Conceptual Design of an in Situ K-Ar Isochron Dating Instrument for Future Mars Rover Missions

Trans. JSASS Aerospace Tech. Japan Vol. 14, No. ists30, pp. Pk_89-Pk_94, 2016

Conceptual Design of an In Situ K-Ar Isochron Dating Instrument for Future Mars Rover Missions

By Yuichiro CHO,1) Shingo KAMEDA,1) Yayoi N. MIURA,2) Yoshifumi SAITO,3) Shoichiro YOKOTA,3) Satoshi KASAHARA,3) Ryuji OKAZAKI,4) Kazuo YOSHIOKA,1) Kazuo SHIBASAKI,1) Takahiro OISHI3) and Seiji SUGITA5)

1) Department of Physics, Rikkyo University, Tokyo, Japan 2)Earthquake Research Institute, The University of Tokyo, Tokyo, Japan 3)Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 4)Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka, Japan 5)Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan

(Received July 30th, 2015)

Age is one of the most important observables in planetary science. Although a sample return mission will provide accurate radiometric age measurements, it may not be launched very often because of its high cost. Thus, in situ radiometric age measurements with a one-way landing mission is very important. We are developing an on-site K–Ar isochron dating instrument for such a mission. This instrument is intended to measure K and Ar abundances with laser-induced breakdown spectroscopy and noble gas mass spectrometry, respectively. In this study, we propose an in situ dating package using mostly flight-equivalent components and a small number of components currently under development by our team. The geochronology instrument suite will contain a pulse laser, a spectrometer, a re-sealable vacuum chamber, a time-of-flight mass spectrometer, and a sample handling system using parallel link arms. Our assessment estimates that such an in situ geochronology package for measuring K–Ar crystallization age with 10–15% of accuracy for rocks with 3 Ga of age would be approximately 240 mm × 240 mm × 400 mm in size and 12.5 kg in mass.

Key Words: Mars Mission, In Situ K–Ar Dating, Laser-induced Breakdown Spectroscopy, Noble Gas Mass Spectrometry

1. Introduction Recently, the NASA Curiosity rover performed the first in situ dating experiment on Martian rock and obtained an Understanding the evolution of a habitable environment age of 4.21 ± 0.35 Ga for a mudstone on the floor of the is one of the primary goals of Mars science. A key Gale crater.7) However, the age of the mudstone does not observable for characterizing the nature of the climate necessarily reflect the age of the geologic unit covering change on ancient Mars is the absolute age of the the Gale crater; it could be crystallization age of Noachian–Hesperian transition.1) However, the geologic surrounding basaltic crustal rocks or could be history of planets including Mars is largely based on sedimentation age of the mudstone. Geologic radiometric age data derived from limited locations on the interpretation of this age measurement is rather difficult, Moon. Furthermore, no radiometric age data with a clear because it is a model age based on whole-rock geologic context have been obtained for Mars. Although a measurement. More specifically, sedimentation age sample return mission will provide highly accurate estimated from this measurement ranges from 1.6 to 4.5 radiometric age measurements, it will be extremely Ga.7) Thus, the absolute age of the Noachian–Hesperian expensive and technically challenging. Thus, there is a boundary is yet to be determined. In such complicated need for an in situ geochronology mission. geological background, isochron dating is particularly K–Ar dating is one of the most common radiometric preferred because it can obtain an accurate age for a rock. dating methods. This method employs the radioactive For example, isochron dating enables us to evaluate the decay of 40K to 40Ar and 40Ca with a total half-life of 1.27 quality of age data (i.e., the contribution of mixture of Gyr.2) This method is easier than other dating methods different origins, and of subsequent degassing by thermal such as Ar–Ar, U–Pb, and Sm–Nd because K is relatively events) from the scatter of the isochron data. The intercept abundant in igneous rocks and Ar can be easily extracted of an isochron can be used for correcting contamination (by simply heating the sample) and requires a simpler from trapped 40Ar. instrumental configuration. Several research groups To accomplish such experiments on Mars, we have been including ours have developed an in situ K–Ar isochron developing an in situ K–Ar isochron dating instrument. dating method for future landing planetary missions.3–6) The ability for local analyses is critical for acquiring an

