The nominal traverse (Figure 1) is wholly contained within the Procellarum KREEP Terrane (PKT) and starts south of and zigzags across the Reiner Gamma formation, heads north through the Marius Hills volcanic complex, across a stretch of Oceanus Procellarum basalts, then over the Aristarchus plateau, around Aristarchus crater, and finishes at an Irregular Mare Patch 45 km from Aristarchus crater. Throughout the 1800 km traverse, the Intrepid rover would investigate these six regions and provide much needed ground truth and key measurements required to retire the Intrepid objectives; this traverse represents the only place on the Moon with the required diversity of units and ages.
Figure 1. A labeled view of the Intrepid traverse path, key regions, planned stops, and nominal timeline.
Reiner Gamma
Reiner Gamma (RG) is an ancient volcanic plain (~3.5 to 3.3 Gyr) that contains one of the strongest so called magnetic anomalies on the Moon and the associated albedo feature known as a swirl. At RG, the main science objectives are to determine the nature of the remanent lunar magnetic field and elucidate its role in moderating space weathering of the regolith and its suitability for radiation protection of surface assets. Space weathering (or maturation) is a process where the physical state, color and albedo of the surface evolves with time (~500 myr time scales) due to exposure to the space environment. These two objectives are achieved with three main observational objectives: 1) characterize the magnetic field across the RG traverse to 2 nT resolution at a 5-meter sampling rate with 50-m spatial location accuracy, 2) measure variations in the strength and composition of solar wind constituents as a function of magnetic field strength and orientation across the RG traverse, and 3) characterize the state of maturation of the regolith across the swirl. The magnetometer and electrostatic analyzer would provide the necessary measurements to retire the first two objectives. The optical maturity of the surface would be measured with two key absorptions in the UV to visible range and the visible to near-infrared. These key measurements would be supplemented with the geochemical measurements provided by the Alpha Particle X-Ray Spectrometer (APXS) and Gamma Ray Neutron Spectometer (GRNS) and mineralogic estimates from spectral reflectance absorption features (300 to 1400 nm) measured with the Point Spectrometer (PS). The RG area consists of basaltic flood lavas with ages estimated as 3.3 Gyr, the oldest volcanic materials to be sampled during the Intrepid mission.
Marius Hills
The Marius Hills (MH) complex is a unique lunar volcanic complex, with the highest concentration of blocky lava flows, domes, and cones on the Moon. While this unique and large grouping of volcanic landforms remains somewhat enigmatic, previous workers generally agree that there were four main volcanic episodes. The first episode built the large scale (250 km) low relief dome that forms the foundation of the complex, then the small domes (1-10 km) were formed, followed by the cones. Finally, these constructs were embayed by mare-like basaltic eruptions. Intrepid would also investigate tectonic landforms (ridges and faults) in this region to elucidate their relation to specific volcanic landforms. Previous studies classified domes and cones based on small-scale roughness and flank slopes, but the cause of these landform distinctions is still debated (compositional, eruption conditions, percent crystallinity etc.). Intrepid would investigate type volcanic landforms within the complex to test hypotheses of eruption conditions and styles vs. magma physical properties as the magma source regions evolved. The ages of these major events are not well-constrained and the area has been subdivided into several units based on visible to near-infrared color properties (Lawrence et al., 2013) and age estimates for these units range from < 1 Gyr to 3.3 Gyr. Because of the small size of the units the uncertainty on these ages is high; however, the largest effusive unit (Flamsteed), which seems to embay the domes and cones, is dated at 2.5 Gyr. Documenting the chemistry of the various units (domes, cones, mare flows) within the MH would help ascertain their relative ages.
Oceanus Procellarum
The Oceanus Procellarum (OP) traverse stretches from MH to the Aristarchus Plateau. Intrepid would traverse mare basalts that erupted over a broad span of lunar history (~2.5 Gyr in MH to 1.9 Gyr (P51) and 1.2 Gyr (P 60) within Oceanus Procellarum. Orbital remote sensing indicates that these flows are of different ages yet have major element chemistries that are indistinguishable with existing observations and thus are only separable by crater density (relative age). The detailed chemistry returned by Intrepid (impossible to obtain from orbital platforms) would show whether the units are compositionally distinguishable, lending greater insight to the evolution of magma source regions over time. In particular, remote sensing is not clear on the possible relationship between extended volcanism and the content of radiogenic heat-producing elements (Th, K), but Intrepid measurements would unambiguously test for a relationship.
Orbital observations of the OP basalt region are confounded by rays from Aristarchus crater that mixed material ejected from the crater into the regolith. Intrepid observations would: 1) show how magma source regions evolved over time (locally and relative to other areas visited by Intrepid), 2) examine the significance of mixing in ray materials, and 3) investigate mare tectonism. Perhaps the most significant contribution of this OP traverse is determining if the basalts are intrinsically rich in KREEP elements, particularly the radiogenic heat-producing elements. Remote sensing observations suggest these OP basalts (both P51 and P60) contain 3 to 6 ppm Th; however, the Th signal may be contamination from Aristarchus crater ejecta, and not intrinsic to the basalts. By measuring the composition of materials in ray shadow areas and excavated from depth at a series of young impact craters (25-200 m diameters) we can test if the Th is native to the basalts or not. Finally, the OP traverse includes a traverse up one of the few lunar shield volcanoes (10 km diameter). The relative paucity of this type of volcanic landform is a mystery, and understanding its origin requires a comparison of its chemistry and physical properties relative to the nearby flood basalt deposits.
Aristarchus Plateau
The traverse then extends across Aristarchus Plateau (AP), which is a crustal block thought to have been uplifted during the formation of the Imbrium basin. The plateau is covered by one of the largest pyroclastic deposits on the Moon and the widest and deepest sinuous rille. The pyroclastic deposit may contain volatile elements in quantities up to several hundred ppm, possibly representing an ore grade deposit. The APXS and GRNS spectra, HLI images, and color stereo observations would: 1) elucidate the range in chemistry of the pyroclastic deposits, 2) measure the range of composition of the basalt beneath the deposit (revealed in rille walls and crater ejecta), and 3) disentangle the mixing of Aristarchus crater ejecta with local material in the ray-formation process (ejecta shadowed areas vs. rays). Characterizing this massive pyroclastic deposit (vertically and laterally) would provide insight to deeper mantle source regions relative to the mare basalt that were sourced from the upper mantle.
Aristarchus Crater
Next, Intrepid would traverse the southeastern rim of Aristarchus Crater (AC), a ~40 km Copernican crater (<300 Myrs age) that excavated material from the crustal block of AP. Of great interest is the unusual layering revealed in the central peak, which originates from more than 5 km depth. The materials seen in the heterogeneous central peak also drape the crater rim and are thus accessible to the full suite of Intrepid instruments, enabling an unprecedented look at crustal variations. Also, while crossing ponded deposits of AC impact melt, Intrepid would investigate the rate of regolith development and document the recent cratering history on the ejecta blanket. By sampling the granular (or blocky) ejecta and impact melt rocks, Intrepid would test the hypothesis that impact melt is a homogenized sample of the target material. AC is one of the best locations on the Moon to test this hypothesis because there are large-scale compositional variations and Intrepid would cross several of these units and sample numerous impact melt deposits. If the hypothesis is correct, all the impact melt deposits would have the same composition and fall on a mixing line of all the granular materials. The most important objective on the AC traverse is determining the nature of the PKT and the associated non-mare rocks (identified from orbit). From the OP traverse Intrepid would have determined if basalts are KREEP-rich or simply contaminated with ejected Aristarchus material. But what is this Aristarchus material? From low resolution (approximately the diameter of the crater) orbital gamma ray observations, it is known that the crater and ejected material contain some of the highest levels of Th on the Moon, potentially in excess of 15 ppm. What is the range of rock types associated with this crater, and which rocks contain Th and other KREEP-rich components? Answering these questions would allow the first constrained test on the origin of KREEP and possibly other petrologically evolved rock types (granite / rhyolite or monzogabbro / monzodiorite). These questions are among the most significant outstanding questions of lunar petrology.
Irregular Mare Patch
The traverse culminates at the Aristarchus Irregular Mare Patch (IMP) that is found within the AC ejecta blanket. More than 70 IMP deposits were identified and proposed to be the youngest volcanic landforms (<100 Myrs) on the Moon. If the young ages of IMPs are confirmed, they would provide clear evidence that internal heat sources persisted significantly past 1 billion years ago. An alternative hypothesis is that the IMPs are composed of a magmatic foam that erupted ~3.5 Gyr and these foams had extreme porosity (90%), which renders IMPs resistant to typical degradation processes. In situ exploration with Intrepid can test the magmatic foam hypothesis with high-resolution images of key landforms, particularly the form of impact craters that are predicted to have different shapes when formed in volcanic foams. As the rover passes over the contact between the AC ejecta and the IMP, compositional and morphologic indicators would reveal the stratigraphic relation between the two units, and thus which is younger. Additionally, images of the landforms at the centimeter scale provide the means to determine the nature of regolith formed in magmatic foams (if the IMP is older) or the eruptive processes of young volcanics and early regolith development (if the IMP is younger).
