Mapping Of Venus Research Articles

Global geological mapping of Venus and the twenty-first-century legacy of William Smith: identification of challenges and opportunities for future research and exploration

Abstract Geological mapping principles have been applied across planet Earth since William Smith's initial 1815 map. During the Space Age, these principles were applied to smaller moons and planets, appropriately adjusted for remote conditions and data, and revealing ‘one plate planets’ recording Earth's missing chapters. Yet what of Venus, Earth's ‘twin’, shrouded in opaque clouds? How can these geological mapping principles be applied to address fundamental questions of planetary history? We outline how Venus's geological mapping over the last 35 years has proceeded towards a successively integrated more global picture that has revealed a surface and history unlike Earth or the smaller planetary bodies! The current 1 : 10 M global geological map documents geological units that all formed in the last c. 20% of Venus's history. The oldest, Fortunian, involved intense deformation and building of thicker crust (tessera). The Guineverian featured near-global emplacement of vast volcanic plains. The Atlian saw prominent rift zones and large shield volcanoes. The three major phases of activity provide a basis for assessing the geological, atmospheric and geodynamical processes operating earlier in Venus's history that led to the preserved record, raising a series of questions to be investigated in the coming decade by an international armada of Venus missions.

Three-dimensional structure of thermal waves in Venus’ mesosphere from ground-based observations

High spectral resolution observations of Venus were obtained with the TEXES instrument at NASA’s Infrared Telescope Facility. These observations focus on a CO2 absorption feature at 791.4 cm-1 as the shape of this absorption feature can be used to retrieve the vertical temperature profile in Venus’ mesosphere. By scan-mapping the planet, we are able to build up three-dimensional temperature maps of Venus’ atmosphere, covering one Earth-facing hemisphere and an altitude range of 60–83 km. A temperature map from February 12, 2019 clearly shows the three-dimensional structure of a planetary-scale thermal wave. This wave pattern appears strongest in the mid-latitudes of Venus, has a zonal wavenumber of 2–4 and the wave fronts tilt eastward with altitude at an angle of 8–15 degrees per km. This is consistent with a thermal tide propagating upwards from Venus’ upper cloud decks. Ground-based observations provide the opportunity to study Venus’ temperature structure on an ongoing basis.

Arecibo Radar Maps of Venus from 1988 to 2020

Abstract Over its 57 yr history, the Arecibo telescope was used to produce radar maps of Venus that pioneered our understanding of surface landforms and geologic processes. The best spatial resolution of 1–2 km achievable with the S-band (2380 MHz) transmitter was first used to map the surface in 1988, providing dual-circular polarization images ahead of the Magellan mission. Along with the 1988 observations, high-resolution images from observing runs in 2012, 2015, 2017, and 2020 have been archived with the NASA Planetary Data System to preserve these legacy data sets. We document the data collection and processing methods for the Arecibo Venus data, discuss unique aspects of the delay-Doppler imaging, and derive relative calibration factors linking the final multilook maps and Magellan data. The observations also allow for derivation of the circular polarization ratio and, potentially, the Stokes vector components. These results are particularly relevant for long-term monitoring of the spin rate, surface change detection, and planning for S-band polarimetry from the EnVision orbiter mission.

The analysis of Venus’ physical surface using methods of fractal geometry

In this paper, the work on investigating fractal structures on Venus was performed on the basis of observations taken by the “Magellan” spacecraft (NASA). The uncertainties in some data produced by “Magellan” were filled by the information that had been collected before – in “Venera 15”, “Venera 16”, and “Pioneer” missions. During the implementation of the work a digital map of Venus’ surface was built, and its spatial model was created. It is worth noting that the choice of basic level surface on Venus is defined by a certain value of potential or a point on its surface through which the geoid passes. The model of Venus’ physical surface was created use the harmonic expansion into spherical functions of altimetry data the “Magellan” mission. In the present paper, for determining and analyzing fractal dimensions the Minkowski mathematical algorithm, which is a simplified option of Hausdorff-Besicovitch dimension and provides high reliability and accuracy, was used. As a result, fractal correlations of Venus’ geoid anomalies in both longitude and latitude as well as the mean value of fractal dimensions were calculated. The following values of mean fractal dimension for Venus surface are obtained: in latitude – Dβ = 1.003; in longitude – Dλ = 0.98. Based on these values, we may conclude that the topographic model of Venus’ physical surface is close to spherical figure. The comparison between the obtained Venus fractal parameters with the ones of the Earth shows the good agreement.

