Lunar Impact Melt Research Articles

Spectral properties of lunar impact melt deposits from Moon Mineralogy Mapper (M3) data

Lunar impact melt deposits have unusual surface properties, unlike any measured terrestrial lava flow. Radar observations suggest that they are incredibly rough at decimeter scales, but they appear smooth in high-resolution, meter-scale optical images. The cause of their unusual surface roughness is unknown. In this work, we investigate the properties of impact melt deposits from seven lunar craters, ranging in size from 7.5 to 96 km in diameter, in an effort to understand the cause of their unique surface texture. We use data from the Lunar Reconnaissance Orbiter's (LRO) Mini-RF instrument to characterize the small-scale roughness of the deposits, data from the LRO Camera (LROC) to characterize their meter-scale morphology, and data from Chandrayaan-1's Moon Mineralogy Mapper (M3) to characterize their composition. This represents the most comprehensive study of the composition of lunar melt deposits completed to date. In particular, we applied a customized spectral unmixing model to the M3 data using laboratory spectra acquired from a range of possible lunar endmembers: pyroxene, olivine, fast-quenched lunar glass simulants, and impact melts and breccias (both synthetic and natural). We found that spectra derived from lunar melt deposits are typically modeled as a mix of the pyroxene and/or impact melts and breccias endmembers. Our modeled results suggest that lunar melt deposits are either crystalline deposits of pyroxene-rich rocks, or a mixture of glassy material and pyroxene minerals. The latter interpretation could explain the roughness observed in the Mini-RF data, if the melt deposits have a glassy surficial layer that shatters during impact gardening to produce decimeter scale blocks.

Physical and compositional properties of impact melts for Jackson and Tycho craters: Implications for space weathering and degradation of lunar impact melts

Impact melts are ubiquitous across the Moon, occurring in settings ranging from massive basin deposits to flows and ponds in and around small craters. Recent high spatial resolution imaging and spectroscopy datasets for the Moon have enhabled the identification and study of impact melt units at increasingly small spatial scales, including on the central peaks of complex craters. Lunar impact melts have unique physical properties at many scales (e.g. smooth appearances in visual imagery, low rock concentration implying smoothness and regolith cover at meter scale, high S-band radar returns implying roughness at decimeter scale, and possible anomalously low OMAT values), which may be due to their unique mode of emplacement or post-emplacement modification processes. We isolate impact melt regions in the crater interior of Jackson and Tycho as well as in the continuous ejecta blanket of Tycho using the geologic maps of Dhingra et al. (2017) and analyze their spectral, compositional, and physical properties utilizing datasets from the SELENE Multiband Imager and Terrain Camera, the LROC Narrow Angle Camera, and Diviner Lunar Radiometer. We leverage the unique high-slope setting of impact melts on the central peaks of these two craters to assess the influence of slope on post-emplacement modification processes, compared with unique physical properties of impact melts, on rock concentration and optical maturation. We find that slope is the primary control on optical maturity, while rock concentration plays a secondary role. We also find that melt units are generally more optically mature than their non-melt counterparts, and that this difference attenuates with distance from the crater center until no difference is noted in the continuous ejecta blanket. This suggests that melt units have different surface properties than the non-melt units in similar settings potentially due to the increased degree of shock they experienced during crater formation.

Impact melt breccia and surrounding regolith measured by Chang'e-4 rover

Chang'e-4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its traverse on the floor of Von Kármán crater within the South Pole Aitken (SPA) basin on the lunar farside. The discovered breccia, being 52 × 16 cm, resembles the lunar impact melt breccia samples 15466 and 70019 that returned by the Apollo missions. It was formed by impact-generated welding, cementing and agglutinating of lunar regolith and breccia. Clods surrounds the breccia-hosting crater were crushed into regolith powders by the rover's wheels, indicating the regolith may be compacted slightly and becomes blocky and friable. Relative mineral fractions are estimated from the in situ measured spectra by a Hapke model-based unmixing algorithm. Unmixing reveals that plagioclase (PLG, 45 ± 6%) is dominant in the regolith, followed by almost equal fractions of pyroxene (PYX, 7 ± 1%) and olivine (OL, 6 ± 2%), indicating the regolith is likely related to noritic rocks. The regolith measured by Chang'e-4 rover was actually a highly mixture of multiple sources, with ejecta from Finsen crater being primary and possible contributions from Alder crater. Finsen and Alder craters are on the margin of the proposed impact melt pool produced by the SPA basin-forming event. Therefore, the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks.

