The Inner SOlar System CHRONology (ISOCHRON) Discovery Mission: # 1110 Returning Samples of the Youngest David S. Draper1, Rachel L. Klima2, Samuel J. Lawrence1, Brett W. Denevi2, and the ISOCHRON science team 1Astromaterials Research Office, ARES Division, NASA Johnson Space Center, Houston, TX (david.draper@.gov), 2Johns Hopkins Applied Physics Laboratory, Laurel, MD

Summary South of Mission Concept Plateau (P-60) Inertial view We propose the Inner SOlar System CHRONology (ISOCHRON) approx. 20° N, 51° W (time of arrival) ’s orbit • SRM Burn Coast, SRM Separation Discovery mission concept: an automated lunar sample return To Sun Launch mission to mare units south of the Aristarchus Plateau estimated to be ~1.5–2.0 Gyr old

Powered Descent • Will address fundamental questions about the time-stratigraphy of Moon arrival lunar magmatic processes and the composition of the lunar crust Earth to Moon Transfer Leg Direct Approach and Landing with implications for all of the terrestrial planets Launch • Land SW of Aristarchus, lunar morning • Safe precision landing with Terrain • Addresses numerous key outstanding questions identified by several Cruise: Low-energy trajectory, Relative Navigation recent community assessments [1-3] ~6 month transit time • Powered descent

