Science Poster for Ames work

1. Laboratory Measurements of Supersaturations Needed to Nucleate
Ice on Martian Dust Analogs in a Simulated Martian Atmosphere
Bruce D. Phebus1,2, Laura T. Iraci2, Anthony Colaprete2, Bradley M. Stone1
1 San Jose State University, San Jose, CA 95192; 2 NASA Ames Research Center, Moffett Field, CA 94035
___________~ Abstract ~___________ ___________~ Data ~___________ ___________~ Results ~___________
Water ice clouds play important roles in both the radiative balance and
........ Silicon Fit
the hydrologic cycle of Mars, and thus the predicted microphysical properties
3.5 _ _ _ Arizona Test Dust
of clouds can greatly affect models of the Mars climate system. Current ____ Clay
simulations rely on parameters that have not been measured for Martian 3.0 Clay Points
conditions.
Calculated (Pliq/Pice)
Saturation Ratio
We have measured the conditions necessary for ice nucleation on dust 2.5 Shet for traditional models
particles at Martian temperatures and water partial pressures. To do so, we
expose a dust sample to water vapor and cool it until ice is observed by
infrared spectroscopy. Our results show that ice nucleation requires much 2.0
greater supersaturation than estimated. Furthermore, we find a strong
temperature dependence which is not predicted by theory.
1.5
We have also observed uptake of water onto clay and Mars simulant
(JSC-1) particles. This observation, coupled with the high supersaturations 1.0
necessary to initiate ice, suggests that supercooled water could be stable Nucleation 155 160 165 170 175 180 185
under Martian atmospheric conditions. As a source of non-frozen, non-gas
phase water, this phenomenon could allow for liquid phase chemistry with Temperature of Nucleation [K]
water on Mars. Equilibrium
___________~ Conclusions ~___________
______~ Introduction to Mars ~______
• Critical saturation ratio needed for nucleation appears to be lower
• Pressure on Mars less than 1% that of Earth for smectite than for ATD
Figures Above show: Above Left shows IR spectra of a nucleation • Scrit is greater that that for traditional models
• Martian atmosphere primarily: ~ CO2 95%, N2 3%, Ar 1.6% experiment on smectite clay. P (H2O) = 5.3x10-6 Torr, T = 169 K,
• Scrit increases at lower temperatures
• The pressure on Mars varies: 3 to 6.5 torr saturation ratio = 1.3. The figure Above Right reports the conditions
for the experiment. Observe that nucleation occurs near 150 • Supercooled water could be present within orange triangles
• Two condensable components: CO2 and H2O (~0.03%) minutes, after which the temperature was adjusted to obtain • Models underestimate saturation ratios needed to nucleate water
• Temperatures for water ice clouds: 100 to 200 K (-173 to -73 °C) equilibrium monitored via the peak area. Steady adsorption of water
is observed leading up to nucleation; note absorbance feature in ice clouds
Reff: http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html 05/01/2008
spectrum Min 142.5 for adsorbed water.
___________~ Implications ~___________
_________~ Experimental ~_________ Adsorption Desorption Figure to the
Left shows:
• Difficulty in nucleating water ice clouds maybe underestimated for
Adsorption &
Desorption of Martian conditions
Figures to Right: 7.0x10-4 torr
9.4x10-2 Pa H2O to JSC-1 • Preliminary modeling suggests drier atmosphere on Mars due to
A schematic of the T = 197.5 K 1.4x10-6 torr Mars Simulant nucleation effects
experiment. RH = 85% 2.3x10-4 Pa
Pressure is held T = 180.6 K • Possible net adsorption of water on lofted dust over many day/night
RH = 4%
constant against continuous cycles
pumping. Cold finger is jacketed
to prevent cold spots. ___________~ Future Work ~___________
Saturation Ratio Defined • Nucleation Conditions for JSC-1 Mars Simulant
The saturation ratio is calculated by dividing the P _________~ Acknowledgements~________
observed pressure by the vapor pressure at the S = obs • Growth Rates for Ice on JSC-1 Mars Simulant, Arizona Test Dust,
observed temperature. VPice Emmett Quigley, Dave Scimeca, Ted Roush
Smectite Clay and Silicon
NASA Planetary Atmospheres Program, Philippe Crane
San Jose State Foundation
Mars Pictures Courtesy NASA/JPL-Caltech