Examining the compositions of impactors striking the Moon using Apollo impact melt coats and anorthositic regolith breccia meteorites

Impactors striking the Moon since the formation of its crust have left an indelible imprint on the lunar surface, in the guise of craters and associated impact rocks. The lunar crust has low intrinsic abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au), at greater than 3000 times lower than in chondrite meteorites. Consequently, during impact, bolides with chondritic or differentiated iron-rich compositions should impart elevated HSE signatures to the lunar crust. Here we examine glassy lunar impact melt coats (IMC) from the outside rims of Apollo 16 cataclastic anorthosites (60015, 65325) and breccias (65035), as well as both fragments and powders of Antarctic anorthositic regolith breccia (ARB) meteorites (Miller Range 090034/36/70/75 and MacAlpine Hills 88105) for their petrography, mineral chemistry and bulk-rock compositions. The HSE concentrations for IMC range from ∼0.001 to 0.1 × CI chondrite, with measured 187Os/188Os between 0.1189 and 0.1366. Anorthositic regolith breccia meteorites, which have components with 2.6–4.1 Ga ages, have similar HSE concentrations to IMC, but typically have lower 187Os/188Os (0.1164–0.1284). These latter Os ratios are generally less radiogenic than those measured in ∼3.8–3.9 Ga Apollo impact melt breccias. The Apollo 16 IMC are not well-dated, but their KREEPy trace-element signatures and associated ages of 3.7 to 3.8 Ga for Apollo 16 glasses might imply, at least in part, an origin from the Imbrium or Serenitatis basin-forming impacts. Within the IMC, metal-schreibersite-troilite assemblages record significant inter-element HSE fractionation which is also reflected in bulk HSE patterns for both IMC and ARB meteorites. Variations in relative and absolute HSE compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies. Collectively, IMC and ARB meteorites exhibit approximately 50% of the variation in Ru/Ir and 187Os/188Os observed in lunar impact melt breccias. These results imply that significant variations in inter-element compositions can occur during impact brecciation and melting and so some impact melt rock HSE compositions may not record the compositions of impactors that struck the Moon with fidelity. Nonetheless, the generally low 187Re/188Os of lunar impact melt rocks means that osmium isotope ratios provide evidence for impact composition, and a change from ordinary to carbonaceous-like impactors either with time – or location – striking the Moon.

Lunar megaregolith mixing by impacts: Spatial diffusion of basin melt and its implications for sample interpretation

The formation ages of lunar impact basins are critical to understanding the late accretion history of the inner solar system. Furthermore, the correct interpretation of the provenance and isotopic dates of basin-derived impact melt (‘basin melt’) is essential for the calibration of lunar chronology function. However, abundances of basin melt in the lunar near-surface are not well understood. Basin melt has been gardened by a long sequence of subsequent impact events, altering its abundance and size distribution. We developed a numerical model to investigate this process by means of the Monte Carlo method in a spatially resolved model. The fraction of melt in ejecta was tracked globally and at the Apollo 14–17 and Luna 20 sampling sites and was compared with K-Ar age distributions of lunar impact melt breccias. It was found that melt produced by the very large SPA basin as well as the relatively late-forming Imbrium basin should be dominant in the near-surface (top one meter). The simulation shows that the melt component at the Apollo 14–17 and Luna 20 sites is strongly affected by nearby mid- to late-forming basins. Imbrium melt should be abundant in Apollo 14–17 samples; Crisium melt is the most significant component of basin-sourced melt in Luna 20 samples; all the Apollo 14–17 and Luna 20 samples could include melt from Serenitatis and the SPA basin; Nectaris melt should occur in Apollo 16, Apollo 17 and Luna 20 samples; and Orientale melt has no significant mixing in the Apollo 14–17 and Luna 20 sampling sites. In general, besides a prominent age peak at 3.9 Ga (related to the Imbrium basin), the model predicts pronounced abundance peaks of older basin melt (>3.9 Ga) which tend to be absent from distributions of K-Ar ages of impactites from landing sites. The diffusion characteristics of basin melt suggest that for future sampling aimed at collecting early basin melt, the re-excavation zones of late impact craters larger than tens of kilometer in diameter inside basins may provide the highest abundances of melt from early basins.

