Looking for Water on Mars:

Phase Change and Thermal Conductivity

Phase changes occur as potential energy is converted to kinetic energy when a material is heated and changed from a solid to liquid, liquid to a gas, or a solid to a gas as in the case of sublimation. Kinetic energy is converted to potential energy when the situation is reversed and the material cools forming a solid from a liquid, a liquid from a gas, or a solid directly from a gas. When a graph is made of time versus temperature over a temperature range that covers the melting and boiling points of the material, plateaus occur where the phase changes actually take place. It is then possible to look for a phase change as a method to determine the possible presence of that material in a given sample.

Thermal conductivity is a property of matter that measures how well heat is transferred through a material. Metals usually have high thermal conductivity while nonmetals often have low thermal conductivity. The thermal conductivity of a given material is altered when the material contains an "impurity" or is a mixture of two different substances.

Use of phase changes and thermal conductivity are two parameters that can be used to determine the presence of water in subsurface soils. During this laboratory activity, the effect of the presence or absence of water in simulated soil samples on thermal conductivity will be examined. The simulated soil samples represent samples taken from dry, cold areas such as the surface of Mars.

Making Samples

Equipment and Materials

  1. Mark a line 3cm from the top of each plastic glass. This line is the fill line for the sand in each plastic glass.
  2. Fold over about 0.5 cm of a straw. Use tape to hold the fold over section next to rest of the straw.
  3. Wrap the straw with wax paper. Seams in the wax paper are to be covered with the transparent tape. Coat the wax paper with a thin layer of petroleum jelly.
  4. Mark each container as containing dry or dampened sand and the identification number for your lab station. The identification number for the sample is given as follows: Period - Lab Station Number - D (for dry) or W (for damp sand).
  5. One person in each group holds the straw in the center of the container while another member of the group carefully adds the dry sand to the fill line on each plastic glass. Use a small spoon to occasionally tap down the sand to compact it and eliminate any air pockets that might form during the filling process.
  6. If needed, trim the straw so that it is even or slightly below the top of the plastic glass.
  7. Repeat the steps #1 to 6 for the "wet" sample. Cover this sample with plastic wrap or seal in a baggie.
  8. Set both samples aside to be placed in a freezer overnight or until the next class.

    Lab Procedure

    Equipment and Materials:

    1. CBL2 *
    2. CBL temperature probes**
    3. TI 83 Plus graphing calculator * with CHEMBIO** program
    4. previously prepared simulated soil samples - one with dry sand and one with wet sand
    5. 1000 ml beaker to hold iced water baths
    6. 400 ml beaker filled about half full
    7. ice and cold water for water baths
    8. hot plate
    9. themistor or themometer for monitoring temperature of the hot water in the 400 ml beaker

      * Texas Instruments **Vernier Software


      Procedure:

    1. DO NOT REMOVE the plastic glasses containing the simulated soil samples from the cooler UNTIL READY TO PLACE each plastic glass in an iced water bath.
    2. Fill the 400 ml beater with water and place on the hot plate. Heat the water to about 40oC.
    3. While the water in the 400 ml beaker is heating, attach a temperature probe to the CBL.
    4. Setup calculator and CBL2.

        Using the "CHEMBIO" program, set up the temperature probe using "USED STORED" for the calibration.

        Choose "TIME GRAPH" from the "DATA COLLECTION" menu. Enter "10" as the time between samples, in seconds. Enter enter "60" as the number of samples.

        Enter "-10" as the minimum temperature (Ymin). Enter "100" as the maximum temperature (Ymax). Enter "5" as the the termperature increment (Yscl).

        Use "stat plot" and set up plots 1 and 2. Plot 1 used "L1" for the x-axis and "L2" for the y-axis. Plot 2 also uses "L1" but "L3" is used for the y-axis. Turn on both plots.

        DO NOT PRESS "ENTER" UNTIL THE TEMPERATURE PROBE IS INSERTED INTO THE SIMULATED SOIL SAMPLE.

    5. Add ice to cold water mixture in the 1000 ml beaker to make an iced water bath. Carefully place one of the plastic glasses with the simulated soil samples in the iced water bath. The ice water bath must extend above the surface of the simulated soil sample BUT NOT ABOVE THE TOP OF THE PLASTIC GLASS.
    6. Place the temperature probes into the heated water. Wait a few minutes for both temperature probes to reach the temperature of the water. (Heating the temperature probes in hot water simulates the heating probes would undergo in order to penetrate frozen soil or would experience if the probes passed through an atmosphere and impacted the surface of a planetary body such as Mars.) DO NOT DISCARD THE HOT WATER until the temperature probes have been inserted into both samples.
    7. Insert each temperature probe into the straw.
    8. Record the temperature every 10 seconds for a total of 10 minutes.
    9. Remove the temperature probes.
    10. Link the calculator to a computer and print out the data table with time and temperatures for each sample.
    11. Graph the data as temperature vs. time. Place the data for both samples on the same graph.


    Looking for Water on Mars: Phase Change and Thermal Conductivity

    Name___________________________________________ Period_____ Date__________________

    Lab Partner(s)_______________________________________________________________________

    1. Turn in one set of data tables and graphs for each lab group.

    2. Each lab partner is to turn in answers to the following questions.

    Questions:

               1. Why must the iced water bath extend at least as high as the top of the sand whether the sand is dry or wet?

                   2. The atmospheric pressure on Mars is 0.007 atm and the temperature could range from -120°C to 20°C. Water sublimes                                (goes directly from a solid to a gas) at that pressure and for most of the temperature range. Would any differences in the
                        observations be expected? If so, why?


                   3. From the graph(s), what conclusions can be drawn about the thermal conductivity of water? Why?

                   4. If a probe penetrates soil to a depth of 15 cm., would the data collected also apply to depths below 15 cm.? Why or why not?

                   5. If a future probe shows that Martian soil behaves similarly to the sand used as the simulated soil, what would be the                                                     observation about the thermal conductivity of the Martian soil if water is present? Why? (Written conclusion - worth 8 points)


                    © Sally Urquhart, 2004

                    Developed with Dr. Mary Urquhart, Univ. of Texas at Dallas