Alpha Particle X-ray Spectrometer Research Articles

Moon Diver: Exploring a pit's exposed strata to understand lunar volcanism

Natural pits on the Moon expose deep cross-sections of the lunar maria, enabling direct investigation of the Moon's volcanic history and providing potential access to subsurface lava tubes. The Moon Diver mission concept seeks to explore the Mare Tranquillitatis pit, which exposes the largest wall of bedrock of the mare pits (∼65 m). The concept is enabled by two innovative capabilities: pinpoint landing near the pit and robotic access to its near-vertical wall with an instrument package to examine the elemental chemistry, mineralogy, and morphology of these bedrock layers. Pinpoint landing uses closed-loop guidance with terrain-relative navigation (TRN), which was advanced by Perseverance landing on Mars, to deliver the lander within a 100-m ellipse. The Axel robotic explorer, which remains tethered to the lander, would egress onto the lunar surface and traverse the relatively flat terrain to the pit's funnel entrance. The lander, which is the data link to Earth, also serves as an anchor and provides power and communication to the rover through its tether. The rover is a novel two-wheeled platform with a trailing boom and a spool that pays out the tether as the rover traverses toward the pit. The 300-m long tether is well margined for the rover to scale the pit wall. The rover carries a surface preparation tool and three additional instrument types: (a) three high-resolution cameras for acquiring context images of the near and far walls with the near-wall pair in a stereoscopic configuration, (b) an alpha-particle-X-ray spectrometer (APXS) for elemental composition, and (c) a multi-spectral microscopic imager (MMI) that uses controlled lighting for mineralogy. The surface-preparation tool removes dust and patina from the rock wall by grinding a small area. This tool, the MMI, and the APXS would be deployed from an instrument bay inside the wheel wells. The rover would independently point each instrument at the same target on the wall with millimeter repeatability. Landing shortly after sunrise, the surface mission timeline is just shy of a lunar daytime (14 Earth days). Beyond the primary mission, the rover would be capable of descending from the overhang and peering into the void that may open to a large cave or lava tube. Lunar pits provide an exciting new target for exploration using innovative robotic capabilities that have been tested with integrated science instruments at multiple terrestrial analogue sites including a pit with basaltic layers in Arizona.

Depth Distribution of Chlorine at Gale Crater, Mars, as Derived From the DAN and APXS Experiments Onboard the Curiosity Rover

AbstractThe subsurface chlorine depth distribution at Gale crater has been studied using measurements from the Alpha Particle X‐ray Spectrometer (APXS) and Dynamic Albedo of Neutrons (DAN) instruments onboard NASA's Curiosity Mars rover. These measurements have been acquired at or near ∼30 drill holes made by Curiosity along its 30 km drive. APXS measured a ∼15 micron surface layer of bedrock and shallow depth of ∼5 cm of drill tailings. DAN is sensitive to the bulk (∼50 cm depth) chlorine abundance and that of other neutron absorbing elements. Joint analysis of APXS and DAN data shows that the chlorine distribution at Gale crater consists of two components: surficial chlorine with concentrations >1 wt.%, possibly controlled by aeolian deposition of Cl‐rich dust and surface alteration, and subsurface chlorine with concentrations <1 wt.%, likely a result of groundwater activity in the past.

Elemental composition of manganese- and phosphorus-rich nodules in the Knockfarril Hill member, Gale crater, Mars

The Mars Science Laboratory rover Curiosity encountered nodules rich in manganese and phosphorus while exploring the Knockfarril Hill member of Gale crater on Mars. Deconvolution of X-ray spectroscopy data acquired by the Alpha Particle X-ray Spectrometer (APXS) at the spectral level indicate P2O5 concentrations possibly in excess of 18 wt% and MnO exceeding 8 wt%. The nodules occur intermittently in ∼mm-thick layers concordant with the sedimentary laminae, extending up to ∼10 cm laterally. Calcium sulfate associated with the nodules is interpreted as having precipitated from fluid that infiltrated between the nodule-bearing bedding planes in a separate and subsequent fluid event. Though the Mn- and P-bearing phase(s) was(were) not definitively identified by X-ray diffraction, evolved gas analyses show that the oxidation state of Mn is most likely 2+.

