i ABSTRACTORSUPPORTINGINFORMATION i Abstract Title: "H20 Adsorption Kinetics On Smectites" RTOP #344-30- I1-02 Aaron Zent Planetary Systems Branch (SST) NASA Ames Research Center M/S 245-3 Moffet Field, CA 94035 tel: (650) 604-5517 fax: (650) 604-6779 Richard C. Quinn Space Science Division (SS) NASA Ames Research Center M/S 239-14 Moffet Field, CA 94035 tel: (650) 604-6501 ]eanie Howard SETI Institute NASA Ames Research Center M/S 239-14 Moffet Field, CA 94035 tel: (650) 604-2295 Abstract Title: "H20 Adsorption Kinetics On Smectites" NASA FORM 1676 (AUG 97'1E PAGE 3 II,O Adsorption Kinetics On Smectites, Aaron P. Zent, NASA Ames Research Center, Moffctt Field, CA, 94035, [email protected], J. Howard, R. C. Quinn SETI Institute, NASA Ames Research Center, Moffett Field, CA, 94035 Introduction: The adsorptive equilibration of H20 atmosphere. Numerical simulations of the PBL with montomorillonite, a smectite clay has been suggest that very substantial amounts of H.,O can measured. At low temperatures and pressures exchange between atmosphere and surface during equilibration can require many hours, effectively the daylight hours, providing that the high- preventing smectites at the martian surface from capacity adsorbent can remain in equilibrium with responding rapidly to diurnal pressure and the rapidly changing conditions of temperature temperature variations. and H20 partial pressure. Smectite clays are the importance of _;mectite Adsorption Kinetics; 30 W|ter Vapor Fl_lx Numerical modeIs of the martian planetary boundary Montmotillo_ile, 100Omlg l layer (PBL) assume instantaneous equilibration 20 15 between adsorbed and vapor phase H20. This . t0 assumption may not be valid for smectite clays. Some ground-based and spacecraft observations ............................................ suggest that atmospheric H20 exchanges with the _ -5- regolith daily. For example, Titov et al. (1995) argued that the Phobos 2 ISM instrument recorded -20 daily variations in the H20 mixing ratio above the -25 flanks of martian volcanoes. They argue that the -3O atmospheric H20 column, observed during the day, is ' ' _ ' ' _ ' _ ' ' l'2 ' ' l_ + ' tlO ' 2't ' _1 _rne (t_r) 5 _+ 2 times the column observed over the favored high-capacity adsorbent for Mars. surrounding plateaus. They advanced the hypothesis The keys to resolving these issues are: a) how that surface clays on the flanks of the volcanoes, much smectite is available, and b) how quickly experiencing significant diurnal exchange with the does is equilibrate with its pore gas? We set out atmosphere, were responsible for the apparent to answer the second question here. increase in the H20 column abundance above the volcanoes. Smectite Adsorption Sprague et al. (1999) have also argued that telescopic A few percent of smectite in the soil will observations, even when corrected for atmospheric dominate H20 adsorptive behavior of the regolith. scattering, show distinct, repeatable, and substantial The reason for that is the availability of interlayer diurnal variations in water abundance. This confirms sites to adsorb a considerable amount of H20 That work produced by the same group, as weIl as by amount of clay is still allowed by available Barker (1997). constraints. Christensen et al. (1998) reported Other observations and models have yielded different upper limits of <20% for clay minerals in the results. Titov et al. (1999), observing the martian Hebes Chasma area. Smectites have interlayer atmosphere from the Pathfinder IMP cameras sites accessible by H20. Typical BET surface reported that water vapor seemed primarily confined areas using H20 ,molecules as probes are in the to a layer in the lowest 1-3 km of the atmosphere, and range of 102 to 103 mz fit. Conversely, the BET showed no significant morning to evening variations surface area using N2,a non-polar molecule that in abundance. Davies (1979) also suggested that no cannot access interlayer sites, are a few 101m2g.,. conclusive evidence of diurnal HzO exchange was Montmorillonite, if it equilibrates rapidly, could observed in Viking data. From a modeling play a significant role in buffering possible large perspective, Zent et al. (1993) argued that the magnitude exchanges of H20. Martian boundary layer must coIIapse in the mid- afternoon, when surface temperatures drop below Fx,aeaim.caa.lX._ atmospheric temperatures. The resulting thermal We are characterizing the kinetics of smectite stabilization of the lowest atmosphere effectively adsorption under Mars-like conditions. We strands most of the atmospheric HzO column at determine this experimentally by first