Polarization and brightness temperature observations of Venus with the GMRT

Venus was observed at frequencies of 1297.67 MHz (23 cm), 607.67 MHz (49 cm) and 233.67 MHz (1.28 m) with the Giant Metrewave Radio Telescope (GMRT) during the period of 25th July and 6th September 2015 when it was close to its inferior conjunction. Values of the brightness temperature (Tb) of Venus from these observations were derived as 622{\pm}43 K, 554{\pm}38 K for 1297.67 and 607.67 MHz frequencies, respectively, are in agreement with the previous observations. The attempt to derive the Tb at 233.67 MHz affirms an upper limit of 321 K which is significantly lower than the previously reported upper limit of 426 K at the same frequency. We also present the dielectric constant ({\epsilon}) values of the Venus surface estimated using the degree of polarisation maps of Venus, derived from the GMRT polarisation observations and theoretical calculations. The {\epsilon} of the Venus surface was estimated to be ~4.5 at both the 607.67 MHz and 1297.67 MHz, close to the reported values of {\epsilon} of 4 to 4.5 from the radar-based observations including the Magellan observations at 2.38 GHz (12.6 cm).

History of the long-wavelength topography of Venus

A comparison of the global geological and topographic maps of Venus allows analysis of the topographic configuration of the major stratigraphic units forming the planet’s surface. The main positive relief features on Venus are manifested as prominences of tectonized units, which usually pertain to the initial episodes in the observable portion of its geological history (the beginning of the Guineverian period). The main morphostructural unit in this category is tessera, the main massifs of which constitute a special class of plateau-like highlands up to several kilometers high and tens to hundreds or thousands of kilometers wide. The younger shield and regional plains are confined to the regional slopes of the highlands formed by ancient tectonized units and are concentrated in regional lowlands. This indicates that regional lowlands (hundreds to thousands of kilometers across) as well as the highlands of ancient tectonized units had been formed in the early stages of the geological history of Venus. The estimates for the absolute model age of regional plains, which fill the lowlands, indicate that the formation of this material unit marks the first third of the observable geological history. Thus, the long-wavelength topographic pattern of Venus had formed, in general, in the early stages of the geological history, during the first third of it. In the final stages, which cover the next two thirds of the observable geological history of Venus (the Atlian period), the formation of long-wavelength topographic features is confined to dome-shaped highs, which make a minor contribution (about 15%) to the overall long-wavelength topographic pattern of Venus. This is evidence of a sharp fall in the level of endogenous activity after the emplacement of regional plains, with the style of the activity having changed from intensive crustal dislocations to extensive yet isolated uplifts of the thick thermal lithosphere.

The resurfacing history of Venus: Constraints from buffered crater densities

Because of atmospheric shielding and endogenic resurfacing, the population of impact craters on Venus is small (about a thousand) and consists of large craters. This population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of Venus, but the nearly spatially random nature of the crater population has complicated efforts to understand this history. Here we utilize the recent 1:15M-scale global geological map of Venus (Ivanov, M.A., Head, J.W. [2011]. Planet. Space Sci. 59, 1559–1600) to help address this problem. The global geological map provides a stratigraphic sequence of units, and known areas where each unit is exposed on the planet. For each crater on Venus we identify the specific geological units predating and postdating the crater. We perform a statistical analysis of this set of observations with a buffered crater density approach, which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and/or postdate more than one unit. In this analysis we consider crater emplacement as random and resurfacing history as determined (although unknown). We obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries. We find that (1) size–frequency distributions of craters superposed on each unit are consistent with each other; (2) regional plains and stratigraphically older units have similar crater retention ages; (3) stratigraphically younger units have a mean crater retention age significantly younger than the regional plains. These findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios. Our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10% of the mean surface age (tens of millions of years). The termination of intensive resurfacing may or may not be synchronous over the planet. Our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images, but obscure radar contrasts between predating lava flows. We do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch.