Exploring the variability of argon loss in Apollo 17 impact melt rock 77135 using high‐spatial resolution40Ar/39Ar geochronology

Abstract40Ar/39Ar incremental heating experiments on whole‐rock lunar samples commonly provide evidence of varying degrees of radiogenic40Ar (40Ar*) loss. However, these experiments provide limited information about whether or not40Ar* is preferentially lost from specific glasses, minerals, or polyphase domains. Ultraviolet laser ablation microprobe (UVLAMP)40Ar/39Ar dating and electron probe microanalysis of mineral clasts and polyphase melt assemblages in Apollo 17 poikilitic impact melt rock 77135 show evidence of geochemical controls on40Ar/39Ar dates. Potassium‐rich glass and K‐feldspar in the mesostasis are the dominant sources for Ar released during low‐temperature steps of published40Ar/39Ar release spectra for this rock, while pyroxene oikocrysts with enclosed plagioclase chadacrysts contribute Ar predominantly to intermediate‐ to high‐temperature steps. Additionally,UVLAMPanalysis of a mm‐scale plagioclase clast demonstrates the potential to use stranded40Ar* diffusive loss profiles to constrain the thermal evolution of lunar impact melt deposits and indicates that the melt component of 77135 cooled quickly. While some submillimeter clasts of plagioclase are distinctly older than the melt, other small clasts yield dates younger than the oldest melt components in 77135, plausibly due to subgrain fast diffusion pathways and/or40Ar* loss during brief episodes of reheating at high temperatures. Our data suggest that integrated petrologic and microanalytical geochronologic studies are necessary complements to bulk sample geochronologic studies in order to fully evaluate competing models for the impactor flux during the first billion years of the Moon's evolution.

Iron Abundances in Lunar Impact Basin Melt Sheets From Orbital Magnetic Field Data

AbstractMagnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and cannot account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon's surface within the impact melt sheet. Here we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt %, with an uncertainty of a factor of about 3. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo.

Lobate impact melt flows within the extended ejecta blanket of Pierazzo crater

Impact melt flows are observed within the continuous and discontinuous ejecta blanket of the 9 km lunar crater Pierazzo, from the crater rim to more than 40 km away from the center of the crater. Our mapping, fractal analysis, and thermal modeling suggest that melt can be emplaced ballistically and, upon landing, can become separated from solid ejecta to form the observed flow features. Our analysis is based on the identification of established melt morphology for these in-ejecta flows and supported by fractal analysis and thermal modeling. We computed the fractal dimension for the flow boundaries and found values of D = 1.05–1.17. These are consistent with terrestrial basaltic lava flows (D = 1.06–1.2) and established lunar impact melt flows (D = 1.06–1.18), but inconsistent with lunar dry granular flows (D = 1.31–1.34). Melt flows within discontinuous ejecta deposits are noted within just 1.5% of the mapping area, suggesting that the surface expression of impact melt in the extended ejecta around craters of this size is rare, most likely due to the efficient mixing of melts with solid ejecta and local target rocks. However, if the ejected fragments (both, molten and solid) are large enough, segregation of melt and its consequent flow is possible. As most of the flows mapped in this work occur on crater-facing slopes, the development of defined melt flows within ejecta deposits might be facilitated by high crater-facing topography restricting the flow of ejecta soon after it makes ground contact, limiting the quenching of molten ejecta through turbulent mixing with solid debris. Our study confirms the idea that impact melt can travel far beyond the continuous ejecta blanket, adding to the lunar regolith over an extensive area.

