Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder

Raina Gough

Current Research

Mars is known to have abundant H2O in the gas (water vapor) or solid (water ice) phases, but until very recently it was not thought that liquid water could exist on Mars. In the past few years, however, orbiting spacecraft and rovers on Mars have collected data suggesting that liquid water may exist on the surface or in the shallow subsurface for at least some portion of the Martian year. Pure liquid water is not thought to be stable due to the low pressures and temperatures found on the surface of Mars; however, salts such as perchlorates and chlorides are present in the Martian soil and may play a key role in stabilizing liquid water.

In the Tolbert group, we experimentally study the formation and stability of liquid (aqueous) salt solutions under low temperature conditions relevant to Mars. We are particularly interested in deliquescence, which is a relative humidity-induced solid-to-aqueous phase transition. A particle deliquesces by absorbing water vapor from the atmosphere when a threshold relative humidity value is reached. This deliquescence relative humidity (DRH) varies with salt composition as well as temperature. The reverse process, the recrystallization of a salt solution into the solid phase, is called efflorescence and occurs at the efflorescence relative humidity (ERH). Kinetic effects inhibit nucleation of a crystalline phase and cause the ERH to be lower than the DRH. This hysteresis effect allows metastable, supersaturated solutions to exist when the humidity is below the DRH but above the ERH. Our experiments use Raman microscopy (see schematic below) to determine the DRH and ERH values of Mars relevant salts and salt mixtures under a range of low temperature conditions. Knowledge of these values is necessary to predict when and where aqueous salt solutions could exist on the surface of Mars.

Thus far, we have found that perchlorate salts are highly deliquescent, absorbing water vapor and forming an aqueous solution at very low DRH values. Sodium perchlorate (NaClO4) is particularly deliquescent (DRH values are as low as 40%) and this salt can exist in a metastable, supersaturated aqueous phase until the ERH of 13% is reached. We have shown that NaClO4, if present on the Martian surface, would likely be in the aqueous (liquid water) phase for 2 to 3 hours per sol (a Martian day). Currently, we are exploring the effect of additional ionic species on the DRH and ERH values.

Schematic of the Raman microscope used to determine the DRH and ERH of Mars relevant salts. Conditions inside the sample cell can be varied from 0 to 100% RH and from 223 to 273 K.

We can determine the phase of a salt particle and thus determine the DRH and ERH of that salt by using Raman spectroscopy and optical microscopy (inset images). Figures A, B and C show a NaClO4 particle deliquescing at 243 K. Figures D, E and F show a NaClO4 particle efflorescing at 243 K. Each image is 40 µm wide. As the RH in the environmental cell increases and decreases, spectral changes appear in the O-H stretching vibration region (broad peak at ~3500 cm-1) and the ClO4- symmetric vibration region (~950 cm-1). Visually, the particle is spherical and darker when liquid (deliquesced) and less spherical and lighter when crystalline (effloresced).

Sodium perchlorate (NaClO4) salts could be deliquesced (i.e.: exist in the aqueous phase) for 2 to 3 hours per day! This figure shows a modeled RH (solid line) and surface temperature (dashed line) as a function of time of day on Mars at the Viking 1 landing site. We use our experimental data to predict when aqueous NaClO4 solution can exist. In the late evening and early morning, ice will exist. In the warm, dry afternoon, the crystalline salt (likely the anhydrous form) will be present. For two periods of time in this particular Martian day, during the late morning and then again during the evening, either stable or metastable aqueous perchlorate solutions can exist (blue shaded regions).