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isochron from a single rock sample. Cho et al.8) developed clay-rich Noachian to the sulfate-rich Hesperian.1) Remote an experimental and analytical method for precise K sensing images and spectral data obtained by the measurements using laser-induced breakdown OMEGA/Mars Express and CRISM/Mars Reconnaissance spectroscopy (LIBS). The combination of LIBS and a Orbiter indicate that there are many attractive landing site mass spectrometric technique yields spot-by-spot K–Ar candidates. Candidate landing sites include Hesperian ages. We have reported preliminary isochron data in lavas on Syrtis Major. A volcanic terrain is preferred as a Ref-4); we measured two gneiss slabs to construct a K–Ar landing site because volcanic rocks are suitable for K–Ar isochron (i.e., 40Ar/36Ar vs. 40K/36Ar). The data points dating experiments. Furthermore, the surface of Syrtis followed a straight line well, strongly suggesting the Major has a crater density comparable to the timing of the feasibility of isochron measurements with our LIBS-mass climatic transition on early Mars.12) spectrometer (MS) approach. From the intercept of the After landing on Mars surface, we need to find rock isochron, we also obtained an initial 40Ar/36Ar ratio of 480 samples suitable for K–Ar measurements and actually pick ± 130, which is comparable to that of terrestrial them up and place in the vacuum chamber to conduct the atmosphere (40Ar/36Ar = 296). This also suggests that the measurements. Here, we describe an expected isotopic composition of trapped Ar is measurable using measurement procedure after a successful landing. An this approach. The ability to measure the initial Ar onboard navigation camera observes the geologic context isotopic ratio in magma is important because such around the landing site to find rocks suitable for dating. measurements will provide insights into the evolution of The rover moves around and stops over the rock, taking the parent magma or trapped Martian atmosphere. Based close-up images of the rock with an imager. The LIBS on the error propagation of K and Ar measurements, our instrument measures the elemental composition of the previous feasibility study4) indicates that the K–Ar ages of rock before the sample acquisition. If ground-penetrating the rocks recording the Noachian–Hesperian transition radar is onboard, it observes the structure in the ground to will be measurable with 10 – 15 % error when the a depth of ~10 m to assess whether the rock is part of the relatively high (1 ‒ 3 wt%) K content of rocks as found in bedrock (hence, autochthonous) or an allochthonous rock the Gale or Gusev craters 9,10) and the crater model age of that originated from another place on Mars. If the rock is the Noachian–Hesperian transition (3.5 Ga) 11) are appropriate for K–Ar dating, a rock core sample is assumed. acquired and delivered to the sample chamber. The However, the instruments used in those experiments chamber is locked and evacuated by a turbomolecular were not optimized for planetary landers. For example, the pump. A blank spectrum is obtained after the evacuation. weight of the Nd:YAG laser and turbomolecular pump LIBS-MS measurements are then carried out on the used in 4) were approximately 68 kg (Surelite I-20, cylindrical surface of the rock core. The LIBS spectra are Continuum) and about 11 kg (TG220F, Osaka Vacuum, obtained for ~1000 laser pulses. The getter purifies the Ltd.), respectively, which were too heavy to use in gases liberated by the laser shots. Then the purified gases planetary missions. Furthermore, the acquisition and are introduced to a time-of-flight mass spectrometer for Ar handling of rock samples were not customized for measurement. The LIBS-MS analyses are repeated for 5 – planetary missions; a rock sample is manually cut into 20 spots for each sample to obtain a K–Ar isochron. In our slabs and introduced into a vacuum chamber by hand. baseline plan, more than five rocks will be measured for These operations must be conducted robotically in dating experiments. The iterative measurements are useful planetary missions. In this context, remodeling the for improving the accuracy and precision of age values. laboratory-based instrument into an actual flight Figure 1 shows a schematic of the instrument suite used instrument is a key step in achieving in situ K–Ar isochron for this experiment. dating missions. The first step is designing the geochronology instrument package. Plasma Thus, in this study, we explore a conceptual design for Electronics Window Sample an instrument package incorporating an automated sample-handling system that can be carried by a Mars Laser Exploration Rover-sized rover. Sample acquisition +Spectrometer & Delivery +Camera

