Fracture toughness measurements on igneous rocks using a high-pressure, high-temperature rock fracture mechanics cell

A sound knowledge of mechanical properties of rocks at high temperatures and pressures is essential for modelling volcanological problems such as fracture of lava flows and dike emplacement. In particular, fracture toughness is a scale-invariant material property of a rock that describes its resistance to tensile failure. A new fracture mechanics apparatus has been constructed enabling fracture toughness measurements on large (60 mm diameter) rock core samples at temperatures up to 750–C and pressures up to 50 MPa. We present a full description of this apparatus and, by plotting fracture resistance as a function of crack length, show that the size of the samples is sufficient for reliable fracture toughness measurements. A series of tests on Icelandic, Vesuvian and Etnean basalts at temperatures from 30 to 600–C and confining pressures up to 30 MPa gave fracture toughness values between 1.4 and 3.8 MPa m1=2. The Icelandic basalt is the strongest material and the Etnean material sampled from the surface crust of a lava flow the weakest. Increasing temperature does not greatly affect the fracture toughness of the Etnean or Vesuvian material but the Icelandic samples showed a marked increase in toughness at around 150–C, followed by a return to ambient toughness levels. This material also became tougher under moderate confining pressure but the other two materials showed little change in toughness. We describe in terms of fracture mechanics probable causes for the changes in fracture toughness and compare our experimental results with values obtained from dike propagation modelling found in the literature

The internal structure of a debris-covered glacier on Mars revealed by gully incision

Viscous flow features (VFFs) in Mars' mid latitudes are analogous to debris-covered glaciers on Earth. They have complex, often curvilinear patterns on their surfaces, which probably record histories of ice flow. As is the case for glaciers on Earth, patterns on the surfaces of VFFs are likely to reflect complexities in their subsurface structure. Until now, orbital observations of VFF-internal structures have remained elusive. We present observations of internal structures within a small, kilometer-scale VFF in the Nereidum Montes region of Mars' southern mid latitudes, using images from the Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) instruments on Mars Reconnaissance Orbiter. The VFF-internal structures are revealed by a gully incision, which extends from the VFF headwall to its terminus and intersects curvilinear undulations and a crevasse field on the VFF surface. Near to the VFF terminus, the curvilinear VFF-surface undulations connect to the VFF-internal layers, which are inclined and extend down to the VFF's deep interior, and possibly all the way to the bed. Similar structures are common near to the termini of glaciers on Earth; they form under ice flow compression where ice thins and slows approaching the ice margin, and ice flow is forced up towards the surface. We performed 3D ice flow modeling which supports this analogy, revealing that the inclined VFF-internal structures, and associated curvilinear structures on the VFF surface, are located in a zone of strong ice flow compression where ice flow deviates upwards away from the bed. The inclined VFF-internal structures we observe could represent up-warped VFF-internal layering transported up to the surface from the VFF's deep interior, or thrust structures representing debris transport pathways between the VFF's bed and its surface. Our observations raise numerous considerations for future surface missions targeting Mars' mid-latitude subsurface ice deposits. Inclined layers formed under flow compression could reduce the requirement for high-cost, high-risk deep drilling to address high-priority science questions. They could allow futures missions to (a) access ice age sequences for palaeoenvironmental reconstruction via shallow sampling along transects of ice surfaces where layers of progressively older age outcrop, and/or (b) access samples of ice/lithics transported to shallow/surface positions from environments of astrobiological interest at/near glacier beds. However, our observations also raise considerations for potential drilling hazards associated with structural complexities and potential dust/debris layers within subsurface ice deposits on Mars. They highlight the importance of characterizing VFF-surface structures, and their relationships to VFF-internal structure and ice flow histories ahead of ice access missions to Mars

Clastic Polygonal Networks Around Lyot Crater, Mars: Possible Formation Mechanisms From Morphometric Analysis

Polygonal networks of patterned ground are a common feature in cold-climate environments. They can form through the thermal contraction of ice-cemented sediment (i.e. formed from fractures), or the freezing and thawing of ground ice (i.e. formed by patterns of clasts, or ground deformation). The characteristics of these landforms provide information about environmental conditions. Analogous polygonal forms have been observed on Mars leading to inferences about environmental conditions. We have identified clastic polygonal features located around Lyot crater, Mars (50°N, 30°E). These polygons are unusually large (> 100 m diameter) compared to terrestrial clastic polygons, and contain very large clasts, some of which are up to 15 metres in diameter. The polygons are distributed in a wide arc around the eastern side of Lyot crater, at a consistent distance from the crater rim. Using high-resolution imaging data, we digitised these features to extract morphological information. These data are compared to existing terrestrial and Martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms. Our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted, clastic polygons formed by freeze-thaw processes. They are too large, do not show the expected variation in form with slope, and have clasts that do not scale in size with polygon diameter. However, the clastic networks are similar in network morphology to thermal contraction cracks, and there is a potential direct Martian analogue in a sub-type of thermal contraction polygons located in Utopia Planitia. Based upon our observations, we reject the hypothesis that polygons located around Lyot formed as freeze-thaw polygons and instead an alternative mechanism is put forward: they result from the infilling of earlier thermal contraction cracks by wind-blown material, which then became compressed and/or cemented resulting in a resistant fill. Erosion then leads to preservation of these polygons in positive relief, while later weathering results in the fracturing of the fill material to form angular clasts. These results suggest that there was an extensive area of ice-rich terrain, the extent of which is linked to ejecta from Lyot crater

