The Mars 2020 Perseverance rover successfully landed on Mars on February 18 and started its search for signs of past life on the red planet. The rover will be characterizing the planet’s geology, including collecting and identifying the composition of Martian rocks and dirt. A Raman spectrometer, which is part of the rover’s on-board laboratory, will help to perform one of the mission’s key goals of analyzing the sulfate content in the Martian soil.
While sulfates have been found in large quantities on Mars’s surface, more experimental data is needed to confirm which sulfates are present and extract information about Mars’s historical hydration levels and the possibility of supporting life.
Raman spectroscopy is an ideal tool for performing the microscopic sample analysis on the rock samples gleaned from the surface of Mars. A Raman spectrum contains a series of peaks that correspond to the different vibrational modes in a molecule. The position and linewidth of these peaks can be so unique to a system that they are considered a “fingerprint” for the molecular species.
The analysis that can be performed with Raman spectroscopy goes beyond just simple chemical classification. For solid samples, the Raman spectrum reflects the material’s crystallinity with a good degree of precision. Often, the Raman spectrum of a molecule will be much sharper and less convoluted than its infrared equivalent as there tends to be fewer combination and overtone bands.
A Raman spectrometer consists of three main components: the light source, optics, and a detector. Narrow-bandwidth laser sources are commonly used to excite Raman spectroscopy samples to acquire spectra with excellent energy resolution. As large and complex chemical species may have large numbers of vibrational modes, an energy resolution of only a few numbers may be required to resolve them for unambiguous identification.
Optics are required to focus and shape the beam from the source to the detector. The complexity of the optical design depends on the particular configuration of the Raman spectrometer. Many of these need to be on moving translation stages for adjustment, such as for scanning the position of diffraction gratings to change the wavelength range focused onto the detector.
Designing instruments for rovers comes with many challenges. First, the instruments need to be incredibly robust to survive their journey to their destined planet. Secondly, the instruments need to be highly automated, as any adjustments, alignment, and measurement optimization will need to be performed remotely.
The Raman spectrometers for Mars had another additional challenge to overcome. The terrain there is incredibly rocky, not to mention the challenges that come with the extreme temperatures on Mars, spanning -125°C in its coldest climates to around 22°C on its warmest sunny days.
Any automated spectrometer relies on motorized translation stages to move gratings, adjust alignment, and focusing. In the spectrometers sent to Mars are Linkam stages, designed for geological applications that are designed to withstand temperatures of < -195 up to 1500°C. The stages can also be temperature controlled for performing Raman measurements on samples that need to be heated for analysis.
The Raman spectrometers on Mars will use the unique Raman signatures of different sulfate species to characterize Mars’ geology, harvested by the coring drills attached to the rover’s front. This mission represents a huge opportunity for understanding how sulfites are formed and in the development of robust and reliable Raman spectrometers with unparallel stability.
Image – As depicted in this artist’s concept, the SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument is located on the end of the robotic arm of NASA’s Perseverance Mars rover. Courtesy NASA/JPL-Caltech.
************
For more information:
