RATES of Al-, Fe-, Mg-, Ca-SULFATES DEHYDRATION UNDER MARS RELEVANT CONDITIONS. Alian Wang, Yuhang Zhou, Dept of Earth and Plan

45th Lunar and Planetary Science Conference (2014) 2614.pdf

RATES OF Al-, Fe-, Mg-, Ca-SULFATES DEHYDRATION UNDER MARS RELEVANT CONDITIONS. Alian Wang, Yuhang Zhou, Dept of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA ([email protected]).

Introduction: Hydrous sulfates are one of the two same grain size range (90-150 µm), whose ID and ho- major types of secondary minerals (the other type is mogeneities were checked by XRD and multi-spot phyllosilicates) that have been found on the surface of laser Raman spectroscopy measurements. Mars, in large quantities and with wide spreading. The dehydration experiments were conducted by Among them, Ca-, Mg-, Fe-sulfates, and recently Al- placing the reaction vials in a vacuumed desiccator. sulfates, have all been identified. The degrees of hy- The atmospheric pressure was maintained by a vacuum dration of these sulfates at the surface and subsurface pump and monitored by a gas-pressure meter. Pressure on Mars would be heavily influenced by environmental from 0.26 to 0.16 mbar was reached during the pro- conditions, which would be subjective to the diurnal gress of dehydration experiments which are lower than and seasonal cycles, and in the long term, to the cycles the average atmospheric pressure on Mars (~ 7 mbar). of Mars obliquity. During MER, MSL, and Mars 2020 The partial H 2O pressure (P H2O ) in vacuum desiccator missions, to determine the hydration degrees of sul- was in the range of 0.25 – 0.14 Pa, near the reported fates and other salts will shed light on the current water PH2O on Mars ~ 0.04 – 0.15 Pa [4]. The dehydration reservoir and the hydrologic evolution of Mars. Both experiments were conducted at three temperatures: 21 of these aspects also relate to the potential generation ± 1°C, 0 ± 0.5 °C, and at -12 ± 2 °C. The temperatures of Martian life (if that ever happened) and the preser- during the experimental periods are recorded by a T- vation of biosignatures. RH-data logger. This study reports two sets of dehydration experi- Depending on the selected temperature, the total ments conducted on crystalline alunogen durations of dehydration experiments vary from 26 Al 2(SO 4)3•17H 2O and melanterite FeSO 4•7H 2O under hours to over 200 hours. During each experiment, 10 Mars relevant pressure (P) and partial H 2O pressure reaction vials (among the total of 15) were taken out (P H2O ) at three temperatures (T). These results are periodically from the vacuumed desiccator at pre- compared with those obtained from a similar set of determined time interval (i.e., 20, 40, 60 min, then 2 h, experiments on crystalline epsomite MgSO 4•7H 2O[1], 4 h, 8 h, 15 h, 30 h, etc), sealed immediately, then to and with published data of Ca-sulfate [2] and Fe 3+ - make gravimetric measurements to monitor the change sulfate [3]. These comparisons revealed the differences in hydration degree. The rest of 5 vials were taken out in the phase transition pathways, and especially in the at larger time intervals (i.e. 40 min, 4 h, 15 h, 30 h, dehydration rates, of these hydrous sulfates under Mars etc), sealed immediately, then to make LRS measure- relevant conditions. ments to monitor the structural changes. Laser Raman Spectroscopy (LRS) has been the ma- Result#1—T-dependent: Comparison among the jor tool that we used to monitor the changes of hydra- dehydration processes of alunogen, melanterite, and tion degrees of these epsomite under Mars relevant five types of major sul- conditions, revealed that they fates. Figure 1 shows are all strongly dependents on the finger-print Raman temperature. spectra of five sulfates Result#2 – dehydration path- with difference cation ways: the dehydration pathways but the highest hydra- and the ending hydration de- tion degrees. grees of three sulfates are very We found that the different, which obviously re- dehydrations of Mg- flect the differences in funda- sulfate and Fe 2+ -sulfate mental compositional and struc- would progress the tural properties of these three fastest on Mars. sulfates. These properties would Dehydration Experi- affect the activation energy val- ments under Mars ues for phase transitions. The relevant conditions: dehydration of melanterite went Both starting sulfates, through rozenite, then amor- Al 2(SO 4)3•17H 2O and phous FeSO4·xH2O (x>1 w in FeSO 4•7H 2O, are in the this study). The dehydration of 45th Lunar and Planetary Science Conference (2014) 2614.pdf

