Laboratory Studies of Heterogeneous Processes Relevant to Mars

Heterogeneous (gas-surface) processes may play an important role in both the atmospheric and surface chemistry of Mars. Atmospheric species may be affected by the chemistry and physical properties of the planetary surface and the surface material may be affected by the components and properties of the atmosphere. In this thesis, several laboratory studies are described which experimentally investigate two types of atmosphere-surface systems likely to exist on Mars. First, experiments were performed to better understand the spatial and temporal variability of atmospheric methane (CH4) on Mars. Reported CH4 plumes in the atmosphere of Mars are difficult to explain using known chemical or physical processes. The observations imply a strong, present-day source and also a rapid yet unknown CH4 sink. We have investigated the potential role of mineral dust in CH4 variability. First, using a Knudsen cell capable of simulating Martian temperature and pressure conditions, we have studied the adsorption of CH4 to a Martian mineral analog as a function of temperature. An uptake coefficient was determined and then applied to the Martian surface-atmosphere system. Our results suggest that adsorption to soil grains could possibly affect the CH4 mixing ratio on a seasonal time scale especially at mid-latitude regions. Additionally, chemical oxidation of CH4 by oxidants thought to exist in the Martian regolith was studied. The Viking mission in the 1970’s found Martian soil was able to oxidize complex organic compounds to CO2. The identity of the oxidant is unknown, but has been proposed to be either hydrogen peroxide or perchlorate salts. We used a gas chromatograph to determine if simulated Mars soil containing these oxidants was able to oxidize CH4 to CO2. However, no CH4 was oxidized within the detection limit of the instrument and only an upper limit reaction coefficient could be reported. Even these upper limit values suggest CH4 could not be removed from the Martian atmosphere rapidly enough to cause variability. We have also studied the interactions of another important trace gas on Mars, water vapor, with perchlorate, a highly deliquescent salt recently discovered in polar soil. A Raman microscope equipped with an environmental cell was used to study phase transitions of the salts. The relative humidity (RH) at which deliquescence (absorption of water vapor by the solid to become an aqueous solution) and efflorescence (crystallization of the aqueous solution) occur were determined as a function of temperature, hydration state and associated cation. We show that the deliquescence RH for perchlorate salts can be low (~40% RH for anhydrous sodium perchlorate, for example). Thermodynamics can predict deliquescence; however, the kinetic inhibition of crystallization causes efflorescence to occur at much lower RH values than deliquescence which allows supersaturated salt solutions to exist in a metastable state. Based on the diurnal RH and temperature cycles on Mars, aqueous solutions could be stable or metastable for several hours a day at the Phoenix landing site. The astrobiological implications of potential liquid H2O on Mars are significant

