their parent body. All but one are relatively young (less than 1.3 billion years old); the single ancient SNC meteorite found to date was apparently formed 4.5 billion years ago. The proportions and isotopic abundance of noble gases trapped in one shergattite resemble the composition of the Martian atmosphere as analyzed by the Viking landers. The current consensus is that the SNC meteorites were ejected from Mars by impacts, and entered orbits around the Sun before falling to Earth.
The following points are made by H.Y. McSween Jr et al (Nature 2001 409:487):
1) The authors point out that observations of Martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been proposed that significant quantities of such water could have been supplied to the Martian surface via outgassing, but this suggestion is apparently contradicted by the low magmatic water content that is generally inferred from chemical analysis of igneous Martian meteorites.
2) The authors now report the distributions of trace elements within pyroxenes of the Shergotty meteorite -- a basalt body apparently ejected 175 million years ago from Mars -- as well as hydrous and anhydrous crystallization experiments that together imply that water contents of pre-eruptive magma on Mars could have been up to 1.8 percent. The authors report they have found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which are believed to have formed at depths in the Martian crust) are enriched in soluble trace elements when compared to the outer rims of the minerals (which are believed to have crystallized on or near to the Martian surface). The authors suggest this implies that water was present in pyroxenes at depth, but was largely lost as pyroxenes were carried to the surface during magma ascent. The authors conclude that ascending magmas possibly delivered significant quantities of water to the Martian surface in recent times, this conclusion reconciling geologic and petrologic views of the outgassing history of Mars.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
nakhlites: A rare type of achondrite meteorite, named after the meteorite that fell at Nakla, Egypt, in 1911, the first known meteorite fall of this type. The nakhlites (also called "augite-olivine achondrites) consist of approximately 80 percent by weight of augite (a type of pyroxene) and approximately 14 percent by weight of iron-rich olivine. The textures of nakhlites suggest they formed within cooling magmas.
chassignites: Another rare type of achondrite meteorite, named after the meteorite that fell at Chassigny, France, in 1815, the only known meteorite fall of this type. Chassignite (also called "olivine achondrite) is an olivine-rich rock: the Chassigny meteorite contains 92 percent olivine.
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PLANETARY SCIENCE: ON MARTIAN WATER
The following points are made by B.M. Jakosky and M.T. Mellon (Physics Today 2004 April):
1) The evolution of the Martian surface and atmosphere are increasingly being seen as connected to the behavior of water on all time scales. Over the course of a single day, even the incredibly small amounts of water in the atmosphere can form clouds that affect the energy balance at the surface, the chemistry of the atmosphere, and the geochemistry of the surface layer. On seasonal time scales, atmospheric water is part of the current climate system: It acts as a tracer of dynamical transport by the winds, and the water vapor can diffuse into (and out of) the subsurface, so that it becomes water ice at high latitudes and adsorbed water at lower latitudes. These seasonal processes, operating over many years, are thought to be responsible for large deposits of ground ice at high latitudes and for the evolution of the polar ice caps. On the million- to billion-year time scales, much larger amounts of water appear to have been present on Mars and to have played a fundamental role in sculpting the surface features that we see.
2) There is tremendous interest in the history and distribution of water because of the important role it plays in making a planet habitable by microorganisms and in potentially allowing life to exist. Because of the evidence for widespread liquid water on Mars throughout its history, the red planet may be the most likely place in our solar system after Earth to find evidence for present-day or past life.
3) The Martian climate and environment are very different from those of Earth, and the history of water cannot be predicted from first principles. Rather, we look to Mars itself to tell us what processes have been important and what role they may have played over time.
4) The Martian atmosphere today is predominantly carbon dioxide, with a pressure of approximately 6 millibars at the surface. Although the CO2 provides a slight greenhouse warming, Mars is about 1.5 times as far from the Sun as Earth and temperatures at the Martian equator average about 220 K. This is well below the freezing point of water, so we don't expect to find stable liquid water at the surface today.(1) Spacecraft- and Earth-based telescopic measurements, however, have identified and mapped water vapor and ice clouds in the atmosphere, ice within the near-surface regions at high latitudes, and ice on the surface in the polar regions.
5) Planetary scientists first detected water vapor in the Martian atmosphere in 1963 using Earth-based telescopic spectroscopy. Although telescopic observations continue today and are being used to understand year-to-year variations in the water cycle, the most detailed measurements of atmospheric water were made from Viking from 1976 to 1978 and from the Mars Global Surveyor (MGS) since 1999. The measurements indicate that the atmospheric water abundance varies both with location and with season in a way that helps elucidate the processes that control it.(2-5)
References (abridged):
1. B. M. Jakosky, R. J. Phillips, Nature 412, 237 (2001)
2. M. D. Smith, J. Geophys. Res. 107, 5115 (2002)
3. M. I. Richardson, R. J. Wilson, J. Geophys. Res. 107, 5031 (2002)
4. L. A. Soderblom, in Mars, H. H. Kieffer, B. M. Jakosky, C. W. Snyder, M. S. Matthews, eds., U. of Ariz. Press, Tucson (1992), p. 557
5. R. B. Leighton, B. C. Murray, Science 153, 136 (1966); M. T. Mellon, B. M. Jakosky, J. Geophys. Res. 100, 11781 (1995); M. T. Mellon, W. C. Feldman, T. H. Prettyman, Icarus (in press).
Physics Today http://www.physicstoday.org
ScienceWeek http://scienceweek.com
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