Manganese Mobility in Gale Crater, Mars: Leached Bedrock and Localized Enrichments

AbstractIn Gale crater on Mars, the rover Curiosity has discovered evidence of fluid mobilization of the redox‐sensitive element manganese. We present results for Mn from Curiosity's Alpha Particle X‐ray Spectrometer (APXS), which show that the average MnO concentration in mudstone‐dominated sedimentary units (0.22 wt%) is about one‐half of the concentration in the average Mars crust (0.44 wt%). Geochemical trends indicate that Mn in the sedimentary bedrock, most of which has a basaltic provenance, was leached by chemical alteration and dissolution. In >350 vertical meters of mudstone‐dominated strata, the apparent leaching of Mn and retention of Fe in Fe‐O‐H phase(s) resulted in the fractionation of Fe and Mn, indicating relatively moderate Eh‐pH fluid conditions that were not highly alkaline, reducing, or oxidizing. Exceptions are fracture‐associated, silica‐rich haloes where both Mn and Fe were leached by low pH fluids. The rover also discovered Mn‐rich veins, nodules, and patchy, dark coatings on rock surfaces, which are variably associated with enrichments in Fe, P, Cl, and/or Zn. These Mn‐rich features represent ∼1% of the 1029 APXS measurements acquired over ∼25 km of rover traverse. A thermochemical model shows that dissolved Mn2+ could have been concentrated via evaporation, sublimation, and/or freezing. Manganese was then likely precipitated in localized features when >99.99% of the Mn2+‐bearing water was removed from the system. These findings indicate that Mn was mobile in Gale crater and therefore bioavailable as a potential energy source for life.

Statistical Analysis of APXS‐Derived Chemistry of the Clay‐Bearing Glen Torridon Region and Mount Sharp Group, Gale Crater, Mars

AbstractThe Glen Torridon stratigraphic sequence marks the transition from the low energy lacustrine‐dominated Murray formation (Mf) (Jura member: Jm) to the more diverse Carolyn Shoemaker formation (CSf) (Knockfarril Hill member: Knockfarril Hill; Glasgow member: Glasgow). This transition defines a change in depositional setting. Alpha Particle X‐ray Spectrometer (APXS) results and statistical analysis reveal that the bulk primary geochemistry of Mf targets are broadly in family with CSf targets, but with subtle compositional and diagenetic trends with increasing elevation. APXS results reveal significant compositional differences between Jura_GT and the stratigraphically equivalent Jura on Vera Rubin ridge (Jura_VRR). The data define two geochemical facies (high‐K or high‐Mg), with a strong bimodal grain distribution in Jura_GT and Knockfarril Hill. The contact between Knockfarril Hill and Glasgow is marked by abrupt sedimentological changes but a similar composition for both. Away from the contact, the Knockfarril Hill and Glasgow plot discretely, suggesting a zone of common alteration at the transition and/or a gradual transition in provenance with increasing elevation in the Glasgow member. APXS results point to a complex history of diagenesis within Glen Torridon, with increasing diagenesis close to the Basal Siccar Point unconformity on the Greenheugh pediment, and with proximity to the beginning of the clay sulfate transition. Elemental mobility is evident in localized enrichments or depletions in Ca, S, Mn, P, Zn, Ni. The highly altered Hutton interval, in contact with the unconformity on Tower butte, is also identified on Western Butte, indicating that the “interval” was once laterally extensive.

