Patches of small smooth plains cover a substantial portion of the Mercurian surface, but their origin and relation to the global evolution are not completely understood. Here, we update the global ...inventory of small smooth plains on Mercury, classifying their spatial distribution, absolute model ages, and possible origins. We reveal that both large and small smooth plains on Mercury were dominantly emplaced within ∼200 million years around 3.7 Ga, and at least ∼24.9% of the global surface was resurfaced during this period. Small smooth plains formed by effusive volcanism are preferentially located around the contemporaneous Caloris, Rembrandt, and Beethoven basins and at thin crust areas. We also report small smooth plains that were likely emplaced by basin ejecta. Together with the distribution of similarly aged large smooth plains, our results reveal that peaked formation of large impact basins may be a major trigger for this short‐term and global‐wide volcanism.
Plain Language Summary
The history of volcanism is a pulse curve of geodynamics of planetary bodies. Large smooth plains on Mercury were formed by effusive volcanism before 3.5 Ga. Small smooth plains occupy a substantial portion of this planet and they have a wider geographic distribution than larger ones, but their possible origins have not been systematically studied. The patchy occurrence of small smooth plains indicates that they may better represent the global thermal evolution. Here, we update the global distribution of small smooth plains and investigate the absolute model age and possible origin for each case. At least 123 of the 315 cataloged small smooth plains were likely emplaced by effusive volcanism that preferentially occurred at thin crust areas. Smooth plains, regardless of origins and sizes, were mainly formed in ∼200 million years around 3.7 Ga, revealing that >24.9% of Mercury surface was emplaced by short‐term effusive volcanism. The preferential occurrence of volcanic smooth plains around the Caloris, Rembrandt, and Beethoven basins suggests a possible trigger by these impact events. We report evidence showing that coeval and collocated small smooth plains can have different origins, as some smooth plains may be ponded ejecta deposits that were emplaced by contemporaneous impact basins.
Key Points
Global inventory of small smooth plains on Mercury is created, with an emphasis on their origins and absolute model ages
Formation of smooth volcanic plains peaked at ∼3.7 Ga, with a preferential distribution around the Caloris, Rembrandt, and Beethoven basins
Smooth plains with unconfirmed volcanic origin were also uniformly formed around 3.7 Ga, some were likely emplaced by basin ejecta
Recent Dark Pyroclastic Deposits on Mercury Xiao, Zhiyong; Xu, Rui; Wang, Yichen ...
Geophysical research letters,
16 May 2021, 2021-05-16, Letnik:
48, Številka:
9
Journal Article
Recenzirano
Understanding the origin of volatiles and identifying opaque phases on the surface of Mercury are important tasks to be completed to understand the planet's building blocks and early evolution. ...Pyroclastic deposits, typically featuring higher and steeper reflectance spectra than the global average, are a bridge connecting the two tasks. We report the first discovery of dark pyroclastic deposits on Mercury, which exhibit identical morphology and geometry with typical reddish pyroclastics but comparable reflectances to low‐reflectance materials (LRM). Two Kuiperian‐aged dark pyroclastic deposits and many potential older ones are discovered. Reflectance spectra for one Kuiperian‐aged pyroclastic deposit exhibits an absorption feature attributable to graphite, indicating incomplete exsolution of C from the pre‐eruption magma and/or a dominance of blasted country rocks in the pyroclastics. Our second Kuiperian‐aged case and also ∼12.5% of the global LRM exhibit no graphite or other absorption features, and metallic iron may be the alternative darkening phase.
Plain Language Summary
The prevailing explanation for the low reflectance of Mercury is graphite, which was possibly differentiated from a hypothesized magma ocean. Graphite is predicted to be stable in Mercury's mantle, and oxidization of graphite may be a major source of volatiles that drove explosive volcanism on Mercury. Typical pyroclastic deposits on Mercury have higher and steeper reflectances than the global average, which were ascribed to the loss of graphite. For the first time, dark pyroclastic deposits are discovered on Mercury, appearing as low‐reflectance and diffuse‐edged thin cover around rimless depressions. We found two candidate dark pyroclastic deposits that were formed in Mercury's most recent history, and the eruption occurred along pre‐existing crustal weaknesses. For one such case, an absorption feature attributable to graphite is visible in both the country rocks and diffuse pyroclastics, indicating that graphite in the magma was incompletely oxidized, or blasted country rocks that contain graphite exist in the pyroclastics. For another candidate, the graphite absorption feature is visible in the country rocks but absent in the pyroclastics. The pyroclastics have redder reflectance spectra, indicating metallic iron is the possible darkening phase. Approximately 12.5% of recognized low‐reflectance materials on Mercury does not exhibit a graphite absorption either.
