Friday, April 12, 2013
Curiosity is on its own for most of this month because the sun is between Earth and Mars, blocking radio signals between the planets.
Instructions were sent to the Mars rover for work during this communication blackout, but we will have to wait until the planet clears the sun for the results.
This month we will look at two of the instruments used to analyze powdered rock and soil samples: the Chemistry and Mineralogy (CheMin) and the Sample Analysis at Mars (SAM).
CheMin, a 22-pound, 10-inch cube inside the rover, uses X-ray diffraction to determine the minerals present in a sample. This is the most definitive method of identification ever sent to Mars.
Curiosity collects rock samples with a percussive drill and soil samples with a scoop.
These tools, along with the sample-processing tool, are located on the end of the robotic arm. The sample processing tool sieves the material and removes large particles.
The robotic arm delivers the sample to a funnel with a removable cover, located near the front of the rover deck, that leads through the deck to a disc-shaped cell in CheMin.
There are 32 of these cells mounted on a rotating sample wheel. Five contain reference samples from Earth to calibrate the instrument; the other 27 are reusable holders for Mars samples.
In X-ray diffraction, a beam of X-rays is directed at the sample and the crystalline structure of the mineral scatters them at predicable angles that depend on the mineral present.
The spacing and intensities of the scattering, like fingerprints, are different for each mineral.
To analyze a sample, the sample wheel rotates to place the X-ray beam on one side of the sample and the detector on the other.
Data is collected for about 10 hours over several nights. CheMin can use this data to determine what minerals are present in the sample in concentrations greater than 3 percent.
Each mineral forms under a specific set of environmental conditions: chemicals present, temperature and pressure.
This means that the minerals identified by CheMin will provide information about the planet’s environment at the time of their formation.
The other instrument we will look at is the Sample Analysis at Mars (SAM), which is actually a suite of three analytical tools. SAM is using these tools to search for the chemistry of life on Mars, primarily carbon-based compounds that are the building blocks of life on Earth. SAM is more sensitive to organics and can identify a greater number of them than any other instrument we have sent to Mars.
SAM examines gases, both those in the Martian atmosphere and the ones released from powdered rock and soil samples that are heated in onboard ovens or dissolved in solvents. Solid samples enter SAM through a funnel system like the one used by CheMin, while atmospheric samples enter through a filtered inlet port on the side of the rover.
SAM’s three tools are a mass spectrometer, a tunable laser spectrometer and a gas chromatograph. The mass spectrometer identifies gases by their molecular weight and electric charge. SAM will use the mass spec to test for several elements, including nitrogen, phosphorous, sulfur, oxygen, hydrogen and carbon. All of these elements are important to life as we know it.
The tunable laser spectrometer uses the absorption of light at specific wavelengths to measure concentrations of methane, carbon dioxide and water vapor in the gases.
It can also determine the ratio of different isotopes of elements in the gases, like carbon-12 and carbon-13. (Isotopes are atoms of the same element with different atomic weights.) These ratios can tell us about past planetary processes, for example, how Mars lost much of its atmosphere.
The third tool is the gas chromatograph, which can separate the different gases in a sample that contains a mixture of gases. These separated gases can then be sent to other instruments, such as the mass spectrometer, for a more specific analysis.
SAM will determine if there are organic molecules on Mars. Finding organic molecules is not conclusive evidence of life, but life as we know it cannot exist without them.
This is an important step in determining the habitability of this site and whether it is capable of preserving evidence of life.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.