Emulation of the curiosity rover alpha particle X-ray spectrometer with accelerator-based PIXE: II

Mono-mineral and ultramafic geochemical reference materials (GRMs) were analyzed using a 2.5 MeV broad-beam proton-induced X-ray emission (PIXE) system designed to emulate the PIXE branch of the Mars Science Laboratory alpha-particle X-ray spectrometer (APXS). These samples supplement the heterogeneous rocks of our earlier emulation work. The results confirm that PIXE provides high-accuracy concentrations for major and minor elements in homogeneous samples, and the results agree well with those from mono-minerals in the original APXS calibration. A second X-ray detector was added to the system to extend the emulation to the X-ray fluorescence branch of the APXS. The GRMs analyzed using this detector have relatively high concentrations of the trace elements copper, zinc, rubidium, strontium or zirconium. Our PIXE concentrations were compared to values provided by both the GRM suppliers and an independent corporation. Agreement was good in most cases, with individual deviations attributed mainly to inaccuracies in these reference concentrations.

Elemental Composition and Chemical Evolution of Geologic Materials in Gale Crater, Mars: APXS Results From Bradbury Landing to the Vera Rubin Ridge

AbstractThe Alpha Particle X‐ray Spectrometer (APXS) on the rover Curiosity has analyzed the composition of geologic materials along a >20‐km traverse in Gale crater on Mars. The APXS dataset after 6.5 Earth years (2,301 sols) includes 712 analyses of soil, sand, float, bedrock, and drilled/scooped fines. We present the APXS results over this duration and provide stratigraphic context for each target. We identify the best APXS measurement of each of the 22 drilled and scooped samples that were delivered to the instruments Chemistry and Mineralogy (CheMin; X‐ray diffractometer) and Sample Analysis at Mars (SAM; mass spectrometer and gas chromatograph) during this period. The APXS results demonstrate that the basaltic and alkali‐rich units in the Bradbury group (sols 0–750) show minimal alteration indicating an arid climate. In contrast, the Murray formation of the Mount Sharp group (sols ∼750–2,301) has compositions indicating pervasive alteration. Diagenetic features are common and show fluid interaction with the sediment after (and possibly during) lithification. A sandstone unit, the Stimson formation, overlies part of the Murray formation. This has a composition similar to the basaltic sand and soil, suggesting a shared source. Cross‐cutting, fracture‐associated haloes are evidence of late‐stage fluid alteration after lithification of the sediment. The APXS dataset, evaluated in concert with the full science payload of Curiosity, indicates that Gale crater was habitable, and that liquid water was stable for extended periods.

Analyses of High‐Iron Sedimentary Bedrock and Diagenetic Features Observed With ChemCam at Vera Rubin Ridge, Gale Crater, Mars: Calibration and Characterization

AbstractCuriosity investigated a topographic rise named Vera Rubin Ridge (VRR) in Gale Crater, for which a distinct hematite‐like signature was observed from orbit. However, the Chemistry and Camera (ChemCam) and Alpha Particle x‐ray Spectrometer (APXS) instruments on board the rover did not record any significant iron enrichment in the bulk of the ridge compared to previous terrains. For this study, we have reverified ChemCam iron calibration at moderate abundances and developed more accurate calibrations at high‐iron abundances using iron‐oxide mixtures in a basaltic matrix in order to complete the ChemCam calibration database. The high‐iron calibration was first applied to the analysis of dark‐toned diagenetic features encountered at several locations on VRR, which showed that their chemical compositions are close to pure anhydrous iron oxides. Then, we tracked iron abundances in the VRR bedrock and demonstrated that although there is no overall iron enrichment in the bulk of the ridge (21.2 ± 1.8‐wt.% FeOT) compared to underlying terrains, the iron content is more variable in its upper section with areas of enhanced iron abundances in the bedrock (up to 26.6 ± 0.85‐wt.% FeOT). Since the observed variability in iron abundances does not conform to the stratigraphy, the involvement of diagenetic fluid circulation was likely. An in‐depth chemical study of these Fe‐rich rocks reveals that spatial gradients in redox potential (Eh) may have driven iron mobility and reactions that precipitated and accumulated iron oxides. We hypothesize that slightly reducing fluids were probably involved in transporting ferrous iron. Mobile Fe2+ could have precipitated as iron oxides in more oxidizing conditions.

