A new chronology for the Moon and Mercury

Icarus, 2013

Standard lunar chronologies, based on combining lunar sample radiometric ages with impact crater densities of inferred associated units, have lately been questioned about the robustness of their interpretations of the temporal dependance of the lunar impact flux. In particular, there has been increasing focus on the ''middle age'' of lunar bombardment, from the end of the Late Heavy Bombardment ($3.8 Ga) until comparatively recent times ($1 Ga). To gain a better understanding of impact flux in this time period, we determined and analyzed the cratering ages of selected terrains on the Moon. We required distinct terrains with random locations and areas large enough to achieve good statistics for the small, superposed crater size-frequency distributions to be compiled. Therefore, we selected 40 lunar craters with diameter $90 km and determined the model ages of their floors by measuring the density of superposed craters using the Lunar Reconnaissance Orbiter Wide Angle Camera mosaic. Absolute model ages were computed using the Model Production Function of Marchi et al. (Marchi, S., Mottola, S., Cremonese, G., Massironi, M., Martellato, E. [2009]. Astron. J. 137,[4936][4937][4938][4939][4940][4941][4942][4943][4944][4945][4946][4947][4948]. We find that a majority (36 of 40) of our superposed crater size-frequency distributions are consistent with the Model Production Function. A histogram of the original crater floor model ages indicates the bombardment rate decreased gradually from $3.8 Ga until $3.0 Ga, implying an extended tail to the Late Heavy Bombardment. For large craters, it also preliminarily suggests that between $3.0 and 1.0 Ga bombardment may be characterized by long periods (>600 Myr) of relatively few impacts (''lulls'') broken by a short duration ($200 Myr) of relatively more impacts (''spike''). While measuring superposed craters, we also noted if they were part of a cluster or chain (named ''obvious secondary''), and analyzed these craters separately. Interestingly, we observe a wide variety of slopes to the differential size-frequency power-law, which demonstrates that there can be considerable variation in individual secondary crater field size-frequency distributions. Finally, four of the small, superposed crater size-frequency distributions are found to be inconsistent with the Model Production Function; possible reasons are: resurfacing has modified these distributions, unrecognized secondary craters, and/or the Model Production Function has incorrect inputs (such as the scaling law for the target terrain). The degraded appearance of the superposed craters and indications of resurfacing suggest that the first cause is the most likely.

Space Sciences Series of ISSI, 2001

The well investigated size-frequency distributions (SFD) for lunar craters is used to estimate the SFD for projectiles which formed craters on terrestrial planets and on asteroids. The result shows the relative stability of these distributions during the past 4 Gyr. The derived projectile size-frequency distribution is found to be very close to the size-frequency distribution of Main-Belt asteroids as compared with the recent Spacewatch asteroid data and astronomical observations (Palomar-Leiden survey, IRAS data) as well as data from close-up imagery by space missions. It means that asteroids (or, more generally, collisionally evolved bodies) are the main component of the impactor family. Lunar crater chronology models of the authors published elsewhere are reviewed and refined by making use of refinements in the interpretation of radiometric ages and the improved lunar SFD. In this way, a unified cratering chronology model is established which can be used as a safe basis for modeling the impact chronology of other terrestrial planets, especially Mars.

Origins of Life and Evolution of Biospheres

If properly interpreted, the impact record of the Moon, Earth's nearest neighbour, can be used to gain insights into how the Earth has been influenced by impacting events since its formation~4.5 billion years (Ga) ago. However, the nature and timing of the lunar impactorsand indeed the lunar impact record itselfare not well understood. Of particular interest are the ages of lunar impact basins and what they tell us about the proposed Blunar cataclysm^and/or the late heavy bombardment (LHB), and how this impact episode may have affected early life on Earth or other planets. Investigations of the lunar impactor population over time have been undertaken and include analyses of orbital data and images; lunar, terrestrial, and other planetary sample data; and dynamical modelling. Here, the existing information regarding the nature of the lunar impact record is reviewed and new interpretations are presented. Importantly, it is demonstrated that most evidence supports a prolonged lunar (and thus, terrestrial) bombardment from~4.2 to 3.4 Ga and not a cataclysmic spike at 3.9 Ga. Implications for the conditions required for the origin of life are addressed.

Reviews in Mineralogy and Geochemistry

Solar System Research, 2020

We compare the number of lunar craters larger than 15 km across and younger than 1.1 Ga to the estimates of the number of craters that could have been formed for 1.1 Ga if the number of near-Earth objects and their orbital elements during that time were close to the corresponding current values. The comparison was performed for craters over the entire lunar surface and in the region of the Oceanus Procellarum and maria on the near side of the Moon. In these estimates, we used the values of collision probabilities of near-Earth objects with the Moon and the dependences of the crater diameters on the impactor sizes. According to the estimates made by different authors, the number density of known Copernican craters with diameters D ≥ 15 km in mare regions is at least double the corresponding number for the remaining lunar surface. Our estimates do not contradict the growth in the number of near-Earth objects after probable catastrophic frag-mentations of large main-belt asteroids, which may have occurred over the recent 300 Ma; however, they do not prove this increase. Particularly, they do not conflict with the inference made by Mazrouei et al. (2019) that 290 Ma ago the frequency of collisions of near-Earth asteroids with the Moon increased by 2.6 times. The number of Copernican lunar craters with diameters not smaller than 15 km is probably higher than that reported by Mazrouei et al. (2019). For a probability of a collision of an Earth-crossing object (ECO) with the Earth in a year equaled to 10-8 , our estimates of the number of craters agree with the model, according to which the number densities of the 15-km Copernican craters for the whole lunar surface would have been the same as that for mare regions if the data by Losiak et al. (2015) for D < 30 km were as complete as those for D > 30 km. With this collision probability of ECOs with the Earth and for this model, the cratering rate may have been constant over the recent 1.1 Ga.

Nature Geoscience

Planetary bombardment histories provide critical information regarding the formation and evolution of the Solar System and of the planets within it. These records evidence transient instabilities in the Solar System's orbital evolution 1 , giant impacts such as the Moon-forming impact 2 , and material redistribution. Such records provide insight into planetary evolution, including the deposition of energy, delivery of materials, and crustal processing, specifically the modification of porosity. Bombardment histories are traditionally constrained from the surface expression of impacts-these records, however, are degraded by various geologic processes. Here we show that the Moon's porosity contains a more complete record of its bombardment history. We find that the terrestrial planets were subject to double the number of ≥20-km-diameter-crater-forming impacts than are recorded on the lunar highlands, fewer than previously thought to have occurred. We show that crustal porosity doesn't slowly increase as planets evolve, but instead is generated early in a planet's evolution when most basins formed and decreases as planets evolve. We show that porosity constrains the relative ages of basins formed early in a planet's evolution, a timeframe for which little information exists. These findings demonstrate that the Solar System was less violent than previously thought. Fewer volatiles and other materials were delivered to the terrestrial planets, consistent with estimates of the delivery of siderophiles 3 and water to the Moon 4. High crustal porosity early in the terrestrial planets' evolution slowed their cooling and enhanced their habitability. Several lunar basins formed early than previously considered, casting doubt on the existence of a late heavy bombardment. The Moon's lack of an atmosphere, lack of plate tectonics, and, excluding the maria, lack of large-scale volcanism make its cratering record one of the most complete in the Solar System 5. The lunar highlands are among the oldest surfaces on the Moon, likely representing portions of the primordial anorthosite flotation crust thought to have differentiated from the Lunar Magma Ocean (LMO) 6 4.29-4.54 Ga 7,8. The cumulative bombardment of these surfaces resulted in the development of the lunar megaregolith 9 , erased the surface expression of massive basins 10 , and generated planetary-scale crustal porosity down to depths on the order of 10s of km 11. Deciphering the total number of impacts responsible for the evolution of a post-LMO era crust, however, has proven difficult 12. Particularly, the lunar highlands may have reached a state of crater equilibrium, such that the formation of every new crater, on average, is accompanied by the erasure of a similar sized, pre-existing crater 13. If the lunar highlands are in a state of equilibrium, then an indeterminate amount of the Moon's visible cratering record has been lost, impeding efforts to use

Astronomy & Geophysics

Vestnik Otdelenia nauk o Zemle RAN, 2011

Most part of the lunar surface relief was formed during the last 5 Ma. This conclusion was received on the basis of detail analysis of large craters of the Moon, Earth, Mars and Mercury. Falling of the galactic comets in the period 5-0.6 Ma, and the tectonomagmatic processes induced by the comets falling played major role in shaping of the Moon topography. Processes of tectonics and volcanism are occurring on the Moon today also. We found volcano in the Tsiolkovsky crater on the reverse side of the Moon that can serve as good example of that. The volcano has a height of 102 m and is located almost in the bottom center of the crater with a diameter of 180 km on a low oval elevation of plume nature 24-26 km in size.

In a running DLR co-funded study extension on radioisotope dating of planetary surface material we are concentrating on the concept development of some mayor critical sub-units, especially the definition and dimensioning of the radioactive source, and on some central sub-units of the mass spectrometer subsystem. Focussing is on the optimization of the neutron flux and on the sensitivity of the mass spectrometer, already studied as integrated part of a lunar lander payload. The time scale of any geologic process determines its very nature. Therefore, one of the highest-priority science goals of planetary exploration is elucidating the absolute chronology like internal differentiation processes or the surface evolution by volcanism and impact cratering. Radioisotope dating of the Apollo samples enabled to link impact crater counting to absolute chronology, not only for the moon but also for other terrestrial planets (Jessberger et al. 1974, Turner 1977, Hiesinger et al. 2000, Neukum e...

Journal of Geophysical Research: Planets, 2018

Flooding of the lunar surface by ancient mare basalts has rendered uncertain the ages of lunar geochemical terranes and several impact basins. Here we combine craters having recognizable surface expressions with craters identified only by their gravitational signatures in Gravity Recovery and Interior Laboratory data to reassess the chronological sequence of lunar impact basins and the ages of major lunar geochemical terranes. Our results indicate that although volcanically flooded regions are deficient in craters with diameters greater than 20 km by more than 50% relative to unflooded regions, craters with diameters greater than 90 km can be readily recognized either by topography or by gravity anomaly. On the basis of the areal density of craters with diameters greater than 90 km we conclude that (1) the Serenitatis basin could be as young as the Imbrium basin; (2) the areal density of craters within the Procellarum KREEP Terrane is significantly lower than that for the South Pole...