Dating the Irregular Mare Patch Ina: Overview and Updates

DATING THE IRREGULAR MARE PATCH INA: OVERVIEW AND UPDATES. F. S. Anderson1, E. B. Bierhaus2, S. E. Braden3, A. L. Fagan4, R. G. Fausch5, J. W. Head III6, K. H. Joy7, J. Levine8, S. Osterman, J. Pernet-Fisher7, R. Tartèse7, P. Wurz5, M. Yant2 1Southwest Research Institute Boulder, CO 80302 USA, 2Lockheed Martin Space Littleton, CO 80127 USA, 3Lunar Scholar Services LLC Aurora, CO 80247 USA, 4Western Carolina University, NC 28723 USA, 5The University of Bern, CH-3012 Bern, CH, 6Brown University, RI 02912 USA, 7The University of Manchester M13 9PL, UK, 8Colgate University, NY 13346 USA. Introduction: The DIMPLE (Dating an Irregular Mare Patch with a Lunar Explorer) payload [1,2] was selected under the NASA Payloads and Research Investigations on the Surface of the Moon (PRISM) program to fly in 2028 as a Commercial Lunar Payload Services (CLPS) payload to the irregular mare patch (IMP) Ina [3-6] (Fig. 1). One of the most striking features of Ina and other large IMPs is that they have very few impact craters, consistent with the hypothesis that they are very young, ~18-66 Ma [4]. This relative youth begs the question how lunar volcanism, thought to have largely ended more than 3 billion years ago, could have remained active into the geologically recent past, and what that means for the geochemical and geothermal history of the Moon. An alternative hypothesis contends that IMPs are in fact ancient, but are comprised of materials that cannot sustain crater-forms over geologic time, such as a volcanic foam or highly vesicular lava [7]. The DIMPLE payload will land on Ina, a 3-km wide volcanic caldera located near the summit of a lunar shield volcano in Lacus Felicitatis. Its goal is to determine whether the surface materials at Ina are young (~33 Myr) or ancient (~3.7 Gyr), and assess the geologic context, geochemistry, nature of the regolith, and vesicularity of rock samples. DIMPLE will distinguish between the young and old hypotheses by landing on a high-standing smooth mound called Mons Agnes (Fig. 1) within the ~50-m deep Ina caldera, and use an instrument called CODEX [8] to measure the elemental composition and Rb-Sr age of local rocks. CODEX uses laser-ablation mass spectrometry to assess composition and adds a resonance ionization stage to provide isobar-free Rb and Sr isotope measurements. Ablation is accomplished with a 266 nm, 100 μJ, 10 ns pulsed laser focused to a ~40 μm spot. Rb resonance ionization is achieved with 778 and 1064 nm, ~100 μJ, 6 ns laser pulses, and Sr with 461, 405, and 1064 nm, ~100 uJ, 6-ns laser pulses. To enhance the efficiency of resonance ionization, CODEX bounces the blue and red lasers through the ablated neutral atoms using a multi-pass optical cell. DIMPLE will acquire a baseline of four ~1.9-3.8 cm diameter, unbrecciated basaltic rocks from: a) the foot of the lander using a robotic arm, b) nearby craters and boulders on the smooth mound using a rover, and c) the nearby rough terrains below the plateau rim (Fig. 1). The team will perform CODEX analysis of up to 2048 spots on each sample, and for those samples which are igneous and have 87Rb/86Sr Fig 1. Oblique view of the Ina caldera looking East. North is to the right. The yellow circles on Mons Agnes show candidate landing sites. Image: LROC NAC M1108203502LR [NASA/GSFC/ASU] > 0.05, provide three (3) or more radioisotopic dates with an uncertainty better than ±930 Myr (Fig. 2), which in conjunction with sample chemistry, sample and context imaging, and 1600-meters of rover traverses (Fig. 3), will allow us to assess the chronology and geologic context of Ina. To prepare each Fig 2. 2600 ± 300 Ma isochron for lunar meteorite LAP 02205, demonstrating sufficient precision to meaningfully date lunar rocks. rock for measurement by CODEX, the arm will pass it over a saw that cuts a smooth, flat surface. CODEX measurements will be supported by cameras on the lander and rover, which provide geological context, as well as close-up imaging of the analyzed rock surfaces [9]. DIMPLE Updates: There have been recent changes to DIMPLE project: 1) the transfer of the DIMPLE rover rake sample acquisition system to CLPS, 2) the transfer of responsibility for the imaging capabilities (previously CLPS-provided cameras) to the DIMPLE team, and 3) the DIMPLE team