Prototype of a Laser-Based Mass Spectrometer for in Situ Dating of Rocks on Planetary Surfaces

Determining ages of planetary materials in situ remains a central, yet unresolved, objective of planetary science. So far, sample-return missions are considered for dating of planetary materials, which are logistics-intensive, expensive, and therefore scarce. Present instruments for in situ dating typically exceed the allocated mass, power, or volume of flight payloads. There are several dating schemes that are suitable for in situ dating on planetary surfaces. We selected the$\mathbf{R b -} \mathbf{S r}$dating system, which has been used for dating of a wide range of terrestrial and extraterrestrial materials, including Martian meteorites. Also, Rb and Sr are relatively immobile compared to other radiogenic systems proposed for flight, and are present at practical abundances in most igneous, metamorphic, and sedimentary rocks. However, separating the${ }^{87} \mathbf{R b}$and${ }^{87} \mathbf{S r}$isotopes needs extremely high mass resolution. To avoid this need we use resonant ionization of$\mathbf{R b}$and$\mathbf{S r}$, respectively, for which present a prototype for a compact mass spectrometry instrument based on laser ablation and ionization that provides the necessary accuracy of the isotope measurements needed for the Rb-Sr system. The first step is pulsed laser ablation of material from the rock (Laser Ablation Mass Spectrometry, LAMS). Analyzing the already ionized species in the ablation plume we determine the chemical composition of the rock, from which we infer its mineralogy. The mineralogy provides the context of the following dating measurements. The second step uses the neutral species in the ablation plume, (Laser Ablation Resonance Ionization Mass Spectrometry, LARIMS), where the$\mathbf{R b}$and$\mathbf{S r}$atoms are selectively ionized by additional lasers using resonant wavelengths, with a temporal offset. This offset separates the${ }^{87} \text{Rb}$and${ }^{87} \text{Sr}$isotopes in time and they are easily measured by time-of-flight mass spectrometry despite their almost identical mass. We report on the realized prototype instrument, with example measurements of the LAMS and the LARIMS modes. The instrument combines a laser-ablation ion source with resonance-ionization mass spectrometry in a shared, single ion mirror off-axis time-of-flight (TOF) analyzer. The instrument operates in two seamlessly switchable modes: in the LAMS-only mode, where the instrument rapidly maps in 2D the chemical composition, enabling identification of mineralogical phases of interest. In dating mode, LARIMS proceeds in two steps: ions from the ablation plume are first deflected from reaching the detector, then lasers ionize neutral$\mathbf{R b}$and$\mathbf{S r}$atoms in the plume by using wavelengths tuned to resonant transitions of the respective atoms. Resonant ionization selectively ionizes only the species of interest, thereby suppressing isobaric interferences in the mass spectra, and thus yields precise${ }^{87} \mathbf{R b}-{ }^{87} \mathbf{S r}$ages. Tests with the prototype system demonstrate (i) the LAMS mode and LARIMS mode within an unmodified geometry and setup, (ii) mass resolutions of 500 for both modes, and (iii) detection limits commensurate with lowppm rubidium and strontium. With a characteristic length below 30 cm, this novel type of instrument represents a practicable solution for landed in situ dating and has been selected for NASA's Dating an Irregular Mare Patch with a Lunar Explorer (DIMPLE) experiment, as part of the Payloads and Research Investigations on the Surface of the Moon (PRISM) and Commercial Lunar Payload Services (CLPS) programs.