2. Measurement Procedures on Mars Chamber

Getter To obtain appropriate isochron ages, it is very important Vac. to find fresh (unaltered) rocks with sufficiently high K gauge concentrations and simple geological context. To find such TOF-MS Rock rocks, we need to assess the elemental compositions, Pump mineralogy, and textures of rock samples. Furthermore, these rocks must represent the geologic unit of interest. Fig. 1. A schematic diagram of our geochronology instrument package. A rock sample in the vacuum chamber is irradiated by laser To achieve the scientific objectives, we propose to land pulses for K–Ar age measurements. on a geologic unit recording the transition from the

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mm × 240 mm × 170 mm. The LIBS package and 3. Geochronology Instrument Suite electronics are separated from the TOF-MS subsystem (Fig. 1). The LIBS package is thermally insulated from the The in situ K–Ar geochronology instrument suite that gas analysis package to prevent thermal damage; the can conduct the experiments described above consists of TOF-MS package can be very cold at night (approximately three subsystems: LIBS for K measurements, a −50°C) because of heat loss through the aperture of the time-of-flight mass spectrometer (TOF-MS) for Ar sample inlet; this package can also become very hot measurements, and rock sampling devices (Table 1). The during the getter reactivation. The size of the LIBS subsystem includes a laser, a spectrometer, and a LIBS-and-electronics box is estimated to be camera for the contextual imaging of the LIBS approximately 230 mm × 220 mm × 150 mm. The mass of the instrument package is another measurement spots. The spatial resolution of imaging is 30 fundamental factor for a rover payload. The mass of the μm to observe the texture of rock samples. The laser pulse flight-equivalent instruments is used to estimate the mass energy is 30 mJ and the laser spot diameter would be ~300 of our geochronology package. The current estimate of the μm. A surface roughness of ± 2 mm is acceptable for LIBS total mass of the package is approximately 12.5 kg (Fig. analyses because of the depth of focus of the objective 3). lens. The laser pulses will evaporate approximately 100 μg Comparison with our previous experiments4) using of a sample. Scanning mirrors are used to control laser bread-board model (BBM) predicts that the instrument spot positions. The detection limit of K is required to be package would yield 10−15% of K−Ar age estimation less than 300 ppm for measuring the K abundances of error for a 3 Ga rock. Both the intensity (~10 GW/cm2) of shergottites. laser pulses and the mass resolution (m/m ~160) of this The TOF-MS subsystem is composed of a TOF-MS, a system proposed in this study are comparable to that used vacuum chamber, a getter to remove active gases, valves, in 4). manifolds, a vacuum gauge, and a turbomolecular pump. The TOF-MS needs to be able to detect 3 × 10−14 mol of Table 1. Candidate components for our in-situ K–Ar dating 40Ar to measure radiogenic 40Ar produced by a 3 Ga rock instrument package. containing 300 ppm of K. Subsystem Component Remarks LIBS Nd:KGW laser Used in ChemCam/Curiosity The sampling subsystem is dedicated to acquiring Spectrometer To be used in RSL/ExoMars appropriate rocks and transferring them to the sample Camera In-house development analysis chamber for in situ dating. This subsystem TOF-MS TOF-MS In-house development includes a coring device and sample delivery robotic arms. Vacuum chamber Based on EXCEED/Hisaki Table 1 summarizes the components used in our Getter Used in EXCEED/Hisaki geochronology instrument suite. In this conceptual study, Valves Used in SAM/Curiosity we use flight equivalent devices (i.e., instruments Turbomolecular Used in SAM/Curiosity previously used in other planetary missions or developed pump for mission projects). Using the heritage of previous Sample Coring device NanoDrill; developed for future missions enables us to construct a realistic instrumental handling planetary missions design, reduce development costs, and shorten the Sample delivery In-house development development period. There are many components that system have been flown in previous missions or designed in detail for near-future missions. For example, a Nd:KGW laser, the Wide Range Pump (Creare, NH., USA), and microvalves (Mindrum Precision Inc, CA., USA) have been used in the Chemistry and Camera (ChemCam) and the Sample Analysis at Mars (SAM) instrument suite of Curiosity. As the sample acquisition tool, NanoDrill (Honeybee Robotics, CA., USA)13) can be used as the corer. In addition, our group has been developing some of the components used in the geochronology instrument suite. These components include a vacuum chamber, a TOF-MS, and a sample delivery system. Descriptions of these devices are provided in the next section. The size of the in situ geochronology package must be minimized for installation on rovers. Considering the size and dimension of these components, we produced a 3-D configuration design for the instrument suite (Fig. 2). The Fig. 2. Conceptual design of the in situ geochronology package. size of the gas analysis package will be approximately 240 The sample handling system is not illustrated here.