Constraining the Potential Liquid Water Environment at Gale Crater, Mars

The Mars Science Laboratory (MSL) Rover Environmental Monitoring Station (REMS) has now made continuous in situ meteorological measurements for several Martian years at Gale crater, Mars. Of importance in the search for liquid formation are REMS’ measurements of ground temperature and inâ air measurements of temperature and relative humidity, which is with respect to ice. Such data can constrain the surface and subsurface stability of brines. Here we use updated calibrations to REMS data and consistent relative humidity comparisons (i.e., with respect to liquid versus with respect to ice) to investigate the potential formation of surface and subsurface liquids throughout MSL’s traverse. We specifically study the potential for the deliquescence of calcium perchlorate. Our data analysis suggests that surface brine formation is not favored within the first 1648 sols as there are only two times (sols 1232 and 1311) when humidityâ temperature conditions were within error consistent with a liquid phase. On the other hand, modeling of the subsurface environment would support brine production in the shallow subsurface. Indeed, we find that the shallow subsurface for terrains with low thermal inertia (Î â ²300 J mâ 2 Kâ 1 sâ 1/2) may be occasionally favorable to brine formation through deliquescence. Terrains with Î â ²175 J mâ 2 Kâ 1 sâ 1/2 and albedos of â ³0.25 are the most apt to subsurface brine formation. Should brines form, they would occur around Ls 100°. Their predicted properties would not meet the Special nor Uncertain Region requirements, as such they would not be potential habitable environments to life as we know it.Plain Language SummaryThe Mars Science Laboratory (MSL) has now made continuous measurements of the local weather at Gale crater, Mars. Such measurements can help guide our search for the formation of liquid water on presentâ day Mars. Specifically, when the right temperature and humidity conditions are met, certain salts can take in water vapor from the atmosphere to produce liquids. Here we use data from MSL along with experimental results on the stability of a Marsâ relevant salt to search for time periods when liquids could potentially form at the surface. Additionally, we use simulations and MSL data to understand the potential to form such liquids in the subsurface. Our results suggest that surface formation of liquids is unlikely throughout MSL’s travels; however, the shallow subsurface may experience conditions that would allow for liquid formation. Not much liquid would form, though, and the properties of these liquids would not permit life as we know it to persist.Key PointsMeasured surface environmental conditions at Gale crater are not favorable to brine formation via deliquescence of calcium perchlorateLiquids may have formed in the shallow subsurface of low thermal inertia units within MSLâ traversed terrainsMSL may best find liquids in the subsurface of units with thermal inertia less than or equal to 175 J mâ 2 Kâ 1 sâ 1/2 and albedo > 0.25 around Ls 100°Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144585/1/jgre20830-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144585/2/jgre20830_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144585/3/jgre20830.pd

Changes in Soil Cohesion due to Water Vapor Exchange: A Proposed Dry-flow Trigger Mechanism for Recurring Slope Lineae on Mars

This file contains the modeling and experimental datasets to accompany the paper "Changes in Soil Cohesion due to Water Vapor Exchange: A Proposed Dry-flow Trigger Mechanism for Recurring Slope Lineae on Mars" currently in review at Geophysical Research Letters. The model datasets are MarsWRF diurnal simulations of the near-surface and surface conditions at Ls = 180 and Ls = 240 degrees at Hale Crater, Mars. The experimental datasets are Raman spectra and microscope images of lab simulations of these same seasons.</p

Electronic repository of raw data and calculated values from the article "Laboratory studies of salt deliquescence kinetics relevant to Mars"

This dataset contains the data associated with the manuscript titled "Laboratory studies of salt deliquescence kinetics relevant to Mars" submited to the Planetary Science Journal. This dataset contains the raw microscope image files collected during experiments, associated experimental conditions that were present when each image file was collected, and calculated kinetic parameters obtained from the image analyses. Each experiment's images are contained in a zip file. &nbsp;</p

Thermal Decomposition Kinetics of the Thermally Stable Jet Fuels JP-7, JP-TS and JP-900

The thermal decomposition kinetics of JP-7, JP-TS and JP-900 were studied, motivated by the need of the hypersonic vehicle community for a fuel that has a high degree of thermal stability. Decomposition reactions were performed at 375, 400, 425 and 450 °C in stainless-steel ampule reactors. In all cases, the pressure before decomposition was 34.5 MPa (5000 psi). Decomposition as a function of time at each temperature was quantified by analyzing the thermally stressed liquid phase using gas chromatography. These results were used to determine global first-order rate constants that approximate the overall decomposition rate of of each fuel. For JP-7, these first-order rate constants ranged from 1.79 × 10^–5 s^–1 at 375 °C to 3.02 × 10^–4 s^–1 at 450 °C. For JP-TS, the rate constants had values between 1.74 × 10^–5 s^–1 at 375 °C to 2.70 × 10^–4 s^–1 at 450 °C. For JP-900, the rate constants ranged from 1.03 × 10^–5 s^–1 at 375 °C to 3.60 × 10^–4 s^–1 at 450 °C. At all temperatures studied, these three fuels have similar rate constants for thermal decomposition; with only one exception, the values of <i>k</i>′ are identical within the combined uncertainty. The rate constants for the decomposition of RP-2, a fuel being considered as a replacement fuel for hypersonic vehicles, are similar in the temperature range studied. Considering the time needed for 1% of the sample to decompose (<i>t</i>_0.01), we find that required instrument residence times range from 16 min at 375 °C to 30 s at 450 °C. The rate constants measured here, as well as the Arrhenius parameters that we calculate, can be used to design and plan physical property measurements at additional temperatures