Origin of discrepancies between crater size-frequency distributions of coeval lunar geologic units via target property contrasts

Recent work on dating Copernican-aged craters, using Lunar Reconnaissance Orbiter (LRO) Camera data, re-encountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements that was observed, but not understood, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus craters, CSFDs of impact melt deposits give significantly younger relative and absolute model ages (AMAs) than impact ejecta blankets, although these two units formed during one impact event, and would ideally yield coeval ages at the resolution of the CSFD technique. We investigated the effects of contrasting target properties on CSFDs and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units. We counted craters with diameters through the transition from strength- to gravity-scaling on two large impact melt deposits at Tycho and King craters, and we used pi-group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets. The new CSFD for the large King Crater melt pond bridges the gap between the discrepant CSFDs within a single geologic unit. Thus, the observed trends in the impact melt CSFDs support the occurrence of target property effects, rather than self-secondary and/or field secondary contamination. The CSFDs generated from the pi-group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets, such that the relative ages of the former are lower relative to the latter. Consequently, coeval impact melt and ejecta units will have discrepant apparent ages. Target property differences also affect the resulting slope of the CSFD, with stronger targets exhibiting shallower slopes, so that the final crater diameters may differ more greatly at smaller diameters. Besides their application to age dating, the CSFDs may provide additional information about the characteristics of the target. For example, the transition diameter from strength- to gravity-scaling could provide a tool for investigating the relative strengths of different geologic units. The magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and/or exposure/degradation ages of the two units. Robotic or human sampling of coeval units on the Moon could provide a direct test of the importance and magnitude of target property effects on CSFDs.

Did the Orientale impact melt sheet undergo large‐scale igneous differentiation by crystal settling?

AbstractSignificant quantities of shock‐induced melt are predicted to form during basin‐scale impact events and may contribute substantially to compositional variation in the lunar crust through differentiation processes. Knowledge of the evolution of impact melt sheets and the cooling and crystallization processes involved is therefore important to understanding the geology and petrology of the Moon. Here we perform a case study on the Orientale basin and investigate the possibility of differentiation of the impact melt sheet by assessing the thermal and physical processes that drive cooling and crystallization, focusing on crystal settling. Results predict that convective processes within the impact melt sheet enhance cooling and solidification and inhibit settling of crystals ~1.5 mm in diameter and smaller, preventing differentiation at or below this size. Impact melt sheet thickness is predicted to have minimal influence on crystallization history, suggesting that crystal size is the primary factor controlling lunar impact melt sheet differentiation.

Terrestrial analogues for lunar impact melt flows

Lunar impact melt deposits have unique physical properties. They have among the highest observed radar returns at S-Band (12.6 cm wavelength), implying that they are rough at the decimeter scale. However, they are also observed in high-resolution optical imagery to be quite smooth at the meter scale. These characteristics distinguish them from well-studied terrestrial analogues, such as Hawaiian pāhoehoe and ʻaʻā lava flows. The morphology of impact melt deposits can be related to their emplacement conditions, so understanding the origin of these unique surface properties will help to inform us as to the circumstances under which they were formed. In this work, we seek to find a terrestrial analogue for well-preserved lunar impact melt flows by examining fresh lava flows on Earth. We compare the radar return and high-resolution topographic variations of impact melt flows to terrestrial lava flows with a range of surface textures. The lava flows examined in this work range from smooth Hawaiian pāhoehoe to transitional basaltic flows at Craters of the Moon (COTM) National Monument and Preserve in Idaho to rubbly and spiny pāhoehoe-like flows at the recent eruption at Holuhraun in Iceland. The physical properties of lunar impact melt flows appear to differ from those of all the terrestrial lava flows studied in this work. This may be due to (a) differences in post-emplacement modification processes or (b) fundamental differences in the surface texture of the melt flows due to the melts’ unique emplacement and/or cooling environment. Information about the surface properties of lunar impact melt deposits will be critical for future landed missions that wish to sample these materials.

