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DEVELOPMENT OF A LUMINESCENCE DATING DEVICE FOR IN SITU DATING OF GEOMORPHOLOGICAL FEATURES ON MARS

Introduction: The surface of Mars has been subject to eolian, fluvial, and periglacial activity. Unfortunately, establishing a chronology on Mars e.g. for dunes, gullies, the north polar layers, and other surface features is difficult. Consequently, techniques to quantify the ages of geomorphological processes on Mars have become an important area of research [1]. Among the techniques proposed is optically stimulated luminescence (OSL) dating, which is well established for age-dating sediments on Earth [2, 3]. We address some of the challenges associated with developing an OSL device for in situ dating of sediments on Mars. Results of experiments, using Martian simulant materials, as well as the design of an OSL instrument will be described. OSL Dating Principles: OSL Dating is a “dosimetric” technique. The time elapsed since deposition of a sediment layer is determined from the radiationdose accumulated in minerals since the last sunlight exposure, and the dose rate due to naturally occurring radioactive nuclides and cosmic radiation. Radiation exposure can be measured by stimulating the sample with light of one wavelength and monitoring the luminescence at another wavelength (OSL). The intensity of the OSL is a function of the absorbed natural radiation dose. If the rate of natural irradiation can be determined, then dividing absorbed dose by dose rate gives a radiation exposure age, i.e. the time elapsed since the last sunlight exposure. Challenges on Mars: An in situ OSL dating method for Martian surface sediments requires new measurement techniques in order to deal with the multiple new challenges not usually found when using OSL to date terrestrial sediments. Among the many challenges are: (1) the use of polymineralic samples, without the benefit of chemical separation, (2) ambient temperatures that are significantly lower than on Earth and highly variable, (3) a solar spectrum that is different from that on Earth, (4) a dose rate that is significantly higher than on Earth and dominated by high energy charged particles from galactic cosmic rays; the dose rate varies with depth, (5) anomalous fading of the OSL signal .......among several other challenges. Mineral composition. Using thermal emission data from the Mars Global Surveyor (MGS) Bandfield et al. [4] and Bandfield [5] identified areas on the Martian surface with mainly two different mineral compositions. The first one is composed of 65% plagioclase and 30 % clinopyroxene, and the second one of 45% plagioclase, only 10 % clinopyroxene and an additional 40 % potassium-rich glass. The plagioclase feldspars have calcium content of 30-70 % [6]. Based on these data we devised two mineral mixtures as surrogates for Martian sediments. The two mixtures are known as OSU-Mars-1 and OSU-Mars-2 and the compositions (in vol. %) are described in the following table: