LANL sleuth on trail of Martian mystery

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When it comes to examining the surface of rocks on Mars with a high-powered laser, five is a magic number for Los Alamos National Laboratory postdoctoral researcher Nina Lanza.
During a poster session today at the 44th Annual Lunar and Planetary Science Conference at The Woodlands, Texas, Lanza described how the laser-shooting ChemCam instrument aboard the Curiosity rover currently searching the surface of Mars for signs of habitability has shown what appears to be a common feature on the surface of some very different Martian rocks during Curiosity’s first 90 days on the Red Planet.
But exactly what that common feature is remains an intriguing mystery — and one that Lanza intends to solve.
The ChemCam instrument uses an extremely powerful laser to vaporize a pinpoint of rock surface. The instrument then reads the chemical composition of the vaporized sample with a spectrometer. The highly accurate laser can fire multiple pulses in the same spot, providing scientists with an opportunity to gently interrogate a rock sample, even up to a millimeter in depth. Many rocks are zapped 30 to 50 times in a single location, and one rock was zapped 600 times.
Members of the ChemCam team generally discard results from the first five laser blasts because of a belief that after the first five blasts.
Instead of tossing out those data, however, Lanza looked at them specifically across a diverse set of Martian rocks. She found that the first five shots had chemical similarities regardless the rock type. What’s more, after five shots, like other scientists had noticed, the spectrum from the vaporized rock stabilized into a representative sample of the rock type below.
“Why is it always five shots?” Lanza wondered.
It could be the first five shots were reading a layer of dust that had settled onto the surface of every rock, but results in laboratories on Earth seem to indicate that the first laser blast creates a tiny shockwave that is very effective at clearing dust from the sample. Therefore, if the first blast is dusting off the rocks, the remaining four blasts could be showing that Martian rocks are coated by a substance, similar in structure if not composition, to the dark rock varnish appearing on Earth rocks in arid locations like the desert Southwest.