NASA is sending a new laser-toting robot to Mars. But unlike the lasers of science fiction, this one is used for studying mineralogy and chemistry from up to about 20 feet (7 meters) away. It might help scientists find signs of fossilized microbial life on the Red Planet, too.
One of seven instruments aboard the Mars 2020 rover that launches this summer, SuperCam was built by a team of hundreds and packs what would typically require several sizable pieces of equipment into something no bigger than a cereal box.
It fires a pulsed laser beam out of the rover’s mast, or “head,” to vaporize small portions of rock from a distance, providing information that will be essential to the mission’s success.
Here’s a closer look at what makes the instrument so special:
A Far Reach
Using a laser beam will help researchers identify minerals that are beyond the reach of the rover’s robotic arm or in areas too steep for the rover to go.
It will also enable them to analyze a target before deciding whether to guide the rover there for further analysis.
Of particular interest: minerals that formed in the presence of liquid water, like clays, carbonates and sulfates.
Liquid water is essential to the existence of life as we know it, including microbes, which could have survived on Mars billions of years ago.
Scientists can also use the information from SuperCam to help decide whether to capture rock cores for the rover’s sample caching system.
Mars 2020 will collect these core samples in metal tubes, eventually depositing them at a predetermined location for a future mission to retrieve and bring back to Earth.
SuperCam is essentially a next-generation version of the Curiosity rover’s ChemCam. Like its predecessor, SuperCam can use an infrared laser beam to heat the material it impacts to around 18,000 degrees Fahrenheit (10,000 degrees Celsius) – a method called laser induced breakdown spectroscopy, or LIBS – and vaporizes it.
A special camera can then determine the chemical makeup of these rocks from the plasma that is created.
Just like ChemCam, SuperCam will use artificial intelligence to seek out rock targets worth zapping during and after drives, when humans are out of the loop. In addition, this upgraded A.I. lets SuperCam point very precisely at small rock features.
Another new feature in SuperCam is a green laser that can determine the molecular composition of surface materials.
This green beam excites the chemical bonds in a sample and produces a signal depending on which elements are bonded together – a technique called Raman spectroscopy.
SuperCam also uses the green laser to cause some minerals and carbon-based chemicals to emit light, or fluoresce.
Minerals and organic chemicals fluoresce at different rates, so SuperCam’s light sensor features a shutter that can close as quickly as 100 nanoseconds at a time – so fast that very few photons of light will enter it.
Altering the shutter speed (a technique called time-resolved luminescence spectroscopy) will enable scientists to better determine the compounds present.
Moreover, SuperCam can use visible and infrared (VISIR) light reflected from the Sun to study the mineral content of rocks and sediments.
This VISIR technique complements the Raman spectroscopy; each technique is sensitive to different types of minerals.
What is Raman Spectroscopy?
Raman spectroscopy is an analytical technique where scattered light is used to measure the vibrational energy modes of a sample. It is named after the Indian physicist C. V. Raman who, together with his research partner K. S. Krishnan, was the first to observe Raman scattering in 1928.1 Raman spectroscopy can provide both chemical and structural information, as well as the identification of substances through their characteristic Raman ‘fingerprint’. Raman spectroscopy extracts this information through the detection of Raman scattering from the sample.
What is Raman Scattering?
When light is scattered by molecule, the oscillating electromagnetic field of a photon induces a polarisation of the molecular electron cloud which leaves the molecule in a higher energy state with the energy of the photon transferred to the molecule.
This can be considered as the formation of a very short-lived complex between the photon and molecule which is commonly called the virtual state of the molecule. The virtual state is not stable and the photon is reemitted almost immediately, as scattered light.
Figure 1 Three types of scattering processes that can occur when light interacts with a molecule.
In the vast majority of scattering events, the energy of the molecule is unchanged after its interaction with the photon; and the energy, and therefore the wavelength, of the scattered photon is equal to that of the incident photon. This is called elastic (energy of scattering particle is conserved) or Rayleigh scattering and is the dominant process.
In a much rarer event (approximately 1 in 10 million photons)2 Raman scattering occurs, which is an inelastic scattering process with a transfer of energy between the molecule and scattered photon.
If the molecule gains energy from the photon during the scattering (excited to a higher vibrational level) then the scattered photon loses energy and its wavelength increases which is called Stokes Raman scattering (after G. G. Stokes).