Inertial view Earth Centered Inertial view Science Goals (time of departure) Moon departure (time of re-entry) • Primary: High-precision radiometric age measurements on young basalts to fill existing gap in age-correlated crater size-frequency To Sun distributions (CSFDs): will greatly improve this widely-used tool for ISOCHRON landing site estimating ages of exposed surfaces on rocky bodies Figure 2. ISOCHRON landing site is Moon arrival • Provide critical insights on lunar thermal and magmatic history from large mare field south of Aristarchus Surface Operations • 4 Earth days max on surface Ascent comprehensive compositional and mineralogical data for direct Plateau, designated P-60 in [7] at 1.2 • Ascent Vehicle Return & EDL/Recovery (notionally Australia) +0.32/-0.35 Gyr; 1.03 to 2.81 Gyr • Collect150g rocklets & regolith • Direct 4.5 day transfer comparison with older, Apollo- and Luna-returned samples w/custom acquisition system launches from Lander across the unit in [8]. • Multiple return launch opportunities • Shed new light on regolith dynamical processes owing to samples’ • Perform imaging formation substantially after the Moon’s heaviest period of bombardment Science Team, Measurements, Deliverables Science Measurement Objectives for Preliminary Subteam(s) Measurements Laboratory Instrumentation Sample Type ISOCHRON science team Examination Team (PET) Landing Site Surface Geochemistry/ Geochronology Sample Context & Background First Chronology/ Petrology/ Impact Science Surface Ops Basalt crystallization age from abundances of radioactive, Measurement of Rb-Sr, Sm-Nd, K-Ca and U-Pb Mass spectrometers (TIMS, SIMS, [MC] ICP- Chronology Surface CSFD Spectroscopy Rocklets Name Last Name Affiliation Role Geology radiogenic isotopes radiochronometers MS) Thermal exposure history from noble gas abundances and • Full range of mare basalt compositions and ages has not yet been Dave Draper JSC PI Measurement of Ar isotopes Noble gas mass spectrometers Rocklets Rachel Klima APL DPI x x isotopic ratios sampled [4,5]. Knowledge of the duration of mare volcanism comes Basalt crystallization age from abundances of radioactive, Mass spectrometers (TIMS, SIMS, Nano- Lu-Hf Chronology and isotope tracer Rocklets Brett Denevi APL PS x x x x immobile radiogenic isotopes SIMS, ICP-MS) from (a) radiometric dating of Apollo and Luna samples and lunar Sam Lawrence JSC PS x x x Mass spectrometers (TIMS, Nano-SIMS, [MC] Zircon or Crystallization age of any phosphates or zircons In Situ measurement of U-Th-Pb radiochronometers meteorites and (b) CSFD analysis of mare surfaces (especially those Lars Borg LLNL Co-I Lead x ICP-MS, SIMS) Phosphate Jeremy Boyce JSC Co-I x x Mineralogy and Petrology Bulk compositions, sub-micrometer characterization of correlated with returned samples) from remote sensing data Basalt origin and KREEP component from in-situ analysis of Bill Cassata LLNL Co-I x structures, elemental abundances, and isotopic XRF, EPMA, SEM, TEM, ICP-MS, RIMS, SIMS Rocklets mineral composition and relationships • Existing CSFD models are calibrated by correlating measured ages of Roy Christoffersen JSC Co-I x x compositions Mark Cintala JSC Co-I x Crystallization and shock history of basalt from analysis of Micrometer-scale mineralogy, petrology, mineral Optical microscope, SEM, EPMA, TEM, SIMS, Rocklets Apollo samples with the crater densities. Models are well-determined Barbara Cohen GSFC Co-I x x Lead x petrographic texture and mineral composition chemistry, and crystallinity ICP-MS, XRF, XRD for ages >~3 Gyr, and reasonably constrained for very recent ages. Steve Elardo University of Florida Co-I x Regolith Evolution, Space Weathering and Exposure History Amy Gaffney LLNL Co-I x x Mass fraction of total Fe in single magnetic domain Is by electron spin/electron paramagnetic Determine integrated sample/sub-sample history of But there is a ~2 Gyr gap in age coverage in these models (Fig. 1) state (nanophase Feo); total Fe mass fraction as FeO; resonance, FeO preferably by non- Ben Greenhagen APL Co-I x x processing by space weathering relative to pristine parent Regolith Intra- and intergrain distribution of nanophase Fe0 in destructive XRF/EPMA or other methods; John Gruener JSC Co-I x mare basalt grain rims and agglutinitic glass. SEM, TEM • Thus CSFD age estimates of ~1-2 Gyr are uncertain to ~1.5 Gyr Harry Hiesinger Universität Münster Co-I Lead x x Determine the composition of lithic and glass fragments Brad Jolliff Washington University Co-I x x x x Micrometer-scale mineralogy, petrology, mineral Optical microscope, SEM, EPMA, TEM, XRF, within the regolith to identify materials transported to the Regolith Katie Joy Manchester Co-I x x chemistry, and crystallinity XRD • Problem exacerbated by landing site via vertical and lateral impact mixing Tom Lapen University of Houston Co-I x x application of lunar CSFD Implanted solar noble gas concentration, grain solar Julie Mitchell JSC Co-I x Sample surface exposure history (topmost surface or near flare track density, width and development of space Mass spectroscopy (type TBD), SEM, TEM Regolith Clive Neal Notre Dame Co-I Lead x surface exposure) model to other bodies weathered grain rims Chip Shearer UNM Co-I x • Need for sample return of Justin Simon JSC Co-I x x Spectroscopy Angela Stickle APL Co-I x x Grain-scale spectroscopy for spectral characterization of Grain-scale spectroscopy for spectral characterization of Regolith or young mare basalts has long FTIR microscope Julie Stopar LPI Co-I x Lead x specific phases specific phases Rocklets been recognized Aileen Yingst PSI Co-I x Lead Bulk spectroscopy of basaltic regolith to ground truth Bulk spectroscopy of basaltic regolith to ground truth UV, NIR, TIR spectrometers Regolith Ryan Zeigler JSC Co-I remote measurements remote measurements • ISOCHRON samples will yield x crucial on lunar • ISOCHRON Science Team brings deep experience in lunar investigations, • Age determinations by leading geochronology laboratories using multiple isotopic and petrologic and geochemical mission operations, sample analysis, and remote sensing noble-gas systems: Concordance will close the ~2 Gyr gap in CSFD models evolution • Diversity and longevity, government + academia + international partnerships • Petrologic, geochemical, mineralogical studies using full range of analytical laboratories for lunar magmatic & thermal evolution, impact & shock processes No samples • ISOCHRON will return samples of • Strong and experienced engineering and hardware partners (proprietary) Studies of regolith dynamical processes and evolution, space weathering, exposure least-mature lunar regolith to • 75% of returned sample mass dedicated for curation & allocation to • constrain gardening dynamics community history through high-resolution analytical and spectroscopic techniques Figure 1. Summary of potential solutions & space weathering processes to lunar CSFDs illustrating gap in data over time coverage. From [6]. References [1] National Research Council (2011) Visions and Voyages: Planetary Science Decadal Survey, DOI: https://doi.org/10.17226/13117. [2] NRC Space Studies Board (2007), The Scientific Context for : Final Report. [3] LEAG (2017) Advancing Science of the Moon SAT report. [6] Hörz F. et al. (1991) Lunar Source Book, 61. [7] Hiesinger H. et al. (2000) J. Geophys. Res. 105, 29239. [8] Stadermann A. C. et al. (2018) Icarus 309, 45.