Scientists believe that the answer to what happened to the missing water may lie locked up in the rocks, and could be discovered by understanding just what the soils are composed of, and learning how much water and carbon dioxide are chemically locked up inside.
To help reveal what happened to Mars' water, the Mars Polar Lander will dig soil samples from the frigid martian terrain and cook them.
A key instrument aboard the spacecraft -- the Thermal and Evolved Gas Analyzer, or TEGA -- will accomplish this task. The instrument houses eight tiny ovens that can each be used to bake a 30-milligram (about a thousandth of an ounce) sample of martian soil to analyze its composition.
By heating soil from its ambient temperature of about minus 75 degrees Fahrenheit (minus 60 Celsius) up to almost 1,750 degrees (950 Celsius), the instrument breaks down the soil components. The oven carefully tracks the temperature and the amount of energy required to heat the sample while a sophisticated laser, called a tunable diode laser, measures the amount of water vapor and carbon dioxide that comes out of the sample.
"By melting whatever is in the oven and flushing whatever gasses are evolved down to the tunable diode laser, we can begin to determine what components make up the martian soil," said Hop Bailey, project manager for the TEGA instrument. The instrument was built entirely at the University of Arizona under the direction of planetary scientist William Boynton, who is principal investigator for the project.
The lander is equipped with a robotic arm that has specialized digging blades and a scoop that will scratch or trench into the polar surface (depending how hard it is), scoop up soil samples and dump them into each one of the TEGA's eight chambers. If the surface is soft enough to allow easy digging, the arm will be able to work down to two feet below the surface.
A miniature camera on the arm can zoom in on sand-size grains of material, allowing scientists on Earth to examine various soil layers to pick the samples they want to test in the TEGA.
When a scoop full of specially-selected dirt is brought over the TEGA, a pair of doors open and the arm dumps into the open chamber below, covering the instrument with excavated soil. The sample material falls through a sifting screen and is funneled down into a tiny cylinder, about the diameter of a ballpoint pen's ink tube.
The bottom inch of that tube is the TEGA's oven. It is wrapped with a heating element and a platinum temperature sensor and encased in three ultra-thin layers of ceramic insulating material.
When enough flakes and flecks and dusty grains of dirt have filled the tube, the oven cylinder rotates upwards. It is capped by another tube that seals the top of the oven and vents the evolved gasses out to the laser sensor when the oven is turned on.
The TEGA's lasers detect water vapor and carbon dioxide because they are tuned to the exact wavelengths where these gasses absorb light. By measuring the amount of laser light that is absorbed as a beam shoots through the gas that comes off a sample, scientists can tell how much of each gas is evolves out of a soil sample at any given instant.
Measuring water and carbon dioxide will not only tell scientists how much of those compounds are frozen in the soil as ice, it will also indicate the amounts and kinds of minerals that make up the soil. That's because these gasses are byproducts of chemical changes that minerals undergo when heated.
Scientists think a lot of the water that used to cover so much of Mars is locked up in minerals called carbonates. Carbonates, like limestone on Earth, can only form in the presence of water and they use up water when they form. When carbonates are heated, they give off carbon dioxide. Members of the TEGA team hope that the carbon dioxide measurements will help them determine just how much of the martian soil is made up of carbonates and what kind of carbonates those are.
But the gas measurements only tell half the story.
The TEGA's heating chambers are actually much more complicated than mere ovens. They are thermal analyzers that measure the amount of energy required to heat each sample through the full range of temperatures up to 1,750 Fahrenheit. The science that measures energy to identify materials is called calorimetry, after calories - the units used to measure amounts of heat.
The TEGA experiment takes advantage of the fact that different materials melt and vaporize at different temperatures, and they each require different amounts of heat to make those transitions.
The amount of energy needed to raise the temperature of a certain mass of liquid water by one degree is about the same regardless of its temperature. This is also true of water ice and vapor. Heating a gram of water by one degree takes about the same amount of energy whether its temperature is 50 degrees Fahrenheit or 150.
It requires much more energy, though, to take water through a phase change - that is, to change it from solid to liquid or from liquid to gas. The amount of heat required to make that phase change is called a material's heat of transition. The science of calorimetry is used widely in laboratories to identify unknown minerals by measuring their heats of transition. Different substances have different heats of transition, so determining a substance's heat of transition can help scientists identify the mineral.
For instance, a soil sample with a lot of water ice mixed in will heat smoothly as it approaches the melting point of water. When it reaches 32 degrees Fahrenheit (zero Celsius), the temperature will stop increasing for a moment and it will plateau until enough energy has been input to cause the ice to melt. Then the sample's temperature will begin increasing smoothly again, and will continue to do so until it reaches a temperature at which another component material changes phase.
By measuring the exact temperature and amount of heat used in each, then coupling that information with the data from the laser, the TEGA team hopes to determine the types and amounts of materials that make up the martian polar surface.
The team has already successfully identified two mystery samples that NASA scientists specially prepared as mock martian soil samples. The team used an engineering model created by TEGA to identify the major component minerals in the sample, and came out "right on the money," Bailey said.
There are about 500 different mineral types that scientists have listed as candidates for what might be found on Mars, Bailey said. TEGA can come very close to pinpointing any mineral in this list, he said.
"In some cases [the TEGA test is] definitive," he said. "You narrow it down to a point where there are no other choices. But in other cases it's not and you end up with maybe two or three choices out of 500 or so."
In that case, team members would propose a further experiment, possibly heating a collection chamber through a second run to extract additional information, Bailey said.
"The belief is that there was a lot of water on Mars at one time, that as much as half of the planet's surface maybe was covered by water, either by an ocean or a shallow lake," Bailey said. "So if we don't find carbonates, there's going to be a lot of people scratching their heads."
He emphasized, though, that the team will not strike out.
"We're going to find a lot of carbonates," he said. "A lot of different types and large quantities. The instrument's going to work. If they get us to the surface and they get us soil, we're going to get some results."
Those results could come near the end of the first week of the polar lander's mission on Mars. The TEGA is scheduled to test its first sample on the sixth and seventh martian days after landing, Dec. 8 and 9 on Earth. Bailey said he expects the team to be ready to make its first announcement about the makeup of the martian polar soil by Dec. 11.
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