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  • Gough RV; Nuding DL; Martínez GM; Rivera-Valentín EG; Primm KM; Tolbert MA. (Mar 2023). Laboratory Studies of Brine Growth Kinetics Relevant to Deliquescence on Mars. , 4(3), 46-46. 10.3847/psj/acbd98
  • Fernanders MS; Gough R; Chevrier VF; Schiffman ZR; Ushijima SB; Martinez GM; Rivera-Valentin EG; Archer PD; Clark J; Sutter B. (Jan 2022). Water uptake by chlorate salts under Mars-relevant conditions. Icarus , 371. 10.1016/j.icarus.2021.114715
  • Ushijima SB; Gough R; Tolbert MA. (Nov 2020). Probing Heterogeneous Efflorescence of Mars-Relevant Salts with an Optical Levitator. ACS Earth and Space Chemistry , 4(11), 1947-1956. 10.1021/acsearthspacechem.0c00161
  • Gough R; Nuding DL; Archer PD; Fernanders MS; Guzewich SD; Tolbert MA; Toigo AD. (Jun 2020). Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry-Flow Trigger Mechanism for Recurring Slope Lineae on Mars. Geophysical Research Letters , 47(11). 10.1029/2020GL087618
  • Meyer, M; Bakermans, C; Beaty, D; Bernard, D; Boston, P; Chevrier, V; Conley, C; Feustel, I; Gough, R; Glotch, T; Hays, L; Junge, K; Lindberg, R; Mellon, M; Mischna, M; Neal, CR; Pugel, B; Quinn, R; Raulin, F; Renno, N; Rummel, J; Schulte, M; Spry, A; Stabekis, P; Wang, A; Yee, N. (Nov 2019). Report of the Joint Workshop on Induced Special Regions. LIFE SCIENCES IN SPACE RESEARCH , 23. 10.1016/j.lssr.2019.09.002
  • Moores JE; Gough R; Martinez GM; Meslin P-Y; Smith CL; Atreya SK; Mahaffy PR; Newman CE; Webster CR. (May 2019). Methane seasonal cycle at Gale Crater on Mars consistent with regolith adsorption and diffusion. Nature Geoscience , 12(5), 321-+. 10.1038/s41561-019-0313-y
  • Gough, RV; Primm, KM; Rivera-Valentin, EG; Martinez, GM; Tolbert, MA. (Mar 2019). Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations. ICARUS , 321. 10.1016/j.icarus.2018.10.034
  • Primm, KM, V Gough, J Wong, G Rivera-Valentin, GM Martinez, JV Hogancamp, PD Archer, DW Ming and MA Tolbert. (Aug 2018). The Effect of Mars-Relevant Soil Analogs on the Water Uptake of Magnesium Perchlorate and Implications for the Near-Surface of Mars. JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS , 123(8). 10.1029/2018JE005540
  • Webster, CR, PR Mahaffy, SK Atreya, JE Moores, GJ Flesch, C Malespin, CP Mckay, G Martinez, CL Smith, J Martin-Torres, J Gomez-Elvira, MP Zorzano, MH Wong, MG Trainer, A Steele, D Archer, B Sutter, PJ Coll, C Freissinet, PY Meslin, RV Gough, CH House, A Pavlov, JL Eigenbrode, DP Glavin, JC Pearson, D Keymeulen, LE Christensen, SP Schwenzer, R Navarro-Gonzalez, J Pla-Garcia, SCR Rafkin, A Vicente-Retortillo, H Kahanpaa, D Viudez-Moreiras, MD Smith, AM Harri, M Genzer, DM Hassler, M Lemmon, J Crisp, SP Sander, RW Zurek and AR Vasavada. (Jun 2018). Background levels of methane in Mars atmosphere show strong seasonal variations. SCIENCE , 360(6393). 10.1126/science.aaq0131
  • Rivera-Valentin, EG, RV Gough, VF Chevrier, KM Primm, GM Martinez and M Tolbert. (May 2018). Constraining the Potential Liquid Water Environment at Gale Crater, Mars. JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS , 123(5). 10.1002/2018JE005558
  • Gough, RV, J Wong, JL Dickson, JS Levy, JW Head, DR Marchant and MA Tolbert. (Oct 2017). Brine formation via deliquescence by salts found near Don Juan Pond, Antarctica: Laboratory experiments and field observational results. EARTH AND PLANETARY SCIENCE LETTERS , 476. 10.1016/j.epsl.2017.08.003
  • Nuding, DL, RV Gough, KJ Venkateswaran, JA Spry and MA Tolbert. (Oct 2017). Laboratory Investigations on the Survival of Bacillus subtilis Spores in Deliquescent Salt Mars Analog Environments. ASTROBIOLOGY , 17(10). 10.1089/ast.2016.1545
  • Primm, KM, RV Gough, VF Chevrier and MA Tolbert. (Sep 2017). Freezing of perchlorate and chloride brines under Mars-relevant conditions. GEOCHIMICA ET COSMOCHIMICA ACTA , 212. 10.1016/j.gca.2017.06.012
  • Urness KN; Gough RV; Widegren JA; Bruno TJ. (Dec 2016). Thermal Decomposition Kinetics of Polyol Ester Lubricants. Energy and Fuels , 30(12), 10161-10170. 10.1021/acs.energyfuels.6b01863
