SHERLOC will look for clues of life on Mars
Was there ever life on Mars? This is one of the questions that the Mars 2020 Rover, Perseverance, is going to try to answer. The mission aims to learn more about the molecules that make up Mars’ surface. As such, there is a particular interest in organic compounds that could be signifiers of life-based processes.
One technique that has not yet been used on Mars is Raman spectroscopy. In chemistry labs, Raman spectroscopy is used to identify organic molecules. This technique is useful because, unlike infrared spectroscopy, it requires no sample preparation and can analyse compounds in their aqueous form. Consequently, it has great potential for use during space missions. Raman Spectroscopy works by shining a UV laser onto a surface. The energy of the light is changed due to an interaction with vibrating molecules and is scattered back off the surface with a different frequency. A Raman spectrometer, the detection machine, measures the frequencies of the light that is reflected. Different molecules have unique vibrations therefore give off different frequencies, enabling us to create a kind of structural fingerprint that allows them to be identified.
On earth, Raman spectroscopy is a tricky technique to perfect, so trying to perform it in space only adds to its difficulty. The effect of the measured vibrations is very weak, so a highly sensitive machine is required to observe them. For this reason, it is important to ensure that the delicate instrument is carefully protected, particularly during launch and take-off. Moreover, it is vital to closely monitor and maintain external conditions, such as temperature, to sustain the instrument’s efficiency and accuracy.
SHERLOC—Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals, a UV Raman and fluorescence spectrometer that is mounted on the rover’s robotic arm—is NASA’s attempt to solve these problems. SHERLOC will be the first time Raman spectroscopy has been used on Mars’ surface. Created by Luther Beegle and his team at the NASA Jet Propulsion Laboratory, the instrument will scan for organic compounds. Dr Beegle hopes that the investigation will “advance the understanding of Martian geologic history and identify its past biologic potential.” To achieve this, SHERLOC will look for molecules that may have been affected by Mars’ past watery environment and are therefore a signifier that Mars might once have supported life.
The instrument is expertly designed to fulfill its goal and overcome potential problems. It is fitted with a particularly long UV laser (248.6nm) so that it can analyse molecules even below Mars’ surface – where many compounds of interest could be found. An autofocus will help SHERLOC obtain the optimum laser focus point and allow for highly sensitive detection of aromatic (cyclic carbon molecules) and aliphatic (straight-chain carbon-hydrogen) compounds. These potentially astrobiologically relevant minerals, known as ‘ARMs’, are molecules formed through aqueous chemistry and could be indicators of Mars’ biological history.
WATSON, or Wide Angle Topographic Sensor for Operations and eNgineering, is SHERLOC’s camera sidekick and will help scientists to build up a greater picture of Mars’ geological secrets. The instrument is based on the technology of MAHLI, the camera on the previous Mars rover, and is able to produce colour microscopic images of the samples that SHERLOC is analysing. This will provide important information on the grain size and textures of the rocky surface – data that is vital for understanding their origin.
The main obstacle that SHERLOC is contending with is the Martian environment. The surface is cold and dusty, and the temperature is prone to fluctuations of 100˚C. In other words, not hospitable conditions for scientific instruments. To overcome this, SHERLOC includes a calibration target which closely monitors the instrument’s performance and will send updates to scientists back on earth. Moreover, WATSON will help to visually monitor both the instrument and the rover as a whole by creating images of wear or damage – paying particular attention to the Rover’s wheel condition.
Another issue is that Mars’ surface contains a high concentration of perchlorates, which are compounds containing the ion (charged particle) ClO4-. On heating above Mars’ normal temperature range, perchlorates will destroy the organic molecules that SHERLOC is interested in. If a high-intensity laser is used (as is the norm in this kind of spectroscopy), it creates heat that could induce a reaction that damages the samples that need to be analysed. However, SHERLOC’s laser has a deep UV wavelength which enhances the signal and results in very high-sensitivity measurements without the need for high intensity. As a consequence, the instrument is remarkably low power and so will not cause a chemical reaction, allowing scientists to study the organic molecules without altering them.
At the moment, SHERLOC is ready and mounted onto the arm of the rover which is set to launch in July. The mission is expected to land on Mars’ Jezero Crater on the 18th of February 2021 and, until then, we wait to see what wonders SHERLOC may find.