Measuring Ripple and Dune Migration in Coprates Chasma, Valles Marineris: A Source to Sink Aeolian System on Mars?

Active aeolian systems are present across the martian surface, with active dune fields commonly found in depressions such as craters and valleys. There are many dune fields within the equatorial region of Mars, including Valles Marineris, but the effects of topography on wind regimes and consequently dune migration in Valles Marineris is poorly understood. We investigated both the ripple and dune migration in a dune field in Coprates Chasma using High Resolution Science Experiment Images (HiRISE) and Context Camera (CTX) images. Migration rates of dune brinks and ripples were measured over varying time scales, between Earth years 2007‐2014. The dunes here are some of the tallest on Mars, with heights of up to 182 m. The dune brinks are migrating eastwards at a rate of 0.1‐0.3 m/EY through the valley due to topographical influences on the local winds. Potential sediment sources for the dune field were identified and investigated by studying thermal inertia and mineralogy. The topographic slope‐related katabatic winds travel down the valley walls and converge with the dominant winds travelling through the center of the valley, causing overall eastwards dune migration. Topography is likely the dominant control on the local wind regime; slope winds travel down the sides of the valley walls and are funneled through the center of the valley. These local winds subsequently facilitate the migration of the large dunes in Coprates Chasma, thus expanding our understanding of local winds in the martian environment

Field measurements of horizontal forward motion velocities of terrestrial dust devils: towards a proxy for ambient winds on Mars and Earth

Dust devils – convective vortices made visible by the dust and debris they entrain – are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites. We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a hundred active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements. Daily (10:00 to 16:00 local time) and two-hour averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-minute smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction. The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in-situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain

Aqueous dune-like bedforms in Athabasca Valles and neighbouring locations utilized in palaeoflood reconstruction

Putative fluvial dunes have been identified within the Athabasca Valles and associated network of channels on Mars. Previous published work identified and measured bedforms in Athabasca Valles using photoclinometry methods on 2–3 m/pixel resolution Mars Orbiter Camera Narrow Angle images, and argued that these were created by an aqueous megaflood that occurred between 2 and 8 million years ago. This event is likely to have occurred due to geological activity associated with the Cerberus Fossae fracture system at the source of Athabasca Vallis. The present study has used higher resolution, 25 cm/pixel images from the Mars Reconnaissance Orbiter HiRISE camera, as well as stereo-derived digital terrain models and GIS software, to re-measure and evaluate these bedforms together with data from newly discovered neighbouring fields of bedforms. The analysis indicates that the bedforms are aqueous dunes, in that they occur in channel locations where dunes would be expected to be preserved and moreover they have geometries very similar to megaflood dunes on Earth. Dune geometries are used to estimate megaflood discharge rates, including uncertainty, which results support previous flood estimates that indicate that a flood with a discharge of ∼2 × 106m3s−1 created these bedforms

Sinuous ridges and the history of fluvial and glaciofluvial activity in Chukhung Crater, Tempe Terra, Mars

International audience<p>We explore the origins of a complex assemblage of sinuous ridges in Chukhung crater (38.47°N, 72.42°W), Tempe Terra, Mars, and discuss the implications of the landsystem for post-Noachian fluvial and glaciofluvial activity in this location [1].</p><p>We produced a geomorphic map of Chukhung crater using a basemap of 6 m/pixel Context Camera (CTX) images and a 75 m/pixel High Resolution Stereo Camera digital elevation model (DEM). We used 25 cm/pixel High Resolution Imaging Science Experiment images, and a 24 cm/pixel DEM generated from CTX stereopair images [2] to aid classifications of sinuous ridges into four morpho-stratigraphic subtypes. We constrained an age envelope of ~2.1–3.6 Ga for Chukhung crater using modelled ages (from crater size-frequency analyses) of units above and below it in the regional stratigraphy. We derived a minimum model age of ~330 Ma for viscous flow features (putative debris-covered glaciers) in southern Chukhung crater.</p><p>Sinuous ridges in southern Chukhung crater emerge from moraine-like deposits associated with the debris-covered glaciers. Sinuous ridges in northern Chukhung crater extend from dendritic fluvial valley networks on the crater wall. The northern sinuous ridges are most likely to be inverted palaeochannels, which comprise subaerial river sediments exhumed as ridges by erosion of surrounding materials.</p><p>Both sinuous ridge subtypes in southern Chukhung crater have numerous esker-like properties. Eskers are ridges of glaciofluvial sediment deposited in meltwater tunnels within or beneath glacial ice. One of the ridge subtypes in southern Chukhung crater is best explained as eskers because these ridges ascend the sides of their host valleys and, in places, escape over them onto adjacent plains. Post-depositional processes can cause inverted paleochannels to cross local undulations in the contemporary topography [3] but the ascent and escape over larger, pre-existing topographic divides is (as yet) not adequately explained by these mechanisms. Eskers, in contrast, form under hydraulic pressure in ice-confined tunnels, and commonly ascend valley walls and cross topographic divides. The esker-like properties of the second sinuous ridge subtype in southern Chukhung crater can also be explained under the inverted palaeochannel hypothesis so the origins of these ridges remain more ambiguous.</p><p>Chukhung crater has undergone protracted and/or episodic modification by liquid water since its formation between the early Hesperian and early Amazonian. This falls after the Noachian period (>3.7 Ga), when most major fluvial activity on Mars occurred. Esker-forming wet-based glaciation in Chukhung crater might have occurred as recently as the mid Amazonian (>330 Ma), when climate conditions are thought to have been cold and hyper-arid. Rare occurrences of eskers associated with Amazonian-aged glaciers in Mars’ mid-latitudes are attributed to transient, localised geothermal heating within tectonic rift/graben settings [4]. The location of Chukhung crater between major branches of the large Tempe Fossae volcano-tectonic rift system is consistent with this hypothesis.</p><p>References: [1] Butcher et al. 2021, Icarus 357, 114131. [2] Mayer and Kite 2016, Lunar Planet. Sci. Conf. Abstract #1241. [3] Lefort et al. 2012, J. Geophys. Res. Planets 117, E03007. [4] Butcher et al. 2017, J. Geophys. Res. Planets 122, 2445–2468.</p&gt

A branching, positive relief network in the middle member of the Medusae Fossae Formation, Equatorial Mars - evidence for sapping?

The Medusae Fossae Formation (MFF) is a geological formation comprising three geological units (members) spread across five principal lobes. It dominates a quarter of the longitudinal extent of the equatorial region of Mars. Positive relief features referred to as ‘sinuous ridges' (commonly interpreted as inverted paleoflow channel or valley fills) have been observed in the lowest member of the western MFF, but have not been identified within the central and eastern portions of the formation, in the middle and upper members. This paper presents the identification and analysis of a branching, positive relief system which occurs in the central lobe of the MFF in what appears to be an exposure of the middle member. A simple geomorphological map of the system is presented, from which we have adopted the working hypothesis that this is an inverted fill of a branching fluvial channel or valley system. A suite of morphological and topographic evidence supporting this hypothesis is presented, including analysis of the network using a~15 m per pixel digital terrain model derived from a Context Imager (CTX) stereo image pair. The evidence supporting this hypothesis includes: 1) The local slope and topography of the upper surface of the network are consistent with a contributory network, 2) The braided, fan-like form at the termination of the branching network is consistent in morphology with it being a depositional fan at the end of a fluvial system, 3) The terminal fan and surrounding deposits show layering and polygonization, 4) There is strong association between the lower order branches and amphitheater shaped scarps in the depression walls. We evaluate the possible origins of this fluvial system and suggest that seepage sapping is the most probable. Two possible models for the evolution of the network and related features are presented; both require melt of ice within the MFF to form liquid water. We conclude that at least some portions of the Medusae Fossae Formation, if not the entire formation, were once volatile-rich. Finally, we note that our observations do not rule out the case that this network formed before MFF emplacement, and has since been exhumed. However, this conclusion would suggest that much of the surrounding terrain, currently mapped as middle-member MFF, is not in fact MFF material at all

Eskers associated with buried glaciers in Mars' mid latitudes: recent advances and future directions

Until recently, the influence of basal liquid water on the evolution of buried glaciers in Mars' mid latitudes was assumed to be negligible because the latter stages of Mars' Amazonian period (3 Ga to present) have long been thought to have been similarly cold and dry to today. Recent identifications of several landforms interpreted as eskers associated with these young (100s Ma) glaciers calls this assumption into doubt. They indicate basal melting (at least locally and transiently) of their parent glaciers. Although rare, they demonstrate a more complex mid-to-late Amazonian environment than was previously understood. Here, we discuss several open questions posed by the existence of glacier-linked eskers on Mars, including on their global-scale abundance and distribution, the drivers and dynamics of melting and drainage, and the fate of meltwater upon reaching the ice margin. Such questions provide rich opportunities for collaboration between the Mars and Earth cryosphere research communities