Evidence for crater ejecta on Venus tessera terrain from Earth-based radar images

We combine Earth-based radar maps of Venus from the 1988 and 2012 inferior conjunctions, which had similar viewing geometries. Processing of both datasets with better image focusing and co-registration techniques, and summing over multiple looks, yields maps with 1–2km spatial resolution and improved signal to noise ratio, especially in the weaker same-sense circular (SC) polarization. The SC maps are unique to Earth-based observations, and offer a different view of surface properties from orbital mapping using same-sense linear (HH or VV) polarization. Highland or tessera terrains on Venus, which may retain a record of crustal differentiation and processes occurring prior to the loss of water, are of great interest for future spacecraft landings. The Earth-based radar images reveal multiple examples of tessera mantling by impact “parabolas” or “haloes”, and can extend mapping of locally thick material from Magellan data by revealing thinner deposits over much larger areas. Of particular interest is an ejecta deposit from Stuart crater that we infer to mantle much of eastern Alpha Regio. Some radar-dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains. We suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites.

The history of volcanism on Venus

The history of volcanism on Venus

SAR interferometry at Venus for topography and change detection

Since the Magellan radar mapping of Venus in the early 1990’s, techniques of synthetic aperture radar interferometry (InSAR) have become the standard approach to mapping topography and topographic change on Earth. Here we investigate a hypothetical radar mission to Venus that exploits these new methods. We focus on a single spacecraft repeat-pass InSAR mission and investigate the radar and mission parameters that would provide both high spatial resolution topography as well as the ability to detect subtle variations in the surface. Our preferred scenario is a longer-wavelength radar (S or L-band) placed in a near-circular orbit at 600km altitude. Using longer wavelengths minimizes the required radar bandwidth and thus the amount of data that will be transmitted back to earth; it relaxes orbital control and knowledge requirements. During the first mapping cycle a global topography map would be assembled from interferograms taken from adjacent orbits. This approach is viable due to the slow rotation rate of Venus, causing the interferometric baseline between adjacent orbits to vary from only 11km at the equator to zero at the inclination latitude. To overcome baseline decorrelation at lower latitudes, the center frequency of a repeated pass will be adjusted relative to the center frequency of its reference pass. During subsequent mapping cycles, small baseline SAR acquisitions will be used to search for surface decorrelation due to lava flows. While InSAR methods are used routinely on Earth, their application to Venus could be complicated by phase distortions caused by the thick Venus atmosphere.

Global geological map of Venus

The surface area of Venus (∼460×106km2) is ∼90% of that of the Earth. Using Magellan radar image and altimetry data, supplemented by Venera-15/16 radar images, we compiled a global geologic map of Venus at a scale of 1:10M. We outline the history of geological mapping of the Earth and planets to illustrate the importance of utilizing the dual stratigraphic classification approach to geological mapping. Using this established approach, we identify 13 distinctive units on the surface of Venus and a series of structures and related features. We present the history and evolution of the definition and characterization of these units, explore and assess alternate methods and approaches that have been suggested, and trace the sequence of mapping from small areas to regional and global scales. We outline the specific defining nature and characteristics of these units, map their distribution, and assess their stratigraphic relationships. On the basis of these data, we then compare local and regional stratigraphic columns and compile a global stratigraphic column, defining rock-stratigraphic units, time-stratigraphic units, and geological time units. We use superposed craters, stratigraphic relationships and impact crater parabola degradation to assess the geologic time represented by the global stratigraphic column. Using the characteristics of these units, we interpret the geological processes that were responsible for their formation. On the basis of unit superposition and stratigraphic relationships, we interpret the sequence of events and processes recorded in the global stratigraphic column. The earliest part of the history of Venus (Pre-Fortunian) predates the observed surface geological features and units, although remnants may exist in the form of deformed rocks and minerals. We find that the observable geological history of Venus can be subdivided into three distinctive phases. The earlier phase (Fortunian Period, its lower stratigraphic boundary cannot be determined with the available data sets) involved intense deformation and building of regions of thicker crust (tessera). This was followed by the Guineverian Period. Distributed deformed plains, mountain belts, and regional interconnected groove belts characterize the first part and the vast majority of coronae began to form during this time. The second part of the Guineverian Period involved global emplacement of vast and mildly deformed plains of volcanic origin. A period of global wrinkle ridge formation largely followed the emplacement of these plains. The third phase (Atlian Period) involved the formation of prominent rift zones and fields of lava flows unmodified by wrinkle ridges that are often associated with large shield volcanoes and, in places, with earlier-formed coronae. Atlian volcanism may continue to the present. About 70% of the exposed surface of Venus was resurfaced during the Guineverian Period and only about 16% during the Atlian Period. Estimates of model absolute ages suggest that the Atlian Period was about twice as long as the Guineverian and, thus, characterized by significantly reduced rates of volcanism and tectonism. The three major phases of activity documented in the global stratigraphy and geological map, and their interpreted temporal relations, provide a basis for assessing the geodynamical processes operating earlier in Venus history that led to the preserved record.

