Mars Base Zero: Carbon Dioxide
Observations and Experiments
By Ray R. Collins
The carbon dioxide (CO2) level in Mars Base Zero has been fluctuating by 100 to 150 parts per million. These fluctuations appear synchronous with the temperature fluctuations, which vary by about 10° Fahrenheit because of a lag in our thermostat. Why does the CO2 fluctuate like this? A second observation is that the trend (over days) in CO2 is up 5-10% as the temperature warms up outside, which might allow for a warmer soil bed (and thus more microbial activity). The rest of this paper examines the possible causes of the short-term (20 minute) cycles in CO2.
Four possible reasons to consider
a) More CO2 is given off by the soil in increased temperatures due to increased soil activity. If this is the case we should observe:
· As more water is added to the soil the CO2 should increase due to more microbial activity—the microbes are no doubt stressed due to water. (The soil in unplanted areas is very dry, for our humidity has been as low as 8%).
· As the temperature of the soil bed increases more CO2 should be given off. THE OPPOSITE OCCURS!
· There should be a delay between the air temperature and the CO2 increase, for it takes time for the temperature to move from the air down into the soil bed. THIS IS NOT OBSERVED!
· Therefore we can rule out increase soil microbial activity as a primary contributor to the short-term CO2 fluctuations.
b) The sensor may be temperature dependent, and give falsely low or high readings as the temperature varies. If this is the case we should observe:
· If the sensor is put in a sealed bottle the CO2 readings should continue to vary with temperature.
c) The plants should absorb more CO2 at higher temperatures, causing the CO2 to fall.
· The number of plants (3 square meters of wheat, 1 square meter of other plants) may not be sufficient to cause this reaction.
· There should be a lag between the time the plants warm up and the low in CO2, for it will take time for the plants to absorb CO2 from the air. This is NOT OBSERVED! There is no lag time.
d) Respiration of animals (chickens, humans) increases at higher temperatures. If this is the case we should observe:
· A low in the CO2 at the optimum temperature for the animals. THIS IS NOT OBSERVED.
· Size of the changes would be dependent on the number of occupants. THIS IS NOT OBSERVED.
· Therefore we can rule this possibility out.
The Experiment
Our observations above indicate that the CO2 is fluctuating due to sensor changes at different temperatures. So the experiment suggested above is done to see if this is the cause of the fluctuations.
Method
Our CO2 sensor is plugged into a Universal Lab Interface and monitored on a 500-megahertz computer using Logger Pro. All software and hardware was purchased in December 1999 from Vernier Software.
The sensor was placed in an airtight bottle. Thus, if the sensor is sensitive to changes in air temperature it will continue to vary with temperature, though a slight delay should be observed as it takes time for the temperature to penetrate the bottle, with somewhat of a flattening effect. (Note: the temperature probe is located outside the bottle.)
The heating system is composed of two parts. The first part, a hot water baseboard loop, is on throughout the duration of this experiment. The second part that determines the temperature in the greenhouse is a 100,000 BTU unit heater. Hot water is always circulating through the unit heater, with a fan that is thermostatically controlled so when heat is needed the fan is turned on. The temperature change is rapid enough so that there is a significant lag between the time the air has warmed up and the time the thermostat warms up, resulting in fairly wide temperature swings between the time the fan comes off and when it shuts off.
This experiment started 1/26/00 at 11:25 am. There was not enough sunlight to cause any variation in inside temperature. At time = 20 minutes (11:45) the sensor was placed in an airtight bottle. The temperature in Mars Base Zero was turned up slightly, to make it cycle more noticeably. At this setting the heater comes on at about 70° F and goes off at about 78° F. The temperature usually continues to rise for a few minutes as the air gets distributed.
Occupants: 1 person, 5 chickens, plants and soil.
The total temperature range during the experiment was 67° F to 84° F, with the average fluctuation of about 10 degrees (70° to 80° F).
Readings of CO2 and temperature were taken every 5 minutes. See graph 1.
At 250 minutes (the time was 15:35) the CO2 monitor was removed from the sealed bottle. The monitoring was continued until 380 minutes.
Results
CO2 fluctuations continued to occur in the sealed glass jar (see Graph 1). The total CO2 range was 517 ppm (parts per million) to 850 ppm, with the average range from about 675 ppm to 800 ppm. Even after the sensor was removed from the bottle no change is observed in the readings.
There is no lag time between the temperature change and the change in CO2 reading, indicating that the jar was not leaking (if it were leaking a CO2 change into the jar, there would be a lag time between the time of the temperature change and the time of the CO2 change. However the expected delay expected due to the time it takes for the temperature change to penetrate the bottle did not occur. This may be because our readings are far enough apart so the temperature changes had time to penetrate the bottle between readings. If several readings were taken per minute the suggested lag may be observed.
Graph 1. Temperature and carbon dioxide swings. The temperature (which is measured in Fahrenheit), was multiplied by 8 to make easy to compared with the carbon dioxide (which is measured in parts per million). Bottom scale is time in minutes from the start of the experiment.
Conclusion and Discussion
Mars Base Zero is a semi-sealed greenhouse in which closed ecological life support experiments are being conducted. As a part of the experiment, we are enriching the atmosphere with CO2 to help promote rapid plant growth. During the time of this experiment we were enriching it by about 300 parts per million, to around 700 parts per million. To make sure we were maintaining the CO2 at the level we desired, we have been taking measurements at 5-minute intervals. These measurements detected a regular swing in CO2 level that appeared coordinated with temperature. A number of theories for this change are examined, and the most likely tested for.
The conclusion from our experiment is that the observed swings in CO2 are related to sensor changes, rather than actual changes in CO2 in the atmosphere. Therefore we conclude that the sensor needs to be maintained at a constant temperature in order to have consistent CO2 readings.