Elisabeth M. Hausrath

Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars

Where Martian rocks have been exposed to liquid water, chemistry versus depth profi les could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profi les constrains the exposure time to liquid water: on Earth, mineral persistence times range from ~10 k.y. (olivine) to ~250 k.y. (glass) to ~1 m.y. ( pyroxene) to ~5 m.y. (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH >~2, glass should persist longer than olivine; therefore, persistence of glass may be a pH indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le. For the profi le measured on the surface of the exposed Martian rock known as Humphrey in Gusev Crater, the calculated exposure time is 22 k.y., which is a minimum due to physical erosion. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profi les should be measured to illuminate the weathering history of Mars.

Elemental release rates from dissolving basalt and granite with and without organic ligands

Bacteria, fungi, lichen and plants all produce organic acids, which can strongly affect weathering by increasing the solubility and mobility of elements. Leaching by organic acids may therefore produce trace element signatures which could record the presence of life in the rock record from early Earth. To elucidate this effect, long term column experiments were performed with powdered granite and basalt with and without 0.01 M citrate at pH=6 for 45 weeks. Both granite (8.44 × 10−13, 3.39 × 10−13) and basalt (2.94 × 10−14, 6.47 × 10−14) dissolution rates mol (Ca, Mg) (m−2 s−1 respectively) were enhanced in the presence of citrate relative to the organic-free controls: granite (3.17 × 10−14, 4.4 × 10−15) and basalt (1.01 × 10−14, 1.04 × 10−14). Enhanced release of individual elements in the presence of citrate was strongly correlated with the stability constant of the citrate-element complex. Elements which might be useful as biosignatures are those elements that showed a strong enrichment in the presence of citrate: Zr, Sc and Mn (basalt), V and Zn (granite), and Y, La, Ce, Th, Ti, Al, P, Pb, Ni and Fe (both basalt and granite). Release of these elements from the rock material in the columns is consistent with dissolution of apatite + Fe sulfides + Fe oxides + augite in basalt and apatite + sphene + hornblende in granite. Similar groups of elements have been reported to be enriched in organic-rich rivers, suggesting leaching of strongly-complexed elements could be useful as biosignatures and may have left mineralogical traces on early Earth.

Short- and Long-Term Olivine Weathering in Svalbard: Implications for Mars

Liquid water is essential to life as we know it on Earth; therefore, the search for water on Mars is a critical component of the search for life. Olivine, a mineral identified as present on Mars, has been proposed as an indicator of the duration and characteristics of water because it dissolves quickly, particularly under low-pH conditions. The duration of olivine persistence relative to glass under conditions of aqueous alteration reflects the pH and temperature of the reacting fluids. In this paper, we investigate the utility of 3 methodologies to detect silicate weathering in a Mars analog environment (Sverrefjell volcano, Svalbard). CheMin, a miniature X-ray diffraction instrument developed for flight on NASAs upcoming Mars Science Laboratory, was deployed on Svalbard and was successful in detecting olivine and weathering products. The persistence of olivine and glass in Svalbard rocks was also investigated via laboratory observations of weathered hand samples as well as an in situ burial experiment. Observations of hand samples are consistent with the inference that olivine persists longer than glass at near-zero temperatures in the presence of solutions at pH approximately 7-9 on Svalbard, whereas in hydrothermally altered zones, glass has persisted longer than olivine in the presence of fluids at similar pH at approximately 50 degrees C. Analysis of the surfaces of olivine and glass samples, which were buried on Sverrefjell for 1 year and then retrieved, documented only minor incipient weathering, though these results suggest the importance of biological impacts. The 3 types of observations (CheMin, laboratory observations of hand samples, burial experiments) of weathering of olivine and glass at Svalbard show promise for interpretation of weathering on Mars. Furthermore, the weathering relationships observed on Svalbard are consistent with laboratory-measured dissolution rates, which suggests that relative mineral dissolution rates in the laboratory, in concert with field observations, can be used to yield valuable information regarding the pH and temperature of reacting martian fluids.

Assessing hydrodynamic effects on jarosite dissolution rates, reaction products, and preservation on Mars

Jarosite flow-through dissolution experiments were conducted in ultrapure water (UPW), pH 2 sulfuric acid, and saturated NaCl and CaCl2 brines at 295–298 K to investigate how hydrologic variables may affect jarosite preservation and reaction products on Mars. K+-based dissolution rates in flowing UPW did not vary significantly with flow rate, indicating that mineral surface reactions control dissolution rates over the range of flow rates investigated. In all of the solutions tested, hydrologic variables do not significantly affect extent of jarosite alteration; therefore, jarosite is equally likely to be preserved in flowing or stagnant waters on Mars. However, increasing flow rate did affect the mineralogy and accumulation of secondary reaction products. Iron release rates in dilute solutions increased as the flow rate increased, likely due to nanoscale iron (hydr)oxide transport in flowing water. Anhydrite formed in CaCl2 brine flow-through experiments despite low temperatures, while metastable gypsum and bassanite were observed in batch experiments. Therefore, observations of the hydration state of calcium sulfate minerals on Mars may provide clues to unravel past salinity and hydrologic conditions as well as temperatures and vapor pressures.