Crystal accumulation in a 4.2 Ga lunar impact melt

The ages of lunar impact basins and the role of fractional crystallization in producing compositional heterogeneity of lunar melt sheets are long-standing problems with significant implications for solar system dynamics and the petrologic evolution of the lunar crust. Here we document the formation of a basin-scale impact on the Moon at 4.20±0.07Ga based on the 147Sm–143Nd isochron age of a magnesian, noritic anorthosite melt rock from lunar breccia 67955. Major element compositions of plagioclase and mafic silicates in the melt rock imply a substantial component of primary Mg-suite cumulates or related lithologies in the pre-impact crustal stratigraphy. Trace element compositions of the plagioclase, including diagnostic ratios such as Sr/Ba, are also mostly similar to those in plagioclase from Mg-suite cumulates, with a small number of grains trending toward compositions observed in ferroan anorthosites. Mineral-melt distribution coefficients applied to trace element compositions of the 67955 plagioclase and pyroxene predict parental melt compositions that contrast strongly with the bulk rock. Compared to the whole rock, parental melts calculated from the plagioclase are enriched in REE (ΣREELa–Yb=131–885, average 619ppm vs. 39.8ppm) and they have more fractionated REE patterns (La/Ybn=1.2–9.8, average 4.9 vs. 1.5) with deep negative Eu anomalies (Eu/Eu∗=0.09–0.40 vs. 1.36). Trace element data for the pyroxenes also imply incompatible-element enriched parental melts. Subsolidus equilibration between the plagioclase and the pyroxene apparently rotated the REE patterns, but the conclusion that the parental melt was highly enriched in REE relative to the whole rock appears robust. Quantitative modeling shows that fractional crystallization of the 67955 whole rock composition cannot reproduce the range of Ba, Sr, Ti, and La concentrations measured in the 67955 plagioclase. Rather, the models require an initial melt composition that was strongly enriched in these elements, and they suggest that fractional crystallization became less efficient as crystallization proceeded. The contrast between the inferred parental melt composition and the bulk rock implies formation of the 67955 noritic anorthosite as a crystal cumulate, and that the cogenetic residual melt was strongly enriched in incompatible elements. If so, this would be the first documented example of fractional crystallization in a lunar impact melt sheet. The petrological and geochemical characteristics of the 67955 noritic anorthosite suggest that it formed by an impact in the Procellarum-KREEP Terrane, and was transported to the Apollo 16 site as Imbrium ejecta. Inheritance of ejecta related to this pre-Imbrium basin may contribute to the common occurrence of ∼4.2Ga 40Ar–39Ar plateau ages in breccia clasts and regolith fragments from the rim of North Ray crater. In that case, those data may provide no constraints on the age of the Nectaris basin, despite its proximity to the Apollo 16 site.

Diverse impactors in Apollo 15 and 16 impact melt rocks: Evidence from osmium isotopes and highly siderophile elements

Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks.The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages <3.84Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages >4.0Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ∼3.9Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ∼3.9Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend.Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core.Pre-4.0Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis, as proposed by Fischer-Gödde and Becker (2012). For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

Distinguishing between basalts produced by endogenic volcanism and impact processes: A non-destructive method using quantitative petrography of lunar basaltic samples