Models of the global cloud structure on Venus derived from Venus Express observations

Spatially-resolved near-infrared spectra from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express have been used to derive improved models of the vertical structure and global distribution of cloud properties in the southern hemisphere of Venus. VIRTIS achieved the first systematic, global mapping of Venus at wavelengths within transparency windows in the 1.6–2.6 μm range, which are sensitive on the nightside to absorption by the lower and middle cloud layers of thermally-emitted radiation from the hot lower atmosphere ( Taylor, F.W., Crisp, D., Bézard, B. [1997]. Venus II: Geology, Geophysics, Atmosphere, and Solar Wind Environment, pp. 325–351). The cloud model used to interpret the spectra is based on previous work by Pollack et al. (Pollack, J., Dalton, J., Grinspoon, D., Wattson, R., Freedman, R., Crisp, D., Allen, D., Bézard, B., de Bergh, C., Giver, L. [1993]. Icarus 103, 1–42), Grinspoon et al. (Grinspoon, D.H., Pollack, J.B., Sitton, B.R., Carlson, R.W., Kamp, L.W., Baines, K.H., Encrenaz, T., Taylor, F.W. [1993]. Planet. Space Sci. 41, 515–542) and Crisp (Crisp, D. [1986]. Icarus 67, 484–514), and assumes a composition for the cloud particles of sulfuric acid and water, with acid concentration as a free parameter to be determined. Other retrieved parameters are the average size of the particles and the altitude of the cloud base in the model. Latitudinal variation in the atmospheric temperature structure was incorporated using data from the Venus Radio Science experiment (VeRa). Values are estimated initially using wavelength pairs selected for their unique sensitivity to each parameter, and then validated by comparing measured to calculated spectra over the entire wavelength range, the latter generated using the NEMESIS radiative transfer and retrieval code (Irwin, P.G.J., Teanby, N.A., de Kok, R., Fletcher, L.N., Howett, C.J.A., Tsang, C.C.C., Wilson, C.F., Calcutt, S.B., Nixon, C.A., Parrish, P.D. [2008]. J. Quant. Spectrosc. Radiat. Trans. 109, 1136–1150). The sulfuric acid concentration in the cloud particles is found to be higher in regions of optically thick cloud. The cloud base altitude shows a dependence on latitude, reaching a maximum height near −50°. The increased average particle size near the pole found by Wilson et al. (Wilson, C.F., Guerlet, S., Irwin, P.G.J., Tsang, C.C.C., Taylor, F.W., Carlson, R.W., Drossart, P., Piccioni, G. [2008]. J. Geophys. Res. (Planets) 113, E12) and the finding of spatially variable water vapor abundance at35–40 km altitude first reported by Tsang et al. (Tsang, C.C.C., Wilson, C.F., Barstow, J.K., Irwin, P.G.J., Taylor, F.W., McGouldrick, K., Piccioni, G., Drossart, P., Svedhem, H. [2010]. Geophys. Res. Lett. 37, L02202) are both confirmed. The implications of these improved descriptions of cloud structure and variability for the chemistry, meteorology, and radiative energy balance on Venus are briefly discussed.

Global mapping of crustal and lithospheric thickness on Venus

We have produced a high‐resolution global admittance map of Venus using a spatiospectral localization technique, calculating 64,800 admittance spectra evenly spaced in latitude and longitude. We grouped the admittance spectra into 35 classes that we inverted for elastic, crustal, and lithospheric thickness using bottom‐loading, top‐loading, and short‐wavelength top‐loading models with long‐wavelength bottom‐loading (“hot spot”) models. Our best fit models identify 17 classes consistent with top loading (40% of the surface), 7 with bottom loading (39%), and 4 with a long‐wavelength bottom‐loading model (15%). The remaining seven classes (6% of the surface) generally had large‐amplitude top‐loading signatures that could not be fit with our models. For the classes that were successfully modeled the estimated range of crustal (0–90 km) and elastic thickness (0–100 km) is larger than results for most previous studies, primarily because of the incorporation of bottom loading. Nearly half of the planet has elastic thickness <20 km, which we cannot distinguish from isostasy, suggesting that these regions are tectonically inactive. A key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions. Both the scale and range of crustal and elastic thickness variations indicate that Venus remains an active planet and that geologic processes may be more complex than previously recognized. In many regions the correlation with surface geology is not obvious, and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation.

