Self-Sterilizing Properties of Martian Soil: Possible Nature & Implications
Total Page:16
File Type:pdf, Size:1020Kb
.- 001CES-391 Self-Sterilizing Properties of Martian Soil: Possible Nature & Implications A. 1. Tsapin, M. G. Goldfeld, K. H. Nealson Jet Propulsion Laboratory, California Instituteof Technology, MS 183-301, 4800 Oak Grove Drive, Pasadena, California 91 009 K. M. Kemner Argonne National Laboratory, Argonne, Illinois B. Moskowitz Institute of Terrestrial Magnetism, University of Minnesota, Minneapolis, Minnesota Copyright 0 2000 Society of Automotive Engineers, Inc. ABSTRACT unaffected.hours, whilelargely remained0, formation These observations were interpreted as an-indication of the As a result of the Viking missions in 1970s, the presence of presence of a strong oxidant on the Martiansurface, or, a strong Oxidant in Martian’Oil was suggested‘ Here we most probably, several different types of oxidants [3-5]. No present a testable, by near-term missions, hypothesis that site on Earth has been foundwith such an oxidantpresent, contributesto that OxidizingPool. strong enough to decomposewater. This is not unexpected, were studied for their spectral and oxidative properties and when one takes intoaccount the abundance of reductants biological activities. Ferrate(V1) has distinctive spectroscopic that make unlikelythe accumulationof such a strong oxidant features making it available for detection by remote sensing anywhereon the surfaceof the Earth. reflectance spectra andcontact measurements via .- Commonoxidation states of iron are (+2) and (+3).How- some dioxygen and ozone. In these circumstances, ever, at certain conditions, higher oxidation states can be ferrate(V1) can serve as a form of stabilization and storage formed, including Fe(lV), Fe(V), andFe(VI). These are quite of activeoxygen, but only if its decomposition is slow unstable under most usual conditions on Earth, where water enoughtopermit the accumulation of the product in is abundant, but they can form and persist in dry systems. question. Besides, Fe(VI) known as tetrahedral Fe0; anion, although a verystrong oxidant, is a well-characterizedchemical FERRATE(V1) STABILITY AND PRESERVATION species, with its standardelectrochemical potentials E" = +2.2 V in acidic, +0.78 V in alkaline solutions[14]. It is rather In aqueoussolutions, ferrate(V1) is ratherstable in alkali stable in strong alkaline solutions in the absence of efficient only, at pH>10, and even then it is readily reduced by most reductants. Ferrates(V1) salts with various cations, such as organic materials. In a pH-E" diagram, where E" is reduction K', Na', Ba", Li', Rb', Cs', Ag', and a few tetralkyVaryl potential,thestability region can be approximately ammonium cations, have been described [15-181. Currently, presented as in Fig. 1. Beyondthat region, ferrate(V1) is there is a burst of interest to ferrate (VI) as a promising reduced according to the following equilibria: oxidizing reagent for organic synthesis [16,19] and material for rechargeablealkaline batteries of increasedcapacity Fe0:- + 8H+ + 3e- w Fe3++ 4H,O POI. Fe0; + 4H,O + 3e- w Fe(OH),(s)+ 50H' In order to assess the possible contribution of ferrate(V1) to the oxidizingpool in the Martiansoil, we address the following questions: 1. Is therean opportunity to form ferrate (VI) onMars, accordingto what is knownabout its surface compo- sition and environmental conditions? T P~oH~' 2. Would such a compound be stableenough to persist and accumulate under those conditions? 3. Would it display the essential reactivity that was found in the samples of Martian soil in the Viking experiments, i.e. produce oxygen gas while moisturized, and 4. Would it produce carbon dioxide when in contact with the organic materials that constituted nutrient solution in those experiments? 5. Would these chemical activities be impaired at heating in a waysimilar to the inactivation seen in Viking's 2 4 6 8 10 12 14 experiments? PH 6. With Marsexploration program in mind, what are the possible approaches to identification of ferrate(Vl), and Fig. 1. Eo-pH stability diagram for iron compoundsin in particularwhat are the spectralfeatures of this aquatic systems. species thatwould permit its characterization by both contact measurements and remote sensing? At a lower pH, not only organic materials, but also water is oxidized. So, thermodynamically one wouldneed a highly HIGHER FERRATES: CHEMISTRYy BIOLOGICAL alkalineenvironment for ferrate(V1) to be stored. This ACTIVITY, AND SPECTRAL FEATURES requirement is not so limiting for Mars soil, as it seems at first glance. Indeed, there are strong reasons to assume that ROUTES TO FERRATE(V1) in the absence of biogeniccalcium carbonatedeposits The regular way to ferrate(V1)is through the wet oxidation of which now serve as a powerful buffer on the Earth