Impact processes play an important role in shaping and reshaping the surfaces of airless planetary bodies. Such processes produce regoliths and generate melts that crystallize and record the homogenization of the geology at the impact site. If the volume of melt is substantial, the resultant crystallized product has an igneous texture and may be free of xenolithic clasts making it difficult to distinguish from melts produced by endogenic magmatic processes. This has been clearly demonstrated during the return of the Apollo samples from the Moon, where Apollo 14 basalt 14310 was initially described as a mare basalt and was only subsequently reclassified as an impact melt following detailed and time consuming crystallization experiments. Another way of distinguishing lunar impact melts from endogenically-derived mare basalts is through the quantification of the highly siderophile elements (HSE: Pd, Rh, Ru, Ir, Pt, Os), which have relatively low abundances in pristine lunar samples but are high in meteorites and, therefore, may be enriched in impact melts. However, these analyses consume relatively large quantities of valuable sample and because of mass constraints cannot be performed on many lunar samples. In this paper we present a quantitative petrographic method that has the potential to distinguish lunar impact melts from endogenically-derived mare basalts using plagioclase and olivine crystal size distributions (CSDs). The slopes and intercepts of these CSDs are used to show that olivine from impact melts displays a steeper CSD relative to olivine from mare basalts. For plagioclase, generally impacts melts display CSDs with shallower gradients than those from endogenous mare basalts and, as for olivines, plot in a distinct field on a CSD slope vs. CSD intercept plot. Using just a thin section to distinguish impact melts from mare basalts enables the goals of future robotic sample return missions to determine the age of the South Pole-Aitken basin in the Moon, because such missions will potentially only return small (2–4mm) “rocklets” for analysis, obviating HSE analyses for impact melt identification.

H2O and O(3PJ) photodesorption from amorphous solid water deposited on a lunar mare basalt

H2O (ν=0) and O(3PJ=2,1,0) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(3PJ) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin–orbit state, H2O exposure, and 157-nm irradiation time. Maxwell–Boltzmann distributions with translational temperatures of 10,000K, 1800K, 400K, and 89K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(3PJ) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10Langmuir (1L=10−6Torrs). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (ν*) and the O(3PJ) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5L H2O, after reaching maximum signal, the cross sections for H2O (ν*) and O(3P2) depletion were measured to be 1.2×10−19cm2 and 6.7×10−20cm2, respectively.

Global distribution of lunar impact melt flows

In this study, we analyzed the distribution and properties of 146 craters with impact melt deposits exterior to their rims. Many of these craters were only recently discovered due to their unusual radar properties in the near-global Mini-RF data set. We find that most craters with exterior deposits of impact melt are small, ⩽20km, and that the smallest craters have the longest melt flows relative to their size. In addition, exterior deposits of impact melt are more common in the highlands than the mare. This may be the result of differing target properties in the highlands and mare, the difference in titanium content, or the greater variation of topography in the highlands. We find that 80% of complex craters and 60% of simple craters have melt directions that are coincident or nearly coincident with the lowest point in their rim, implying that pre-existing topography plays a dominant role in melt emplacement. This is likely due to movement during crater modification (complex craters) or breached crater rims (simple craters). We also find that impact melt flows have very high circular polarization ratios compared to other features on the Moon. This suggests that their surfaces are some of the roughest material on the Moon at the centimeter to decimeter scale, even though they appear smooth at the meter scale.

Photodissociation of water and O(3PJ) formation on a lunar impact melt breccia

AbstractPhotodissociation of water deposited on an impact melt breccia collected during Apollo 16 was studied by measuring O(3PJ = 2,1,0) photoproducts detected with resonance‐enhanced multiphoton ionization. For each spin‐orbit state, the oxygen atom time‐of‐flight (TOF) spectrum was measured as a function of H2O exposure and 157 nm irradiation time. Four Maxwell‐Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 102 K were required to fit the data. The most likely formation mechanisms are molecular hydrogen elimination following ion‐electron recombination, secondary recombination of hydroxyl radicals, and photodissociation of adsorbed hydroxyls. The irradiation time required to reach maximum oxygen signal suggests that water clusters into islands when adsorbing on the lunar impact melt breccia. After enough irradiation for the oxygen atom yield to reach its maximum, the slowly decreasing signal was fit with an exponential curve to obtain a cross section that represents the rate of surface hydroxyl depletion. For 0.1, 1, and 5 Langmuir (1 L = 10−6 Torr s) H2O exposure, respectively, the measured O(3P) depletion cross sections were 4.9 × 10−20, 6.6 × 10−20, and 4.6 × 10−20 cm2. These results imply that photodissociation of water on the lunar surface cannot account for the large mass‐16 (±1 amu) signal observed in the lunar atmosphere. Unless another significant source of oxygen atoms is present, this unexpectedly large signal is likely due to CH4 or OH.