APXS‐Derived Compositional Characteristics of Vera Rubin Ridge and Murray Formation, Gale Crater, Mars: Geochemical Implications for the Origin of the Ridge

AbstractThe resistant ~50 m thick Vera Rubin ridge (VRR) situated near the base of Mount Sharp, Gale crater, Mars, has been deemed a high priority science target for the Mars Science Laboratory mission. This is because of (1) its position at the base of the 5 km layered strata of Mount Sharp and (2) the detection of hematite from orbit, indicating that it could be the site of enhanced oxidation. The compositional data acquired by the Alpha Particle X‐ray Spectrometer (APXS) during Curiosity's exploration of VRR help to elucidate questions pertaining to the formation of the ridge. APXS analyses indicate that VRR falls within the compositional range of underlying lacustrine mudstones, consistent with a continuation of that depositional environment and derivation from a similar provenance. Lower Fe concentrations for VRR compared to the underlying strata discounts the addition of large amounts of hematite to the strata, either as cement or as detrital input. Compositional trends are associated with VRR cross‐cut stratigraphy, indicating postdepositional processes. Higher Si and Al and lower Ti, Fe, and Mn than the underlying mudstone, particularly within distinct patches of gray/blue bedrock, are consistent with the addition of Si and Al. Lateral and vertical compositional variations suggest enhanced element mobility and fluid flow (possibly via multiple events) through VRR, increasing toward the top of the ridge, consistent with the action of warm (~50–100°C), locally acidic saline fluids as inferred from the mineralogy of drilled samples.

Dust cover on Curiosity's Mars Hand Lens Imager (MAHLI) calibration target: Implications for deposition and removal mechanisms

We assessed dust coverage on the Mars Science Laboratory Mars Hand Lens Imager (MAHLI) and Alpha Particle X-Ray Spectrometer (APXS) calibration targets from 14 MAHLI images acquired at <100 μm/pixel, between Mars Science Laboratory Curiosity rover sols 34 and 2248, in order to place constraints on dust accumulation and removal on two endmember orientations (vertical and horizontal). Dust coverage was estimated by (1) determining reflectance ranges for image pixels covered by dust; (2) using the ratio of calibrated MAHLI red band data to blue band data as a proxy for the concentration of dust on the calibration target; and (3) manually counting dust-covered pixels in representative areas of the MAHLI calibration target. The results of each method are consistent within uncertainties, but the reflectance method provided the most efficient and effective way to measure dust cover on each target. Mean and median dust coverage is ~4.7% and 3.9% for the MAHLI target, 9.4% and 8.9% for the APXS target, and 51.9% and 63.4% for the REMS UV sensor. Maximum dust coverage (during the dust storm) is 49% and 42% for the MAHLI and APXS calibration targets respectively, and 80% for the REMS UV sensor. In modeling dust accumulation and removal, the best fit for the MAHLI and APXS targets is one that assumes 2–4% dust removal per sol and 20–40% efficiency in collection, while for the REMS UV sensor, it is one that assumes near 0 removal and 7% of nominal accumulation. Results indicate that the vertically-mounted MAHLI and APXS targets accumulated less dust overall during the mission than horizontally-mounted hardware such as the REMS UV sensor; this was true even during the 2018 global dust event. In addition, while the vertical orientation did not protect the targets from dust deposition during that event, dust removal following the event was more effective on these targets than on horizontally-mounted hardware. Because conditions cannot be monitored continuously, these studies cannot fully discriminate among the potential causes of this dust removal. However, the results suggest that vertical mounting is a reasonable dust mitigation strategy for hardware for which short-term dust accumulation is not a risk factor.

