Impact Melt Deposits Research Articles

Orientale Basin as a Guide for Identifying Lunar Basin Datable Impact Melt and Assessing Impact Melt Differentiation

Abstract Two fundamental questions face lunar scientists: (1) What is the absolute age of each lunar impact basin and thus the early impact flux curve? (2) To what degree did basin impact melt seas undergo differentiation? We compiled a 1:200,000-scale geological map of the lunar Orientale basin, focusing on identifying the most widespread and accessible occurrences of impact melt deposits from the basin-forming impact to help guide sample-return missions to Orientale and especially to other undated lunar basins using the identification and interpretation strategies for Orientale. We assess the size of craters excavating through basalt cap rock that may have exhumed datable basin impact melt, and we assess the possibility of impact melt sampling and melt differentiation for the large complex crater Maunder. We also provide guidance for distinguishing impact melt produced by larger complex craters from excavated basin melt and determining whether such craters may have also sampled through the entire melt deposit. Our analysis finds six such sites that are predicted to have the same age—that of the Orientale-forming event—and provides guidance for assessing possible melt differentiation. Future missions could collect samples from these sites for in situ age dating and petrologic assessment and/or for return to Earth and subsequent age dating and analysis. By sampling and dating impact melt of known provenance from the Moon’s dozens of large basins, future work can anchor the chronostratigraphy of the Moon’s formative years. Such information could be scaled to infer Earth’s large impactor flux around the time of life’s first emergence.

Organic Input to Titan's Subsurface Ocean Through Impact Cratering.

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 1011 mol/Gyr for HCN hydrolysis and from 0 to 1014 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼103 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

A hybrid geological map of Sibelius Crater on Mercury, and its associated ejecta and impact melt deposits

Abstract Using MESSENGER Mercury Dual Imaging System data, we produced three new maps of Sibelius Crater, Mercury. Geomorphological and spectral maps were combined into a single hybrid map containing units associated with ejecta deposits, crater floor landforms and impact melt ponding. Spatial measurement of these units shows that ∼50% of the mapped melt pond area lies within a large, degraded impact crater (crater B), beyond the significantly lower northern Sibelius rim, with a potential melt flow to a smaller, degraded impact structure further north (crater C). Freshly processed spectral data from the eight-colour Map Projected Multispectral Reduced Data Record data highlight the emplacement of multiple uplifted ejecta units with distinct spectral properties. A new, high-resolution digital elevation model was created to help define and analyse crater floor uplift features and disrupted crater rims and to create detailed cross-sections. These illustrate a proposed location of B's central uplift structure exposed in the northern wall slopes of Sibelius. Small features at the limit of visibility, such as a groove possibly associated with a rolling or sliding mega-boulder and lobate melt flow on the crater floor with accompanying channel opening, are highlighted for future investigations by BepiColombo's instruments once it reaches orbit.

Modeling the formation of Menrva impact crater on Titan: Implications for habitability

Titan is unique in the solar system: it is an ocean world, an icy world, an organic world, and has a dense atmosphere. It is a geologically active world as well, with ongoing exogenic processes, such as rainfall, sediment transportation and deposition, erosion, and possible endogenic processes, such as tectonism and cryovolcanism. This combination of an organic and an ocean world makes Titan a prime target for astrobiological research, as biosignatures may be present in its surface, in impact melt deposits and in cryovolcanic flows, as well as in deep ice and water ocean underneath the outer ice shell. Impact craters are important sites in this context, as they may have allowed an exchange of materials between Titan's layers, in particular between the surface, composed of organic sediments over icy bedrock, and the subsurface ocean. It is also possible that impacts may have favored the advance of prebiotic chemical reactions themselves, by providing thermal energy that would allow these reactions to proceed. To investigate possible exchange pathways between the subsurface water ocean and the organic-rich surface, we modeled the formation of the largest crater on Titan, Menrva, with a diameter of ca. 425 km. The premise is that, given a large enough impact event, the resulting crater could breach into Titan's ice shell and reach the subsurface ocean, creating pathways connecting the surface and the ocean. Materials from the deep subsurface ocean, including salts and potential biosignatures of putative subsurface biota, could be transported to the surface. Likewise, atmospherically derived organic surface materials could be directly inserted into the ocean, where they could undergo aqueous hydrolysis to form potential astrobiological building blocks, such as amino acids. To study the formation of a Menrva-like impact crater, we staged numerical simulations using the iSALE-2D shock physics code. We varied assumed ice shell thickness from 50 to 125 km and assumed thermal structure over a range consistent with geophysical data. We analyze the implications and potential contributions of impact cratering as a process that can facilitate the exchange of surface organics with liquid water. Our findings indicate that melt and partial melt of ice took place in the central zone, reaching ca. 65 km depth and ca. 60 km away from the center of the structure. Furthermore, a volume of ca. 102 km3 of ocean water could be traced to depths as shallow as 10 km. These results highlight the potential for a significant exchange of materials from the surface (organics and ice) and the subsurface (water ocean), particularly in the crater's central area. Our studies suggest that large hypervelocity impacts are a viable and likely key mechanism to create pathways between the underground water ocean and Titan's organic-rich surface layer and atmosphere.

Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander

Abstract NASA’s Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan’s prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based “life as we know it” (as might occur in the interior mantle ocean, potential cryovolcanic flows, and/or impact melt deposits) and potential “life, but not as we know it” that might use liquid hydrocarbons as a solvent (within Titan’s lakes, seas, and/or aquifers). Consideration of both of these solvents simultaneously led to our initial landing site in Titan’s equatorial dunes and interdunes to sample organic sediments and water ice, respectively. Ultimately, Dragonfly's traverse target is the 80 km diameter Selk Crater, at 7° N, where we seek previously liquid water that has mixed with surface organics. Our science goals include determining how far prebiotic chemistry has progressed on Titan and what molecules and elements might be available for such chemistry. We will also determine the role of Titan’s tropical deserts in the global methane cycle. We will investigate the processes and processing rates that modify Titan’s surface geology and constrain how and where organics and liquid water can mix on and within Titan. Importantly, we will search for chemical biosignatures indicative of past or extant biological processes. As such, Dragonfly, along with Perseverance, is the first NASA mission to explicitly incorporate the search for signs of life into its mission goals since the Viking landers in 1976.

Emplacement conditions of lunar impact melt flows

Hypervelocity impacts on terrestrial bodies have the potential to rapidly heat and redistribute silicate target material to form impact melt flows. On the Moon, a subset of impact melt deposits exited the craters as laterally moving flows that moved away from the crater rim under the influence of gravity. These impact melt flows exhibit similarities to lava flows, but have the potential to be superheated by the impact cratering process. To estimate the initial temperature (T0) and thickness (H0) of these flows we combine new remote sensing analyses, experimentally-derived rheological relationships for three lunar analog materials, and numerical forward models of impact melt flow emplacement to identify combinations of parameters that yield flows matching the observed length of impact melt deposits on the Moon. We focus on the impact melt flows of four craters, which span a range of target materials and crater diameters (D): Giordano Bruno (D = 22.1 km), Necho (D = 36.9 km), Tycho (D= 85.3 km), and Copernicus (D= 96.1 km). We modeled three melt compositions—anorthosite, norite, and basalt—emplaced under cold ambient temperatures (Tambient= –180 °C) on the Moon, to place a minimum bound on cooling-limited flow emplacement. Results show that observed flow lengths can be achieved by subliquidus melts emplaced within a laminar flow regime. For high-density (0% vesicularity) melts, best-fit inversions for T0 and H0 have mean values of 1253.3 ± 149.8 °C and 21.6 ± 8.7 m, respectively; for low-density (64% vesicularity) melts, mean T0 and H0 values are 1343.3 ± 87.8 °C and 27.9 ± 5.7 m—with uncertainties reported at 1 standard deviation. Model results also show that under cold ambient temperatures, impact melt flows with 0 to 64% vesicularity would be expected to have mean emplacement durations of 76, 78, 174, and 186 h for Giordano Bruno, Necho, Tycho, and Copernicus, respectively. Therefore, initial impact melt temperatures do not have to be in the super-liquidus range to form gravity-driven flow deposits matching observed lengths, and emplacement times are long enough for the majority of self-secondary fragments to land before the impact melt flows are fully emplaced. This suggests that some impact melt flow may be lava-like in their emplacement dynamics and that differences in crater populations on impact melt flows and adjacent ejecta may differ significantly, with impact melts providing a more robust estimate of the emplacement age of the young craters due to the presence of fewer secondaries. Impact melts could still have super-liquidus temperatures at the time of their initial formation, but turbulence would facilitate the rapid entrainment of external clasts, quickly lowering melt temperatures to generate flows that are similar to the products of effusive volcanic eruptions.

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.

