In 1999, NASA’s Lunar Prospector revealed concentrated hydrogen signatures detected in permanently shadowed craters at the lunar poles. While scientists have long speculated about the source of vast quantities of hydrogen at the poles, recent discoveries made by NASA’s Lunar CRater Observing and Sensing Satellite (LCROSS) are shedding new light on the question of water on the Moon.
Water and other compounds found on the Moon represent potential resources that could sustain future lunar exploration. In situ resources are very important since they do not have to be launched out of Earth’s gravitational well. It costs about $50,000 per pound to launch a payload to the Moon. And since water is one of the resources that will have to be resupplied to a manned lunar outpost, water would be part of the total cost of the payload taken to the Moon. One ton of water and one ton of oxygen per year would be required for the early stages of a manned outpost. This makes it necessary that a water extraction process will be developed for use at an outpost. And once the water is extracted, oxygen can be obtained from the water by electrolysis.
Microwave processing to extract water has unique advantages over other processes. Because of the high vacuum, the thermal conductivity of lunar soil is very low. Additionally, microwave energy is advantageous because it heats from the inside out. This means that the excavation of lunar soil could be unnecessary, thereby minimizing Moon dust and the negative aspect of perhaps having to strip-mine the Moon.
The basic components of the microwave extraction system include a microwave source, waveguides to deliver the energy to the soil, and a cold trap to capture the water vapor (Figure 2). First, the microwave energy penetrates and heats the soil and, since ice is relatively transparent to microwave energy, heat is transferred from the soil particles to the water ice condensed onto the surface of the soil. On the Moon, water ice transforms directly to water vapor by sublimation. Once in the cold trap, the water vapor will transform back to ice. In addition to the system components, a power source and a rover to transport the extraction system will be necessary.
Since the microwave processing parameters and hardware requirements for water extraction is a complex multiphysics problem, NASA employed simulation to address the challenges. COMSOL Multiphysics is being used to calculate the microwave penetration into, and heating of, simulated lunar soil. The properties of the simulant are approximated by complex electric permittivity and magnetic permeability measured in the lab. Calculations can be performed on different geometries, for a range of microwave frequencies and different power levels, for the simulated lunar soil. Since the temperature varies with time as the soil heats, temperature-dependent soil dielectric properties can be incorporated into the model along with temperature-dependent thermal conductivity of the soil.
For the simulation, NASA used the software’s RF Module to model the microwave power penetration and attenuation into the soil (Figure 3). When the model was running without error, the physics of heating and heat flow were added. A transient analysis was used to determine heating as a function of time. (An AVI movie is available at http://science.nasa.gov/science-news/science-at-nasa/2009/07oct_microwave/ that shows lines of constant temperature as the heating progresses.)
Development of an early experiment payload for a lunar lander mission requires the specification of the microwave frequency, power, and method of delivery of power. Developing experiments with several different microwave frequencies would require a significant investment of resources, manpower, and time to perform experiments. COMSOL permits the calculation of microwave penetration and heating that could be expected with different experiment scenarios. This can reduce the time, labor, and cost to narrow the hardware requirements for the experiment.
This work was done by Dr. Edwin Ethridge of Marshall Space Flight Center using COMSOL Multiphysics software. For more information, Click Here