Susan J. Conway

Long‐lived explosive volcanism on Mercury

The duration and timing of volcanic activity on Mercury are key indicators of the thermal evolution of the planet and provide a valuable comparative example for other terrestrial bodies. The majority of effusive volcanism on Mercury appears to have occurred early in the planets geological history (~4.1–3.55 Ga), but there is also evidence for explosive volcanism. Here we present evidence that explosive volcanism occurred from at least 3.9 Ga until less than a billion years ago and so was substantially more long-lived than large-scale lava plains formation. This indicates that thermal conditions within Mercury have allowed partial melting of silicates through the majority of its geological history and that the overall duration of volcanism on Mercury is similar to that of the Moon despite the different physical structure, geological history, and composition of the two bodies.

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 planets 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.

The indication of Martian gully formation processes by slope–area analysis

Abstract The formation process of recent gullies on Mars is currently under debate. This study aims to discriminate between the proposed formation processes – pure water flow, debris flow and dry mass wasting – through the application of geomorphological indices commonly used in terrestrial geomorphology. High-resolution digital elevation models (DEMs) of Earth and Mars were used to evaluate the drainage characteristics of small slope sections. Data from Earth were used to validate the hillslope, debris-flow and alluvial process domains previously found for large fluvial catchments on Earth, and these domains were applied to gullied and ungullied slopes on Mars. In accordance with other studies, our results indicate that debris flow is one of the main processes forming the Martian gullies that were being examined. The source of the water is predominantly distributed surface melting, not an underground aquifer. Evidence is also presented indicating that other processes may have shaped Martian crater slopes, such as ice-assisted creep and solifluction, in agreement with the proposed recent Martian glacial and periglacial climate. Our results suggest that, within impact craters, different processes are acting on differently oriented slopes, but further work is needed to investigate the potential link between these observations and changes in Martian climate.

Decameter thick remnant glacial ice deposits on Mars

On Mars, a smooth, draping unit—the “latitude-dependant mantle” (LDM), believed to comprise meter thick layers of dust and ice—extends from the midlatitudes to the poles, covering at least 23% of the surface. We show that the LDM can be 30 m deep on pole-facing crater walls, and by measuring the erosional and depositional volumes of small gullies that incise these LDM deposits, we show that it must contain between 46% and 95% ice by volume. Extrapolating to a global scale, these deposits account for ~104 km3 of near-surface ice, doubling previous LDM volume estimates. Thick LDM deposits can be emplaced during the many orbital variation-driven climate excursions that occurred during the Amazonian period. We suggest that LDM deposits are similar to ice sheets composed of massive ice with a surface lag.

Earth-like aqueous debris-flow activity on Mars at high orbital obliquity in the last million years

Liquid water is currently extremely rare on Mars, but was more abundant during periods of high obliquity in the last few millions of years. This is testified by the widespread occurrence of mid-latitude gullies: small catchment-fan systems. However, there are no direct estimates of the amount and frequency of liquid water generation during these periods. Here we determine debris-flow size, frequency and associated water volumes in Istok crater, and show that debris flows occurred at Earth-like frequencies during high-obliquity periods in the last million years on Mars. Results further imply that local accumulations of snow/ice within gullies were much more voluminous than currently predicted; melting must have yielded centimetres of liquid water in catchments; and recent aqueous activity in some mid-latitude craters was much more frequent than previously anticipated.

Identifying Martian gully evolution

Abstract Martian gullies are small-scale, geologically recent features characterized by the alcove-channel-apron morphology associated with flows with a component of liquid water. Theories advanced to explain Martian gully formation include groundwater processes and melting of near-surface ice due to climate variation. Gullies are often associated with ‘mantling terrain’ that drapes topography at mid to high latitudes and which has been proposed to be ice-rich. We have morphologically classified Martian gullies into four groupings according to whether they form solely within the mantle (Type A), erode into ‘bedrock’ (Type B), and by how well developed they appear (1 or 2). Orientation, length, geological setting and latitude were also recorded, as well as whether more than one generation of gullies formed on a given slope (labelled ‘reactivated’). About 25% of gullies form solely within the mantle; these are generally shorter than gullies that erode bedrock and the morphologically simplest gullies (A1) are the shortest. We present latitude and orientation trends for the most recent episode of gully formation. We suggest that this recent activity is probably controlled by either deposition of ice-rich material or degradation of pre-existing ice-rich material.

