Ring-Moat Dome Structures (RMDSs) in the Lunar Maria:Statistical, Compositional, and Morphological Characterization and Assessment of Theories of Origin

Ring-moat dome structures (RMDSs) are positive morphologic features found clustered across many mare regions on the Moon, of which only a few isolated examples have been previously reported. Our continuing survey has expanded the known locations of the RMDSs from ~2,600 to over 8,000, indicating that RMDSs are more common geological features than previously thought. This work presents a detailed geomorphological analysis of 532 RMDSs identified in several mare basins. The combination of detailed elemental mapping, morphological and morphometric analyses, spatial distribution relationships with other geologic structures, and comparison with terrestrial analogs lead us to conclude that (1) RMDSs represent low circular mounds with diameters of a few hundred meters (average about 200 m) and a mean height of 3.5 m. The mounds are surrounded by moats ranging from tens to over 100 m in width and up to several meters in depth; (2) there is a wide variation of titanium abundances, although RMDSs are more commonly found in mare regions of moderate-to-high titanium content (>3 wt% TiO2); (3) RMDSs are found to occur on or around fractures, graben, and volcanic edifices (small shields and cones); (4) a spatial association between RMDSs and Irregular Mare Patches (see Braden et al., 2014, https://doi.org/10.1038/ngeo2252) is observed, suggesting that both may form from related lava flows; (5) comparisons between RMDSs and lava inflationary structures on Earth support an inflation-related extrusive nature and a genetic relationship with host lava flow processes; and (6) RMDS embayment relationships with craters of different degradation ages superposed on the host mare, and regolith development models, produces conflicting age relationships and divide theories of RMDS origin into two categories, (1) synchronous with the emplacement and cooling of the host lava flows ~3–4 Ga and (2) emplaced substantially after the host mare lava unit, in the period ~0–3 Ga. We outline the evidence supporting this age conundrum and implications for the different theories of origin and describe future research avenues to help resolve these outstanding questions. ©2020. American Geophysical Union. All Rights Reserved

The Lunar Mare Ring-Moat Dome Structure (RMDS) Age Conundrum:Contemporaneous With Imbrian-Aged Host Lava Flows or Emplaced in the Copernican?

Ring-moat dome structures (RMDSs) are small circular mounds of diameter typically about 200 m and ∼3–4 m in height, surrounded by narrow, shallow moats. They occur in clusters, are widespread in ancient Imbrian-aged mare basalt host units and show mineralogies comparable to those of their host units. Based on these close associations and similarities, a model has been proposed for the formation of RMDS as the result of late-stage flow inflation, with second boiling releasing quantities of magmatic volatiles that migrate to the top of the flow as magmatic foams and extrude through cracks in the cooled upper part of the flow to produce the small RMDS domes and surrounding moats. In contrast to this model advocating a contemporaneous emplacement of RMDSs and their host lava flows, a range of observations suggests that the RMDS formed significantly after the emplacement and cooling of their host lava flows, perhaps as recently as in the Copernican Period (∼1.1 Ga to the present). These observations include: (a) stratigraphic embayment of domes into post-lava flow emplacement impact craters; (b) young crater degradation age estimates for the underlying embayed craters; (c) regolith development models that predict thicknesses in excess of the observed topography of domes and moats; (d) landform diffusional degradation models that predict very young ages for mounds and moats; (e) suggestions of fewer superposed craters on the mounds than on the adjacent host lava flows, and (f) observations of superposed craters that suggest that the mound substrate does not have the properties predicted by the magmatic foam model. Together, these observations are consistent with the RMDS formation occurring during the period after the extrusion and solidification of the host lava flows, up to and including the geologically recent Late Copernican, that is, the last few hundreds of millions of years of lunar history. We present and discuss each of these contradictory data and interpretations and summarize the requirements for magma ascent and eruption models that might account for young RMDS ages. We conclude with a discussion of the tests and future research and exploration that might help resolve the RMDS age and mode of emplacement conundrum

What is the Oxygen Isotope Composition of Venus? The Scientific Case for Sample Return from Earth’s “Sister” Planet

