Low temperature fluorescence excitation and emission spectra of molecules relevant to Mars: Chlorobenzene, benzoic acid, phthalic acid, mellitic acid, and benzene in water ice solutions at temperatures between 78 K and 273 K

Abstract

The discovery of chlorobenzene detected in soil samples obtained in Mars has been the subject of several interpretations. The original sample was subjected to pyrolysis before the gas chromatography-mass spectrometry (GC-MS) separation and identification with the Sample Analysis at Mars (SAM) instrument on the Curiosity rover. It is believed that chlorobenzene detected at concentrations above the background level of the instrument was a product of other organic molecules reacting with perchlorates of the Martian soil. Mars surface temperatures may reach a high of about 293 K at noon, at the equator, and a low of about 120 K at the poles. In this paper, characteristic fluorescence excitation and emission spectra are presented for chlorobenzene and the suggested precursor molecules: benzoic acid, phthalic acid, and mellitic acid in transparent hexagonal water ice solutions at temperatures between 273 K and 78 K. In addition, the liquid solution spectra at 292 K. Spectra of benzene in water ice solutions have also been included. The experimental results presented in this paper are to be compared with fluorescence experiments of Mars missions that will explore the polar regions and other regions where water ice is expected to be found. Limits of detection at part per billion by mass (ng/g) concentrations at room temperature are discussed based on our results and the literature. The application of fluorescence for low temperature studies of Mars samples is also discussed with a proposed miniature instrument with a laser source to increase the sensitivity of the detection and could be used for solid or liquid samples.