dehydrating altitude, and prevents significant diurnal exchange. samples of SWy-I a Na-montmorillonite, and then One possibility for establishing rapid diurnal equilibrating them to Mars-like conditions for variations in the atmospheric H20 column is to use a prescribed periods of time, and noting approach to very high-capacity adsorbent at the base of the equilibrium load. Approximatel1y00mgsamplewserebakedoutat appears to equilibrate in under 1hour. However, 2200Cfor I hour undervacuumbeforethe at 2IlK with RH of 32%, the sample requires experimenttosr,emovaesmuchH20aspossible.. approximately 26 hours to reach its equilibrium Asinstandaraddsorptiomneasuremetnhtess,amples adsorbate population. Additional data are being arethenequilibrateadtlowTwithwatervaporW. e acquired at temperatures of 273 K, but with controtlhepartiaplressuroefwaterinthemanifold extremely low RH (approx. 0.05) to see how bybufferingiceatatemperatusrleightlybelowthat rapidly equilibration occurs under those ofthesoil,providingRHontheorderof0.2- 0.5. conditions which are representative of the near Soil and ice temperaturesarecontrolledby surface regolith during the day. We will also be addressing the kinetics of desorption in samples under Mars-like conditions. SWy-1Equilibration Kinetics Discussion and Summary ._ .273K i _" P.122 P6 Plainly, numerical models of the PBL will need to i Rm.2o% 1 , i j account for the kinetic barriers to equilibration in 002 ..........i i " smectite clays, should they prove to be present in 0 even small percentages in the Martian regolith. eo#.015 f I ! In particular, there remains concern about whether smectite clays undergo interlayer under Mars-like O.Ol P-O. tIPi conditions. It has been argued (Koster van Groos ...... L.... !............=...... I........_" and Guggenheim, 1984) that once interlayer collapse occurs in smectites that they are virtually TO 2g 30 40 impossible to rehydrate. Therefore, the state of Time (Hours) the interlayer H20 in smectites under mars-like conditions becomes a significant question. immersion coolers, which have stability no more than In any event, smectite clays are unlikely to + 0.5 C. Therefore, we cannot achieve RH greater account for putative large diurnal variations in than about 50% without risking saturation in the soil atmospheric H20. In the first place they do not sample. Samples are allowed to equilibrate for appear to equilibrate rapidly enough with the pore various amounts of time, then are sealed and removed gases at Mars-like conditions. Secondly, any from the manifold. Samples are carried to gas putative concentration of clay-like minerals would chromatograph without breaking seal, heated to 220° quickly lose it's charge of H20 due to atmospheric C and trapped under LN 2for 1hour. This effectively transport to other, more clay-poor sites. transfers all the adsorbed H20 from the soiI to a clean The favored hypothesis for the diurnal variations glass trap. The traps are then heated with wire wrap observed by Titov (1995) and Sprague et al and manifold heaters to approximately 100 °C to (1999). Colburn et al (1989) reported diurnal vaporize all of the H20. On injection, the column variations in optical depth, which supports the temperature is 140°C, and the thermal conductivity hypothesis that a measurable fraction of the detector is at 160°C. atmospheric H20 column condenses diurnally on During calibration, soil samples were baked out, and aerosols. Further supporting this hypothesis is the trapped with no exposure to H20. Typical counts observation by Sprague et al (1999) that the were on the order of 103. In experiments with apparent diurnal H20 variation is maximized equilibration, signals were on the order of 104 to 105, during heavy atmospheric dust loading. a full order of magnitude (at a minimum) above the References background. In order to calibrate the system, Mg, Barker E. SProc. Mars Telescopic Obs. Wkshp 11,LPI., 1997. Na, and Li chloride solutions were used. These Christensen et al. Science, 279, 1692-1898.1998. solutions provide number densities in the vapor phase Colburn et al Icarus, 79, 159-189,1989 that are similar to the expected values from the Koster van groos and Guggeheim, Amer. Mineral., 69, 872- adsorbate. 879, 1984. Sprague, A. L., et al., DPS 31, 1190, 1999. Titov, D.M. et al., Adv. Space Res. 16, 23-33, 1995. Equilibration Timescales Titov et al, J. Geophy. Res., 104, 9019-9026, 1999. The results to date are shown in Figure 2. There is Zent et al. Z Geophys. Res., 98, 3319-3337, 1993 clearly a kinetic barrier to equilibration as a function of temperature. At 273 K, and RH = 0.2, the system