Looking for Water on Mars:
Phase Change and Thermal
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
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.
Equipment and Materials
- sand (already dampened with water for those
groups observing the sand and water samples)
- wax paper strip
- transparent tape
- markers to label the containers
- petroleum jelly
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?
- Mark a line 3cm from the top of each plastic
glass. This line is the fill line for the sand in each plastic glass.
- Fold over about 0.5 cm of a straw. Use tape to hold the fold over section next to rest of the straw.
- 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
- 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).
- 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
- If needed, trim
the straw so that it is even or slightly below the top of the plastic
- Repeat the
steps #1 to 6 for the "wet" sample. Cover this sample with plastic wrap
or seal in a baggie.
- Set both
samples aside to be placed
in a freezer overnight or until the next class.
Equipment and Materials:
- CBL2 *
- CBL temperature probes**
- TI 83 Plus graphing calculator *
with CHEMBIO** program
- previously prepared simulated soil
samples - one with dry sand and one with wet sand
- 1000 ml beaker to hold iced water baths
- 400 ml beaker filled about half full
- ice and cold water for water baths
- hot plate
- themistor or themometer for monitoring
temperature of the hot water in the 400 ml beaker
* Texas Instruments **Vernier Software
- 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.
- Fill the 400 ml beater with water and
place on the hot plate. Heat the water to about 40oC.
- While the water in the 400 ml beaker is
heating, attach a temperature probe to the CBL.
- 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.
- 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.
- 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.
- Insert each
temperature probe into the straw.
- Record the
temperature every 10 seconds for a total of 10 minutes.
- Remove the
- Link the
calculator to a computer and print out the data table with time and
temperatures for each sample.
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
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.
2. The atmospheric pressure on Mars is 0.007 atm and the temperature
could range from -120°C to 20°C. Water sublimes
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
© Sally Urquhart, 2004
Developed with Dr. Mary Urquhart, Univ. of Texas at