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reflectron TOF-MS that can be carried by a rover. The Laser head TOF-MS will have a length of 180 mm and a diameter of 0.6 Spectrometer 0.6 Optics 0.2 100 mm excluding the ion source and the electronics. The Actuators 0.3 mass of the TOF-MS would be approximately 3 kg, Arms & motors CMOS 0.01 Baseplates including the ion source and electronics. Figure 4 is a 1.9 0.5 schematic diagram of the reflectron TOF-MS and ion Sample NanoDrill LIBS trajectories. Neutral gases extracted from sample rocks are handling 1.0 3.5 Electronics 2.9 ionized by electron impacts. The ions are introduced to the Gauge 0.1 TOTAL 1.3 TOF chamber as an ion beam, accelerated by a voltage of 12.5 Pump 0.7 about 4 kV and detected by a microchannel plates (MCP). Getter, TOF-MS Numerical simulations with SIMION package indicate that Valves, 6.1 Manifolds a mass resolution of 160 will be achieved with this setup, 1.0 40 Chamber TOF-MS which is sufficiently high to separate Ar peak from peaks 3.0 1.3 at 39 Da and 41 Da.

Fig. 3. Expected masses of the components. The unit is kilograms.

4. Status of In-house Development

This section outlines our development of the three components used in the instrument suite. 4.1. Vacuum chamber The vacuum chamber consists of a chamber body, a lid, a latch to lock/unlock the lid, a motor to move the lid to the “closed” position, and two paraffin actuators to push a latch from two sides. In the “closed” position, the latch holds the chamber lid to keep it closed. When the paraffin actuator at the right-hand side pushes the latch, the latch rotates and unlocks the lid. Then, the lid opens spontaneously by a spring. This “opening operation” has Fig. 4. Schematic of the setup of the reflectron TOF-MS. Incoming been demonstrated by the EXCEED instrument on board ions are accelerated by electrodes at the entrance. After travelling in the Hisaki spacecraft.14) To close the lid and seal the the drift tube, the ions are reflected by the reflectron and detected by an MCP. vacuum chamber, a motor moves the lid to the “closed” position. Then, the other paraffin actuator generates a 4.3. Sample acquisition and handling sealing force by pushing the latch from the left-hand side. We also designed a sample handling system for age In addition to the open/close function, vacuum sealing measurement protocol. Our design has the advantage that is an essential function of the chamber. In terrestrial it can avoid the following problems expected for sample laboratories, noble gas analyses are performed using metal handling operations on Mars: (i) dust adhesion on the gaskets to achieve high vacuum. The Curiosity mission O-rings and (ii) a rock core becoming stuck in the has proven the capability of metal gasket seals. However, chamber. deforming a metal gasket for vacuum sealing requires a A rock core is carried to the sample chamber by using large force (e.g., 667–1350 N for Curiosity).15) Moreover, two robotic arms sequentially; one arm has a corer on its multiple uses of metal gaskets for vacuum seal are end and the other arm transports the core into and out of difficult. In contrast, elastomeric O-rings are easier to use the chamber using a sample holder. Both arms use a because they require a much smaller force than metal parallel link mechanism. First, the corer obtains a rock gaskets. Moreover, elastomeric O-rings allow for multiple core. Then, the parallel link arm moves the corer to a times of opening and closing. position where the corer ejects the rock core into a sample However, O-ring seals have a few potential holder through a sample inlet above the holder. The rock disadvantages as well: (i) degassing from elastomers; (ii) core in the sample holder is then carried by the other permeation of Martian air; and (iii) limited tolerance to parallel link arm and placed into the chamber (Fig. 5). The low temperatures, which are expected during a Mars holder helps to prevent rock particles adhering to the mission (approximately −50°C). Work in progress sample from falling onto the vacuum seal. The robotic arm includes the noble gas experiments and the sets the sample holder in the chamber before the chamber low-temperature thermal cycle tests using the elastomeric lid is closed. After geochronology experiments, the arm O-rings to evaluate the degree of these potential problems picks up the holder again to replace the sample. The rock for O-rings. core in the holder is discarded by another small arm with a 4.2. Time-of-Flight Mass Spectrometer clamp on its end. This operation assures that a sample is We are currently developing a small, lightweight