Ground calibration of Alpha Particle X-ray Spectrometer (APXS) on-board Chandrayaan-2 Pragyaan rover: An empirical approach

Alpha Particle X-ray Spectrometer (APXS) was one of the scientific experiments on-board the Pragyaan rover of the Chandrayaan-2 mission to Moon, with the objective to determine the elemental composition of the lunar surface near the landing site in the south polar region of the Moon. APXS estimates the abundances by measuring the characteristic X-ray lines of the constituent elements present in the target surface, when irradiated by the onboard Cm-244 radioactive source. X-ray spectral measurement in the energy range of 0.8–25 ​keV with an energy resolution of ~135 eV at 5.89 ​keV enables APXS to detect all the major rock-forming elements, such as Na, Mg, Al, Si, Ca, Ti, and Fe. This paper describes the calibration of APXS as an X-ray spectrometer, provides a procedure for quantitative estimation of elemental concentrations based on the measurements of standard geochemical reference materials and provides the detection limits of the major elements. It is shown that, following this procedure, the concentration of the major elements in any sample can be determined with typical accuracy of better than 10% of the actual value.

Particle Induced X-ray Emission spectrometry (PIXE) of Hawaiian volcanics: An analogue study to evaluate the APXS field analysis of geologic materials on Mars

The Alpha Particle X-ray Spectrometer (APXS), a field instrument onboard four martian rovers, measures largely unprepared, in situ samples on Mars. The APXS has high precision that enables the determination of elemental concentrations in a wide range of geologic materials. However, lack of sample preparation can lead to heterogeneous matrix effects, and understanding the associated uncertainty is essential for interpreting APXS data. Here we use Particle Induced X-ray Emission spectrometry (PIXE) to analyze a suite of geologic samples from Hawai'i as an analogue study to better understand APXS analyses of martian samples. Wavelength-Dispersive X-ray Fluorescence (WDXRF) analyses of fused glass beads establish higher-accuracy standards for the Hawaiian samples. Sulfate-silicate mixtures were made to evaluate sulfur analysis by PIXE. Results show that the PIXE concentrations for most major elements have 2–6% accuracy, which is comparable to the APXS. However, the PIXE concentrations are systematically high in Al and low in Mg, resulting in lower accuracy (13% and 20%, respectively). Olivine-phyric lavas and most of their altered products have the largest discrepancies with Al concentrations up to 25% high and Mg up to 35% low. Sulfur is systematically high (up to 30% in a basalt matrix) compared to gravimetric S concentrations in the sulfate-silicate mixtures. These systematic deviations in Mg, Al, and S are linked to heterogeneous matrix effects, because PIXE and APXS analyses assume all atoms in a sample to be homogeneously mixed on the sub-micrometer scale, which is not the case. Two key implications for APXS results are: (1) Olivine-bearing samples likely have reported concentrations of Mg that is too low and Al that is too high. Thus, olivine-phyric basalts in Gusev crater and the basaltic sand and soil at three landing sites may have Mg and Al concentrations closer to those of the olivine-phyric shergottites and modelled martian crust than previously thought. (2) Sulfate-silicate mixtures may have overestimated S concentrations reported, resulting in greater uncertainty in the stoichiometry of Ca-sulfates, which is used to deduce the geochemical associations of sulfur in samples.

Alpha Particle X-ray Spectrometer onboard Chandrayaan-2 Rover

Alpha Particle X-ray Spectrometer (APXS) is one of the two scientific experiments on Chandrayaan-2 rover named as Pragyan. The primary scientific objective of APXS is to determine the elemental composition of the lunar surface in the surrounding regions of the landing site. This will be achieved by employing the technique of X-ray fluorescence (XRF) spectroscopy using in situ excitation source 244 Cm emitting both X-rays and alpha particles. These radiations excite characteristic X-rays of the elements by the processes of particle induced X-ray emission and XRF. The characteristic X-rays are detected by the ‘state-of-the-art’ X-ray detector known as Silicon Drift Detector, which provides high energy resolution, as well as high efficiency in the energy range of 1–25 keV. This enables APXS to detect all major rock forming elements such as, Na, Mg, Al, Si, Ca, Ti and Fe. The flight model of the APXS payload has been completed and tested for various instrument parameters. The APXS provides energy resolution of ~135 eV at 5. 9keV for the detector operating temperature of about –35°C. The design details and the performance measurement of APXS are presented in this paper.

