Stand-Off Detection of Organic Compounds on Mars Using Ultraviolet Raman Spectroscopy and Time-Resolved Laser-Induced Fluorescence

Recent discoveries of organic carbon and methane on Mars have continued to motivate the search for complex organics on the Martian surface. Instrumentation that directly identifies and characterizes organic carbon is a high priority for the Mars 2020 rover, the successor to the Curiosity rover. Although no Raman instrument has been used in a planetary setting other than Earth, Mars 2020 will contain both SHERLOC, a 248 nm Raman instrument, and a 532 nm Raman system as part of the SuperCam instrument.

A 266 nm Raman spectrometer incorporating time-resolved fluorescence capabilities was developed at York University in order to investigate the performance of an ultraviolet Raman system in detecting organic material on Mars. A range of pure organic compounds, mineral-organic mixtures, and complex Mars analogue rocks from extreme environments on Earth were selected for study. This diverse sample suite allowed investigations that established the detection capabilities of the instrument using controlled samples, and demonstrated the potential for 2D mapping across rough unprepared surfaces to detect traces of organics embedded in a complex mineral matrix. While visible excitation wavelengths suffer from interfering fluorescence that can overwhelm the Raman signal, 266 nm was found to reduce or remove fluorescence in the Raman window in all samples that were tested, increasing the sensitivity to organic carbon.

Many organic compounds exhibit strong fluorescence outside the Raman window when excited with 266 nm radiation. This fluorescence was investigated to determine if it could improve the ability to detect organic material. A method was developed for measuring fluorescence lifetimes with sub-ns precision, and the fluorescence decay profiles of a wide range of aromatic organic molecules were characterized. Incorporating time-domain capabilities improved both the ability to discriminate between organic compounds and the ability to separate organic from mineral fluorescence. The capabilities investigated in this work may be useful when interpreting the measurements that will be returned from the Mars 2020 mission, and for developing the next generation of flight instruments.