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CHEMICAL WEATHERING ON MARS.
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Analyzes planet's geologic processes compared to Earth.... More...
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Paper Abstract:
Analyzes planet's geologic processes compared to Earth.
Paper Introduction:
Chemical Weathering on Mars
Recent high pressure experiments have provided insight on the chemical composition of Mars' mantle (11:2239). Such studies indicate that 15.7 percent of the planet is composed of metal a large proportion of which is iron (11:2239). It is estimated that partial melting of this mantle produces an iron rich basaltic magma which generally reaches the planet's surface via massive shield volcanos and fissure eruptions (5:90). The extrusion of this turbulent, low viscosity lava has resulted in thinly disseminated iron sulfide mineralization over large areas of Mars (5:90). In addition to these iron rich flows, the red planet's surface is also characterized by large accumulations of basaltic ash (7:2237). These deposits some of which are meters thick have undergone variable degrees of weathering (10:24). It
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99:28-41; 1992.1 . 2. The most popular models includea combination of a variety of minerals (12:22 )--the predominant ones beingeither palagonite or crystalline smectites (9:32). Furthermore, this aqueous environment may also have been acidic(13:28 ). It is a subject that could, perhaps, elucidatethe geologic history of the martian surface, yet remains incompletelyunderstood. Investigations have revealed that significantweathering can occur in extremely cold and arid climates (2:6 ). Itsfeatures have resulted from a steady stream of volcanic material(pyroclastic flow) which has produced massively bedded tuff deposits(7:2237). On Earth, basaltic ash is commonly associated withhydrovolcanism (7:2237). In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. Before X-ray diffractionanalysis, clays were thought to be amorphous (8:154). Anotherterrestrial environment that has been compared to Mars is that ofAntarctica (2:6 ). For example, on Earth, clays generally form in moist,temperate environments (2:6 ). Chemical Evolution and Oxidative Weathering of Magmatic Iron Sulfides on Mars. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. Its crystal size, however, is very small: eachparticle has only short-range crystalline structure (12:219). Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. It is estimated that partial melting of thismantle produces an iron-rich basaltic magma which generally reaches theplanet's surface via massive shield volcanos and fissure eruptions (5:9 ).The extrusion of this turbulent, low viscosity lava has resulted in thinlydisseminated iron sulfide mineralization over large areas of Mars (5:9 ).In addition to these iron-rich flows, the red planet's surface is alsocharacterized by large accumulations of basaltic ash (7:2237). Inaddition, the Viking Lander spacecraft employed X-ray fluorescence (XRF)spectroscopy and various other chemical analyses in an effort to elucidatethe exact chemical nature of the martian soils (12:219). Illuminating though they may be, they fail toaccount for all of the factors operating in extraterrestrial chemicalweathering. Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. Such low pH solutions would dissolve significant concentrationsof silica, Al, Mg, Fe, and other ionic species (13:28 ). It is thought that these meteorites (SNCs) might berock samples from the volcanic plains of Mars that have been ejected intospace by one or more impact-cratering events (9:28). 7. Viking Lander XRF measurements on Martian soil notedhigh concentrations of S and Cl (13:279). Thesedeposits--some of which are meters thick--have undergone variable degreesof weathering (1 :24). First described byvon Walterhausen in 1845, the composition and the chemical processes whichproduce palagonite are poorly understood (7:2237). Furthermore, researchers have also hypothesized that this water mayhave been particularly caustic (5:89). It is the iron--oxidized to the ferric state--that accounts forMars' reddish hue (11:2239). Jakosky, B.M. These basaltic tephras tend to be an orange to yellowish brownfrom their variable states of alteration and their inherent abundance offerric iron (7:2237). It is thought that the brightest regions of Marscorrespond to regions covered with highly weathered ash deposits (1 :24),whereas darker areas consist of essentially unweathered basalt particles(1:56). Suchwater would have interacted hydrothermally with Mars' basalts to formweathering products (6:91). These elements have beeninterpreted as the chlorides and sulfates of evaporites (13:279). In fact, the degree ofcrystallinity of a clay can sometimes provide clues as to the conditions ofits formation (8:154). Analysis of Poorly Crystalline Clay Mineralogy: Near Infrared Spectrometry Versus X-Ray Diffraction. 