1.1

The LIBS spectroscopy

The LIBS benefits from six years of ChemCam operation on the Martian surface. This technique uses powerful laser pulses [6], focused on a small spot on target rock and soil samples within 7 m of the rover to ablate atoms and ions in electronically excited states, from which they decay, producing illuminated plasma. The plasma light is collected by a telescope, and focused onto the end of an optical fiber. The fiber carries the light to a demultiplexer and three spectrographs that record the spectrum over a range from 240 – 850 nm (Figure 3). The spectra yield emission lines of elements present in the samples. Typical rock and soil analyses yield detectable quantities of Na, Mg, Al, Si, Ca, K, Ti, Mn, Fe, H, C, O, Li, Sr and Ba. Other elements that may be observed in soils and rocks include F, Cl, S, N, P, B, Ni, Zn, Cu and Rb.

Figure 3:

LIBS spectrum of GYP-A

1.2

The Raman and fluorescence spectroscopy

Standoff Raman spectroscopy provides point detection of many minerals and potential detection of different organic compounds if they exist on Mars [3]. To perform Raman spectroscopy, the laser beam is frequency doubled and is directed towards the target in a 532 nm pulsed beam of low dispersion. The target is thus illuminated at lower power density than LIBS, such that the molecules at the surface are vibrationally excited, resulting in scattered light that is modulated by the vibrational frequency of the material. This “Raman” scattered light can be detected if the much stronger Rayleigh-scattered light at the laser wavelength is blocked. Like LIBS, the telescope collects the Raman signal; it is transferred via a fiber cable to a transmission spectrometer in the rover body. The signal is intensified and recorded. The intensifier allows the exposure to be gated to a very short duration, removing background ambient light. Time-resolved fluorescence can also be recorded. Organic and biological molecules produce fluorescence both with UV and visible laser excitation with very short lifetimes (< 1 ns to 200 ns). SuperCam will be able to distinguish short-lived organic fluorescence from that of longer-lived (μs-ms) minerals and rocks on Mars, thus identifying targets that have biological molecules embedded in them. In the absence of biological materials, SuperCam Raman will not have interference from short-lived fluorescence backgrounds.

Figure 4 shows a Raman spectrum measured with the EQM on a rhodonite rock.

Figure 4:

Raman spectrum of a rhodinite rock measured with the EQM

1.3

The visible and infra-red spectroscopy

VISIR passive spectroscopy (Figure 5) has demonstrated its powerful capability in the detection and identification of mineral phases through characteristic absorption features related to vibrational stretching and/or bending of characteristic molecular bounds. [4] The SuperCam wavelength range (0.4–0.85 μm, 1.3–2.6 μm) provides easy identification of most minerals to be found in the Mars geological record:

It might provide a tool to identify complex organic compounds from absorptions at 1.7 and 2.3-2.5 μm.

For atmospheric measurements, SuperCam records CO₂, CO, H₂O, O₂ (IR and 700-850 nm) and O₃ (UV). The full spectral range is used to measure scattered light diagnostic of aerosol size distribution, composition, and opacity.

Figure 5:

IR spectra of lab targets measured with the EQM

1.4

The context imaging

The Remote Micro-Imager [5] places the chemical and mineralogical analyses in their geomorphological context, as well as independently remotely imaging small details without needing to drive up to the samples. The RMI will help determine which samples within the vicinity of the rover are of sufficient interest to use the contact and in-situ instruments for further characterization. It will also provide primary science in and of itself, in providing analyses of samples that are inaccessible to contact and in-situ instruments, such as vertical outcrops in canyon or crater walls that might display strata of geological interest. Analysis of these strata may provide information on the climate history of Mars. Figure 6 shows a rock with a vein pictured with the RMI on the EQM.

Figure 6:

Rock measured with the EQM

1.5

The Microphone:

The SCM Primary science objective is to support the LIBS investigation to obtain unique properties of Mars rocks and soils through their coupling with the LIBS laser.

In addition, other opportunistic science objectives are to monitor various artificial sounds (rover sounds), and to contribute to basic atmospheric science: wind, convective vortices, dust devils at close distance, or saltation (wind-blown sand).

1.6

On-board Calibration:

In addition, a set of calibration targets mounted nominally 1.56 m from the Mast Unit will enable periodic calibration of the instrument. Experience on ChemCam showed that these calibration targets are invaluable for LIBS, and SuperCam will fly more targets. The SCCT includes targets for Raman, VISIR spectroscopy, and RMI, as well.

3.1

Spectral resolution

The spectral resolution has been measured using a monochromator monitored during the measurement with a lab spectrometer. For sampled wavelengths and at different temperatures of the spectrometer the IRS response is measured. A Gaussian fit of the measurement is applied to calculate the FWHM response. A deconvolution of the monochromator response is calculated to extract the IRS spectral resolution.

Figure 8 shows the IRS response to the monochromator stimuli and the extracted spectral resolution of the instrument at different temperatures. The spectral resolution meets the requirement at all wavelengths and performance temperature range.

Figure 8:

IRS response to different spectral stimuli by a monochromator (left) and calculated spectral resolution at different spectrometer temperatures (right)

3.2

Spectral registration

The scientific requirement shows that the spectral registration needs to be known better than ±1nm in the performance temperature range. In order to measure the registration, a Fabry-Perot interferometer associated with a calibrated lab spectrometer is used. The Fabry-Perot spectrum measured with the lab spectrometer is convolved by the IRS resolution. The positions of spectral lines of the Fabry-Perot are calculated for the lab spectrometer measurement. Simultaneously, the positions of the spectral lines measured with the IRS are calculated. The registration map is then extracted by comparison of both measurements.

Figure 9 shows the superposition of the Fabry-Perot spectrum measured with the lab spectrometer and its spectrum measured with the IRS using the calculated registration map. Relative registration with the spectrometer temperature is also shown. The registration knowledge meets requirement in the completed performance spectrometer temperature range.

Figure 9:

Fabry-Perot measurement with lab spectrometer and IRS (left) and relative spectral registration with spectrometer temperature (right).

3.3

Signal to noise ratio

The signal to noise ratio requirement is at least 60 assuming 80 s to acquire a spectrum and background of 86 spectral elements, an irradiance of 300W/m² and a 30% Lambertian target albedo. This requirement needs to be met in the performance temperature range of the spectrometer.

In order to verify this requirement, noise measurements have been done with variation of the different instrument parameters such as the integration time and at different spectrometer temperatures. Depending on the parameters, a noise limit has been calculated to meet the requirement and the measured noise is compared to this limit.

Figure 10 shows the comparison with the requirement of the measured noise in all the parameters conditions. The IRS meets the SNR requirement for all parameters values and spectrometer temperatures.

Figure 10:

Noise for both photodiodes for all parameter conditions compared with the requirement

3.4

Real target measurements

On the engineering qualification model at LANL spectra of real rocks have been measured. The target is illuminated with a high infrared radiance lamp. The reflected flux is measured with the IRS integrated to the Mast Unit. The complete instrument is cooled down to -10°C. The reflectance of the target is derived using a reference measurement (spectrally flat target).

Figure 11 compares the reflectance of gypsum measured with a 5nm resolution lab spectrometer, the same measurement convolved with the IRS resolution and the IRS measurement. This high correlation of those measurements shows that the IRS is correctly calibrated in terms of spectral registration and spectral resolution.

Figure 11:

Comparison of gypsum reflectance measured with the IRS and with a lab spectrometer.