We seek to address questions regarding the temperatures, time scales, nature of water-rock interaction, and chemical characteristics of ancient aqueous systems with the final goal of assessing their suitability for sustaining life. The technical approach to these questions consists of measurements of light stable isotopes (H, C, O, S) of extraterrestrial materials, terrestrial analogs, and laboratory experiments.
The group is particularly interested in the interaction of carbon and water and the products of this interaction. This includes carbon dioxide gas, carbonate minerals and organic compounds.
This theme has been the major emphasis of our research and we address it using a broad spectrum of different approaches.
Stable Isotopic Analysis of Martian Meteorites: We performed thee the first ion microprobe measurements of carbon isotopes in the ALH 84001 meteorite and established strong constraints on previously proposed formation models (Niles et al. 2005). In addition these analyses provide a window into the carbon chemistry on Mars indicating a complex relationship between the carbonates and different carbon reservoirs including the atmosphere. We have also collaborated with Marc Thiemens at UCSD to perform detailed Δ17Ο measurements in ALH 84001 carbonates to estimate the δ13C composition of the ancient martian atmosphere (Shaheen et al. 2015).
Measurements of Martian Atmospheric CO2: While working on the Phoenix Mars lander mission we measured the stable isotopic composition of the martian atmosphere using the magnetic sector mass spectrometer in the TEGA payload, which was published in Science (Niles et al. 2010). The values of δ13C and δ18 Ο measured by our study were -2.5‰ and +31.0‰ respectively which are broadly similar to the values of CO2 in the terrestrial atmosphere. This indicates that the CO2 is likely in equilibrium with liquid water and that volcanic degassing and carbonate formation have been important and ongoing processes on Mars. This conclusion is also consistent with results from the martian meteorites which now come into much clearer focus with the data from the modern atmosphere. We have also been acting as science team member on the Mars Science Laboratory where we have collaborated on similar measurements of martian CO2 (Webster et al. 2013).
Orbital Measurements of Mars: In collaboration with Joe Michalski we found layered phyllosilicate-carbonate rich rocks in the central peak of a 60 km wide impact crater near the Syrtis Major volcanic complex (Michalski and Niles 2010). We interpret these layered rocks to come from 5-6 km depth and thus indicate the presence of a subsurface hydrothermal environment on Mars. This is the first discovery of a subsurface hydrothermal environment on Mars which has strong astrobiological implications because subsurface environments may provide protection from the very cold, oxidizing conditions at the surface.
Chemical Modeling: Equilibrium thermodynamic modeling is a useful technique to apply to aqueous geochemical problems but it is tempered by the fact that many of the geochemical environments we are interested in are typically low temperature and dominated by kinetic factors - a famous problem with carbonate chemistry. Thus, we have utilized equilibrium thermodynamic modeling in creative ways to address geochemical problems. We performed a study where we looked at the formation of Mg-,Fe-rich solutions through interaction with CO2-rich fluids and ultramafic rocks (Niles et al. 2009). This focused on the formation of Mg- and Fe-rich fluids in the context of the ALH 84001 carbonates using the unique chemical compositions of the carbonates to constrain the problem and thermodynamic modeling to show that lower temperature (< 100°C) CO2-rich fluids were necessary to form the ALH 84001 carbonates.
Field Work, Laboratory Work and Terrestrial Analogs: We also address the formation of carbonates in extreme environments, focusing on the large micro-scale isotopic variations created by kinetic processes such as freezing or evaporation. Rapid degassing of CO2 from aqueous solutions can create large kinetic isotope enrichments in carbon isotopes and smaller variations in oxygen isotopes. These have been well studied in cave environments on Earth, but also play an important role in forming small carbonate crusts in desert and polar environments. Our research group has collected samples in Arizona and obtained samples from the Canadian Arctic to study micro-scale stable isotope variations in these crusts and caliche deposits. Additionally we have performed laboratory experiments which synthesized carbonate minerals through freezing processes under martian conditions. These carbonates also show strong kinetic enrichments in carbon and oxygen isotopes. A substantial portion of this work was performed by undergraduate interns working through the Lunar Planetary Institute Summer Intern Program. All of this work has been outlined in a series of abstracts (Socki et al. 2009; Fu and Niles 2010; Socki et al. 2010; Volk et al. 2011).
Martian Rover Data: We have formulated an alternative explanation for the formation of the sediments found on Mars at Meridiani Planum by the Opportunity rover (Niles and Michalski 2009). This hypothesis suggests that the sediments are a lag deposit from a massive ice-dust deposit formed during obliquity variations early in Mars' history. Thus, the sulfate-rich sediments found by the Opportunity rover were weathered inside massive deposits of ice, atmospheric dust, and volcanic aerosols. When the climate of Mars changed and the ice sublimed away, it left behind a lag deposit of highly hydrated, sulfur-rich material. This mechanism could be the primary mechanism for forming sulfates on Mars.
Laboratory Experiments: We have begun a set of experiments to better understand the geochemistry of sulfuric-acid brines in ice deposits. We have examined the weathering rates of olivine at temperatures below 0°C down to -50°C in sulfuric-acid brines (Worsham et al. 2010). The influence of atmospheric chemistry on the chemistry of the sediments of Mars will be a major growth area in martian geochemistry, and our long term research plans are to develop an 17Ο measurement capability to better understand this weathering process. Careful isolation and measurement of the 17Ο composition of materials from martian meteorites, laboratory experiments, and terrestrial samples will provide substantial insight into weathering processes in the martian atmosphere.
Field Work: Hydrated sulfate minerals have recently been discovered on the planet Mars. Of particular interest within the context of these discoveries is the role water and/or cryogenic fluids played during their formation. Resent research we have embarked on includes completing a series of analyses of the stable isotope composition of a variety of samples collected from evaporite mounds and associated moraine materials from the Lewis Cliff Ice Tongue, Antarctica. These evaporite mounds consist almost entirely of two related Na-sulfate minerals, mirabilite (Na2SO4.10H2O) and thenardite (Na2SO4). In addition to the sulfate minerals we also collected liquid water from several shallow moraine lakes, glacial ice, secondary glacial ice (ice lenses), and precipitation (snow). Isotope analyses of these samples include δ18Ο and δD of precipitation, ice, and lake water, and δ34S, δ18Ο and Δ17Ο of the sulfates. Our data suggest that secondary glacial ice and moraine lake water influenced the growth of these evaporite minerals and mineral formation likely occurred sub-glacially (Socki et al. 2009).
Laboratory Experiments: We have been exploring reaction pathways for the abiotic synthesis of hydrocarbons through hydrothermal experiments at high temperatures (600°-800°C) and pressures (0.5 - 2 GPa) to understand what effects pressure and catalysts have on abiotic synthesis of hydrocarbons. We have found that one criteria used for distinguishing the abiogenic origin of hydrocarbons, the inverse correlation of δ13C and δ2H values with increasing molecular mass of n-alkanes, is in fact reversed at higher pressures. We think this is due to differences in the reaction pathway and intermediaries under different experimental conditions (Fu and Niles 2010). We utilize an innovative analytical technique by connecting a Pyroprobe to a GC-MS/GC-IRMS to perform compound specific isotope measurements of gases and solids (Socki et al. 2011). This allows us to examine intermediate compounds left behind on the surfaces of the catalyst minerals.
The objective of this work was to utilize the unique set of analytical tools on the MSL lander to assess carbon bearing species and better understand Mars climate evolution, the nature of ancient martian aqueous systems, and the potential for supporting life on Mars. Data analysis continues as we work on laboratory analogs to understand chemical and isotopic measurements from the lander.