4. 8. Theweathered basaltic ash, or sideromelane, on Mars is thought by some tocorrespond to that on Earth and consist primarily of palagonite (7:2237).A number of different methods are employed to make this analysis. In arelatively short time geologically, Antarctic's fine-grained basalts havebecome strikingly pitted by exposure to minimal snow and wind erosion(2:6 ). Finally, one other area that has been used in an attempt to decipherMars' geologic processes is the Pavant Butte region of Utah (7:2237). Of these minerals, palagonite is the leastwell defined petrologically (12:219). The Geochemical Model of Mars: An Estimation From The High Pressure Experiments. 19:2239-2242; 1992.12. Scientific Results of the NASA- Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. Icarus. Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. Mars Volatile Evolution: Evidence From Stable Isotopes. This is in marked contrast to the cold andaridity characteristic of the present-day Martian surface (2:6 ). Gooding, J.L. Finally, though, moretechnologically advanced means of examining the planet include groundbasednear-infrared and remote sensing reflectance spectroscopy (12:219). Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. These tuff deposits consist predominantly ofbasaltic glass or sideromelane (8:154). Farrand, W.H.; Singer, R.B. 89-9 . Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. Boston, P.J.; Ivanov, M.V.; McKay, C.P. Bell, J.; McCord, T. A member of the smectite group of clays, montmorillonite, forexample, often results from the alteration weathering of volcanic ash inprehistoric lakes or estuaries (12:219). The mechanisms of weathering onMars, however, represent a different sort of problem. Soil Mineralogy and Chemistry on Mars: Possible Clues From Salts and Clays in SNC Meteorites. 67-68. In addition, martian hematite is alsocrystalline (12:219). Their occurrence and theircomposition have important implications for the planet's geologic history,a history which is subject to a number interpretations (8:154). Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. Can Iron Oxide/Oxyhydroxide Minerals Be Identified on the Martian Surface From Groundbased VIS-NIR Spectra. Icarus. Mossbauer Spectra of Olivine-Rich Weathered Achondrites: II. Nine meteorites--five shergottite, three nakhlite,and one chassignite--collected between 1815 and 1979 at various locationsall over the world represent a geochemically- and isotopically-relatedgroup of rocks (9:28). The exact composition and the mechanisms of formation for thesemartian soils, however, are not fully understood (7:2237). It has long beenknown that the minerals of Mars' surface contain high quantities of iron(11:2239). Allen, C.C.; Conca, J.L. 95:3 -3 8; 1992. Farrand, W.H.; Singer, R.B. The result would be theconversion of such ferromagnesian minerals as pyrrhotite to pyrite (5:89).Another proposed mechanism for chemical weathering on Mars involves brinesolutions (13:279). This solution would stabilize andmobilize a number of complex iron ions (5:89). One area that has been studiedis Hawaii. Ferrolysis of Iron-Bearing Martian Brines: Origin of Dust-Storm Particulates on Mars. Limits on the Compositional Variability of the Martian Surface. 3. It is caused, in part, by the presence of solar ultraviolet light on Mars,particularly the wavelengths 19 to 3 nm (4:3 1). Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. Using this evidence, various mineralogical models for the martiansediments have thus been proposed (9:32). 6. It is believed, in fact, that the weathered materialon Mars also contains large quantities of ferric iron (4:3 1). Straub, D.W.; Burns, R.G. Scientific Results of the NASA- Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. Orenberg, J.; Handy, J. Thus, the term "clay" includes materials which have beensubjected to a range of weathering intensities. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. This explosive interaction between basalticmagmas and surface (or near-surface) water typically produces tuff ringsand tuff cones (8:154). Burns, R.G.; Fisher, D.S. As sideromelane is weathered, itis transformed to the clay mineral, palagonite (8:154). Hawaiian soils--formed by subaerial secondary alteration ofmafic volcanic ash to palagonite under semi-arid circumstances (12:219)--are good spectral analogs of weathered areas on Mars (7:2237). Thus, it is difficult to make comparisons between the processes ofearth and those of Mars. Chemical Weathering of Basaltic Rocks Under Cold Arid Conditions. Burns, R.G.; Martinez, S.L. Suchformations imply that oxidative reactions in an aqueous environment musthave occurred at sometime during martian geologic history (13:279). Icarus. Few environments onEarth even approximate martian conditions. Thus, thesegeologic features are called palagonite tuffs (8:154). 154- 155. On Earth, clays are the most abundant group of minerals occurring atthe surface (8:154). Reflectance Spectroscopy of Palagonite and Iron-Rich Montmorillonite Clay Mixtures: Implications for the Surface Composition of Mars. On Earth, the ozonelayer acts to shield the planet from much of this radiation (4:3 1). 18:2237-224 ; 1991. On The Possibility of Chemosynthetic Ecosystems in Subsurface Habitats on Mars. These ions--precursors to the clay silicates, oxyhydroxides, and hydroxo-sulfatematerials--would then form as precipitates in response to changes oftemperature, pH, and salinity (13:28 ). Geophysical Research Letters. Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. It is not clear whether this water occurred on or belowthe planet's surface, but it may have been generated by the melting of apresent-day, subsurface permafrost (5:89). Such studies indicate that15.7 percent of the planet is composed of metal--a large proportion ofwhich is iron (11:2239). 96:219-225; 1992.13. After equilibrating with oxygen inthe atmosphere--even at present-day Martian partial pressures--it may haveformed an acidic solution (5:89). 94:14-31; 1991.11. As aresult, most hypotheses assume that at some point in Mars' geologic historywater was present. Literature Cited 1. Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. Chemical Weathering on Mars Recent high pressure experiments have provided insight on thechemical composition of Mars' mantle (11:2239). Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles. 5. 55-56. Spectral Analysis and Mapping of Palagonite Tuffs of Pavant Butte, Millard County, Utah. In addition, theseestimates typically also include a significant quantity of nanophasehematite (12:219) and secondary weathering products such as ferric ionsulfates and possibly other, more exotic, poorly crystalline ironoxide/oxyhydroxides (3:67). 59-6 . 91- 92. This highlyoxidized state, however, results from factors not present on Earth (4:3 1). Researchers have hypothesized that a similar phenomenon might alsooccur on the red planet (2:6 ). Adams, J.B.; Smith, M.O. For this reason, the existence of these minerals on Mars has been atopic of considerable debate (8:154). The smectite clays, in contrast,though, have a distinct crystalline structure characterized by anoctahedral sheet in coordination with two tetrahedral sheets in whichoxygen atoms are shared (12:219). 9. Brachina, Chassigny, Alha 77 5, and Nakhla. The heat necessary for suchmelting could have been generated by volcanism or impact (2:6 ). On earth, these minerals typically form under a variety ofconditions. Geophysical Research Letters. The mosttraditional investigative tool is probably the telescope. In: Solomon, S.C.; Sharpton, V.L.; Zimbelman, J.R., eds. It is now known,however, that any given clay specimen lies somewhere on a continuum betweena highly crystalline end member and a completely amorphous endmember(8:154). 279-28 .----------------------- 7 They generally form in either hydrothermal alterationzones or sedimentary environments (8:154). Icarus. It basically resultsfrom the hydration of sideromelane and is probably best described as anamorphous, ferric iron silica gel (12:219). Houston, TX: Lunar and Planetary Institute; 199 ; Technical Report Number 9 - 6: pp. Ohtani, E.; Kamaya, N. A more indirect analytic technique, though, involves the use ofmeteorites (9:28-39). Thus, there are several differenthypotheses relating to the compositions and the modes of formation of theweathered soils on Mars.
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