Constructional Volcanic Edifices on Mercury: Candidates and Hypotheses of Formation

Mercury, a planet with a predominantly volcanic crust, has perplexingly few, if any, constructional volcanic edifices, despite their common occurrence on other solar system bodies with volcanic histories. Using image and topographical data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, we describe two small (< 15 km‐diameter) prominences with shallow summit depressions associated with volcanically flooded impact features. We offer both volcanic and impact‐related interpretations for their formation, and then compare these landforms with volcanic features on Earth and the Moon. Though we cannot definitively conclude that these landforms are volcanic, the paucity of constructional volcanic edifices on Mercury is intriguing in itself. We suggest that this lack is because volcanic eruptions with sufficiently low eruption volumes, rates, and flow lengths, suitable for edifice construction, were highly spatiotemporally restricted during Mercury's geological history. We suggest that volcanic edifices may preferentially occur in association with late‐stage, post‐impact effusive volcanic deposits. The ESA/JAXA BepiColombo mission to Mercury will be able to investigate further our candidate volcanic edifices, search for other, as‐yet unrecognized edifices beneath the detection limits of MESSENGER data, and test our hypothesis that edifice construction is favored by late‐stage, low‐volume effusive eruptions

Prolonged eruptive history of a compound volcano on Mercury: volcanic and tectonic implications

A 27 × 13 km ‘rimless depression’ 100 km inside the southwest rim of the Caloris 19 basin is revealed by high resolution orbital imaging under a variety of illuminations to 20 consist of at least nine overlapping volcanic vents, each individually up to 8 km in 21 diameter. It is thus a ‘compound’ volcano, indicative of localised migration of the site 22 of the active vent. The vent floors are at a least 1 km below their brinks, but lack the 23 flat shape characteristically produced by piston-like subsidence of a caldera floor or 24 by flooding of a crater bottom by a lava lake. They bear a closer resemblance to 25 volcanic craters sculpted by explosive eruptions and/or modified by collapse into void 26 spaces created by magma withdrawal back down into a conduit. This complex of 27 overlapping vents is at the summit of a subtle edifice at least 100 km across, with 28 flank slopes of about only 0.2 degrees, after correction for the regional slope. This is 29 consistent with previous interpretation as a locus of pyroclastic eruptions. 30 Construction of the edifice could have been contributed to by effusion of very low 31 viscosity lava, but high resolution images show that the vent-facing rim of a nearby 32 impact crater is not heavily embayed as previously supposed on the basis of lower 33 resolution fly-by imaging. Contrasts in morphology (sharpness versus blurredness of 34 the texture) and different densities of superposed sub-km impact craters inside each 35 vent are consistent with (but do not prove) substantial differences in the age of the 36 most recent activity at each vent. This suggests a long duration of episodic 37 magmagenesis at a restricted locus. The age range cannot be quantified, but could be 38 of the order of a billion years. If each vent was fed from the same point source, 39 geometric considerations suggest a source depth of at least 50 km. However, the 40 migration of the active vent may be partly controlled by a deep-seated fault that is 41 radial to the Caloris basin. Other rimless depressions in this part of the Caloris basin 42 fall on or close to radial lines, suggesting that elements of the Pantheon Fossae radial 43 fracture system that dominates the surface of the central portion of the Caloris basin 44 may continue at depth almost as far as the basin rim

Hollows on Mercury: materials and mechanisms involved in their formation

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking. In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles

Mechanisms of explosive volcanism on Mercury: implications from its global distribution and morphology

