Small scale tectonism on Venus: An experimental and image based study

Closely spaced parallel lineations in the plains of Venus are interpreted as extensional rubble-filled fractures because they show no structure and are radar-bright irrespective of look angle. Their formation was investigated using a new methodology which combines material science and fracture mechanics principles together with experimental measurement and the analysis of Magellan data. Mapping in the Guinevere and Sedna Planitia regions shows that the closely spaced parallel fractures (CSPF) follow a concentric pattern around the edge of the large topographic rise of Western Eistla Regio. 13 spacing profiles show that most of the CSPF have spacings of between 0.8 and 1.2km. Using a new fracture mechanics apparatus designed to simulate Venusian surface conditions (90bar of CO2, 450°C), the fracture toughness of basalt was measured from atmospheric to 200 bar confining pressure and from room temperature to 600°C. 1 /2 Fracture toughness was found to increase from -2.4 MPam1/2 at ambient pressure to about -3.0 MPam1/2 at 50 bar confining pressure. Higher confining pressures have no further effect. Fracture toughness shows no clear trend with temperature, rising from an ambient level of -2.4MPam1/2 to -3.0MPa1/2 at 150°C and returning to -2.4MPam1/2 at higher temperatures. A new, two-dimensional model based upon fracture mechanics is described. The depth of the CSPF is controlled by the stress intensity factor but their spacing is controlled by the initiation of new cracks. Application of a faulting criterion to limit the conditions under which the CSPF can form shows that the spacing is consistent with a regional tensile stress of 5.5-8.5MPa. This stress could have resulted from uplift of Western Eistla Regio by -2km

Geological mapping of the Hokusai (H05) quadrangle of Mercury: Status update

[Introduction] MESSENGER data are being used to construct ~1:3M scale quadrangle geological maps of Mercury. Here, we present our progress mapping the Hokusai (H05) quadrangle

Post-deposition (and ongoing?) modification of Caloris ejecta blocks

Mercury’s circum-Caloris region hosts numerous kilometer-scale knobs. If these land-forms, peculiar to Caloris, are its ejecta, then they can provide insight into the deep materials of the planet

Wind-Stress Dust Lifting in a Mars Global Circulation Model: Representation across Resolutions

The formation of Martian dust storms is believed to be driven by dust lifting by near-surface wind stress (NSWS). Accurately representing this dust lifting within Mars Global Circulation Models (MGCMs) is important in order to gain a full understanding of the Martian dust storm cycle. Parameterisations of dust lifting by NSWS exist within several MGCMs; implementations differ but they all follow a similar design, so progress within one model is relevant to the entire field. Few studies have explored in detail how the results of these parameterisations can be affected by changing the horizontal resolution of the model. An accurate parameterisation of dust lifting by NSWS will lift a representative dust mass, reproducing characteristic dust optical depths in the atmosphere. The geographical distribution of the dust lifting by NSWS will also change throughout the year, affecting patterns of dust storm formation and development. Currently, suitable values for dust lifting parameters must be identified at every new model resolution. Resolutions of ~5° latitude x ~5° longitude are often used to model the Martian climate, as thermal tides and long-term weather patterns can be well represented at this resolution. However, smaller scale phenomena (such as near-surface winds driven by local topography) cannot be accurately depicted at this resolution. We use the LMD-UK MGCM to complete multi-year simulations across multiple model resolutions. Our experiments range from ‘low’ resolution ~5° lat x ~5° lon to ‘high’ resolution ~1° lat x ~1° lon. In experiments with fixed, constant lifting parameters, we find that higher resolution simulations lift more dust, but that this trend is asymptotic. At low resolutions, dust lifting increases proportionately with the increase in number of horizontal gridboxes. However, at high resolutions, doubling the number of gridboxes results only in a 30% increase in the total dust mass lifted. Geographical and temporal distributions of dust lifting are investigated, as well as the total dust lifted, in order to assess the optimum parameters for each resolution, and to develop a calibration scheme for this dust lifting across model resolutions. The scheme is verified through comparison with spacecraft observations of dust optical depths and dust storm locations

How Do Martian Dust Devils Vary Throughout the Sol?