The anatomy of fresh complex craters on the mid‐sized icy moons of Saturn and self‐secondary cratering at the rayed crater Inktomi (Rhea)

AbstractCassini mapping of Saturn’s mid‐sized icy moons of well‐preserved complex craters in the 45–95 km size range provides insight into cratering processes at lower surface gravity and on icy targets. These craters are characterized by steep rim scarps, rugged hummocky floor deposits of curvilinear ridges and scarps, and rugged conical central peaks. Ponded impact melt or related deposits are not observed on any floor or ejecta units of any of these complex craters, indicating that melt production may be much lower than predicted. Mantling ejecta units drape over pre‐existing topography, grading into concentric zones of secondary craters at ~1 crater diameter from the rim, demonstrating that secondaries occur on mid‐sized icy planetary bodies. The largest and youngest bright‐ray system is the 49 km central peak crater Inktomi, target of a mapping campaign down to 34 m pixel scales. In addition to classical secondary craters up to 3 km, several hundred small craters <1 km in size form an unusual densely spaced cluster across the eastern floor and ejecta deposit. The terrain‐indiscriminate distribution of these eastern floor craters indicates they are not explosive volatile release pits, such as mapped on Vesta or Mars, but self‐secondary craters formed by the fallback of ejected blocks back into the crater itself during crater formation. Self‐secondaries likely formed by an irregular and stochastic but poorly understood impact process, such as spallation influenced by irregular surface topography. Self‐secondary craters of this type could influence the interpretation of crater counts on large fresh impact craters and basins elsewhere.

The Global Distribution of Lunar Light Plains From the Lunar Reconnaissance Orbiter Camera

AbstractWe present the first globally consistent map of lunar light plains from Lunar Reconnaissance Orbiter Camera mosaics 100 m/pixel. Based on this map, ~70% of light plains are likely related to the Orientale and Imbrium basins; however, many light plains deposits originated with local‐ and regional‐scale impacts, nonmare volcanics (i.e., the Apennine Bench Formation), as well as ancient impact melt deposits that lack diagnostic small‐scale features. Compositional evidence suggests that the light plains associated with basins (i.e., Cayley plains) form through substantial mixing with local materials, implying that ballistic sedimentation is a primary formation mechanism. Based on the distribution of light plains with respect to the Orientale and Imbrium basins, the stratigraphic extent of an individual basin extends to at least four basin radii. As such, the South Pole‐Aitken basin modified at least 80% of the lunar surface. This work suggests that the entire lunar surface was modified to varying degrees at the time of large basin formation. This has implications for sample interpretations because any highland site within four radii of a large basin could potentially contain primary basin material or more local material that was redeposited by secondaries, neither of which is necessarily representative of the terrain upon which they are found.

Geologic Analyses of the Causes of Morphological Variations in Lunar Craters Within the Simple‐to‐Complex Transition

AbstractThe diameter range of 15 to 20 km is within the transition from simple to complex impact craters located on the Moon. This range spans roughly a factor of 3 in impact energy for the same impactor speed, composition, and trajectory angle. We analyzed the global population of well‐preserved craters in this size range in order to assess the effects of target and impactor properties on crater shapes and morphologies. We observed that within this narrow diameter range, simple craters are confined to the highlands, and complex craters are more abundant in the mare. We found unusually deep craters around the highlands‐mare boundaries and favor the hypothesis that they form by impact cratering on high‐porosity terrain. We infer that target properties primarily contribute to the observed morphological variations in the craters. Simple crater formation is favored by a terrain that is more homogeneous in strength and topography, while transitional and complex crater formation is aided by heterogeneity in lithology, topography, or strength, or a combination of these parameters. Clearly visible impact melt deposits in a small percentage of simple craters, and two cases of craters differing in morphologies from their nearest neighbors in similar geologic settings, suggest that variation in impactor properties such as impact velocity and impactor size may have some role in causing morphological differences between similar‐sized craters.

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.

Strategies for Detecting Biological Molecules on Titan.

Saturn's moon Titan has all the ingredients needed to produce "life as we know it." When exposed to liquid water, organic molecules analogous to those found on Titan produce a range of biomolecules such as amino acids. Titan thus provides a natural laboratory for studying the products of prebiotic chemistry. In this work, we examine the ideal locales to search for evidence of, or progression toward, life on Titan. We determine that the best sites to identify biological molecules are deposits of impact melt on the floors of large, fresh impact craters, specifically Sinlap, Selk, and Menrva craters. We find that it is not possible to identify biomolecules on Titan through remote sensing, but rather through in situ measurements capable of identifying a wide range of biological molecules. Given the nonuniformity of impact melt exposures on the floor of a weathered impact crater, the ideal lander would be capable of precision targeting. This would allow it to identify the locations of fresh impact melt deposits, and/or sites where the melt deposits have been exposed through erosion or mass wasting. Determining the extent of prebiotic chemistry within these melt deposits would help us to understand how life could originate on a world very different from Earth. Key Words: Titan-Prebiotic chemistry-Solar system exploration-Impact processes-Volcanism. Astrobiology 18, 571-585.

