The Age Of Ina And The Thermal History Of The Moon
Introduction: Apollo 15 photography revealed the presence of an enigmatic landform, known as Ina [1-3]. Due to its small size (<3 km) and inadequate observations, a conclusion about its formation mechanism has remained elusive, but hypotheses included effusive and explosive eruptions [1-3]. LROC provides 0.25-1 m scale imaging under a variety of lighting conditions, which along with stereo-based topography has allowed renewed investigation of Ina [4,5]. Ina’s morphology consists of two units: smooth mounds (SM) and uneven terrain (U) (Figure 1). Crater size–frequency distributions (CSFD, diameters ≥10 m) from Ina indicate an age of <100 My , a result inconsistent with interpretations of lunar thermal evolution models . However, the mound materials may be significantly more porous (and nearly strengthless) compared to typical mare regolith, resulting in smaller than expected craters and thus an anomalously young CSFD model age [7,8]. Settling the controversy over Ina’s age is critical for understanding the thermal evolution of the Moon (and indirectly its bulk composition). Detailed knowledge of Ina’s geochemistry would provide insight for the late-stage compositional evolution of the mantle (if the <100 My age date is correct). Science Goals: The primary science goal at Ina is to determine the relative and absolute ages of the SM and U materials, as well as the surrounding mare plains and thus disambiguate the age of what may be some of the most recent volcanism on the Moon . Secondary goals include determining: 1) the major and minor element chemistry of Ina units, 2) their physical properties and 3) their emplacement mechanisms. If Ina is indeed young, its age and properties will allow revised models of lunar thermal evolution. If Ina is not young , investigation of the physical properties of the different materials would shed light on the effects of target properties on crater diameters, with significant implications for the application of CSFD techniques to craters formed in the strength-scaling regime [8, 9]. Measurement Requirements: Meeting the science goals requires the following observations (in order of increasing complexity): 1) High resolution stereo images (<10 cm/pixel over >50% of Ina; <2 cm/pixel over >10%), to constrain the morphology of craters, tectonic features, and volcanic landforms. 2) Compositional characterization (sample area smaller than individual SM; APXS, LIBS, etc.), to confirm composition of both units. 3) Direct testing of regolith mechanical and physical properties. 4) Absolute age dating and trace element analyses. Mission Options: Meeting the primary science goal requires sample return (~1 kg) or in-situ age dating (±250 My). Testing the basaltic composition, formation mechanisms, and relative ages of SM and U can be achieved with either a lander or rover. Lander. Downward-facing cameras could acquire the stereo imagery during descent for the morphologic investigation. Landing on the U would allow direct investigation of its physical and chemical properties, while the (topographically higher) SM could be observed at a distance. Landing in the morning and observing with a well-calibrated camera for a full lunar day would allow photometric analysis to determine surface properties of both units . Rover. A rover would allow detailed observations at several locations of the contacts between landforms, as well as tectonic features, which would improve the investigation of emplacement mechanisms. Landing on the eastern side of Ina (18.661°, 5.331°E) would allow the rover to traverse between the SM and U, testing the physical and chemical characteristics of both. Sample Return. The most accurate and precise absolute ages and geochemical characterization can be obtained with a sample return, which in turn would benefit from a rover to investigate the physical properties of key landforms and collect documented samples from key locations, although a rover is not required to meet the key science goals.