Rheological and petrological implications for a stagnant lid regime on Venus

AbstractVenus is physically similar to Earth but with no oceans and a hot dense atmosphere. Its near-random distribution of impact craters led to the inferences of episodic global resurfacing and a stagnant lid regime, and imply that it is not currently able to lose proportionately as much heat as Earth. This paper shows that a CO2-induced asthenosphere and decoupling of the mantle lid from the crust, caused by the elevated surface temperature, enables lid rejuvenation. Global hypsography implies a rate of 4·0±0·5km²a−1 and an implied heat loss rate of 32·8±3·6TW, ~90% of a scaled Earth-like rate of heat loss of 36TW. Estimates of the rate of lid rejuvenation by plume activity – 0·07 to 0·09km²a−1– imply that ten times the number of observed plumes are required to equal this rate of heat loss. However, lid rejuvenation by convection allows Venus to maintain a stable tectonic regime, with subcrustal horizontal extension (half-spreading) rates of between 25 and 50mma−1 determined from fits to topographic profiles across the principal rift systems. While the surface is largely detached from these processes, the association of rifting and other processes does imply that Venus is geologically active at the present day

VenSAR: A multi-functional S-band radar for the EnVision mission to Venus

The EnVision science case requires an instrument capable of providing global stereo images at 10-50 m resolution, phase information from at least 20% of the surface for interferometry, as well as the ability to provide 1-10 m resolution images of specific targets in the C- to S-band range (X-band does not penetrate through the atmosphere to the surface of Venus). VenSAR is adapted from the individual phase centre design of NovaSAR-S, which offers much greater flexibility that can be optimised for Venus science. In addition, its S-band wavelength offers an acceptable compromise between InSAR resolution and atmospheric stability. The use of an off-the-shelf system, adapted for use at Venus, saves cost and provides for directly comparable data from Venus and Earth at a resolution two orders of magnitude better than Magellan, for the first time allowing the direct measurement of rates of tectonic and volcanic processes on another planet

The geological context and evidence for incipient inversion of the London Basin

A reappraisal of ground investigation data across London reveal that a range of unexpected ground conditions, encountered in engineering works since Victorian times, may result from the effects of ongoing inversion of the London Basin. Site investigation borehole data and the distribution of river terrace deposits of the Thames and its tributaries reveal a complex pattern of block movements, tilting and dextral transcurrent displacement. Significant displacements (~10 m) observed in Thames terrace gravels in borehole TQ38SE1565 at the Lower Lea Crossing, showing that movement has occurred within the last ~100 ka. Restraining bends on reactivated transcurrent faults may ex-plain the occurrence of drift filled hollows, previously identified as fluvially scoured pingos, by faulting and upward migration of water on a flower structure under periglacial conditions. Mapping the location of these features constrains the location of active transcurrent faults and so helps predict the likelihood of encountering hazardous ground conditions during tunnelling and ground engineering

A novel orbiter mission concept for venus with the envision proposal

In space exploration, planetary orbiter missions are essential to gain insight into planets as a whole, and to help uncover unanswered scientific questions. In particular, the planets closest to the Earth have been a privileged target of the world’s leading space agencies. EnVision is a mission proposal with the objective of studying Earth’s closest neighbor. Designed for Venus and competing for ESA’s next launch opportunity, the proposal already went through the selective technical review for the M4 launch opportunity, and was submitted to the M5 call, incorporating feedback from ESA. The main goal is to study geological and atmospheric processes, namely surface processes, interior dynamics and atmosphere, to determine the reasons behind Venus and Earth’s radically different evolution despite the planets’ similarities. To achieve these goals, the operational orbit selection is a fundamental element of the mission design process. The design of an orbit around Venus faces specific challenges, such as the impossibility of choosing Sun-synchronous orbits. In this paper, an innovative genetic algorithm optimization was applied to select the optimal orbit based on the parameters with more influence in the mission planning, in particular the mission duration and the coverage of sites of interest on the Venusian surface. After summarizing the EnVision proposal’s mission concept for Venus, the optimization and innovation of the operational orbit design will be analyzed in terms of its benefits to the mission

Engineering geology and tunnelling in the Limmo Peninsula, East London

The Limmo Peninsula site has some of the most complex geology of London's Crossrail project and was the launching point for four tunnel boring machines (TBMs) to allow construction of Crossrail's eastern running tunnels. It is located in East London, c. 2 km east of the Canary Wharf business district, adjacent to the River Lea. It consists of a ventilation shaft, an auxiliary shaft, two sprayed concrete lining (SCL) tunnels interconnecting the shafts and four SCL adits for assisting in the launching of the TBMs. As part of the design requirements, some geological formations had to be depressurized from surface wells. The site is geologically complex: it is in the vicinity of a drift-filled hollow and it is located within the area of influence of several tectonic features. A geological ground model developed from important new information obtained during the design stage ground investigations and from direct observations conducted during construction stages reveals an inverted transtensional flower structure (i.e. it is now a transpressional restraining bend). Of special interest are the unusually low values of undrained shear strength of the London Clay associated with the tectonic setting

VenSAR on EnVision: taking Earth Observation radar to Venus

Venus should be the most Earth-like of all our planetary neighbours: its size, bulk composition and distance from the Sun are very similar to those of Earth. How and why did it all go wrong for Venus? What lessons can be learned about the life story of terrestrial planets in general, in this era of discovery of Earth-like exoplanets? Were the radically different evolutionary paths of Earth and Venus driven solely by distance from the Sun, or do internal dynamics, geological activity, volcanic outgassing and weathering also play an important part? EnVision is a proposed ESA Medium class mission designed to take Earth Observation technology to Venus to measure its current rate of geological activity, determine its geological history, and the origin and maintenance of its hostile atmosphere, to understand how Venus and Earth could have evolved so differently. EnVision will carry three instruments: the Venus Emission Mapper (VEM); the Subsurface Radar Sounder (SRS); and VenSAR, a world-leading European phased array synthetic aperture radar that is the subject of this article. VenSAR will obtain images at a range of spatial resolutions from 30 m regional coverage to 1 m images of selected areas; an improvement of two orders of magnitude on Magellan images; measure topography at 15 m resolution vertical and 60 m spatially from stereo and InSAR data; detect cm-scale change through differential InSAR, to characterise volcanic and tectonic activity, and estimate rates of weathering and surface alteration; and characterise of surface mechanical properties and weathering through multi-polar radar data. These data will be directly comparable with Earth Observation radar data, giving geoscientists unique access to an Earth-sized planet that has evolved on a radically different path to our own, offering new insights on the Earth-sized exoplanets across the galaxy

The composition and internal structure of Europa

The bulk density of Europa, as derived from Voyager data, indicated a bulk composition similar to the Moon or Mars but with the addition of 5 to 10% water. The accepted view was that this water is distributed as a layer of ice ~150 km thick [1] at the surface. Debate has continued as to whether this layer is partly liquid. Ransford [2] was the first to suggest that this model may be too simplistic and proposed that there may also be a hydrated silicate layer. However, a full cosmochemical treatment of Europa (or any Galilean satellite) has not be conducted. This abstract presents the results of a simple model of Europa derived from the composition of a chondrite appropriate to its position in the proto-jovian nebula. This model assumes full differentiation into core, mantle, crust and cryosphere and that the internal temperature distribution is such that hydrated silicates are stable to their pressure limited phase boundary (at about 2 GPa). The model has been refined to fit the within the uncertainty limits of mass and radius but is not unique and other possible compositions have yet to be tested. The bulk composition used in the model is listed in Table 1 by weight percent and mole fraction

Ice Crust Thickness and Internal Composition of Europa

The radius (1569 km), mass (4·868 x 1022 kg) and I/MR2 (0·264, E4; 0·347, E6), have been determined from data returned Galileo encounters, and provide useful constraints on bulk composition models of Europa. I present new physical and chemical models derived from known meteoritic compositions expected to be representative of Europa’s bulk composition. A core is differentiated, along with a mantle, crust and cryosphere. Using a modified CIPW norm method, the mineralogy of the crust and of the depleted and fertile mantle is derived, and hence the density of each layer is found. The thickness of the crust and cryosphere are adjusted to match the observed mass, radius and moment of inertia of Europa. Radiogenic and tidal energy production rates are calculated and used to define two possible thermal structures of the crust and mantle