Forsterite dissolution rates in Mg‐sulfate‐rich Mars‐analog brines and implications of the aqueous history of Mars

High salinity brines, although rare on Earths surface, may have been important in the geologic history of Mars. Increasing evidence suggests the importance of liquid brines in multiple locations on Mars. In order to interpret the effect of high ionic strength brines on olivine dissolution, which is widely present on Mars, 47 new batch reactor experiments combined with 35 results from a previous study conducted at 25°C from 1 < pH < 4 in magnesium sulfate, sodium sulfate, magnesium nitrate, and potassium nitrate solutions with ionic strengths as high as 12 m show that very high ionic strength brines have an inhibitory effect of forsterite dissolution rates. Multiple linear regression analysis of the data suggests that the inhibition in dissolution rates is due to decreased water activity at high ionic strengths. Regression models also show that mMg up to 4 m and mSO4 up to 3 m have no effect on forsterite dissolution rates. The effect of decreasing dissolution rates with decreasing aH2O is consistent with the idea that water acts as a ligand that participates in the dissolution process. Less available water to participate in the dissolution reaction results in a slower dissolution rate. Multiple linear regression analysis of the data produces the rate equation logr=−6.81−0.52pH+3.26logaH2O. Forsterite in dilute solutions with a water activity of one dissolves twice as fast as those in brines with a water activity of 0.8.

Natural Fumarolic Alteration of Fluorapatite, Olivine, and Basaltic Glass, and Implications for Habitable Environments on Mars

Fumaroles represent a very important potential habitat on Mars because they contain water and nutrients. Global deposition of volcanic sulfate aerosols may also have been an important soil-forming process affecting large areas of Mars. Here we identify alteration from the Senator fumarole, northwest Nevada, USA, and in low-temperature environments near the fumarole to help interpret fumarolic and acid vapor alteration of rocks and soils on Mars. We analyzed soil samples and fluorapatite, olivine, and basaltic glass placed at and near the fumarole in in situ mineral alteration experiments designed to measure weathering under natural field conditions. Using synchrotron X-ray diffraction, we clearly observe hydroxyl-carbonate-bearing fluorapatite as a fumarolic alteration product of the original material, fluorapatite. The composition of apatites as well as secondary phosphates has been previously used to infer magmatic conditions as well as fumarolic conditions on Mars. To our knowledge, the observations reported here represent the first documented instance of formation of hydroxyl-carbonate-bearing apatite from fluorapatite in a field experiment. Retreat of olivine surfaces, as well as abundant NH4-containing minerals, was also characteristic of fumarolic alteration. In contrast, alteration in the nearby low-temperature environment resulted in formation of large pits on olivine surfaces, which were clearly distinguishable from the fumarolic alteration. Raman signatures of some fumarolically impacted surfaces are consistent with detection of the biological molecules chlorophyll and scytenomin, potentially useful biosignatures. Observations of altered minerals on Mars may therefore help identify the environment of formation and understand the aqueous history and potential habitability of that planet.

Synthesis and characterization of the Mars-relevant phosphate minerals Fe- and Mg-whitlockite and merrillite and a possible mechanism that maintains charge balance during whitlockite to merrillite transformation

Abstract Merrillite [Ca9NaMg(PO4)7] occurs as a dominant primary Ca-phosphate mineral in martian meteorites and therefore presumably also on Mars. The mineral is an important phase in exploring differences in geologic processes between Earth and Mars, and also has astrobiological implications due to its potential role as a significant source of the bio-essential nutrient phosphate. Merrillite does not occur terrestrially as a discrete mineral phase, making it difficult to obtain for Mars-relevant studies. It can, however, be synthesized from a similar terrestrial mineral, whitlockite (natural or synthetic), through dehydrogenation. Here we present methods for synthesizing relatively large quantities (0.5 g or greater per batch) of coarse crystalline (75 μm+) Mg-whitlockite, Fe-whitlockite, mixed Fe/Mg-whitlockites, and from these synthesized minerals produce Mg-merrillite, ferrous and ferric Fe-merrillite, and ferrous and ferric mixed Fe/Mg-merrillite. Chemistry and atomic structures of synthesized Fe- and mixed Fe/ Mg-whitlockite and ferrous and ferric Fe- and mixed Fe/Mg- merrillite resulting from single-crystal X-ray diffraction, infrared spectroscopy, and electron microprobe analyses are presented. We also present a mechanism for maintaining charge balance during the formation of merrillite from whitlockite. Our results shed light on these mineral structures for future martian studies, and provide methods for creating coarse crystalline merrillite for use in Mars-relevant thermodynamic, kinetic, soil/dust simulant, crystallographic, astrobiological, and other studies.

