Laboratory Studies of Water Ice Cloud Formation under

Martian Conditions

Laura T. Iraci, Anthony ColapreteNASA Ames Research Center

Bruce Phebus, Brendan Mar, Brad StoneSan Jose State University

Alexandria BlanchardMichigan 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

3

1

2

3

ln44 r

v

SkTrG

ππσ −=Δ

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 ~ 1.2 - 1.3; RHice ~ 120 - 130%) 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 KPH2O = 2 x 10-7 – 9 x 10-5 Torr (2.7 x 10-7 - 1.2 x 10-4 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 Scrit (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

-5.E-04

5.E-04

2.E-03

3.E-03

4.E-03

5001000150020002500300035004000

Frequency (cm-

Absorbance

• 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:PH2O = 8.6 x 10-6 TorrT = 166.9 KScrit = 2.8

€

Scrit =PPobservedVPequilibrium

0.00

0.10

0.20

0.30

90 100 110 120 130

Time (min)

Peak Area 3500-3000 cm

-1

166

168

170

172

Temperature (K)

Pe

ak

Are

a 3

00

0-3

50

0 c

m-1

Te

mp

era

ture

(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:PH2O = 8.6 x 10-6 TorrT = 166.9 KScrit = 2.8

€

Scrit =PPobservedVPequilibrium

Ice Nucleation on Silicon (blanks)

• Scrit depends on Tnucl

• Need S as large as 3 (RH = 300%) to start ice formation at coldest temperatures.

x-axis is 100% RHi 1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

Ice Nucleation on ATD(Arizona Test Dust)

• volume mean diameter = 5 m, 68-76% SiO2, 10-15% Al2O3, and 2-5 % Fe2O3 by wt.

• Nucleation on ATD is not much easier than on silicon (dashed line)

Sa

tura

tion

Ra

tio,

Scr

it

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

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 Scrit) than for ATD or Si.• Still pretty tough at T < 168 K !• Particle type matters… and what if a mixture?

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

JSC Mars-1 Regolith Simulant• Simulant has a known spectral similarity to bright regions of Mars

• Quarried, weathered volcanic ash (Pu’u 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

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

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 fractionCourtesy of O. Marcu and M. Sanchez, NASA ARC

Ice Nucleation on Dark (Heavy) Fraction

• Dark fraction behaves like whole sample

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

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?

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

1.0

1.5

2.0

2.5

3.0

3.5

4.0

155 160 165 170 175 180 185

Tnucleation

Saturation Ratio, S

crit

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 Scrit 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?

-0.010

0.000

0.010

0.020

2700290031003300350037003900

frequency (cm-1)

absorbance

S25 142,544

S25 165,917

S25 164,232

S25 162,759

S25 159,811

S25 99,377

Adsorption & Desorption from Clay

7.2x10-4 torr9.5x10-2 PaT = 196.5 KRH = 95%

1.7x10-6 torr2.3x10-4 PaT = 179.3 KRH = 0.2%

Adsorption Desorption

Adsorption & Desorption JSC-1 Mars Simulant

7.0x10-4 torr9.4x10-2 PaT = 197.5 KRH = 85%

1.4x10-6 torr2.3x10-4 PaT = 180.6 KRH = 4%

DesorptionAdsorption

Conclusions

• Martian ice clouds don't form at 100% RH. If it’s 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 Scrit

• Influence of particle shape on Scrit

• Growth rate and accommodation coefficient

• Effect of CO2 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