Pressurized groundwater outflow experiments and numerical modeling for outflow channels on Mars

The landscape of Mars shows incised channels that often appear abruptly in the landscape, suggesting a groundwater source. However, groundwater outflow processes are unable to explain the reconstructed peak discharges of the largest outflow channels based on their morphology. Therefore, there is a disconnect between groundwater outflow processes and the resulting morphology. Using a combined approach with experiments and numerical modeling, we examine outflow processes that result from pressurized groundwater. We use a large sandbox flume, where we apply a range of groundwater pressures at the base of a layer of sediment. Our experiments show that different pressures result in distinct outflow processes and resulting morphologies. Low groundwater pressure results in seepage, forming a shallow surface lake and a channel when the lake overflows. At intermediate groundwater pressures, fissures form and groundwater flows out more rapidly. At even higher pressures, the groundwater initially collects in a subsurface reservoir that grows due to flexural deformation of the surface. When this reservoir collapses, a large volume of water is released to the surface. We numerically model the ability of these processes to produce floods on Mars and compare the results to discharge estimates based on previous morphological studies. We show that groundwater seepage and fissure outflow are insufficient to explain the formation of large outflow channels from a single event. Instead, formation of a flexure-induced subsurface reservoir and subsequent collapse generates large floods that can explain the observed morphologies of the largest outflow channels on Mars and their source areas.

Water induced sediment levitation enhances downslope transport on Mars

On Mars, locally warm surface temperatures (~293 K) occur, leading to the possibility of (transient) liquid water on the surface. However, water exposed to the martian atmosphere will boil, and the sediment transport capacity of such unstable water is not well understood. Here, we present laboratory studies of a newly recognized transport mechanism: “levitation” of saturated sediment bodies on a cushion of vapor released by boiling. Sediment transport where this mechanism is active is about nine times greater than without this effect, reducing the amount of water required to transport comparable sediment volumes by nearly an order of magnitude. Our calculations show that the effect of levitation could persist up to ~48 times longer under reduced martian gravity. Sediment levitation must therefore be considered when evaluating the formation of recent and present-day martian mass wasting features, as much less water may be required to form such features than previously thought.Downslope sediment transport on Mars is reported, but the transport capacity of unstable water under low pressures is not well understood. Here, the authors present a newly discovered, highly reactive transportation mechanism that is only possible under low pressure environments.

New slope-normalized global gully density and orientation maps for Mars

Abstract We reanalyse the global distribution of gullies in order to provide a set of observational constraints that models of gully formation must explain. We validate our results derived from the global data with four detailed case studies. We show that the availability of steep slopes is an essential factor to consider when assessing the spatial distribution and abundance of gullies. When the availability of steep slopes is taken into account, it reveals, with a few exceptions, that gullies are found almost uniformly across the whole 30°–90° latitude band. Our analysis also reveals that massive ice deposits are anti-correlated with gullies, and that the undulations in the equatorwards limits of the gully distribution could be explained by longitudinal variations in maximum surface temperatures (controlled by variations in surface properties, including thermal inertia and albedo). We find a sharp transition in both hemispheres between pole-facing gullies, which extend from 30° to 40°, to a more mixed, but dominantly equator-facing orientation of gullies polewards of 40°. We have no definitive explanation for this transition but, based on previous studies, we suggest it could be linked to the availability of near-surface ice deposits.

Time will tell: temporal evolution of Martian gullies and palaeoclimatic implications.

Abstract To understand Martian palaeoclimatic conditions and the role of volatiles therein, the spatiotemporal evolution of gullies must be deciphered. While the spatial distribution of gullies has been extensively studied, their temporal evolution is poorly understood. We show that gully size is similar in very young and old craters. Gullies on the walls of very young impact craters (less than a few myr) typically cut into bedrock and are free of latitude-dependent mantle (LDM) and glacial deposits, while such deposits become increasingly evident in older craters. These observations suggest that gullies go through obliquity-driven degradation–accumulation cycles over time, controlled by: (1) LDM emplacement and degradation; and (2) glacial emplacement and removal. In glacially-influenced craters, the distribution of gullies on crater walls coincides with the extent of glacial deposits, which suggests that the melting of snow and ice played a role in the formation of these gullies. Yet, present-day activity is observed in some gullies on formerly glaciated crater walls. Moreover, in very young craters, extensive gullies have formed in the absence of LDM and glacial deposits, showing that gully formation can also be unrelated to these deposits. The Martian climate varied substantially over time, and the gully-forming mechanisms are likely to have varied accordingly.