Detecting Life on Mars

Abstract

The objective of NASA JPL's latest mission, the Mars 2020 Rover, is to detect extinct life on Mars and to cache samples for a future Mars sample return mission. SHERLOC, an instrument on the arm of the rover, accomplishes the former byidentifying organic compounds using Deep UV Fluorescence and Raman Spectrometry. SHERLOC is designed to search for in situ organics. However, given that such biomolecules exist to be found, will SHERLOC be able to detect them? What is its detection limit? What sort of resolution will the Raman spectrum obtain? Is there a danger of false positive or irregular Raman detection? To investigate these questions, I created biological samples mixed with Martian soil simulant, sealed in Martian gas and subjected to long term heating to mimic the characteristics of organic biomarkers that SHERLOC might find on Mars. I then performed spectrometry on these samples at NASA JPL to reveal possible fluorescence and Raman maps SHERLOC would produce on Mars with compositionally similar soil. The samples were degenerated significantly with high temperature and low pressure to mimic the billion year long diagenesis on the Martian surface and to test the limits of detection of the instrument. The spectral analyses of these lab samples can then be used as a standard to confirm organic biomarkers on Mars. The compounds identified by the instrument were cross-checked with DNA extraction, X-Ray diffraction, and High Performance Liquid Chromatography to assess its accuracy and sensitivity. In Week 0 of the experiment, SHERLOC was able to detect a partial number of the organic compounds present in the experimental samples, such as, phosphate ion, amino acids, C-C, benzene ring, C=C, C-H, and aromatic hydrocarbons. The spectral signal weakened significantly in Week 2 with C-H and phosphate ion bonds no longer detected. No peaks were present by Week 4. No false positives, irregular raman spectra or DNA were detected by SHERLOC. No DNA could be extracted in the lab samples after Week 0. There was a steady racemization of aspartic acid over the duration of the experiment, with the D/L ratios increasing from 0 to 0.8. The detection limit of the instrument based on correlating aspartic acid concentration and absence of Raman peaks was found to be 0.144 ppm.