Dehydration at 323K, 31%RH, 1 atm epsomite went Dehydrations at 294K & Pmars Fe4.67(SO4)6(OH)2.20H2O MgSO4.7H2O FeSO4.7H2O MgSO4.7H2O FeSO4.7H2O Al2(SO4)3.17H2O through hexa- 1.2 hydrite, then 1.2 alunogen ferricopiapite amorphous 1.0 1 MgSO4·xH2O Meta-alunogen (14w) epsomite (x changes 0.8 0.8 20w + 7w from 3 w to 1.8 melanterite Rozenite (4w) 4w 0.6 w, [1]). In 0.6 epsomite comparison, 0.4 alunogen lost 0.4 its hydrogen- Am(1w)+ melanterite 0.2 0.2 1w bonded struc- Am (2w) O in structure, n(t)/n(0), normalized structure,in n(t)/n(0), O 2

tural H 2O much H O in structure, n(t)/n(0) , normalized structure,, in n(t)/n(0) O 0.0

0 2

H 1 10 100 1000 10000 100000 faster than ep- 1 10 100 1000 10000 somite and Dehydration duration (minutes) Hours in dehydration melanterite, Figure 2. Number of H 2O per sulfate molecule, as Figure 3. Number of H 2O per sulfate molecule, as n(t)/n(0)m (Fig. 2) sug- n(t)/n(0)m (%) vs. experimental duration in minutes, (%) vs. experimental duration in minutes, during the gesting an high- during their dehydration processes under Mars relevant process of dehydration at 1 atm, 31%RH, and 323K. Only P and P . Amorphous species occurred. crystalline-to-crystalline phase transformation happened. ly unstable H2O Implication to sulfate dehydrations on Mars: An structure, but. meta-alunogen Al 2(SO 4)3·12H 2O has a much stable structure than hexahydrite and rozenite. comparison between recent Mars obliquity cycle and The comparison of dehydration processes of the low T dehydration rate of epsomite made by Wang melanterite and epsomite are complicate because both et al [5] suggests that Mg-sulfates with mid-high hy- can have two different pathways, involving crystalline- dration degree could remain in subsurface of Mars to-crystalline or crystalline-to-amorphous phase transi- equatorial regions until today. On the other hand, the tions. Current experiements revealed following stabil- experimental results reported here indicating that ep- ity orders: (1) ferrohexahydrite is much less stable than somite has the fastest dehydration rates when com- hexahydrite; (2) rozenite is less stable than starkeyite; pared with other four major type of sulfates. Therefore, if Al 3+ -, Fe 3+ -, and Ca 2+-sulfates formed during high (3) amorphous MgSO 4·xH 2O is less stable than rozenite. obliquity periods in sub-surface salt-rich layers, we Result#3 – dehydration rates: When comparing the anticipate an even higher (than in Mg-sulfates) % of dehydration rates, the dehydration pathways have to be structural H 2O in them would remain until today. The considered. Figure 2 revealed that under the same Mars existence of ferric sulfates with highest hydration de- relevant P, PH2O , Ts, the dehydration of epsomite gree at subsurface in Gusev was demonstrated by the (when going through amorphous phases) has the fastest color change of Fe 3+ -sulfates-rich subsurface regolith rate. Figure 3 shows when only crystalline-to- after their exposure by Spirit rover to Mars surface crystalline phase transformations involved, melanterite [6,7]. has a faster dehydration rate than epsomite. Further- The current experimental study supports the hy- more, Robertson and Bish [2] reported a laboratory pothesis that subsurface hydrated sulfates are the major investigation on the phase transition among three host for the high Water-Equivalent-Hydrogen (WEH) CaSO 4·xH 2O (x=2, 0.5, 0). In their experiment, the values observed at two equatorial regions on Mars starting of dehydration from gypsum to bassanite was [8,9,10]. observed after two weeks into the reaction at 323K in Acknowledgement: NASA-MFRP NNX10AM89G vacuum of 15 Pa. If compare their observation (20160 References: [1] Wang et al., 2009, JGR, 114, E04010; minutes) with the starting time of alunogen, epsomite, [2] Robertson and Bish (2013, Icarus, 223, 407-417; and melanterite (20 - 40 minutes) at similar atmospher- [3] Wang et al., 2012, Icarus,218, 622-643; [4] Smith ic pressure but lower temperature at 294K (Fig. 2), it 2002, JGR, 107, 5115; [5] Wang et al., 2013, Icarus, can be readily concluded that gypsum has the lowest 226, 980-991; [6] Wang et al., 2008, JGR, 113, dehydration rate. Conclusion: Among the five major types of hydrous E12S40; [7] Wang et al., 2011, JGR, 116, E00F17; [8] sulfates, Al-, Fe 2+ -, Fe 3+ -, Mg- and Ca-sulfates with the Feldman et al., 2004, GRL, highest hydration degrees, malenterite has the highest doi:10,1029/2004GL020181; [9] Fialips et al., 2005, dehydration rate through crystalline-to-crystaline path. Icarus, 178, 74-83; [10] Jakosky et al., 2005, Icarus, When amorphorzation happens, epsmoite has the high- doi:10.1016/j.icarus.2004.11.014. est dehydration rate.