Clay mineral formation under oxidized conditions and implications for paleoenvironments and organic preservation on Mars

Clay mineral-bearing locations have been targeted for martian exploration as potentially habitable environments and as possible repositories for the preservation of organic matter. Although organic matter has been detected at Gale Crater, Mars, its concentrations are lower than expected from meteoritic and indigenous igneous and hydrothermal reduced carbon. We conducted synthesis experiments motivated by the hypothesis that some clay mineral formation may have occurred under oxidized conditions conducive to the destruction of organics. Previous work has suggested that anoxic and/or reducing conditions are needed to synthesize the Fe-rich clay mineral nontronite at low temperatures. In contrast, our experiments demonstrated the rapid formation of Fe-rich clay minerals of variable crystallinity from aqueous Fe3+ with small amounts of aqueous Mg2+. Our results suggest that Fe-rich clay minerals such as nontronite can form rapidly under oxidized conditions, which could help explain low concentrations of organics within some smectite-containing rocks or sediments on Mars.In the Gale Crater on Mars, organic matter has been detected, but in much lower concentrations than expected. Here, the authors conduct clay mineral synthesis experiments which suggest that clay minerals may rapidly form under oxidized conditions and thus explain the low organic concentrations in Gale Crater.

Dissolution rates of amorphous Al- and Fe-phosphates and their relevance to phosphate mobility on Mars

Abstract Phosphate is an essential nutrient for life on Earth, and therefore if life exists or ever existed on Mars it may have required phosphate. Amorphous Al- and Fe-phosphates rapidly precipitate from acidic solutions and amorphous Al-phosphates likely control phosphate concentrations in some natural waters on Earth. The amorphous fraction of martian soils has also been shown to be enriched in P, and amorphous phosphates are therefore also likely important in the phosphate cycle on Mars. Despite this importance, however, few dissolution rates exist for amorphous Al- and Fe-phosphates. In this study, dissolution rates of amorphous Al- and Fe-phosphates were measured in flow-through reactors from steady state concentrations of Al, Fe, and P. A pH-dependent rate law, log R = log k - npH was determined from the dissolution rates, where R is the dissolution rate, k is the intrinsic rate constant, and n is the reaction order with respect to H+. For amorphous Al-phosphate, log k = -6.539 ± 1.529 and n = 2.391 ± 0.493. For amorphous Fe-phosphate, log k = -13.031 ± 0.558 and n = 1.376 ± 0.221. The amorphous Al-phosphate dissolves stoichiometrically under all experimental conditions measured, and the amorphous Fe-phosphate dissolves non-stoichiometrically, approaching stoichiometric dissolution as pH decreases, due potentially to Fe oxyhydroxides precipitating and armoring grain surfaces. Perhaps due to these effects, amorphous Al-phosphate dissolution rates are approximately three orders of magnitude faster than the amorphous Fe-phosphate dissolution rates measured under these experimental conditions. Amorphous Al-phosphate dissolution rates measured in this study are also faster than published dissolution rates for the crystalline Al-phosphate variscite. The rapid dissolution rates measured in this study therefore suggest that, if these phases are present on Mars, phosphate would be rapidly released into acidic environments.

Modeling background radiation using geochemical data: A case study in and around Cameron, Arizona

This study compares high resolution forward models of natural gamma-ray background with that measured by high resolution aerial gamma-ray surveys. The ability to predict variations in natural background radiation levels should prove useful for those engaged in measuring anthropogenic contributions to background radiation for the purpose of emergency response and homeland security operations. The forward models are based on geologic maps and remote sensing multi-spectral imagery combined with two different sources of data: 1) bedrock geochemical data (uranium, potassium and thorium concentrations) collected from national databases, the scientific literature and private companies, and 2) the low spatial resolution NURE (National Uranium Resource Evaluation) aerial gamma-ray survey. The study area near Cameron, Arizona, is located in an arid region with minimal vegetation and, due to the presence of abandoned uranium mines, was the subject of a previous high resolution gamma-ray survey. We found that, in general, geologic map units form a good basis for predicting the geographic distribution of the gamma-ray background. Predictions of background gamma-radiation levels based on bedrock geochemical analyses were not as successful as those based on the NURE aerial survey data sorted by geologic unit. The less successful result of the bedrock geochemical model is most likely due to a number of factors including the need to take into account the evolution of soil geochemistry during chemical weathering and the influence of aeolian addition. Refinements to the forward models were made using ASTER visualizations to create subunits of similar exposure rate within the Chinle Formation, which contains multiple lithologies and by grouping alluvial units by drainage basin rather than age.