Inversely, if the molecule loses energy by relaxing to a lower vibrational level the scattered photon gains the corresponding energy and its wavelength decreases; which is called Anti-Stokes Raman scattering.
Quantum mechanically Stokes and Anti-Stokes are equally likely processes. However, with an ensemble of molecules, the majority of molecules will be in the ground vibrational level (Boltzmann distribution) and Stokes scatter is the statistically more probable process. As a result, the Stokes Raman scatter is always more intense than the anti-Stokes and for this reason, it is nearly always the Stokes Raman scatter that is measured in Raman spectroscopy.
Figure 2 Jablonski Diagram showing the origin of Rayleigh, Stokes and Anti-Stokes Raman Scatter
It is clear from the above, that the wavelength of the Raman scattered light will depend on the wavelength of the excitation light. This makes the Raman scatter wavelength an impractical number for comparison between spectra measured using different lasers. The Raman scatter position is therefore converted to a Raman shift away from excitation wavelength:
Figure 2 shows that Raman spectroscopy measures the energy gap between the vibrational levels of the molecule. The ladder of vibrational levels shown in Figure 2 is for a single vibrational mode of the molecule. Polyatomic molecules will contain many vibrational modes, each with their own ladder of vibrational levels.
For non-linear molecules with N atoms, the number of vibrational modes is given by:
3N – 6
The 3N is the total degrees of freedom of the molecule and the translational 3 degrees of freedom, and 3 rotational are then subtracted which leaves 3N – 6 vibrational modes. For linear molecules, there is one less rotational degree of freedom and the number of vibrational modes is therefore:
3N – 5
Not all vibrational modes can be detected using Raman spectroscopy. For a vibrational mode to be measured it must be ‘Raman Active’ which occurs when the molecular polarisability changes during the vibration.
Raman Spectrum of CCl4
An example Raman spectra is that of Carbon Tetrachloride (CCl4) and is shown in Figure 3. CCl4 is a tetrahedral molecule with three pronounced Raman active vibrational modes in the 100 cm-1 to 500 cm-1 wavenumber region (there is an additional peak at ~780 cm-1 which is not shown).
In the centre of the spectrum is the Rayleigh scatter peak at the laser wavelength. This peak is millions of times more intense than the Raman scatter and is therefore normally blocked by a notch or edge filter in the Raman spectrometer but was included here for clarity.
Symmetrically placed on either side of the Rayleigh peak are the three Stokes and three Anti-Stokes peaks corresponding to the three most intense Raman active vibrations of CCl4.
It can be seen that the Anti-Stokes lines are much weaker than the Stokes due to the larger population of molecules in the ground vibrational level of each mode. CCl4 has one of the simplest Raman Spectra but the same principle applies for all samples: Raman spectroscopy is used to measure the unique vibrational fingerprint of the sample and from that information chemical, structural and physical properties can be determined.
Figure 3 Raman Spectrum of CCl4 measured using a 532 nm laser
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Laser With a Mic Check
SuperCam includes a microphone so scientists can listen each time the laser hits a target. The popping sound created by the laser subtly changes depending on a rock’s material properties.
“The microphone serves a practical purpose by telling us something about our rock targets from a distance.
But we can also use it to directly record the sound of the Martian landscape or the rover’s mast swiveling,” said Sylvestre Maurice of the Institute for Research in Astrophysics and Planetary Science in Toulouse, France.
The Mars 2020 rover marks the third time this particular microphone design will go to the Red Planet, Maurice said.
In the late 1990s, the same design rode aboard the Mars Polar Lander, which crashed on the surface. In 2008, the Phoenix mission experienced electronics issues that prevented the microphone from being used.
In the case of Mars 2020, SuperCam doesn’t have the only microphone aboard the rover: an entry, descent and landing microphone will capture all the sounds of the car-sized rover making its way to the surface. It will add audio to full-color video recorded by the rover’s cameras, capturing a Mars landing like never before.
SuperCam is led by Los Alamos National Laboratory in New Mexico, where the instrument’s Body Unit was developed. That part of the instrument includes several spectrometers, control electronics and software.
The Mast Unit was developed and built by several laboratories of the CNRS (French research center) and French universities under the contracting authority of CNES (French space agency). Calibration targets on the rover deck are provided by Spain’s University of Valladolid.
JPL is building and will manage operations of the Mars 2020 rover for the NASA Science Mission Directorate at the agency’s headquarters in Washington.