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removed from the chamber even when the rock is (a) Top fractured into fragments during age measurements. Arm To prevent the O-rings from catching dusts floating in the Martian air, we use a double-lid system. The entire O-rings sample handling system is placed in a rover and separated from the outside by a lid (sample introduction lid). The Clamp-2 rock core sample is inserted to the sample holder through this lid. The vacuum chamber lid remains closed during the sample introduction lid is open; the chamber lid is Chamber opened only when the sample introduction lid is closed. Thus, the chance of catching dust is reduced because the O-rings are not exposed to the outside. Moreover, we may (b) Side Sample holder Clamp-1 carry a gas tank to blow off the dusts, and/or use an electrically charged pole for collecting the dusts before the chamber lid is opened. Rock core Chamber lid 4.4. Future work Building an actual instrument suite based on this design has challenges because of the complexity of the measurement system. To construct the instrument package, future studies will focus on (i) measuring the K Clamp-2 concentrations using a low-energy (15–30 mJ) pulse laser, (ii) measuring the Ar amounts using the newly developed (c) Front Clamp-1 TOF-MS (see section 4.2), and (iii) building the sample handling system that can acquire, set, and discard a rock core automatically. Work in progress includes building a breadboard model of the K–Ar dating package that can be Clamp-2 carried by a rover. Experimental results using the instrument package will be addressed elsewhere.

5. Conclusion Fig. 5. Conceptual diagram of the sample delivery system. The rock core acquired by a corer is inserted into the sample holder We made a conceptual design for an in situ K–Ar (shown at left in (a) and (b)). The holder is then delivered to the chamber by the parallel link arm. After the sample holder is placed isochron dating instrument package. We used a in the chamber, the clamp releases the holder from the arm. The arm combination of flight-equivalent devices and devices for the corer is not shown. currently developed by our team. The current best estimate indicates that our package would be approximately 240 mm × 240 mm × 400 mm in size and 12.5 kg in mass. As References part of the efforts to develop the geochronology instrument suite, a reusable vacuum chamber, a sample 1) Bibring, J. P., et al.: Global Mineralogical and Aqueous Mars handing system using the parallel link mechanism, and a History Derived from OMEGA/Mars Express Data, Science, reflectron TOF-MS are being developed. Future studies 312 (2006), pp. 400-404. include constructing an actual instrument package based 2) Steiger, R. H. and Jäger, E.: Subcommission on on the design proposed in this study. Geochronology: Convention on the Use of Decay Constants in Geo- and Cosmochronology, Earth Planet. Sci. Lett., 36 (1977), pp. 359-362. Acknowledgments 3) Cho, Y., Miura, Y. N. and Sugita, S.: Development of a Laser Ablation Isochron K-Ar Dating Method for Landing Planetary Advices from Dr. Genya Ishigami of Keio University Missions, 2011 PERC Planetary Geology Field Symposium and Dr. Ryuta Hatakenaka of JAXA are greatly Abstracts, (2011), pp. P30. 4) appreciated. This study was supported by funds from the Cho, Y., Miura, Y. N. and Sugita, S.: Development of an in-situ K-Ar Isochron Dating Method 2: Validation Measurements ISAS Mars EDL Working Group. This study was with Natural Rocks, Proceedings of Lunar and Planetary supported by JSPS Grant-in-Aid for Young Scientists (B) Science Conference, 45 (2014), p. 1205. Grant Number 15K17796, JSPS Grant-in-Aid for Research 5) Cohen, B. A., Miller, J. S., Li, Z.-H., Swindle, T. D. and Activity Start-up Grant Number 26887040, and JSPS French, R. A.: The Potassium-Argon Laser Experiment (KArLE): In Situ geochronology for Planetary Robotic KAKENHI 26247092. Missions, Geostand. Geoanal. Res., 38 (2014), pp. 421-439. 6) Cohen, B. A., Swindle, T. D. and E., R. S.: In Situ Geochronology on the Mars 2020 Rover with KArLE

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