Laboratory Studies of Water Ice Cloud Formation under Martian Conditions Laura T. Iraci, Anthony Colaprete NASA Ames Research Center Bruce Phebus, Brendan Mar, Brad Stone San Jose State University Alexandria Blanchard Michigan Technological University

Outline Experimental methods Ice onset conditions (four different materials) Preliminary GCM results Water uptake before ice Conclusions & future work

Introduction Water ice clouds are observed on Mars important role in the radiative balance and hydrologic cycle probably form on suspended dust

Homogeneous ice nucleation Classical Nucleation Theory Nucleation is the onset of an energetically stable phase Germs are tiny packets of the new phase within the metastable phase Heterogeneous ice nucleation

Introduction Water ice clouds are observed on Mars important role in the radiative balance and hydrologic cycle probably form on suspended dust Some GCMs address microphysics of cloud formation commonly use m = 0.95 (S ~ ; RH ice ~ %) for onset recent retrievals from PFS on Mars Express and reanalysis of TES data suggest that atmosphere is drier than models wrong input parameters for model may explain discrepancy Our goal: measure onset conditions for ice nucleation dust samples representing probable particle types appropriate temperatures & pressures T = 155 – 185 K P H2O = 2 x – 9 x Torr (2.7 x x mbar)

Mars Cloud Chamber

Making "Clouds" on Si Substrate

Bruce and Alexandria took all the data!

Experimental Procedure Place dust on Si substrate, evacuate chamber Set water pressure Lower temperature until nucleation is observed (IR) Calibrate T by establishing equilibrium Calculate S crit (the RH at which ice began) Repeat for different water pressures, different dust materials Ice vapor pressure taken from Murphy & Koop, 2005, QJRMS

Infrared Signature of Ice sharp absorbance feature at ~3 um indicates water ice

Representative Experiment Start with desired water pressure, cool in steps until ice forms. Red Line: Temperature Black Line: Amount of Ice Nucleation after 110 min At nucleation: P H2O = 8.6 x Torr T = K S crit = 2.8 Peak Area cm -1 Temperature (K).

Representative Experiment Start with desired water pressure, cool in steps until ice forms. Red Line: Temperature Black Line: Amount of Ice Nucleation after 110 min At nucleation: P H2O = 8.6 x Torr T = K S crit = 2.8

Ice Nucleation on Silicon (blanks) S crit depends on T nucl Need S as large as 3 (RH = 300%) to start ice formation at coldest temperatures. x-axis is 100% RH i

Ice Nucleation on ATD (Arizona Test Dust) volume mean diameter = 5 m, 68-76% SiO 2, 10-15% Al 2 O 3, and 2-5 % Fe 2 O 3 by wt. Nucleation on ATD is not much easier than on silicon (dashed line) Saturation Ratio, Scrit

Collected Clay from Sedona, AZ Smectite-rich ~50% have d < 1.5 m Surface area dominated by largest 5-10% of particles Micrograph courtesy of O. Marcu and M. Sanchez, NASA ARC

Ice Nucleation on Clay (Sedona, AZ) Nucleation on clay particles is easier (smaller S crit ) than for ATD or Si. Still pretty tough at T < 168 K ! Particle type matters… and what if a mixture?

JSC Mars-1 Regolith Simulant Simulant has a known spectral similarity to bright regions of Mars Quarried, weathered volcanic ash (Puu Nene) <1 mm size fraction

Ice Nucleation on JSC-1 Mars Simulant Nucleation on JSC-1 simulant shows same trend: harder at colder T Need S ~3.5 to start ice at 155 K Comparable to clay and ATD

JSC-1 Sample Separation Ground in a mortar & pestle with water Centrifuged to compact light fraction & allow for easy sample separation Fractions separated by pipetting Light & dark fractions kept for experiments

Micrographs 1 mm Whole JSC Light fraction Dark fraction Courtesy of O. Marcu and M. Sanchez, NASA ARC

Ice Nucleation on Dark (Heavy) Fraction Dark fraction behaves like whole sample

Ice Nucleation on Light Fraction Light fraction nucleates ice more easily than whole sample May nucleate as easily as S = 2 at cold temperatures Why does this portion behave differently when separated?

Ice Nucleation (Summary) Use of m = 0.95 may be quite wrong for T < ~ 170 K S values for m = 0.95

Implications of High S crit Values

Changes in Cloud Particle Radius Often, cloud particles are larger with new m(T) Model predicts smaller in some places/times m Difference in Cloud Mean (by mass) Radius: Standard - New

Changes in Water Vapor Column Red areas are drier with lab parameters in model In general, atmosphere is drier Difference in water vapor column: Standard - New

Difference in Water Surface Frost Redistribution of polar frost New parameters suggest south polar cap smaller, thicker Difference in water frost: Standard - New

MGCM Results Summary Using the T-dependent lab observations results in: Significant differences in cloud particle size and mass In general, at latitudes below 60 deg, cloud particles are larger Larger particles lead to a drying out of the inter-hemisphere circulation Overall drying of the atmosphere by 20-50% The percent difference in total planetary water vapor ( black ) and clouds ( blue ) = (1-Standard/Constrained) x100

Water Uptake before Ice Formation

Clay Takes Up Water Before Ice Water uptake before ice growth Probably taken up into clay lattice - known phenomenon Is this why clay nucleates ice a bit better than silicate?

Adsorption & Desorption from Clay 7.2x10 -4 torr 9.5x10 -2 Pa T = K RH = 95% 1.7x10 -6 torr 2.3x10 -4 Pa T = K RH = 0.2% AdsorptionDesorption

Adsorption & Desorption JSC-1 Mars Simulant 7.0x10 -4 torr 9.4x10 -2 Pa T = K RH = 85% 1.4x10 -6 torr 2.3x10 -4 Pa T = K RH = 4% DesorptionAdsorption

Conclusions Martian ice clouds don't form at 100% RH. If its cold enough, they don't even form at 300% RH! models may be oversimplifying m can be considerably smaller than 0.95 Ice nucleation conditions are temperature-dependent Most dust materials show comparable behavior clay is best, JSC-1 simulant next best light fraction of JSC-1 may be much better than anything else? Models are needed to evaluate implications several feedbacks, esp. through particle size and sedimentation nucleation conditions may affect atmospheric water vapor, cloud distribution, and even surface frost location and quantity Clay and JSC-1 show uptake and retention of water slow to equilibrate in either direction not fully reversible??

Future Work Characterize separated fractions of JSC-1 Influence of dust size on S crit Influence of particle shape on S crit Growth rate and accommodation coefficient Effect of CO 2 bath gas Role of Australians in US Politics

Acknowledgements NASA Planetary Atmospheres NASA Undergraduate Student Research Program Chamber Design and Assembly: Dave Scimeca, Rosi Reed, Emmett Quigley, Tricia Deng, Rachel Mastrapa Technical Assistance: Oana Marcu and Max Sanchez; Ted Roush; Orlando Santos, Tsege Embaye, & Linda Jahnke Helpful Conversations: Lou Allamandola, Rachel Mastrapa; Bob Haberle, Jeff Hollingsworth Supporting Players: Janice Stanford & Melody Miles; Barrie Caldwell; Sandra Owen, Brett Vu ; Ben Oni, Olivia Hung, Maricela Varma & Brenda Collins; San Jose State Foundation