Detection of organic carbon in Mars-analog paleosols with thermal and evolved gas analysis

Published: 17 December 2021| Version 1 | DOI: 10.17632/bkvcff9dw8.1
Contributor:
Adrian Broz

Description

This study examined the organic carbon content of ~30 million Mars-relevant paleosols with an instrument configured to operate like the Sample Analysis at Mars - Evolved Gas Analysis (SAM-EGA) instrument onboard Curiosity Mars Rover. The files in this data set include the raw TG-DSC-EGA data generated with two instruments, and serve as supplemental materials to the corresponding manuscript. A Setaram Labsys Evo differential scanning calorimeter (DSC) / thermal gravimeter (TG) connected to a Pfeiffer Omnistar quadrupole mass spectrometer (QMS) was configured to operate similarly to the SAM evolved gas analyzer. The SAM instrument does not have TG/DSC capabilities, but these components permit a better understanding of phase transitions and chemical reactions in laboratory experiments. Approximately 50 mg ± 3 mg of ground paleosol sample were placed in an Al2O3 sample crucible. The sample crucible and an identical empty reference crucible were placed in the furnace and then the system was purged twice with helium gas and set to a pressure of 30 mbar. Helium was chosen as a carrier gas because it is inert and because it used as a carrier gas in the SAM instrument. The crucibles were heated from approximately 35 °C to 1000 °C at a heating rate of 35°C/min and at a flow rate of 10 sccm. Volatiles ranging from mass/charge (m/z) 1 - 100 were measured. Next, total organic carbon (TOC) content was determined using a Netzsch TG/DSC coupled to a Pfeiffer QMS. An Al2O3 sample crucible and an identical reference crucible were placed in the furnace. The instrument was purged twice with ultra-high purity O2 and set to a pressure of 1000 mbar prior to sample analyses to remove any contamination in the system. Oxygen was chosen as a carrier gas because it encourages complete combustion of all organic and inorganic carbon in samples. The crucibles containing samples were heated from approximately 35 °C to 1000 °C at a heating rate of 35°C/min and at a flow rate of 19 ml O2/min. A series of three blanks were analyzed before and after each group (n=10) of samples. A calibration curve for CO2 was created by analyzing a calcite standard (Iceland sparry calcite 40 ųM) at eight sample masses ranging from 0.01 – 4 mg. This calibration curve was used to calculate the amount of CO2 evolved from each sample, and these values were used to calculate total carbon in each sample. Carbon was considered organic between 150-550° C and inorganic from ~700-900° C. Total organic carbon was quantified by deconvolving CO2 peaks if a carbonate-C peak was present. This was done by determining the relative percentage of peak area from inorganic carbon-evolved CO2 (~700-900° C) then subtracting this value from total carbon-evolved CO2 peak area to solve for TOC. All evolved gas plots were background-corrected to account for possible atmospheric contamination.

Files

Steps to reproduce

Data were imported into Igor Pro 7.0 and background-corrected to account for atmospheric composition. All EGA plots were generated in Igor. CO2 peak area was calculated by importing data into Origin Pro. Total organic carbon was quantified by deconvolving CO2 peaks if a carbonate-C peak was present. This was done by determining the relative percentage of peak area from inorganic carbon-evolved CO2 (~700-900° C) then subtracting this value from total carbon-evolved CO2 peak area to solve for TOC.

Institutions

NASA Johnson Space Center, Jacobs Engineering Technology and Science Group, University of Oregon, Purdue University

Categories

Planetary Science, Thermal Analysis, Paleosols, Mars, Robotic Planet Rover, Soil Organic Carbon

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