  • Gough, RV, VF Chevrier and MA Tolbert. (Oct 2016). Formation of liquid water at low temperatures via the deliquescence of calcium chloride: Implications for Antarctica and Mars. PLANETARY AND SPACE SCIENCE , 131. 10.1016/j.pss.2016.07.006
  • Nuding, DL, RD Davis, RV Gough and MA Tolbert. (Mar 2015). The aqueous stability of a Mars salt analog: Instant Mars. JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS , 120(3). 10.1002/2014JE004722
  • Nuding, DL, EG Rivera-Valentin, RD Davis, RV Gough, VF Chevrier and MA Tolbert. (Nov 2014). Deliquescence and efflorescence of calcium perchlorate: An investigation of stable aqueous solutions relevant to Mars. ICARUS , 243. 10.1016/j.icarus.2014.08.036
  • Rummel JD; Beaty DW; Jones MA; Bakermans C; Barlow NG; Boston PJ; Chevrier VF; Clark BC; de Vera J-PP; Gough RV. (Nov 2014). A New Analysis of Mars "Special Regions": Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2). Astrobiology , 14(11), 887-968. 10.1089/ast.2014.1227
  • Gough RV; Widegren JA; Bruno TJ. (May 2014). Thermal Decomposition Kinetics of the Thermally Stable Jet Fuels JP-7, JP-TS and JP-900. Energy and Fuels , 28(5), 3036-3042. 10.1021/ef500338n
  • Gough, RV, VF Chevrier and MA Tolbert. (May 2014). Formation of aqueous solutions on Mars via deliquescence of chloride-perchlorate binary mixtures. EARTH AND PLANETARY SCIENCE LETTERS , 393. 10.1016/j.epsl.2014.02.002
  • Burger JL; Lovestead TM; Gough RV; Bruno TJ. (Apr 2014). Characterization of the Effects of Cetane Number Improvers on Diesel Fuel Volatility by Use of the Advanced Distillation Curve Method. Energy and Fuels , 28(4), 2437-2445. 10.1021/ef5000547
  • Gough RV; Chevrier VF; Baustian KJ; Wise ME; Tolbert MA. (Feb 2014). Laboratory studies of perchlorate phase transitions: Support for metastable aqueous perchlorate solutions on Mars (vol 312, pg 371, 2011). Earth and Planetary Science Letters , 387, 169-169. 10.1016/j.epsl.2013.11.042
  • Harvey BG; Meylemans HA; Gough RV; Quintana RL; Garrison MD; Bruno TJ. (Jan 2014). High-density biosynthetic fuels: the intersection of heterogeneous catalysis and metabolic engineering. Physical Chemistry Chemical Physics , 16(20), 9448-9457. 10.1039/c3cp55349c
  • Gough RV; Widegren JA; Bruno TJ. (Jun 2013). Thermal Decomposition Kinetics of 1,3,5-Triisopropylcyclohexane. Industrial and Engineering Chemistry Research , 52(24), 8200-8205. 10.1021/ie400903z
  • Burger JL; Gough RV; Bruno TJ. (Feb 2013). Characterization of Dieseline with the Advanced Distillation Curve Method: Hydrocarbon Classification and Enthalpy of Combustion. Energy and Fuels , 27(2), 787-795. 10.1021/ef301692p
  • Gough RV; Bruno TJ. (Jan 2013). Composition-Explicit Distillation Curves of Alternative Turbine Fuels. Energy and Fuels , 27(1), 294-302. 10.1021/ef3016848
  • Gough, RV, VF Chevrier, KJ Baustian, ME Wise and MA Tolbert. (Dec 2011). Laboratory studies of perchlorate phase transitions: Support for metastable aqueous perchlorate solutions on Mars. EARTH AND PLANETARY SCIENCE LETTERS , 312(4-Mar). 10.1016/j.epsl.2011.10.026
  • Gough, RV, JJ Turley, GR Ferrell, KE Cordova, SE Wood, DO DeHaan, CP Mckay, OB Toon and MA Tolbert. (Feb 2011). Can rapid loss and high variability of Martian methane be explained by surface H2O2?. PLANETARY AND SPACE SCIENCE , 59(3-Feb). 10.1016/j.pss.2010.09.018
  • Meslin, PY, R Gough, F Lefevre and F Forget. (Feb 2011). Little variability of methane on Mars induced by adsorption in the regolith. PLANETARY AND SPACE SCIENCE , 59(3-Feb). 10.1016/j.pss.2010.09.022
  • Gough, RV, MA Tolbert, CP McKay and OB Toon. (May 2010). Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere. ICARUS , 207(1). 10.1016/j.icarus.2009.11.030
  • Hatch, CD, RV Gough, OB Toon and MA Tolbert. (Jan 2008). Heterogeneous nucleation of nitric acid trihydrate on clay minerals: Relevance to Type Ia polar stratospheric clouds. JOURNAL OF PHYSICAL CHEMISTRY B , 112(2). 10.1021/jp075828n
  • Hatch, CD, RV Gough and MA Tolbert. (Jan 2007). Heterogeneous uptake of the C-1 to C-4 organic acids on a swelling clay mineral. ATMOSPHERIC CHEMISTRY AND PHYSICS , 7(16). 10.5194/acp-7-4445-2007