surface, Fe(lll) with hypochlorite [15, 161: we used this approach in bothpre-Cambrian Earth, and Mars at some stage of its ourexperiments. However, formation of ferrate(V1)was geological history, were covered with a "soda ocean" of its reported in dry, elevated-temperaturereactions of iron- pH>9 [23, 241. containingmaterials with somealkaline peroxides and Different scenarios, with ratheracidic evolution pathways superoxides [21, 221. On the other hand, there is a strong havebeen suggested forMars [25]. However, in the beliefthat active oxygen speciessuch as peroxides, absence of conclusive pH measurements, the only experi- superoxides,singlet oxygen, hydroxyl radicals, and ozone mentalevidence now available, from the same Viking UV are formed under irradiation in Martian atmosphere, and program, as Oyamapointed out [l], favorsan alkaline affect its soil [6-111. Overall, Martian atmosphere is highly environment:indeed, the solutionsproduced by mixing oxidized, dominated by carbon dioxide and with presence of samples of Martiansoil with water displayed a short-term I. absorption of carbon dioxide, as any basic solution would do. From simulation experiments, Quinn and Orenberg [26] also concluded that Martian soilmaterial is mostlikely at -16.177% least mildly alkaline. Ferrates of alkaline metals are unstable in the presence of 1 moisture.However the Martian surface is dry andcold. -8.75646 Thus, evenalkaline ferrates couldbe stabilized there. Besides, otherferrates, such as barium ferrate(Vl), are much less water soluble and consequently fairly stable in 295.81C humid milieu. It is essential that ferrate(V1) anions are not sensitive to light [27]. Thus, overall, there is enough reason to suggest that the formation of ferrate (VI) and its preservation in soil are consistent with the present knowledge of chemical compo- sition and environmental conditionsat the Martian surface. CHEMICAL PROPERTIES OF FERRATE(V1) In the Viking Gas Exchange Experiments, it was found that samples of Martiansoil released oxygen gas upon introduction of watervapor into the sample cell [1,2]. Oxygen was also released from a sample of soil that had been pre-heated at 145"C, although the amount of gas was substantiallyreduced. When treated with a "nutrient" solution, i.e. an aqueous mixture of organic acids and amino acids,carbon dioxide was first rapidly released, thenthe rate of its production decreased after a small fraction of all organic carbon had been oxidized. Carbon dioxide release Fig. 2. Thermal decomposition of &FeO,: was completely prevented by preheating the soil samples at Thermogravimetry (TGA), and differential scanning 160°C, and a consumption was observed instead. In light of calorimetry (DSC). Heating rate: 5"C/min. these results,thermal decomposition of ferrate(V1) is relevant to the problem in hand. Oxvaen Release in the Reaction with Water Thermal Decomposition Dry ferrate is not immediately reactive with aprotic solvents Potassiumferrate(V1) as a dry powder is stable at room such as ether, chloroform, or benzene, which permits their temperature. However, on heating it decomposes, releasing applicationfor removal of traces of waterfrom ferrate oxygen gas. Decomposition slowly proceeds, starting about preparations. Neither is ferrate(V1) soluble in any of those 50°C, and is complete by 300°C. Its rate strongly depends solvents. on the traces of water: dry samples are less sensitive to heating.Thermal decomposition is a complex,multi-stage Upon addition of water to potassium ferrate (VI), the mixture process, as can be seen fromthermogram and DSC bubblesindicating an intense gas evolution. In Viking's observations (Fig. 2). One may speculate that transient experiments, no attemptwas made exploreto the lower oxidation states of iron, such as Fe(+5) and Fe(+4), interaction of soil samples with pure liquid water.Oxygen are first produced,before the final product, Fe,O,, forms. evolution was recorded, however, when water vapor from These intermediate forms still display high oxidative power. the nutrient solution accessed the solid soil. The nature of A catalyticeffect of Fe(lll) on ferratedecomposition is thereaction was not explored further, and it remained anticipated and may significantly complicate the results of unclear if it were a stoichiometricoxidation of water,or DSC andthermogravimetry analysis. These results are decomposition of the solid material, catalyzed or initiated by essential to the understanding of Viking's data since they water. show that "sterilization" might result in a set of various iron products in a poorly predictable manner, depending on the The reaction of water with potassium ferrate, however, is a other components of the soil and possiblyon the rate of truewater oxidation: GC-MS analysis of the gas product heating as well. after reaction of potassium