Sublimation Rate of CO2 in Simulated Mars Conditions with Various Grain Sizes

1 2 1 Sublimation Rate of CO2 in Simulated Mars Conditions K.F. White and D.W.G. Sears , Department of Physics & Astronomy, Ball State University, Muncie, IN 47306, USA , 2Department of Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA, .

Introduction: The search for water on Mars The Andromeda Chamber is an experimental extended from the observation of gullies in the chamber which can simulate Mars to the closest mid-1800’s to the recent theory of possible frozen and most basic conditions: atmospheric pressure, reserves of water (H2O) below the carbon dioxide atmospheric composition, atmospheric (CO2) ice caps. temperature, and solar radiation. The solar The southern pole is thought to have a reserve radiation feature was not utilized in the of frozen H2O. This was confirmed after high experiments over the ten week period of research. hydrogen concentrations, associated with water, The Andromeda Chamber could not reach a Mars were detected over the poles by using the gamma- surface polar temperature of -195°C3 because the ray spectrometer on Mars Odyssey1. This theory weight balance could only operate above -12°C. was supported by Durham (1999) in that solid CO2 All experiments, therefore, were conducted at a could not be strong enough to be the only constant temperature of -10°C. constituent in the poles, especially the southern cap An atmosphere of CO2 was held at a near where the elevation and bulge conditions are constant pressure of 7 mbars3, varying +/- 0.2. 2 against high CO2 ice strength . However, a fine Each sample was photographed before and after powder or small gravel or pebble sized CO2 ice entering the chamber and by this grain size and blocks be more of a possibility to stability on the surface area was calculated. The data collected of poles. mass loss over time was graphed and analyzed. During the summer of 2007, a series of This mass reading was recorded every 30 seconds experiments were conducted using the Andromeda due to the speed of the sublimation of CO2. Simulation Chamber located at the University of Five of the twelve experiments were thrown out Arkansas’s Department of Space and Planetary of the final analysis due to various conditions Sciences at the W.M. Keck Laboratory. From the which would severely skew the study, see Table 2. data concerning mass loss over time collected, a Sample Reason series of sublimation rates at various CO2 ice grain #1 Inaccurate surface area and ratio of sizes (fine powder, various gravel sized cylinders, grain size to container not taken into and solid blocks) were determined and compared to account, geometric affect skewed the recession and reformation of the polar caps of data. Mars. These rates and comparisons were to help #2 Inaccurate surface area and ratio of determine the granular composition of both Mars grain size to container not taken into poles and if the rates differ. In so doing, the grain account, geometric affect skewed sizes of the north pole (Planum Boreum) and south data. pole (Planum Australe) can be determined. #3 Ratio of grain size to container not Methods: In the course one month, a series of taken into account, geometric affect twelve sampled experiments (see Table 1) which skewed data. involved various grain sizes were conducted using #4 Ratio of grain size to container not the Andromeda Chamber. taken into account, geometric affect Sample Date Comments skewed data. #1 06/01/07 Gravel blocks of CO2 ice. #8 Humidity in chamber high. #2 06/01/07 Gravel blocks of CO ice. 2 Table 2: Tabulation of Abandoned Data #3 06/08/07 Solid Cylinder of CO2 ice.

#4 06/08/07 Solid Cylinder of CO2 ice. Two files due to computer Results: Using a graph of mass loss (in grams) malfunction. over time (in minutes), the slope of that curve was

#5 06/18/07 Gravel sized CO2 ice. used to determine the rate (g/min) of loss during

#6 06/18/07 Gravel sized CO2 ice. that experiment, later to be converted into g/hr. In

#7 06/19/07 Fine Powdered CO2 ice. Figure 1, a graph of each g/hr slope with their #8 06/25/07 Gravel Sized CO2 ice. corresponding grain size reveals a nearly horizontal #9 06/25/07 Fine Powdered CO2 ice. slope, suggesting that grain size has little affect on #10 07/02/07 Gravel sized CO2 ice. rate of mass loss, regardless of surface area. #11 07/02/07 Fine Powdered CO2 ice.