Mechanisms and cross sections for water desorption from a lunar impact melt breccia

AbstractDesorption of H2O (ν = 0) following 157‐nm irradiation of amorphous solid water on a lunar impact melt breccia was measured with resonance‐enhanced multiphoton ionization. Photofragments of vibrationally excited water were detected with nonresonant ionization. The average cross section for H2O (ν = 0) removal and destruction at 0.1 Langmuir (1 L = 10−6 Torr∙s) H2O exposure was measured to be (7.1 ± 1.9) × 10−19 cm2. Cross sections were also measured at 0.3, 1, 5, and 10 L exposures. Because these cross sections increase drastically with decreasing water coverage, water is not expected to remain intact as H2O on the sunlit lunar surface. Instead, photons are likely to cause H2O to desorb or dissociate. The OH+ fragment of H2O (ν*) increased in intensity with increasing irradiation as hydroxyl groups built up on the surface and then recombined. The OH+ signal eventually began to decrease after a dose of 5 × 1018 photons cm−2. Under these conditions, the cross section for H2O (ν*) photodesorption was measured to be 6.4 × 10−20 cm2 for an initial exposure of 5 L H2O.

Impact melt differentiation in the South Pole-Aitken basin: Some observations and speculations

The stratigraphy of the South-Pole Aitken basin (SPA) interior is consistent with that of a massive impact melt sheet that differentiated to form cumulates. Spectroscopic and geophysical constraints on the stratigraphy of SPA suggest a ~12.5km thick layer of norite above ultramafic pyroxenite and dunite layers. A similar stratigraphy is produced from differentiation by crystal settling of a ~50km thick impact melt sheet (lunar impact melt sheets >10km thick likely undergo differentiation by crystal settling) formed by an oblique impact (and thus containing ~20vol. % crustal material). We propose that impact melt differentiation can account for geophysical (nonzero crustal thickness) and geochemical (~2ppm Th) anomalies in SPA.

Initial observations of lunar impact melts and ejecta flows with the Mini‐RF radar

The Mini‐RF radar on the Lunar Reconnaissance Orbiter spacecraft has revealed a great variety of crater ejecta flow and impact melt deposits, some of which were not observed in prior radar imaging. The craters Tycho and Glushko have long melt flows that exhibit variations in radar backscatter and circular polarization ratio along the flow. Comparison with optical imaging reveals that these changes are caused by features commonly seen in terrestrial lava flows, such as rafted plates, pressure ridges, and ponding. Small (&lt;20 km) sized craters also show a large variety of deposits, including melt flows and ponds. Two craters have flow features that may be ejecta flows caused by entrained debris flowing across the surface rather than by melted rock. The circular polarization ratios (CPRs) of the impact melt flows are typically very high; even ponded areas have CPR values between 0.7 and 1.0. This high CPR suggests that deposits that appear smooth in optical imagery may be rough at centimeter‐ and decimeter‐ scales. In some places, ponds and flows are visible with no easily discernable source crater. These melt deposits may have come from oblique impacts that are capable of ejecting melted material farther downrange. They may also be associated with older, nearby craters that no longer have a radar‐bright proximal ejecta blanket. The observed morphology of the lunar crater flows has implications for similar features observed on Venus. In particular, changes in backscatter along many of the ejecta flows are probably caused by features typical of lava flows.