Investigation of radiation damage due to particle irradiation on Silicon Drift Detector for Chandrayaan-2 mission

This paper presents the work carrie dout in support of Silicon Drift Detector (SDD) characterization for the Chandrayaan-2 instruments namely Solar X-ray Monitor (XSM) and Alpha Particle X-ray Spectrometer (APXS). The XSM and APXS instruments use SDD to detect X-rays from the Sun and fluorescent X-rays from the lunar surface respectively. The aim of this study is to understand and quantify any spectroscopic performance degradation of these X-ray spectrometers due to space radiation in the SDD during its operational period. Any radiation damage to the SDD leads to increase in the detector leakage current and thus degrades the spectroscopic performance, mainly the energy resolution. The expected end-of-life (EOL) 10 MeV equivalent proton fluence was modeled using SPENVIS simulation software. The Silicon Drift Detector was irradiated with 10 MeV protons for the doses up to 24 krad in logarithmic steps and measured the energy resolution and the leakage current at each dose. The spectrometer energy resolution degraded from ∼ 142 eV at 5.9 keV to ∼ 250 eV for the cumulative proton dose of ∼ 11 krad for the mission life of one year. This satisfies the performance requirement of the SDD based X-ray spectrometers onboard Chandrayaan-2 mission.

Mars Science Laboratory Alpha Particle X-ray spectrometer trace elements: Situational sensitivity to Co, Ni, Cu, Zn, Ga, Ge, and Br

Alongside major and minor elements, trace elements in Martian samples quantified through geochemical analyses by the Mars Science Laboratory (MSL, Curiosity) Alpha Particle X-ray Spectrometer (APXS) provide insight into ancient environments on Mars. APXS spectrum simulations facilitated the characterization of trace elements measured by the APXS under experimental situations commonly encountered on Mars. The precise quantification limits (PQL) obtained under ideal conditions are consistent with limits of detection in the literature and increase with degrading spectral quality. Major trace element species, such nickel, zinc, and bromine, with typical concentrations in the 100s of μg/g (or less), are quantifiable even with short (e.g., 20 min) early morning measurements between drives given the typical concentrations at the MSL site, Gale Crater. Target compositions currently in the public domain with major trace element species below the derived PQL specific to each measurement are noted. Quantification of copper, gallium, and germanium at 30 ppm and <10% uncertainty can be achieved with measurement durations of 6 h, 3 h, and 4 h respectively. In areas of local enrichment, such as the Murray formation, germanium abundance can be assessed with <10% uncertainty in 30 min. The detection and quantification of cobalt is hindered by the presence of overlapping peaks associated with the ubiquitous iron and nickel present in Mars samples. Only a single sample interrogated by Curiosity's APXS up to sol 2003 contains cobalt at detectable levels, coinciding with a composition similar to terrestrial ferromanganese crusts and nodules formed under inorganic aqueous conditions. The work presented herein emphasizes the need for acquiring quality APXS measurements where reliable trace element quantification is desired and offers guidance to improve the tactical operation of the MSL APXS instrument.

Multiple ionization X-ray satellites of Ti and its binary oxides in alpha particle PIXE

High energy resolution X-ray emission spectroscopy was used to study the multiple ionization satellites in the K X-ray spectra of titanium induced by target irradiation with MeV alpha particles. Beam energies of 3–5 MeV were used to explore the beam energy dependence. In addition, several titanium binary oxides excited with 5 MeV beam were measured to study also the chemical bonding effects on the multiple ionization satellites. Energy shifts and intensities of the satellites corresponding to one and two L-shell vacancies were determined from the measured spectra. The results will be used to refine the existing experimental alpha particle induced multiple ionization satellite database which is needed to improve the accuracy of PIXE analysis using alpha particle beams, including the PIXE mode of the Mars alpha particle X-ray spectrometers.