#12 07/03/07 Gravel sized CO2 ice. Table 1: Tabulation of Experiments Grams/Hour vs. Grain Size The two poles mass losses over time were very y = 0.059x + 22.314 similar, a result seen in the lab in Figure 1, when 30 ) r

h comparing grain sizes and grams lost per hour on

/ 25 g (

r 20 Mars. u o 12 H

s Planum Boreum: 1.388 x 10 g/hr s o 10 12 L

s Planum Australe: 1.969 x 10 g/hr s

M 0 Then the sublimation rate, using the mass lost per 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 hour above, was calculated using Equation 1 and Grain S ize (cm) two estimated results were found: Planum Boreum: 0.000118 mm/hr Figure 1: Mass loss over time slope versus the Planum Australe: 0.000946 mm/hr grain size. It is clear that despite grain size, the These numbers are much lower than those mass lost appears to be constant. produced in the Andromeda experiments. This is due to the limitations of the chamber’s weight Variables on the graph in Figure 1 may be balance, causing the atmospheric temperature to be caused by inconsistencies in atmospheric pressure -10°C, not the polar atmospheric temperature of (allowing 1% variation), atmospheric temperature -195°C3. (allowing less than 0.1% variation), and mass Conclusion: It cannot be determined how balance errors due to chamber conditions (allowing different the Mars sublimation rates are from the 5% error). In attempt to minimize this error in the ones found in lab due to the temperature difference mass balance, the first 20 minutes of the hour and between the chamber and Mars; however using the twenty minute experiment run were removed from Andromeda Chamber to simulate near Mars consideration to allow the balance to stabilize. conditions produced interesting results. The grain (g/hr) x (1/CO2 ice Density) x (1/Surface Area) size does not appear to affect the mass loss Equation 1: Sublimation Rate Formula overtime but does affect the sublimation rate. It is Using Equation 1, a graph was produced estimated by referencing Figure 2 that the seasonal comparing the calculated sublimation rates to their Planum Boreum CO cap most likely consists of a corresponding grain size, see Figure 2. 2 finer grain size than that of Planum Australe, Sublimation Rate vs. Grain Size though their grain sizes are likely to be similar. y = 71.392x + 129.25 500.00 Further analysis of these values as well as )

r exploring the fuller spectrum of grain sizes to

h 400.00 / m

m sublimation rates should be attempted. It is also ( 300.00

R suggested to determine how a thin layer of Martian

200.00 n o i t dust and/or layers of H2O ice beneath sample can a 100.00 m i l b 0.00 affect these rates. Many factors can and do govern m u S 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 over the sublimation of CO2 ice on the Martian Grain Size (cm) polar caps, all of which should be thoroughly explored to better understand the state in which the Figure 2: Sublimation Rates versus Grain Size. A poles have been formed. slow increase in rate as grain size, and thus surface Acknowledgements: The W.M. Keck Foundation area, increases. funded the KECK Laboratory for Space Simulation at the Arkansas Space and Planetary Science at the Discussion: Using the sublimation rates Department of University of Arkansas’s calculated, see Figure 2, the data was compared to Andromeda Simulation Chamber which was Mars polar rates calculated by using the summer donated by the Jet Propulsion Lab of Pasadena, and winter diameters of the north and south poles. California. The 2007 REU Internship research The non-permanent portion of the Planum Boreum opportunity is funded by National Aeronautics and ice cap is approximately one meter deep and the Space Administration (NASA) and hosted by the non-permanent portion of the Planum Australe 4,1 University of Arkansas and the it’s Department of polar ice cap is eight meters deep . These non- Space and Planetary Sciences. permanent depths are of the parts of the caps that References: are sublimated away during summer, not depths of [1] Boynton, W.V., et al., Distribution of the permanent caps themselves. Hydrogen in the Near Surface of Mars: Evidence Planum Boreum is 2,500 miles wide during the 6 for Subsurface Ice Deposits, Science Express Vol. winter and 600 miles wide during the summer . 297, DOI 10.1126/science 1073722, pp. 81-85 Planum Australe is approximately 1,000 miles (2002). [2] Durham, W.B., et al, Steady-State flow during the winter and only 200 miles during the of Solid CO : Preliminary Results, Geophy. Res. summer5. 2 Lett. 26 (1999). [3] Croswell K., Magnificent (2003) [5] Darling, D.