Key Points
We describe candidate dark pyroclastic deposits that were formed by vulcanian eruptions in Mercury's Kuiperian age
One deposit has an absorption attributable to graphite, suggesting a dominance of country rock or incomplete exsolution of magmatic carbon
A second deposit exhibits no graphite or other absorption features, but metallic iron is a possible darkening phase
Self-secondaries are a population of background secondaries, and they have been observed on top of impact melt and ballistically emplaced ejecta deposits on various planetary bodies. Self-secondaries ...are formed by impacts of sub-vertically launched ejecta, but the launch mechanism is not confirmed. The potential threat of self-secondaries to the theoretical and applicable reliability of crater chronology has been noted, but not constrained. Hitherto discovered self-secondaries were located around complex impact craters, but their potential existence around simple craters has not been discovered. Here we report the first discovery of self-secondaries around lunar cold spot craters, which are an extremely young population of simple craters formed within the past ~1 million years on the Moon. Self-secondaries are widespread on layers of cascading flow-like ejecta deposits around cold spot craters. The spatial density of self-secondaries dwarfs that of potential primary craters. The spatial distribution of self-secondaries is highly heterogeneous across the ejecta deposits. With respect to the impactor trajectory that formed cold spot craters, self-secondaries formed at the downrange of the ejecta deposits have the largest spatial density, while those at the uprange have the smallest density. This density pattern holds for all cold spot craters that were formed by non-vertical impacts, but self-secondaries do not exhibit other systematic density variations at different radial distances or at other azimuths with respect to the impactor trajectory. Among known mechanics of ejecting materials to the exterior of impact craters, impact spallation is the most likely scenario to account for the required large ejection velocities and angles to form self-secondaries. The production population of self-secondaries is estimated based on the highly diverse crater size-frequency distributions across the ejecta deposits of cold spot craters. For a better understanding of the impact history on the Moon, a systematic investigation for the effect of self-secondaries on lunar crater chronology is required.
Hollows are the key to understand the composition and evolution of volatiles in the planet Mercury. A fundamental question about the formation mechanism of hollows is the true reflectance spectra of ...the possible volatile compounds. We use the high‐resolution images and reflectance spectra returned by the MESSENGER spacecraft to investigate the nature and origin of hollows on Mercury. Hollows are divided as different geomorphological facies according to their morphology and possible degree of devolatilization, and reflectance spectra for different facies of hollows are extracted. Large sample analyses reveal that the spectra of hollow floors and background terrains are the two end‐members, whose mixtures can account for the observed wide ranges of reflectance spectra of hollows. Spectral analyses favor that carbon might be a volatile compound that formed hollows, although direct spectral observations for the hollow‐forming volatiles are prohibited because volatiles are mainly lost from steep hollow walls. The initiation and growth of hollows are controlled by structural heterogeneities in the subsurface, such as widespread cooling fractures that were developed in melt sheets of complex craters. A maximum model age of 103 (+200, −96) thousand years is derived for the global population of hollows, yielding an average growth rate >1,000 times that of previous estimations. Based on the geometry of the different morphological facies of all visible hollows on Mercury, the conservation of mass predicts a minimum volume of lost volatiles of 1,266 km3. We predict that an active rejuvenation of volatiles has occurred in the shallow crust of Mercury.
Plain Language Summary
Surprisingly, the surface of Mercury turns out to be volatile rich, and the loss of volatiles have formed the mysterious hollows. Reflectance spectra in visible‐to‐near‐infrared were frequently used to deduce the possible volatile species that formed hollows, but inconsistent interpretations widely existed. A major reason is that previously reported spectra are for mixtures of different morphological facies of hollows (i.e., bright haloes, hollow floors, and background terrains), but these facies have gone through of different degrees of devolatilization. Here we carefully extracted the spatially correlated reflectance spectra for different facies of hollows, showing that observed spectra cannot represent those of hollow‐forming volatiles. Some background terrains feature a weak absorption band at ~500–600 nm, suggesting that graphite might be the source volatile. For the global population of hollow, <10 m/pixel images reveal one potential impact crater on hollow floors, yielding a maximum model age of 103 thousand years. We further provide a solid constraint on the thickness of halo materials and estimate that the formation of the observable hollows exhausted > ~1,266 km3 volatiles. This volume is a substantial portion of the global budget of C and S in the shallow crust of Mercury, implying ongoing rejuvenation of volatiles in Mercury's recent history.