The identification of widespread pyroclastic vents and deposits on Mercury has important implications for the planet's bulk volatile content and thermal evolution. However, the significance of pyroclastic volcanism for Mercury depends on the mechanisms by which the eruptions occurred. Using images acquired by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse-edged spectral anomaly, a configuration previously used to identify sites of explosive volcanism. We find that these sites cluster at the margins of impact basins and along regional tectonic structural trends. Locally, pits and deposits are usually associated with zones of weakness within impact craters and/or with the surface expressions of individual thrust faults. Additionally, we use images and stereo-derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider, thinner deposit. These eruptions were therefore likely driven by a relatively high concentration of volatiles, consistent with volatile concentration in a shallow magma chamber prior to eruption. The colocation of sites of explosive volcanism with near-surface faults and crater-related fractures is likely a result of such structures acting as conduits for volatile and/or magma release from shallow reservoirs, with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases. Our findings suggest that widespread, long-lived explosive volcanism on Mercury has been facilitated by the interplay between impact cratering, tectonic structures, and magmatic fractionation

On the asymmetry of Nathair Facula, Mercury

Nathair Facula is the largest and most spectrally distinct of nearly 200 ‘bright red’ spots (faculae) on Mercury’s surface, most of which are accepted to be deposits from explosive volcanic eruptions. Like most of Mercury’s faculae, it hosts a non-circular central pit (in this case nearly 40 km wide and 3 km deep). However, the center of this facula does not coincide with its central pit’s midpoint. Quantitative analysis of two sets of spectral data shows that the facula’s midpoint is offset by 10-30 km northwards or northeastwards, and probably lies outside the pit. The pit area is almost certainly a ‘compound vent’, within which the locus of eruption has migrated between eruptive episodes. The asymmetry of the facula and the texture of the vent floor are consistent with the most energetic and/or the most recent eruptions having occurred from the northeastern part of the compound vent, but evidence that the center point of the facula lies outside the vent indicates that it may be necessary to invoke an additional factor such as asymmetric eruption fountains

Modification of Caloris ejecta blocks by long-lived mass-wasting: A volatile-driven process?

The Caloris basin is the largest well-preserved impact basin on Mercury. As such, Caloris ejecta afford us an opportunity to study material from Mercury’s deep interior with remote sensing. We have made observations of the geomorphology, colour, distribution, and flank slopes of the circum-Caloris knobs. Our observations suggest that these circum-Caloris knobs are modified ejecta blocks from the Caloris impact. High-resolution MESSENGER images show that knobs are conical and relatively uncratered compared with the surrounding plains, which implies the knobs have undergone resurfacing. We have observed material that has sloughed off knobs superposing impact craters that demonstrably postdate the Caloris impact, which requires some knob modification to have been more recent. We have observed hollows, depressions in Mercury’s surface generally believed to have been caused by volatile-loss, on and closely associated with several knobs, which indicates that many knobs contain volatile material and that knob modification could extend into Mercury’s recent past. Our measurements show that knob flanks typically have slopes of ∼21°, which is steep for a mound of unconsolidated material that was originally emplaced ∼3.8 Ga. The conical shape of knobs, their steep slopes, the dearth of superposed craters on knobs, and knob superposition relationships with other landforms suggest that Caloris ejecta blocks of arbitrary original shape were modified into their present shapes by long-lived mass-wasting. Mass-wasting must have dominated over impact gardening, which would have produced domal morphologies only. We suggest that mass-wasting was probably driven by volatile-loss, in a manner analogous to terrestrial landforms called ‘molards’. If the circum-Caloris knobs are analogous to molards, then they represent a landform and a process hitherto undocumented on Mercury, with implications for the volatile content of the planet’s interior. These knobs therefore are prime targets for BepiColombo, which could search for fresh failures and volatile exposures in the knobs

Planet Mercury: Volcanism in a theatre of global contraction, with examples from the Hokusai quadrangle

Mercury's geological history has been dominated by global contraction caused by secular cooling of the planet's interior. This cooling has had a profound effect on the expression of the planet's volcanism and tectonism, and the expressions of these two surface evolutionary processes are deeply intertwined. Here, we use case studies from the Hokusai quadrangle of Mercury to gain insight into the interplay between Mercury's volcanism and tectonism, which we review throughout this paper. We perform the first crater size–frequency analysis of the southernmost extent of Borealis Planitia, Mercury's largest expanse of volcanic plains, and find that it formed ~3.8–3.7 Ga. We discuss the importance of “intermediate plains”, a widespread unit in the Hokusai quadrangle, as the manifestation of relatively low-volume effusions with an uncertain stratigraphic relationship with Borealis Planitia. Finally, we detail the formation of the Suge Facula pitted ground during the geological history of Rachmaninoff crater, and hypothesise that such textures probably formed more widely on Mercury but have often either been buried by thick lava flows or otherwise obscured. Unanswered questions in this work can be used to drive the next phase of Mercury exploration and research with the arrival of the BepiColombo mission

Map of tectonic shortening structures in Chryse Planitia and Arabia Terra, Mars

We present a 1:4,000,000 scale map of tectonic landforms in Chryse Planitia and Arabia Terra, on either side of Mars’ dichotomy. Our study area is a ∼3 million km2 region, transitional between Mars’ highlands and lowlands including Oxia Planum, the landing site of the ExoMars rover. Using a structural mapping approach, we digitised all kilometre-scale tectonic structures at a scale of 1:50,000 using high-resolution data (∼6 m/pixel). Although this region is represented as sparsely tectonised on global tectonic maps, we find evidence of widespread tectonic shortening structures across the region. The shortening structures have a dominant N-S orientation and occur in all globally identified geological units. The structural map contributes to a broader understanding of the geological history of the region and Mars’ wider tectonic history

Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage

We used the instantaneous growth rate method to determine the effects of food, temperature, krill length, sex, and maturity stage on in situ summer growth of krill across the southwest Atlantic sector of the Southern Ocean. The main aims were to examine the separate effects of each variable and to generate a predictive model of growth based on satellite-derivable environmental data. Both growth increments in length on moulting (GIs) and daily growth rates (DGRs, mm d-1) ranged greatly among the 59 swarms, from 0.58–15% and 0.013–0.32 mm d-1. However, all swarms maintained positive mean growth, even those in the low chlorophyll a (Chl a) zone of the central Scotia Sea. Among a suite of indices of food quantity and quality, large-scale monthly Chl a values from SeaWiFS predicted krill growth the best. Across our study area, the great contrast between bloom and nonbloom regions was a major factor driving variation in growth rates, obscuring more subtle effects of food quality. GIs and DGRs decreased with increasing krill length and decreased above a temperature optimum of 0.5°C. This probably reflects the onset of thermal stress at the northern limit of krill’s range. Thus, growth rates were fastest in the ice edge blooms of the southern Scotia Sea and not at South Georgia as previously suggested. This reflects both the smaller size of the krill and the colder water in the south being optimum for growth. Males tended to have higher GIs than females but longer intermoult periods, leading to similar DGRs between sexes. DGRs of equivalent-size krill tended to decrease with maturity stage, suggesting the progressive allocation of energy toward reproduction rather than somatic growth. Our maximum DGRs are higher than most literature values, equating to a 5.7% increase in mass per day. This value fits within a realistic energy budget, suggesting a maximum carbon ration of ~20% d-1. Over the whole Scotia Sea/South Georgia area, the gross turnover of krill biomass was ~1% d-1

Widespread small grabens consistent with recent tectonism on Mercury

Since Mariner 10 first imaged Mercury in 1974, tectonic landforms dominated by shortening structures have been extensively documented. Contractional tectonism on Mercury is thought to have begun early in the planet’s history and is theorized to have continued throughout Mercury’s geological history, but observational evidence for recent tectonism is limited. Here we report the widespread occurrence of relatively young grabens on Mercury from global mapping of tectonic features using MESSENGER imagery. The identified grabens are about 10 to 150 m deep, tens of kilometres in length and generally less than 1 km wide. We find that the grabens occur as secondary tectonic features on larger compressional tectonic structures, which indicates continued activity of the parent structure. We estimate that they must be ~300 million years old or younger; otherwise, impact gardening would have masked their signature by burial and infilling. The widespread distribution of grabens and their young age supports the continued activity of Mercury’s shortening structures into geologically recent times and is consistent with thermochemical evolution models for a slowly cooling planetary interior and prolonged global contraction