Dust devils are vortices of air made visible by entrained dust particles. Dust devils have been observed on Earth and captured in many Mars lander and orbiter images. Martian dust devils may be important to the global climate and are parameterised within Mars Global Circulation Models (MGCMs). We show that the dust devil parameterisation in use within most MGCMs results in an unexpectedly high level of dust devil activity during morning hours. In contrast to expectations, based on the observed behaviour of terrestrial dust devils and the diurnal maximum thermal contrast at the surface, we find that large areas of the modelled Martian surface experience dust devil activity during the morning as well as in the afternoon, and that many locations experience a peak in dust devil activity before mid-sol. Using the UK MGCM, we study the amount of surface dust lifted by dust devils throughout the diurnal cycle as a proxy for the level of dust devil activity occurring. We compare the diurnal variation in dust devil activity with the diurnal variation of the variables included in the dust devil parameterisation. We find that the diurnal variation in dust devil activity is strongly modulated by near-surface wind speeds. Within the range of daylight hours, higher wind speeds tend to produce more dust devil activity, rather than the activity simply being governed by the availability of heat at the planet's surface, which peaks in early afternoon. We compare our results with observations of Martian dust devil timings and obtain a good match with the majority of surface-based surveys. We do not find such a good match with orbital observations, but these data tend to be biased in their temporal coverage. We propose that the generally accepted description of dust devil behaviour on Mars is incomplete, and that theories of dust devil formation may need to be modified specifically for the Martian environment. Further dust devil observations are required to support any such modifications

Geological Mapping of the Debussy Quadrangle (H-14) - Preliminary Results

Geological mapping of Mercury is essential to build an understanding of the history of the planet and to set the context for observations by BepiColombo [1]. Geological mapping of the Debussy quadrangle (H-14) is now underway as part of a program to map the entire planet at a scale of 1:3M using MESSENGER data [2]. The quadrangle is located in the southern hemisphere of Mercury at 0o – 90o E and 22.5o – 65o S. This will be the first high resolution map of the quadrangle as it was not previously imaged by Mariner 10

Regolith-atmosphere exchange of water in Mars' recent past

We investigate the exchange of water vapour between the regolith and atmosphere of Mars, and how it varies with different orbital parameters, atmospheric dust contents and surface water ice reservoirs. This is achieved through the coupling of a global circulation model (GCM) and a regolith diffusion model. GCM simulations are performed for hundreds of Mars years, with additional one-dimensional simulations performed for 50 kyr. At obliquities ε = 15° and 30°, the thermal inertia and albedo of the regolith have more control on the subsurface water distribution than changes to the eccentricity or solar longitude of perihelion. At ε = 45°, atmospheric water vapour abundances become much larger, allowing stable subsurface ice to form in the tropics and mid-latitudes. The circulation of the atmosphere is important in producing the subsurface water distribution, with increased water content in various locations due to vapour transport by topographically-steered flows and stationary waves. As these circulation patterns are due to topographic features, it is likely the same regions will also experience locally large amounts of subsurface water at different epochs. The dustiness of the atmosphere plays an important role in the distribution of subsurface water, with a dusty atmosphere resulting in a wetter water cycle and increased stability of subsurface ice deposits

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

Comparison of Global-Scale and Mesoscale Modelling of Vertical Profiles in the Martian Atmosphere: How Does Model Resolution Impact Predictions of Conditions at Mission Landing Sites?