Characterization of the Effects of Cetane Number Improvers on Diesel Fuel Volatility by Use of the Advanced Distillation Curve Method

The cetane number (CN) is a measure of the ignition quality of a fuel for compression-ignition engines according to the self-ignition delay. If the CN of a fuel is too low, chemical compounds known as CN improvers may be added to increase both the CN and performance of the fuel. The addition of CN improvers is dependent upon the detailed properties of the particular fuel. While many fuel properties are important for design, the vapor–liquid equilibrium, as described by volatility, is very sensitive to composition. In this work, we measured blends of diesel fuel with the following CN improvers: amyl nitrate, isoamyl nitrate, isoamyl nitrite, 2-ethylhexyl nitrate, and the multi-component CN improver PM-1, in diesel fuel by use of the advanced distillation curve (ADC) method to determine the amount of CN improver in the various distillate volume fractions. Tracking the CN improver throughout the volatility profile of diesel fuels provides valuable information for determining structural property relationships, and moreover, it provides the basis for the development of equations of state that can describe the thermodynamic properties of these complex mixtures, with specific attention paid to additives. We have found that the addition of CN improvers significantly decreases the temperature at which boiling begins and that the majority of the CN improver is thermolytically degraded before the first drop can be collected. These observations are supported by low-pressure ADC, where the CN improver was found in fractions up to 30%. These results have implications in the prediction of thermophysical properties of diesel fuel with CN improvers

Thermal Decomposition Kinetics of Polyol Ester Lubricants

Synthetic lubricants are widely used for applications that require high thermal and oxidative stability. In order to facilitate new designs and applications for these fluids, we are measuring a suite of thermophysical and transport properties for lubricant base fluids and mixtures. As part of the property measurements, here, we report the global thermal decomposition kinetics of four polyol ester lubricant base oils, in addition to a fully qualified (MIL-PRF-23699) formulation. The fluids were heated in stainless steel ampule reactors and the extent of decomposition was measured by gas chromatography coupled with flame ionization detection (GC-FID), from which pseudo-first-order rate constants were derived. The rate constants for decomposition ranged from 1 × 10^–8 s^–1 at 500 K to 2 × 10^–4 s^–1 at 675 K. Arrhenius parameters across this temperature regime are also reported. Other techniques for chemical characterization applied in this work include gas chromatography with mass spectrometry (GC-MS), nuclear magnetic resonance (NMR) spectroscopy, and Karl Fischer titration

Laboratory Studies of Brine Growth Kinetics Relevant to Deliquescence on Mars

Although previous studies have shown that the near-surface environmental conditions on Mars may permit salt deliquescence and therefore brine production, there is significant uncertainty in the kinetics of the process. Indeed, experimental studies have shown that deliquescence is either very rapid or too slow to be relevant to Mars. To resolve this uncertainty, we performed laboratory experiments to investigate the growth rate of Mars-relevant calcium perchlorate brines over a range of temperatures (184–273 K) and water vapor pressures (0.2–220 Pa). We show that the brine growth is faster at higher water vapor pressures and lower temperatures and for smaller particles. From our data, we determined a temperature-dependent net uptake coefficient for gas phase water molecules colliding with a perchlorate brine surface in the range of 3.8 × 10 ^−4 at 185 K to 4.2 × 10 ^−6 at 273 K. These values suggest that deliquescence on Mars is likely to be slow even when conditions thermodynamically permit a brine to form. We find that along the Curiosity rover traverse at Gale Crater, the near-surface conditions would only allow particles <1 μ m to fully deliquesce over a typical sol. At the higher-latitude Phoenix landing site, deliquescence may be 30% faster due to the higher water vapor pressures, but still, only micron-scale salt grains or coatings would be expected to deliquesce during a typical sol. These results suggest that brines formed via deliquescence on the surface of Mars are likely only present on small scales that may not be readily detected using conductivity or imaging techniques