A re-examination of the fundamental parameters approach to calibration of the Curiosity rover alpha particle X-ray spectrometer

In terrestrial instruments for X-ray emission analysis (e.g. X-ray fluorescence, electron microprobe) the angle of excitation and the angle of characteristic X-ray emission by samples are typically well-defined. This is not the case for the Mars rovers’ alpha particle X-ray spectrometers, necessitating use of “effective” angles in any fundamental parameters approach to spectrum fitting and derivation of element concentrations. A new Monte Carlo computation of expected characteristic X-ray yields has enabled us to examine the choice of effective angles in detail and to compare with our previous estimates; the mean angles taken over all detected events appear to be the most satisfactory approximation. These were used in our GUAPX code to process APXS spectra from ten mono-mineralic geochemical reference materials. Agreement between our concentration values and those determined by the specialist company ActLabs was excellent.

Empirical simulations for further characterization of the Mars Science Laboratory Alpha Particle X-ray Spectrometer: An introduction to the ACES program

Simulation software was developed that accurately emulates the response of the Mars Science Laboratory (MSL) Alpha Particle X-ray Spectrometer (APXS) under variable experimental conditions including temperature, standoff, measurement duration, and target composition. Around 450 spectra collected on Mars provide the spectral library for calibration of the “APXS Characterization by Empirical Simulation” (ACES) software. Output from the ACES software package is presented alongside discussion of its development. Analyses of simulated spectra demonstrate major species compositions are well within instrument accuracy for the theoretical concentrations input in the simulation. The work presented herein provides the foundation and support for future scientifically motivated investigations including limits of detection for trace elements (i.e. atomic number 27+) under various experimental conditions.

Multiple ionization X-ray satellites of magnesium, aluminum and silicon in alpha particle PIXE

Abstract Wavelength-dispersive spectroscopy was used to study the multiple ionization satellites in the K X-ray spectra of magnesium, aluminum and silicon bombarded by alpha particles having energies of 3, 4 and 5 MeV. The measured energy shifts and intensities relative to the diagram lines of the groups with one, two and three L-shell vacancies were extracted from the spectra. These results compare well with the values in an interim database which was recently assembled from the limited existing literature. The present work is the first step in refining this database so that it may be used to improve the accuracy of PIXE analysis using alpha particle beams, including the PIXE mode of the Mars alpha particle X-ray spectrometers. The effects of oxide bonding are also determined and compared to those observed when excitation is effected by photons and by electrons.

Seasonal Atmospheric Argon Variability Measured in the Equatorial Region of Mars by the Mars Exploration Rover Alpha Particle X‐Ray Spectrometers: Evidence for an Annual Argon‐Enriched Front

AbstractThe Mars Exploration Rover Opportunity (MER‐B) has been exploring the surface of Mars since landing in 2004. Its Alpha Particle X‐ray Spectrometer (APXS) is primarily used to interrogate the chemical composition of rocks and soil samples in situ. Additionally, the APXS has measured the atmosphere of Mars with a regular cadence, monitoring the change in relative atmospheric argon density. Atmospheric measurements with the MER‐B APXS span over six Mars years providing an unprecedented level of statistics for careful study of the ubiquitous APXS spectral background. Several models were applied to high‐frequency long‐duration Spirit rover atmospheric APXS measurements. The most stable model with the least uncertainty was applied to the MER‐B data set. Seasonal variation of 10–15% in equatorial atmospheric argon density was observed ‐ in agreement with existing literature and global climate models. Unseen in previous work and global climate models, an abrupt deviation from the model‐predicted annual mixing ratio was measured by the MER‐B APXS around Ls 150. The sharp change, ~10% over 10° Ls, provides strong evidence for a northward migrating front, enriched in argon, sourced from the south pole at the end of southern winter. A similar weaker front is possibly observed around Ls 325, sourced from the northern polar region.