Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC)
The Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) is an arm-mounted, Deep UV (DUV) resonance Raman and fluorescence spectrometer utilizing a 248.6-nm DUV laser and <100 micron spot size. The laser is integrated to an autofocusing/scanning optical system, and co-boresighted to a context imager with a spatial resolution of 30 µm.
SHERLOC operates over a 7 × 7 mm area through use of an internal scanning mirror. The 500-micron depth of view, in conjunction with the MAHLI heritage autofocus mechanisms, enables arm placements from 48 mm above natural or abraded surfaces without the need for rover arm repositioning/movement. Additionally, borehole interiors, after sample core removal, can be analyzed as a proxy for direct core analysis.
In addition to the combined spectroscopic and macro imaging component, SHERLOC also integrates a “second-eye” with a near-field-to-infinity imaging component called WATSON (Wide Angle Topographic Sensor for Operations and eNgineering), which is used for engineering science operations and science imaging. WATSON is a build-to-print camera based on the Mars Science Laboratory (MSL) Mars Hand Lens Imager (MAHLI). Integration is enabled by existing electronics within SHERLOC.
Deep UV-induced native fluorescence is very sensitive to condensed carbon and aromatic organics, enabling detection at or below 10-6 w/w (1 ppm) at <100 µm spatial scales. SHERLOC's deep UV resonance Raman enables detection and classification of aromatic and aliphatic organics with sensitivities of 10-2 to below 10-4 w/w at <100 µm spatial scales. In addition to organics, the deep UV Raman enables detection and classification of minerals relevant to aqueous chemistry with grain sizes below 20 µm grains.
SHERLOC's investigation combines two spectral phenomena, native fluorescence and pre-resonance/ resonance Raman scattering. These events occur when a high-radiance, narrow line-width, laser source illuminates a sample. Organics that fluoresce absorb the incident photon and reemit at a higher wavelength. The difference between the excitation wavelength and the emission wavelength indicates the number of electronic transitions, which increases with increasing aromatic structures (i.e. number of rings). This phenomenon is highly efficient, with a typical cross section 105x greater than Raman scattering, and enables a powerful means to find trace organics.
The native fluorescence emission of organics extends from ~270 nm into the visible. This is especially useful, because it "creates" a fluorescence-free region (from 250 – 270 nm) where Raman scattering can occur. With SHERLOCs narrow-linewidth 248.6 nm DUV laser, additional characterization by Raman scattering from aromatics and aliphatic organics and minerals can be observed. Furthermore, excitation with a DUV wavelength enables resonance and pre-resonance signal enhancements (>100 to 10,000×) of organic/mineral vibrational bonds by coupling of the incident photon energy to the vibrational energy. This results in high-sensitivity measurements, with low backgrounds, without the need of high-intensity of lasers, and avoids damage or modification of organics by inducing reactions with species such as perchlorates.
SHERLOC Ops: An Example Measurement on Fig Tree
Using the SHERLOC testbed, an analysis of a piece of the astrobiologically interesting chert obtained from the Fig Tree Group is shown.
A context image of the sample is acquired. Using the internal scanning mirror, a 50-micron laser spot is systematically rastered over the surface. On the same CCD, spectra in the range 250-360 nm are obtained. Analysis of the fluorescence region (>270 nm) identifies regions where organic material is present. Analysis of the fluorescence spectra identifies number of aromatic rings present, and identifies regions of high organic content. In order to achieve higher specificity, a longer integration can be used to collect deep UV Raman spectra. The Raman spectra shown on the right are from the two circles shown in the context image.
By studying the fluorescence and Raman data we can conclude that our analysis indicates that:
- The chert has not been altered uniformly--pressure/temperature exposures are evident from carbon maturity variation.
- The majority of matrix is thermally mature carbon--anthracitic to sub-bituminous.
- An intrusion of silica with much younger carbon invaded the main matrix.
Potential for biosignature preservation in the matrix is low due to thermal history of the sample, with high preservation in the thermally unaltered vein material.