Thursday, September 6, 2012

Ultrafast Optical Pulses and Hip Hop Play a Critical Role on Mars Rover

Artist Rendering of ChemCam Laser
Analysis on Mars Science Laboratory. From
What do front man for the Black Eyed Peas,, and ultrafast optical pulses have in common? They are both playing crucial role on the newest Mars rover mission. On August 28,'s song "Reach for the Stars" was the first musical composition to be transmitted to Earth from another planet, in this case from Curiosity, twelve days after its  Seven Minutes of Terror landing, complete with state-of-the-art supersonic parachute and sky-crane. I'm still a bit shocked at this science fictionesque feat of impressive engineering seeming to border on hubris. Really, a sky-crane? Really?

While's  interplanetary music transmission is playing a critical role in science and engineering outreach as part of google+ and Lockheed Martin sponsored initiative SYSTEM (Stimulating Youth for Science Technology Engineering and Math), ultrafast optics is playing a critical role for analyzing the geology of the martian surface. On August 19, ChemCam, an instrument that is a part of the Mars Science Laboratory on board Curiosity, ablated part of a rock with ultrafast optical laser pulses and performed chemical analysis on the emitted plasma to determine rock and soil composition, a first for exogeology. Though the technique, laser induced break-down spectroscopy (LIBS), is almost as old as the laser itself, it has never been performed on another planet. What makes LIBS so useful for Mars exploration is that as an active remote sensing technique, no physical contact needs to be made with the rock or soil under test, including cleaning the sample area.

The previous Mars rovers required a rock abrasion tool to remove dust and outer layers to analyze the more interesting unweathered interior of rock and soil samples. On Curiosity, initial pulses "clean" the area and subsequent pulses create the plasma of interest whose spectrum is to be analyzed. For this instrument standoff distances can be as far as 7 m. The LIBS instrument has been combined with a Remote Micro-Imager (RMI) to give contextual information around the approximate 0.5 mm LIBS interrogation points in a single instrument called ChemCam. The figure below shows the precision of the laser system as well as the resolution of the Micro-Imager at 3 m stand-off.  The choice to burn precision holes in the U.S. dollar and Euro (near Toulouse, France on the Euro map) is in homage to locations of the collaborating institutions  Los Alamos National Laboratory, Centre National d'Etudes Spatiales, and Centre National de la Recherche.

Demonstration of ChemCam's shooting accuracy and micro imager resoltion at 3 m
standoff after ablating holes in U.S. and European currency respectively. The inset
 (lower left) shows the difference image. Image from poster "Progress on Calibration
of the ChemCam LIBS Instrument on the Mars Science Laboratory Rover,"
by principle investigator R.C. Weins, 2010. 

Besides the ultrafast laser system, ChemCam is a goldmine of optical engineering and instrumentation. There is honestly something for almost any kind of optical scientist on this instrument. Details can be found both on the ChemCam website and in a review of the instrument suite (an easy geeky read which I had trouble putting down). The laser and imaging optics reside in the mast of ChemCam (the seeming periscope-like eye of the rover) and the spectrometers and supporting equipment live in the body unit. The mast and body are connected by optical fiber.

Schematic of ChemCam. From "The ChemCam Instrument Suite on the Mars
Science Laboratory Rover: Body Unit and Combined System Tests," Space
Sci. Rev., DOI 10.1007/s11214-012-9902-4, (2012).

As a laser scientist, when I read the initial news of ChemCam, I wanted to know as much as possible about the ultrafast laser on board. What kind of pulses do you need to in order to create a plasma from rock? What power? What width? What wavelength or wavelength range is required for LIBS analysis? Was it a fiber laser or solid state version?

The laser on ChemCam is a neodymium potassium gadolynium tungstate (Nd:KGW) at a center wavelength of 1067 nm made by Thayles Optronique. Pulses are 5 ns in duration with more than 10 mJ of energy in order to deliver 10 MW of power per square millimeter to the target. The repetition rate is very low, 1-10 Hz, in order to maximize pulse energy. It turns out that the wavelength is not very special and could be anything from visible to near IR- the field strength is what is most important for creating the plasma. The ChemCam team chose a laser in the 1.0 micron region due to the simplicity and practicality of obtaining the necessary energy density for LIBS. However, another advantage to choosing excitation light at 1.0 microns is that it is longer than the reddest wavelength of expected characteristic emission lines from the plasma. These lines range from 240-850 nm.

ChemCams first spectrum using data collected from a rock near the landing site dubbed,
 "Coronation". Image from Andy Shaner's ChemCam blog.

So far preliminary data from ChemCam's  LIBS instrument, show clean "textbook" spectra. The first spectrum (shown above) is consistent with basalt, a type of volcanic rock which is known to be present on Mars. The carbon peak in the spectrum comes from the carbon dioxide-rich martian atmosphere. Hydrogen disappeared after the first shot, indicating it was only on the surface, and the concentration of magnesium became less with subsequent lasr shots. ChemCam began ablating rocks on the martian surface August 19, and has since been taking more target practice. The latest picture from ChemCam on Augusts 25, (see below) shows a 5 x 1 raster scan to investigate chemical variability across the sample.

A before and after photo of a 5x1 raster scan during an August 25, chemical variability analysis.

The feats being accomplished by Curiosity are truly amazing. It seems the sky is the limit when it comes to what this rover can wait, to quote "Reach for the Stars",

"Why do they say the sky is the limit when I've seen footprints on the moon?...let's reach for the stars"

I take that back, the stars are the limit. Thanks

1 comment: