Possible Detection of Nitrates on Mars by the Sample Analysis at Mars (SAM) Instrument

Abstract

International audience

Introduction: Planetary models suggest that nitrogen was abundant in the early Martian atmosphere as dinitrogen (N 2). However, it has been lost by sputtering and photochemical loss to space [1, 2], impact erosion [3], and chemical oxidation to nitrates [4]. Nitrates , produced early in Mars' history, are later decomposed back into N 2 by the current impact flux [5], making possible a nitrogen cycle on Mars. It is estimated that a layer of about 3 m of pure NaNO 3 should be distributed globally on Mars [5]. Nitrates are a fundamental source for nitrogen to terrestrial microorganisms. Therefore, the detection of soil nitrates is important to assess habitability in the Martian environment. The only previous mission that was designed to search for soil nitrates was the Phoenix mission but was unable to detect evolved N-containing species by TEGA and the MECA WCL [6]. Nitrates have been tentatively identified in the Nakhla meteorite [7]. The purpose of this work is to determine if nitrates were detected in first solid sample (Rocknest) in Gale Crater examined by the SAM instrument. Materials and Methods: Samples collected from Rocknest, located in Gale Crater, which consists of an inactive, sandy wind drift mantled with dust, were analyzed by the SAM instrument. Prior to sample analysis, a blank was run using an empty quartz cup to characterize the background of the SAM instrument. The quartz cup was sealed inside the pyrolysis oven and heated to ~840°C at a rate of 35°C/min under a He carrier gas flow rate of 1.5 cm 3 /min and at an oven pressure of ~30 mbar. A small fraction of the gas released from the cup was measured directly by electron impact quadrupole mass spectrometry (QMS mass range 2-535 Da, resolution 0.1 Da). Then four samples of Rocknest bedform materials < 150 μm diameter size fraction (~20 mg) were examined by SAM. A thermal analyzer (Netzsch STA 449 F1 Jupiter Simultaneous TG/DSC) coupled to a mass spectrometer (Netzsch QMS 403 C Aeolos) was used in the laboratory to heat samples upto 1200°C at a rate of 20°C/min under a He