A Miniaturised Raman Spectrometer for Life Detection in Ice

Abstract

The objective of this MSc thesis was to design a miniaturised Raman spectrometer that can be used in terrestrial deep ice applications of the IceMole probe. This probe is a manoeuvrable subsurface system for clean in situ analysis and sampling, developed at FH Aachen University of Applied Sciences since 2008, that is now part of a bigger DLR collaborative initiative, called Enceladus Explorer (EnEx). It combines melting with a hollow rotating ice screw and it now presents a cross sectional area of 80mm x 80mm.

A Raman spectrometer is a promising non-destructive technique that allows to identify both mineral and biological compounds with a minimum amount of sample, by measuring the inelastic scattering of light. In order to achieve an appropriate design for the instrument, the project followed a systems engineering approach.
The first step was the derivation of the requirements related to the technical constraints dictated by the miniaturised design of the probe and to the performances that allow the spectrometer to detect potential biosignatures.

The design process was carried out by successive choices so as to propose a final configuration, developed as a CAD model in the software CATIA™. The instrument performs its measurements through a sapphire window in the IceMole probe, looking directly into the ice. It excites the sample with a laser wavelength of 532nm transmitted via a dichroic beamsplitter. The scattered light is collected by a long-pass edge filter and a 30° off-axis parabolic mirror that focuses the light into a collimator. The light is then delivered to a CCD spectrometer via an optical fibre. The spectrometer is able to measure a spectral range of 546.9-700nm with an accuracy of at least 0.4nm, combination of the characteristics of the optical components. The final result is a Raman spectrometer design modelled by the use of off-the-shelf components, selected through systems engineering trade-offs. The final dimensions of the instrument are 65mm x 65mm x 150mm, with a mass of about 1.1kg.

The outcome of this thesis will have to be verified and validated in order to ensure the correct working of the system, the compliance with the derived requirements and the validity of the use of this instrument to satisfy the customers’ needs and expectations. Therefore, a preliminary verification plan was proposed in accordance to systems engineering.

Eventually, the future of such a Raman spectrometer will be space exploration and life detection missions as payload of the subsurface probe. The space targets for those missions are the ones identified as most promising for harbouring potential living micro-organisms: the polar caps of Mars, Jupiter’s moon Europa and Saturn’s moon Enceladus.