Evidence for self-secondary cratering of Copernican-age continuous ejecta deposits on the Moon

Crater size-frequency distributions on the ejecta blankets of Aristarchus and Tycho Craters are highly variable, resulting in apparent absolute model age differences despite ejecta being emplaced in a geologic instant. Crater populations on impact melt ponds are a factor of 4 less than on the ejecta, and crater density increases with distance from the parent crater rim. Although target material properties may affect crater diameters and in turn crater size-frequency distribution (CSFD) results, they cannot completely reconcile crater density and population differences observed within the ejecta blanket. We infer from the data that self-secondary cratering, the formation of impact craters immediately following the emplacement of the continuous ejecta blanket by ejecta from the parent crater, contributed to the population of small craters (< 300m diameter) on ejecta blankets and must be taken into account if small craters and small count areas are to be used for relative and absolute model age determinations on the Moon. Our results indicate that the cumulative number of craters larger than 1km in diameter per unit area, N(1), on the continuous ejecta blanket at Tycho Crater, ranges between 2.17 × 10−5 and 1.0 × 10−4, with impact melt ponds most accurately reflecting the primary crater flux (N(1) = 3.4 × 10−5). Using the cratering flux recorded on Tycho impact melt deposits calibrated to accepted exposure age (109 ± 1.5 Ma) as ground truth, and using similar crater distribution analyses on impact melt at Aristarchus Crater, we infer the age of Aristarchus Crater to be ∼280 Ma. The broader implications of this work suggest that the measured cratering rate on ejecta blankets throughout the Solar System may be overestimated, and caution should be exercised when using small crater diameters (i.e. < 300m on the Moon) for absolute model age determination.

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.

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.

Geological mapping of impact melt deposits at lunar complex craters Jackson and Tycho: Morphologic and topographic diversity and relation to the cratering process

High resolution geological mapping, aided by imagery and elevation data from the lunar reconnaissance orbiter (LRO) and Kaguya missions, has revealed the scientifically rich character of impact melt deposits at two young complex craters: Jackson (71km) and Tycho (85km). The morphology and distribution of mapped impact melt units provide several insights into the cratering process. We report elevation differences (>200m) among large, coherent floor sections within a single crater and interpret them to be caused by crater wall collapse and/or large scale structural failure of the floor region. Clast-poor, smooth melt deposits are correlated with floor sections at lower elevations and likely represent ponded deposits sourced from higher elevation regions (viz. crater walls). In addition, these deposits are also located in the inferred downrange direction of the impact. Melt-coated large blocks spanning several kilometers are common on the crater floors and may represent collapsed wall sections or in some cases, subdued sections of the central peaks. Spatial trends in the mapped impact melt units at the two craters provide clues to decipher the conditions during each impact event and subsequent evolution of the crater floor.

Geomorphologic mapping of the lunar crater Tycho and its impact melt deposits

Using SELENE/Kaguya Terrain Camera and Lunar Reconnaissance Orbiter Camera (LROC) data, we produced a new, high-resolution (10m/pixel), geomorphological and impact melt distribution map for the lunar crater Tycho. The distal ejecta blanket and crater rays were investigated using LROC wide-angle camera (WAC) data (100m/pixel), while the fine-scale morphologies of individual units were documented using high resolution (∼0.5m/pixel) LROC narrow-angle camera (NAC) frames. In particular, Tycho shows a large coherent melt sheet on the crater floor, melt pools and flows along the terraced walls, and melt pools on the continuous ejecta blanket. The crater floor of Tycho exhibits three distinct units, distinguishable by their elevation and hummocky surface morphology. The distribution of impact melt pools and ejecta, as well as topographic asymmetries, support the formation of Tycho as an oblique impact from the W-SW. The asymmetric ejecta blanket, significantly reduced melt emplacement uprange, and the depressed uprange crater rim at Tycho suggest an impact angle of ∼25–45°.

Zumba crater, Daedalia Planum, Mars: Geologic investigation of a young, rayed impact crater and its secondary field

Zumba is a ∼2.9km diameter rayed crater on Mars located on extensive lava plains in Daedalia Planum to the southwest of Arsia Mons. It is a well-preserved young crater with large ejecta rays that extend for hundreds of kilometers from the impact site. The rays are thermally distinct from the background lava flows in THEMIS daytime and nighttime thermal infrared data, a unique characteristic among martian rayed craters. Concentrated within the rays are solitary or dense clusters of secondary craters with associated diffuse dark-toned deposits along with fewer secondary craters lacking dark-toned deposits. Using CTX images, we have mapped secondary craters with dark-toned deposits, collectively termed “secondary fields”, to investigate their distribution as a function of distance from the impact site. The mapped secondary field was then used to investigate various aspects of the crater-forming event such as the surface angle and direction of the projectile, the effect of secondary craters on surface age estimates, and the number of secondary craters produced by the impact event. From our mapping, a total of 13,064 secondary fields were documented out to a 200km radial distance beyond a 15km-wide non-secondary zone around Zumba crater. Results show that the highest areal coverage of secondary fields occurs within 100km of Zumba and within its rays, decreasing radially with distance to a background scattering of small secondary fields that are <10km2 in size (∼91.9% of the total population). The strong east–west asymmetry of crater rays and low areal coverage of fields to the south of Zumba correlate well with studies of Zumba impact melt deposits, indicating a moderately oblique impact projectile coming from the south. Using primary craters in a ∼101km2 sample region and all craters (primaries and secondaries) from 43 select secondary fields in two map sectors in the study area, we obtain ages of ∼580±100Ma and ∼650±70Ma, respectively, for the lava flows into which Zumba impacted. These ages are consistent with and intermediate to 0.1–1Ga volcanic flow units within and near Daedalia Planum. For craters within the secondary fields, a log differential plot of the data shows a pronounced downward deflection of the binned points for craters <100m in diameter. This is an indication that possible aeolian erosion and deposition, in combination with emplacement and remobilization of Zumba ejecta, have modified the lava flow surfaces in Daedalia Planum over time. Our crater count results also show that even in the presence of large numbers of secondary craters, the age of the primary crater surface can still be identified from the data for craters >100m. We estimate that ∼352,000 secondary craters were produced by the Zumba impact event.

Ancient and recent clay formation on Mars as revealed from a global survey of hydrous minerals in crater central peaks

AbstractClay minerals on Mars have commonly been interpreted as the remnants of pervasive water‐rock interaction during the Noachian period (&gt;3.7 Ga). This history has been partly inferred by observations of clays in central peaks of impact craters, which often are presumed uplifted from depth. However, combined mineralogical and morphological analyses of individual craters have shown that some central peak clays may represent post‐impact, possibly authigenic processes. Here we present a global survey of 633 central peaks to assess their hydrous minerals and the prevalence of uplifted, detrital, and authigenic clays. Central peak regions are examined using high‐resolution Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment data to identify hydrous minerals and place their detections in a stratigraphic and geologic context. We find that many occurrences of Fe/Mg clays and hydrated silica are associated with potential impact melt deposits. Over 35% of central peak clays are not associated with uplifted rocks; thus, caution must be used when inferring deeper crustal compositions from surface mineralogy of central peaks. Uplifted clay‐bearing rocks suggest the Martian crust hosts clays to depths of at least 7 km. We also observe evidence for increasing chloritization with depth, implying the presence of fluids in the upper portions of the crust. Our observations are consistent with widespread Noachian/Early Hesperian clay formation, but a number of central peak clays are also suggestive of clay formation during the Amazonian. These results broadly support current paradigms of Mars' aqueous history while adding insight to global crustal and diagenetic processes associated with clay mineral formation and stability.

Rembrandt impact basin: Distinguishing between volcanic and impact-produced plains on Mercury

The surface of Mercury has been heavily modified by impact cratering and volcanic plains since the initial formation of its lithosphere and crust. As in the case of the Moon, the origin of early plains deposits has been difficult to determine because ponded impact basin ejecta and impact melt deposits can mimic the topography and texture of extrusive volcanic plains. In order to understand better the role that impact basins play in resurfacing on Mercury, we used MESSENGER Mercury Dual Imaging System (MDIS) data to map the distribution of plains deposits within and around the relatively young Rembrandt impact basin (∼720km in diameter). There are two different Rembrandt plains units: (1) high-reflectance interior and exterior plains, and (2) low-reflectance exterior plains. The morphology, crater size–frequency distribution, and N(20) crater density values were analyzed for each deposit. Observations of the two high-reflectance plains deposits are consistent with previous studies of volcanically produced smooth plains on Mercury. The low-reflectance exterior plains are older than the other high-reflectance plains and comparable in age to the Rembrandt basin impact event. On the basis of age, areal distribution, and albedo relationships with basin materials, the low-reflectance plains are interpreted as basin impact melt deposits. The large areal extent of the low-reflectance plains suggests that basin impact melt deposits may have played an important role in resurfacing Mercury, especially during the early geologic history of the planet when impact rates were higher.