Issue Date: March 31, 2014
Zapping Rocks On Mars
“It’s been a good year on Mars,” said Roger C. Wiens, speaking at a symposium at Pittcon, held earlier this month in Chicago. Wiens, a scientist at Los Alamos National Laboratory, is principal investigator for ChemCam, an instrument on the Curiosity rover, the latest vehicle to explore the surface of Earth’s next-door neighbor. During the past 18 months, ChemCam has acquired more than 120,000 spectra, which are helping to elucidate the geologic history of Mars.
ChemCam stands for Chemistry & Camera. Mounted on Curiosity’s mast, the instrument captures high-resolution images of the rover’s surroundings. It also determines the chemical composition of rocks and soil on the martian surface to identify promising samples for more detailed analysis by other instruments on the rover. But unlike those other instruments, ChemCam can analyze rocks and soils up to 7 meters away. To do that, it uses a technique called laser-induced breakdown spectroscopy, or LIBS.
LIBS is a spectroscopic technique in which laser pulses ablate material from the surface of a sample and generate a plasma that breaks the material down into its constituents—mostly atoms but some molecules—which emit light at characteristic wavelengths. Depending on the range of the spectrometer used, LIBS can detect any element.
In ChemCam’s LIBS system, a laser beam is expanded through a telescope and zaps target rocks. Sample emission comes back to the instrument through the same telescope. The instrument’s spectrometer is actually three simple spectrometers—optimized for ultraviolet, violet, and visible and near-infrared—in one box. LIBS works better on Mars than it does in terrestrial environments, Wiens said, because the martian atmosphere is less dense than Earth’s.
For much of the time since landing in the Gale Crater on Mars in August 2012, Curiosity has been heading toward a geologic feature dubbed Mount Sharp. It’s top speed is 150 meters per day, but the martian terrain has required it to travel more slowly and take a few unintended detours. All told, it has traveled about 6 km. It has about 5 km more to go.
Wiens commended NASA’s Jet Propulsion Laboratory, which has the job of aiming ChemCam. Hitting desired spots on Mars’s surface with ChemCam’s laser “is like throwing a dime 8 feet away and targeting different spots up and down Roosevelt’s head,” he said.
LIBS data are revealing much about the Red Planet. For example, the sampled dust on Mars is hydrated. ChemCam spectra show that the hydration is ubiquitous, and the onboard suite of analytical instruments has quantified the abundance of water at about 2–4% by weight. When shooting at most rocks, hydrogen appears in only the first laser shot. That hydrogen comes from “water in the dust that gets blown off the surface of the rock,” Wiens said.
The LIBS images also reveal signs of hydrothermal alteration on Mars. The scientists see veins of hydrated calcium sulfate on fractured rocks. On raised ridges in an area called Yellowknife Bay, the levels of magnesium and lithium go up, indicating that magnesium-rich clay was injected through cracks in rocks.
Most recently, Wiens said, LIBS data have provided the first evidence of fluorine on Mars. Scientists had assumed that there’s fluorine on Mars, but this is the first time it’s actually been detected.
The fluorine results were reported in more detail at the Lunar & Planetary Science Conference, held in The Woodlands, Texas, two weeks after Pittcon. At least 20 locations analyzed by ChemCam contain bands that match laboratory-acquired spectra showing calcium-fluoride molecular emissions. Two candidates for the fluorine-containing minerals are fluorite (CaF2) and fluorapatite (Ca5(PO4)3F).
And there’s more to come. Curiosity continues its trek across a small patch of Mars. “We hope to get to the base of Mount Sharp sometime around the end of the year,” Wiens said.
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