  THERMAL INETIA OF MARTIAN SEDIMENTS AT VARIOUS LEVELS OF SATURATION.   A. Baker1, R. Kang2, C. Vilen3, R. Perentis1, T.J. Williamson1, E. Yang1, A. Walling1, 1Durham Academy Upper School, Durham, NC 27705. 2East Chapel Hill High School, Chapel Hill, NC 27514. 3Chapel Hill High School, Chapel Hill, NC 27514   Introduction:  For hundreds of years, the question of whether or not life exists on planets other than our own dominated the scientific community. In recent years, collected data has concludes that Mars once held similar properties to Earth that are vital to the existence of life. One such property is water. In 2008, the Phoenix Lander confirmed that ice existed beneath the surface of Mars [1]. The presence or absence of a water table just beneath the surface may potentially be inferred from the thermal inertia of the planet's surface. Thermal inertia is a measure of a material's resistance to heating and cooling.  Today, orbiting instruments like THEMIS measure the temperature of surfaces on Mars. These data can be used to determine the thermal inertia of the sediments in different places. If groundwater is discovered, many suspect that these reservoirs could harbor biological specimens or evidence of specimens that once existed on Mars, thus confirming the existence of extraterrestrial life.  The MONS (Mars Outreach for North Carolina Students) program conducted an experiment that tested how the presence of different amounts of water under the surface of sediments affected its thermal inertia. This information could be used to interpret the THEMIS data to try to locate water under the surface of Mars. The purpose of this study was to determine whether the depth of the water table affects the thermal inertia of a sediment sample. Analytical Approach: To test how different water table depths affect the thermal inertia of sediment, we recorded the surface temperature of samples with different depths of simulated water table as they heated and cooled. The sediment samples were gathered from Eno River in North Carolina and were sifted to separate two different grain sizes, 'gravel' and 'sand,' for experimentation—course gravel being characterized as 3mm to 2cm in diameter. This report will focus on the gravel experiment.  To ensure that there was no interference with the temperature data, the samples were held in cylindrical plastic containers 10 cm deep and 20 cm in diameter. This was to satisfy the NASA guidelines, which stated that there could be no other materials within 10 cm of the point of measurement.  Before each run of the experiment, a sediment sample was put in an aluminum pan and placed in an oven for several hours to remove any possible moisture. After the sample cooled to room temperature, gravel was poured into the sample container to the depth of the desired saturation level, measuring from the bottom. Room-temperature water was poured on this sediment until it was saturated. Saturation, in this study, was defined by adding a sufficient amount of water until small pools began to form on the top of the sample. This saturated layer simulated a water table. Dry gravel was added on top of this until the sample was 10 cm deep in total.  The lamp used as our heat source provided 630 watts per square meter in order to simulate the 590 watts per square meter on Mars. The lamp also provided a wide range of electromagnetic energy, including infrared and UV in addition to visible, in order to better simulate the nature of sunlight on Mars.  The lamps were set up 50 cm directly above the surface of the sediment and were pointed at the center of the surface. The infrared thermometer was set on a stand 40 cm away from the surface of the sample horizontally and 50 cm above the surface of the sample vertically. It was angled to read the temperature of the center of the surface of the sample, taking continuous readings (see Figure 1-1).   Before the heat lamp was turned on, we recorded three infrared thermometer readings of the sample at 30-second increments to find the initial temperature. Once the lamp was turned on, the sample temperature was recorded every 30 seconds until the temperature was stable. The temperature was "stable" when it did not fluctuate more than 1 degree Celsius for 5 minutes, or 10 consecutive readings. At this point, the heat lamp was turned off, and temperature readings continued every 30 seconds until stable again. This process was repeated with different levels of saturation, or simulated water table: 10 cm (all) dry; 8 cm dry, with the bottom 2 cm wet; 6 cm dry, 4 cm wet; and 4 cm dry, 6 cm wet. Each different level of saturation was run at least twice. Figure 1-1. Experimental Set Up  Results: Initially, we found that moisture within a gravel sample decreases the time until stabilization during heating. We found that the cooling temperatures most closely resemble an inverse exponential graph. Through Logger Pro 3, we calculated the heating and cooling curves of the gravel at different saturations. The equations (Figure 1-2) were determined to be natural logs with the X being the time and Y being the temperature of the gravel. With these equations, we were able to find the heating and cooling curves of each gravel trial (Figure 1-3).     This experiment revealed several trends. In general, the deeper the saturation layer, the longer the sample took to stabilize, and the hotter it got before doing so. This trend was evident in the results for the runs that were all dry, 4 cm saturated from the bottom, and 6 cm saturated from the bottom. One exception to this general rule is the setup with 8 cm dry and 2 cm wet, which stabilized more quickly than the others. However, it still followed the trend in terms of the temperature it reached before stabilizing. In general, the data shows that the closer the water table is to the surface, the higher the thermal inertia of the sediment.  Discussion: An important characteristic of the sediments that exist on Mars is cohesion. The cohesion of the materials determines the interparticle force between the sediment grains. Small grain size means that there is less space between grains, causing more cohesion and more interparticle force. The interparticle force and capillary action is what allows water to seep between sediment grains. Based on this and water's high heat capacity, the gravel took longer to heat up because the water was absorbing the heat from the gravel at the same time the gravel was heating up from the heat source.  This experiment showed that the thermal inertia of gravel is increased by the presence of water beneath the surface. Furthermore, the depth of moisture below the surface determines how much the thermal inertia is raised. Models created from the data may provide a better understanding of water on Mars based on satellite temperature data. If models derived from this experiment are applied to THEMIS data, and the identity of the sediments in question are known, one could investigate the presence of liquid water on the surface of Mars. Sources of Error. When running this experiment, there were several sources of error. There was a constant fluctuation in our data due to sudden flows of cold air from the air-conditioning and the opening and closing of the door. To solve these problems it would be best to relocate to an area away from air conditioning vents and doorways.  Future Work:  In the future, thermal sensors could be used to take temperature readings under the surface of the sediment throughout the experiment. Also, water may evaporate during the experiment and affect the results. This could be investigated by weighing the sample before and after testing. Finally, sediment that is more similar to that found on Mars or with a wider variety of grain sizes and saturation levels could be tested. Acknowledgements:  We would like to thank Howard Lineberger, Samuel Fuerst, and Charles Payne for coordinating the MONS program. We also thank Dr. Jeff Moersch of the University of Tennessee, who is conducting similar research to design a model to best interpret THEMIS infrared data, and Nathan Sanders who compiled the experimental data and created the graphs. Finally, we would like to thank the Burroughs Welcome Fund for funding the MONS program. References: [1] Thompson, Andrea. "Did Phoenix Mars Lander Find Liquid Water?" Msnbc.com. Msnbc Digital Network, 11 Mar. 2009. Web. 21 Dec. 2012.  Figure 1-2 equation 1-- 10 cm of dry Gravel from surface  equation 2-- 8 cm of dry Gravel from surface equation 3-- 4 cm of dry Gravel from surface equation 4--6 cm of dry Gravel from surface  Figure 1-3 Blue- 10 cm dry Gravel from surface Turquoise/green- 8 cm dry Gravel from surface Green- 4 cm dry Gravel from surface Red- 6 cm dry Gravel from surface 
