Lunar Surface Model Age Derivation: Comparisons Between Automatic and Human Crater Counting Using LRO‐NAC and Kaguya TC Images

Abstract Dating young lunar surfaces, such as impact ejecta blankets and terrains associated with recent volcanic activities, provides critical information on the recent events that shaped the surface of the Moon. Model age derivation of young or small areas using a crater chronology is typically achieved through manual counting, which requires a lot of small impact craters to be tediously mapped. In this study, we present the use of a Crater Detection Algorithm (CDA) to extract crater populations on Lunar Reconnaissance Orbiter—Narrow Angle Camera (LRO‐NAC) and Kaguya Terrain Camera images. We applied our algorithm to images covering the ejecta blankets of four Copernican impact craters and across four young mare terrains, where manually derived model ages were already published. Across the eight areas, 10 model ages were derived. We assessed the reproducibility of our model using two populations for each site: (a) an unprocessed population and (b) a population adjusted to remove contaminations of secondary and buried craters. The results showed that unprocessed detections led to overestimating crater densities by 12%–48%, but “adjusted” populations produced consistent results within <20% of published values in 80% of cases. Regarding the discrepancies observed, we found no significant error in our detections that could explain the differences with crater densities manually measured. With careful processing, we conclude that a CDA can be used to determine model ages and crater densities for the Moon. We also emphasize that automated crater datasets need to be processed, interpreted and used carefully, in unity with geologic reasoning. The presented approach can offer a consistent and reproducible way to derive model ages

Automatic Mapping of Small Lunar Impact Craters Using LRO‐NAC Images

Abstract Impact craters are the most common feature on the Moon’s surface. Crater size–frequency distributions provide critical insight into the timing of geological events, surface erosion rates, and impact fluxes. The impact crater size–frequency follows a power law (meter‐sized craters are a few orders of magnitude more numerous than kilometric ones), making it tedious to manually measure all the craters within an area to the smallest sizes. We can bridge this gap by using a machine learning algorithm. We adapted a Crater Detection Algorithm to work on the highest resolution lunar image data set (Lunar Reconnaissance Orbiter‐Narrow‐Angle Camera [NAC] images). We describe the retraining and application of the detection model to preprocessed NAC images and discussed the accuracy of the resulting crater detections. We evaluated the model by assessing the results across six NAC images, each covering a different lunar area at differing lighting conditions. We present the model’s average true positive rate for small impact craters (down to 20 m in diameter) is 93%. The model does display a 15% overestimation in calculated crater diameters. The presented crater detection model shows acceptable performance on NAC images with incidence angles ranging between ∼50° and ∼70° and can be applied to many lunar sites independent to morphology

Seismic efficiency and seismic moment for small craters on mars formed in the layered uppermost crust

Seismic activity generated by impacts depends on impact conditions and properties of the impact site. Here, we combined mapping of the regolith thickness with numerical impact simulations to better estimate the seismic efficiency and seismic moment generated in small impact events in the uppermost crust on Mars. We used mapping of crater morphology to determine the regolith thickness that craters formed in. We found that local regolith thickness in the late Amazonian units is between 4 and 9 m. Combined with previous estimates for the NASA InSight landing site, we composed a more realistic uppermost crust analog and implemented it in numerical impact simulations. We estimated the seismic efficiency and seismic moment for small craters on Mars impacting a non-porous or fractured bedrock overlaid by 5, 10, or 15 m thick regolith. Seismic energy showed more dependence on target properties. Three orders of magnitude more energy were produced in stronger targets. The seismic moment does not depend on target properties, and we confirm that seismic moment is almost proportional to impact momentum. The resulting seismic moment is in agreement up to a factor of 4 between different target types. We improved the scaling relationships developed from numerical simulations used in seismic moment approximations by constraining its dependence on more realistic target properties

The Lomonosov Crater Impact Event: A Possible Mega‐Tsunami Source on Mars

International audienceRecent research suggests that major meteorite impact events into a Late Hesperian/Early Amazonian ocean likely produced a mega‐tsunami that would have resurfaced coastal areas in northwestern Arabia Terra. The orientations of the associated lobate deposits, a conspicuous type of landforms called Thumbprint Terrain, suggests that if an impact event triggered the mega‐tsunami, the most likely location of the source crater is within the northern plains regions situated north of Arabia Terra. This study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami. We selected 10 complex impact craters, based on their diameters, location, and geomorphic characteristics. Of those, the Late Hesperian ~120‐km‐diameter Lomonosov crater exhibits a unique topographic plan view asymmetry (compared to other similar‐sized and similar‐aged craters in the northern plains such as Micoud, Korolev, and Milankovic). We attribute its broad and shallow rim, in part, to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity. The likely marine formation of the Lomonosov crater, and the apparent agreement in its age with that of the Thumbprint Terrain unit (~3 Ga), strongly suggests that it was the source crater of the tsunami. These results have implications for the stability of a late northern ocean on Mars

Impact and habitability scenarios for early Mars revisited based on a 4.45-Ga shocked zircon in regolith breccia

After formation of a primordial crust, early impacts influenced when habitable conditions may have occurred on Mars. Martian meteorite Northwest Africa (NWA) 7034 is a regolith breccia that contains remnants of the earliest Martian crust. The paucity of shock deformation in NWA 7034 was previously cited as recording a decline in giant impacts by 4.48 billion years and evidence for habitable Mars by 4.2 billion years ago. We present new evidence of high-pressure shock effects in a 4.45–billion year–old zircon from the matrix of NWA 7034. The zircon contains {112} shock twins formed in the central uplift of a complex impact structure after 4.45 billion years and records impact pressures of 20 to 30 gigapascals. The zircon represents the highest shock level reported in NWA 7034 and paired rocks and provides direct physical evidence of large impacts, some potentially life-affecting, that persisted on Mars after 4.48 billion years