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Laser probes planetary surfaces

DOE/Los Alamos National Laboratory : 08 May, 2007  (Technical Article)
With a spark from a small laser, researchers from the Department of Energy's Los Alamos National Laboratory can analyze soils and rocks from more than 50 feet away.
The prototype instrument they are developing for planetary exploration combines a laser the size of a small flashlight, optics and a spectral analyzer into a compact, low-power package.

'We can fire the laser every five seconds and within three minutes get enough data for an accurate measurement,' said Los Alamos' David Cremers, principal investigator on the NASA-funded development effort. 'The scientific returns for planetary exploration increase dramatically when you can conduct such rapid analyses.'

In a related effort, a Los Alamos team led by Roger Wiens is combining a laser and a mass spectrometer to conduct standoff analyses of asteroids or other airless bodies.

Cremers and Wiens provided reports on their development efforts at the Lunar and Planetary Science Conference in Houston.

Cremers' laser-induced breakdown spectroscopy technology has been under development at Los Alamos for 18 years, but up to now has been applied to Earth-based purposes. It has been field tested for elemental analysis of rocks, soils, gases and airborne aerosol particles. A private company is developing a field-portable LIBS unit for mining and environmental monitoring applications.

LIBS works by firing a brief, intense laser pulse at the surface of an object. The laser heats and vaporizes a small spot, about as wide as a pencil eraser, on the surface. A small telescope co-mounted with the laser captures light from the glowing vapor and feeds it into the spectral analyzer. Elements create unique spectral signatures that signal their presence; with correct calibration, the intensity of the emissions reveals the relative abundance of the elements.

Cremers and his colleagues have shown in lab tests they can get accurate measurements for a variety of key elements from a sample some 60 feet away, and expect the technique would be even more effective in the thin atmosphere of Mars.

'We didn't know going in how well we'd do with such a small laser,' Cremers said. 'This time nature was nice to us.'

The LIBS technique could be especially useful for planetary exploration because of its ability to conduct analyses at a distance. A rover would not have to cross hazardous terrain to sample important rocks or strata. LIBS could reach up to cliff faces or across craters or peek inside cracks and crevices.

The laser also can blast through the weathered veneer on a rock and reveal the true composition hidden beneath.

The end product of the three-year, $1.1 million development effort will be a prototype LIBS instrument for field tests in the Mohave Desert. A flight model of the instrument would require additional development work to reduce the instrument's size and power requirements and increase its ruggedness.

Wiens leads a related development effort for LIMS, or laser ionization mass spectrometry. In this system, the laser ablates material from the surface of an object and the freed, ionized atoms enter a mass spectrometer, which can provide compositional measurements based on the mass of an ion. The LIMS and LIBS techniques are complementary and both can use the same laser to ablate a sample for analysis.

Los Alamos has a history of advances in lightweight, low-power mass spectrometers and currently has such instruments flying on NASA's Cassini and Deep Space 1 instruments.

'In fact, our development work will make use of spares from the space plasma mass spectrometers on Cassini and Deep Space 1,' Wiens said.

The Los Alamos team has installed rock and soil targets and a mass spectrometer in a vacuum system and started gathering initial data. A more complicated spectrometer will be used starting in a few months.

The team will use the test results to optimize the overall system for effective collection of ions kicked free by the laser and high-resolution mass measurements. The team also will study tradeoffs between the distance at which the instrument can make reliable measurements and overall mass and cost.

'Our intention through this effort is come up with an instrument that eventually could be incorporated into a lander craft for the moon, an asteroid or an outer solar system body,' Wiens said.

NASA funding for the LIMS effort totals $360,000 over two years.

Others involved in these development efforts include James Blacic, Monty Ferris, Herb Funsten, Andrew Knight, Jane E. Nordholt and Steve Ritzau of Los Alamos and Wendy Calvin of the U.S. Geological Survey.
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