In-situ mapping of monocrystalline regions on Mars

Elemental quantification instruments for planetary missions provide a capability for in-situ identification of mineral phases via stoichiometry, an essential step in petrological investigations. X-ray fluorescence (XRF) has been employed for this purpose by multiple generations of Mars rovers (i.e., Pathfinder, Spirit and Opportunity, Curiosity and Perseverance). The Planetary Instrument for X-ray Lithochemistry (PIXL) aboard Perseverance rasters a micro-focused X-ray beam to generate micron-mm-sized maps illustrating variations in elemental composition and allowing mission scientists to identify rock components (i.e., sedimentary grains, veins and igneous crystals). Energy-dispersive X-ray diffraction can also be detected with PIXL and can be used as an additional constraint on component boundaries, providing PIXL with the capability to map monocrystalline regions in-situ. Here we introduce and apply a new method where each diffraction peak is partitioned independently according to its energy, using the instrument geometry to inform consistent partitioning. Applying this method to datasets acquired from the Dourbes abrasion patch in the Séítah formation of Jezero crater, Mars, reveals monocrystalline regions that were hidden using previous methods. This application of the technique allows faster and more accurate visualization of petrographic textures in future PIXL datasets, in particular those with rock components that are not easily separable using stoichiometry alone.

行星探测任务中的元素定量仪器可通过化学计量学（stoichiometry）实现矿物相的原位识别，这是岩石学研究的关键环节。X射线荧光（X-ray fluorescence, XRF）已被多代火星漫游车——包括火星探路者、勇气号、机遇号、好奇号与毅力号——用于该类研究。毅力号（Perseverance）搭载的X射线岩石化学行星仪器（Planetary Instrument for X-ray Lithochemistry, PIXL）以微聚焦X射线束开展扫描，生成微米至毫米级的元素组成变化图谱，助力任务科学家识别岩石组分（即沉积颗粒、脉体与火成晶体）。PIXL还可检测能量色散X射线衍射（energy-dispersive X-ray diffraction）信号，该信号可作为约束组分边界的额外依据，使PIXL具备原位绘制单晶区域图谱的能力。本文提出并应用了一种新方法：依据衍射峰的能量对其进行独立分区，并借助仪器几何参数确保分区的一致性。将该方法应用于火星杰泽罗撞击坑塞伊塔地层（Séítah formation）杜尔贝磨损点（Dourbes abrasion patch）采集的数据集后，可识别出此前方法未能发现的单晶区域。该方法的应用可使未来PIXL数据集的岩相纹理可视化工作更高效、更精准，尤其适用于仅通过化学计量学难以区分组分的岩石样本。