Detailed modelling of the Martian atmosphere is completed for every spacecraft designed to land on the planet’s surface. This provides the most complete picture of the environment that the descending module will be entering and travelling through, and facilitates planning of the Entry, Descent and Landing (EDL) phase of the mission. The selected resolution of an atmospheric model can impact the results of the experiments performed. The complexities of atmospheric modelling also require models of different scales to best represent the behaviour of different scale atmospheric phenomena. Comparisons between multiple model results and in situ data are crucial for improving future environmental predictions for missions landing on Mars. This work describes how changes in model scale and resolution (horizontal and vertical) can impact experimental results, using as a case study the selected landing site of the European Space Agency (ESA) Schiaparelli module. Schiaparelli was part of ESA’s ExoMars 2016 mission; the module descended through the Martian atmosphere on 19th October 2016. Experiments were completed that encompassed the period of Schiaparelli’s descent, using both a global-scale and a mesoscale model. The global model used in this work is the UK version of the LMD (Laboratoire de Météorologie Dynamique) Mars Glob-al Circulation Model (“the MGCM”), a 3D multi-level spectral model of the Martian atmosphere up to an altitude of ~100 km [1]. The mesoscale model used in this work is the LMD Martian Mesoscale Model (MMM) [2]; in these experiments an altitude of ~50 km was modelled in the mesoscale. Multiple resolution experiments were completed using the MGCM; results range from a ‘low’ resolution ~5° latitude x ~5° longitude (a resolution typically used for Martian climate modelling) to a ‘high’ resolution ~1° lat x ~1° lon. The vertical dimension is modelled using a set number of vertical layers; in these experiments the number of vertical layers selected was between 23 and 100. Experiments were run for a simulated year, starting from initial conditions based upon prior atmospheric observations, thus providing an independent prediction of conditions through the period of this case study. The MMM experiments were com-pleted in a set of nested resolutions, ranging from the outer, lowest resolution results at 63 km x 63 km, to the inner, highest resolution results at 7 km x 7 km. MMM experiments were completed using 60 vertical layers. Previous comparisons of global-scale and meso-scale modelling have focused on areas containing small-scale topographical variation that is not present in the global scale models. This work considers the relatively flat topography of the Schiaparelli site – a location that is more representative of the majority of historical Martian landing sites than areas that contain severe, small-scale topographical variation. Initial analysis has focused on constructing vertical profiles from the model output at both experimental scales, following preliminary information on the descent trajectory of the Schiaparelli module. An example comparison of atmospheric profiles constructed from MGCM results at different model resolutions. The plot displays atmospheric temperature obtained from experiments completed at different vertical resolutions: 23 and 100 vertical levels. There is a good match between the results, with a root mean square deviation (RMSD) of 9.83 K be-tween the results for the full height of the profiles; the RMSD reduces to 2.05 K when considering only the lowest ~10 km of the profiles (approximately one scale height). A comparison of vertical atmospheric temperature profiles from MGCM and MMM results. While the trend in the results is similar, the results differ by ~10 K between the models through most of the profile, down to a height of ~3 km above the surface. Between 50 and 3 km above the surface the RMSD of the profiles is 9.79 K; below 3 km (down to the lowest MGCM model lay-er) the match is closer, with an RMSD of 2.59 K. Further comparisons have been completed between the MGCM and MMM results, such as wind speed and direction, including consideration of the wider topographical and atmospheric context of Schiaparelli’s landing site and EDL period. These results show that, for the region considered within this case study, changing the horizontal or verti-cal resolution used in MGCM experiments does not greatly impact the results obtained. Similarly, the MMM results do not vary more than ~4 K with chang-ing horizontal resolution. In both cases, lower resolu-tions results (which are quicker and less computationally expensive to complete) are a good approximation of higher resolution results. Additionally, the similarity of the trends seen in the results from the different scale models suggests that global-scale model results are a reasonable approximation for mesoscale model results, for a number of potential landing locations on Mars. The module successfully transmitted some data that was captured during its descent, primarily from engineering sensors; this data includes the module's trajectory and attitude during the mission’s EDL phase. The ExoMars AMELIA (Atmospheric Mars Entry and Landing Investigations and Analysis) team aim to use the data returned by Schiaparelli during descent, combined with dynamic modelling of the module's motion, to reconstruct atmospheric profiles of density, pressure, temperature and wind speed [3]. Upon the release of the Schiaparelli data, the results from both the MGCM and MMM experiments will be compared with the data, supporting the work of the AMELIA team. References: [1] Forget et al. (1999) JGR, 104, E10. [2] Spiga et al. (2009) JGR, 114, E2. [3] Ferri et al. (2012) 9th International Planetary Probe Workshop