added a standard to the inside of the mass spectrometer aperture flight cover, separate from the design of the standard and reference mineral assembly. The rover rake. The transfer of the rover rake sample acquisition system was requested by NASA to simplify the assembly, integration, and testing with the CLPS rover, which will be delivered significantly after the delivery of DIMPLE. Cameras on the lander. In the original proposal, DIMPLE requested the capability for microscopic images of samples from a distance of 15 cm and the capability to image the lunar surface to see potential rock samples within reach of DIMPLE’s robotic arm. At the time, cameras providing these capabilities were not considered part of the DIMPLE payload. As the project progressed, there were concerns that the relatively late delivery and integration of CLPSprovided cameras compared to the DIMPLE design and delivery would complicate testing to verify meeting the DIMPLE science requirements. Thus, NASA decided to transfer responsibility for these cameras to the DIMPLE team, simplifying the integration schedule between the DIMPLE payload and the CLPS lander. Figure 3. Notional traverses, #1-4 starting from landing circle “A”. The CLPS-provided rover will travel up to 1600 meters in total distance and retrieve ~1.9-3.8 cm diameter rocks to bring back to the lander for sample preparation and CODEX analysis. Golden shading represents the loss of communication zone calculated with an estimated lander and rover height; actual CLPS configuration may vary. Standards and References: The DIMPLE Standards and Reference Mineral Assembly is designed to be mounted to the robotic arm. Before and during CODEX analysis of a lunar sample, the robotic arm will move the assembly in front of the CODEX aperture to take calibration data. The assembly includes the follow standards: NIST SRM-1264a steel and NIST BHVO-2G glass, as well as reference minerals including anorthosite, ilmenite, augite, and forsterite. Characterization and analysis of the reference minerals is underway at the University of Manchester. In addition to the standards and references mounted on the robotic arm, a piece of reference material will be mounted to the inside of the mass spectrometer aperture cover. During cruise or after landing, the team will perform CODEX analysis of the reference sample attached to the cover to demonstrate functionality of CODEX independently of lunar sample availability or robotic arm performance. Science and Operations Preparations: In the past year and half the DIMPLE science team has worked alongside the engineering team to prepare the DIMPLE Operations Center [10], increase the detail and accuracy of our Concept of Operations (ConOps) [11], develop our methodology for identifying and measuring basaltic sample vesicularity [12], and better understanding the key statistical drivers of CODEX measurements [13], among other activities. Overall, the DIMPLE program has passed its initial Systems Requirements Review (SRR), and is expecting to hold the Preliminary Design Review (PDR) in mid 2025. Conclusion: Using innovative instruments like CODEX on small payloads like DIMPLE, we can obtain new scientific results for well-posed in-situ scientific problems, while also driving the scientific need for new sample returns by identifying exciting samples in advance. Through collaboration and partnership between NASA PRISM, CLPS, and our international colleagues, DIMPLE is poised to open a new chapter in planetary in-situ age dating. Acknowledgments: This work is supported through PRISM grant 80NSSC24M0001. The UK Co-Is are supported by UKRI STFC funding. RF and PW are supported by the Swiss National Science Foundation and Swiss Space Office. References: [1] Anderson et al. (2024) LPSC Abs #2336. [2] Anderson et al. (2024) LPSC Abs #2547. [3] Strain and El-Baz (1980) LPSC 2437-244. [4] Braden et al. (2014) Nature Geosci. 7, 787. [5] Garry et al. (2012) JGR, 117, E00H31. [6] Schultz et al. (2006) Nature, 444, 7116. [7] Qiao et al. (2019) J. Geophys. Res.-Planets, 124, 1100-1140 [8] Levine et al. (2023) Planet. Sci. J. 4, 92. [9] Fagan et al. (2024) GSA 218-7. [10] Alexander et al. (2025) SpaceOps 18 Abs. [11] Anderson et al. (2025) LEAG Annual Meeting Abs #5081. [12] Fagan et al. (2025) LPSC Abs #2473. [13] Levine et al. (2025) LPSC Abs #1172.