Venus is Earth’s closest planetary neighbour and both bodies are of similar size and mass. As a consequence, Venus is often described as Earth’s sister planet. But the two worlds have followed very different evolutionary paths, with Earth having benign surface conditions, whereas Venus has a surface temperature of 464 °C and a surface pressure of 92 bar. These inhospitable surface conditions may partially explain why there has been such a dearth of space missions to Venus in recent years.The oxygen isotope composition of Venus is currently unknown. However, this single measurement (Δ17O) would have first order implications for our understanding of how large terrestrial planets are built. Recent isotopic studies indicate that the Solar System is bimodal in composition, divided into a carbonaceous chondrite (CC) group and a non-carbonaceous (NC) group. The CC group probably originated in the outer Solar System and the NC group in the inner Solar System. Venus comprises 41% by mass of the inner Solar System compared to 50% for Earth and only 5% for Mars. Models for building large terrestrial planets, such as Earth and Venus, would be significantly improved by a determination of the Δ17O composition of a returned sample from Venus. This measurement would help constrain the extent of early inner Solar System isotopic homogenisation and help to identify whether the feeding zones of the terrestrial planets were narrow or wide.Determining the Δ17O composition of Venus would also have significant implications for our understanding of how the Moon formed. Recent lunar formation models invoke a high energy impact between the proto-Earth and an inner Solar System-derived impactor body, Theia. The close isotopic similarity between the Earth and Moon is explained by these models as being a consequence of high-temperature, post-impact mixing. However, if Earth and Venus proved to be isotopic clones with respect to Δ17O, this would favour the classic, lower energy, giant impact scenario.We review the surface geology of Venus with the aim of identifying potential terrains that could be targeted by a robotic sample return mission. While the potentially ancient tessera terrains would be of great scientific interest, the need to minimise the influence of venusian weathering favours the sampling of young basaltic plains. In terms of a nominal sample mass, 10 g would be sufficient to undertake a full range of geochemical, isotopic and dating studies. However, it is important that additional material is collected as a legacy sample. As a consequence, a returned sample mass of at least 100 g should be recovered.Two scenarios for robotic sample return missions from Venus are presented, based on previous mission proposals. The most cost effective approach involves a “Grab and Go” strategy, either using a lander and separate orbiter, or possibly just a stand-alone lander. Sample return could also be achieved as part of a more ambitious, extended mission to study the venusian atmosphere. In both scenarios it is critical to obtain a surface atmospheric sample to define the extent of atmosphere-lithosphere oxygen isotopic disequilibrium. Surface sampling would be carried out by multiple techniques (drill, scoop, “vacuum-cleaner” device) to ensure success. Surface operations would take no longer than one hour.Analysis of returned samples would provide a firm basis for assessing similarities and differences between the evolution of Venus, Earth, Mars and smaller bodies such as Vesta. The Solar System provides an important case study in how two almost identical bodies, Earth and Venus, could have had such a divergent evolution. Finally, Venus, with its runaway greenhouse atmosphere, may provide data relevant to the understanding of similar less extreme processes on Earth. Venus is Earth’s planetary twin and deserves to be better studied and understood. In a wider context, analysis of returned samples from Venus would provide data relevant to the study of exoplanetary systems

Grooves of Phobos as Seen on Rectified Images Taken by the Mars Express High Resolution Stereo Camera

In this work, we analyze images taken by the HRSC camera onboard Mars Express to revisit the problem of the origin of Phobos’ grooves, numerous linear features, often turning to chains of small craters, which criss-cross its surface

Grooves of Phobos as seen on the MEX HRSC Rectified Images and Comparisons with Planetary Analogs

Here we analyze the HRSC images of Phobos revisiting a problem of origin of grooves, numerous linear features, often turning to chains of small craters. They were originally assumed to be fractures resulted of the large impacts or from tidal stresses. Also were suggested other hypotheses, the most discussed of which are a suggestion that the grooves were formed by rolling blocks of Phobos crater ejecta or they are chains of coalescing secondary craters formed by ejecta from large craters of Mars

Geologic interpretation of the near-infrared images of area SW of Beta Regio taken by the Venus Monitoring Camera

We analyze night-time near-infrared (NIR) images of Beta-Phoebe region obtained with the 1-μm channel of the Venus Monitoring Camera (VMC) onboard Venus Express. Comparisons with the results of the Magellan radar survey and the model NIR images show that the night-time VMC images provide reliable information on spatial variations of the NIR surface emission. Here we consider if tessera terrain has the different NIR emissivity (and thus mineralogical composition) in com- parison to the surrounding basaltic plains. This is done through the study of an area SW of Beta Regio where there is a massif of tessera terrain, Chimon-mana Tessera, surrounded by supposedly basaltic plains. Our analysis showed that 1-μm emissivity of tessera surface material is by 15 – 35 % lower than that of relatively fresh suppos- edly basaltic lavas of plains and volcanic edifices. This is consistent with hypothesis that the tessera material is not basaltic, maybe felsic, that is in agreement with the results of analyses of VEX VIRTIS and Galileo NIMS data. If the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of Venus. We have found that the surface materials of plains in the study area are very variegated in their 1-μm emissivity, which probably reflects variability of degree of their chemical weathering. We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit