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A Martian dust devil roughly 12 miles (20 km) high was captured winding its way along the Amazonis Planitia region of northern Mars on March 14, 2012, by the High-Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter. Despite its height, the plume is little more than three-quarters of a football field wide.
Above: A Martian dust devil roughly 12 miles (20 km) high was captured winding its way along the Amazonis Planitia region of northern Mars on March 14, 2012, by the High-Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter. Despite its height, the plume is little more than three-quarters of a football field wide.

TLS Development

Towards a Smaller More Sensitive TLS

Erik Alerstam
Amazing discoveries have come from the tunable laser spectrometer (TLS) aboard the Curiosity Mars rover, which is part of NASA's Mars Science Laboratory mission (MSL). since it landed in August 2012. Among these was the detection of methane on the Martian surface MSL’s TLS is a remarkable instrument, but in the years since its deployment, JPL has made great strides towards smaller laser spectrometers—1 kg or less—that are more sensitive and more robust, and that possess expanded capabilities. Among these are small atmospheric probes, landers, rovers, and smallsat/CubeSat implementations where gas abundances and isotope ratio measurements are needed. Making a smaller, robust, and more sensitive TLS is key to understanding the other worlds in our solar system.

The sensitivity of TLS instrumentation is limited not by light source intensity, since MDL’s semiconductor lasers produce high output power, but rather by optical interference fringes within the multipass cell. Moving away from the traditional Herriott cell optomechanical configurations in which spot-pattern overlap enhances optical fringes, new solutions were sought that still produce long pathlengths (meters) but with equidistant spot pattern spacings that lead to improved sensitivity.

Engineers from outside MDL working with the TLS team developed two sample cell configurations: one to address the need for improved sensitivity and a second to reduce the sample gas amount needed (e.g., for comet sampling). The first configuration uses a long pathlength comparable to the original TLS, but with optical beam spots more evenly spaced to avoid optical fringes that otherwise limit sensitivity. This first configuration was tested both at room temperature and at Mars conditions in a thermal-vacuum chamber, to reveal that the optical fringes observed were a factor of 10 less than those in TLS so that this new configuration is 10 times more sensitive to gas detection. The complete instrument weighs only ~1 kg, but outperforms MSL’s TLS by a factor of 10 for methane detection, being able to detect 0.2 parts per billion in 1 hour. The second sample cell design used a cylinder-within-a-cylinder configuration to leave only a small volume in between, that was sampled by the circular optical beam pattern. In theory, this spectrometer will need only 1/100th the gas amount for a measurement.

This miniature version is being considered as a payload addition for Mars mission concepts, including MarsDrop, microprobes designed to piggyback on larger landers. It is also a candidate for the SpaceX Red Dragon payload. Other potential applications include Discovery- and New Frontiers-class mission concepts that rely on multiple atmospheric probes to examine places like Venus and Saturn. Continuing miniaturization and improvements in sensitivity and reliability are the future of planetary exploration, and instruments such as the miniature TLS will revolutionize the exploration of distant worlds.
A miniature open-path TLS that uses an advanced optical scheme with an MDL-produced tunable laser for Mars methane sensing.
A miniature open-path TLS that uses an advanced optical scheme with an MDL-produced tunable laser for Mars methane sensing.
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A CAD rendering of the fore-optics of the miniature open-path TLS. It uses flight heritage components/subassemblies in a compact, robust, and passively athermalized configuration.
IA CAD rendering of the fore-optics of the miniature open-path TLS. It uses flight heritage components/subassemblies in a compact, robust, and passively athermalized configuration.
+ Larger image
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