Key Points
Hollow floor and regolith on background terrain are two end‐members of hollow spectra, published spectra are for devolatilized materials
Global population of observable hollows are less than 103 kyr, >1,266 km3 of volatiles are lost, larger growth rate supports C as the volatile
Volatiles are lost from hollow walls and an active feeding system of volatiles should exist in the subsurface
The crust of the Moon records the complete history of collisions by different-sized projectiles from various sources since its early solidification. Planetary bodies in the inner Solar System ...experienced similar sources of impactors, and the Moon is an ideal witness plate for the impact history. Impact flux on the Moon connects planetary endogenic evolution with orbital dynamics of celestial bodies, and the resulting crater chronology enables remote age estimation for geological units on extraterrestrial bodies. Therefore, defining the lunar impact history has long been a core pursuit in planetary sciences. Ubiquitous impact structures on the Moon and their widespread impact melt deposits are the major agents used to untangle lunar crater chronology. Anchored by 10 successful sample return missions from the Moon, cumulative crater densities were derived for 15 geological units based on their interpreted exposure ages (~3.92 Ga to 25 Ma) and superposed crater densities. Afterword, crater production rates in the entire history of the Moon were constructed on the basis of hypothesized change patterns of impact flux. Following this commonly adapted strategy, it has been a consensus that impact flux in the first billion years of the lunar history was orders of magnitude larger than that afterward, and the latter was not only more or less stable but also punctuated by discrete spikes. However, different versions of lunar crater chronology exist because of insufficient constraints by available anchor points and widespread disagreements on both sample ages and crater densities of existing anchor points. Endeavors from various disciplines (e.g., sample analyses, remote observation, and modeling crater formation and accumulation) are making promising progresses, and future sample return missions with both optimized sampling strategy and analyzing techniques are appealed to fundamentally improve the understanding of lunar impact flux.
The Chang’E‐4 mission has been exploring the lunar farside. Two scientific targets of the rover onboard are (1) resolving the possible mineralogy related to the South Pole‐Aitken basin and (2) ...understanding the subsurface processes at the lunar farside. Publications to date that are based on the reflectance spectra and radar data obtained by the rover have shown a persistent inconsistency about the local stratigraphy. To explain both the abnormal surface topography at the landing site and the unexpected radargram observed by the rover, the Alder crater has been frequently reported to be older than the mare basalts at that landing site. However, this argument is not supported by earlier geological mapping nor recent crater statistics. Resolving this controversy is critical for a full understanding of the geological history of the landing area and for correct interpretations of the scientific data returned. Employing detailed crater statistics, rigorous statistical analyses, and an updated crater chronology function, this study is determined to resolve the relative ages of the Alder crater, Finsen crater, and the mare basalts on the floor of Von Kármán. Our results reveal that while background secondaries and local resurfacing have widely occurred in the study area, affecting age determinations, the statistics are significant enough to conclude that the Alder crater is the oldest among the three targets. This independent constraint is consistent with both the crosscutting relationships of different terrains in this area and global stratigraphic mapping. Our results exclude Alder as a possible contributor of the post‐mare deposits at the landing site, appealing for a more systematic stratigraphy study to resolve the provenances of these deposits.
Key Points
Detailed crater statistics were performed for key stratigraphic markers at the Chang’E‐4 landing region.
Alder is older than the mare basalts at the landing site, and Finsen is the youngest.
Minor contributions of ejecta by Alder should exist at the shallow subsurface.
The radar equipment carried by the Chang'E-3 (CE-3) mission marked the first deployment of rover-mounted ground-penetrating radar (GPR) to observe the lunar surface. This provided an unparalleled ...opportunity for a high-resolution investigation into the fine structure of the lunar regolith. This article has revealed the presence of multiple discrete layers within the top 4 m of the lunar regolith using high-frequency radar data from the CE-3 Yutu rover. Subsequently, we have established realistic models of the lunar regolith to obtain the radar simulation calculated by finite-difference time-domain (FDTD) technology. Thus, we compare the simulated radar data with actual observational data to comprehensively confirm the existence of multiple discrete layers within the lunar regolith. Taking into consideration the geological context of the CE-3 landing site and the principles of impact crater formation, we infer that the origin of the multiple discrete layers within the top 4 m of the CE-3 landing site is likely the by-product of multiple depositions of ejecta from nearby small craters. Our findings suggest the possibility of the widespread existence of multiple discrete layers within the lunar regolith and emphasize the significant contribution of ejecta from small impact craters to the accumulation of local lunar regolith thickness on the Moon's surface.
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