On observing the compositional variability of the surface of Venus using nightside near‐infrared thermal radiation

There are several windows in the near‐infrared wavelength range (∼1 μm), in which thermal radiation emitted from the planetary surface penetrates through the thick Venus atmosphere and clouds. In this study we develop an improved method to estimate the surface emissivity on the basis of near‐infrared thermal emission from the nightside Venus, which is observed through these windows. A simple radiative transfer model demonstrates that multiple reflection of thermal radiation between the atmosphere (including clouds) and the solid surface has a significant influence on the observed radiance under the condition of Venus, where reflectivity of overlying atmosphere and clouds is high. Thus it is necessary to take the effect of the reflection by the planetary surface into account in order to estimate accurately the variation in the surface emissivity on the basis of near‐infrared observation of Venus. The net effect of multiple reflection of surface thermal radiation between the atmosphere and the surface is to significantly reduce the spatial contrast in thermal radiation due to surface compositional variation. The model calculation demonstrates that despite this effect, detection of granitic rocks on the Venus surface using near‐infrared windows is feasible. Since granitic and basaltic rocks have dramatically different 1 μm emissivities, granitic rocks are distinguishable from basaltic rocks by ground‐based telescopic observation. A Venus orbiter that measures both the near‐infrared thermal radiation and the surface altitude with great accuracy will provide us with a reliable surface emissivity map of Venus, which is a very valuable tool to detect granitic (i.e., Earth‐like continental) rocks on Venus.

Geologic units on Venus: evidence for their global correlation

Detailed geologic mapping of approximately 30% of the surface of Venus has revealed a stratigraphic sequence that appears to be repeated in widely separated areas on the planet. This sequence shows a transition from oldest highly deformed terrain units, the tessera, to a series of widespread volcanic plains units, each with recognizable morphologic characteristics and interpreted eruption style, and finally to individual eruptive centers associated with edifices and rift zones. This sequence of events is accompanied by characteristic and repetitive patterns of tectonic deformation. In order to test the validity and broader applicability of this sequence, we compare it with local, regional, and global mapping published in 17 analyses by other authors. This comparison shows that the sequence of units, and thus the relative time sequence of the corresponding geologic events, is generally the same in virtually all of the widely distributed areas that have been studied, comprising about one-half the surface of Venus. We then address several alternative explanations for the observed sequence: (1) that the similar units and sequences correspond to events that occurred in different areas at different times (repetitive in different places but not time correlative); (2) that the sequences occurred generally synchronously in different places (repetitive and time correlative); or (3) that the situation is intermediate between the two cases. In order to distinguish among these possibilities, we analyze evidence for the lateral global continuity of units, the chronology of emplacement on the basis of the density of superposed craters, and the consistency of stratigraphic relationships based on these data. We find that this analysis supports the hypothesis that the individual sequences are repetitive in different parts of the planet, and generally time correlative between these locations. As a further test of this stratigraphy, we outline a global model for the geological evolution of Venus and assess the duration of emplacement of units, the timing and style of tectonic phases, and the volumes and implied fluxes of volcanic deposits. We find that the morphologically recognizable part of the history of Venus comprises only the last 10–20% of its total history. Emplacement of tessera-forming material and its deformation into tessera terrain are the major geologic events of the initial Fortunian time. Several stages of areally extensive volcanism occurred subsequently, burying vast areas of tessera and forming regional plains during Sigrunian, Lavinian, and Rusalkian times. The average global rate of volcanism was a few km 3 y −1, and the emplacement of plains was accompanied by alternating episodes of contraction and extension. The last global-scale distributed tectonic episode, the formation of an extensive network of wrinkle-ridges, happened near the end of the Rusalkian Period, and marked the transition to the present stage of the history of Venus, the Atlian Period. Atlian times are characterized by a predominance of regional rifting focused at several broad rises, and localized rift-associated volcanism in the form of shield volcanic constructions and lobate volcanic plains-forming units, and is the longest time duration among the stratigraphic units considered, although the resulting tectonic and volcanic features and deposits cover only 10–20% of the surface of Venus. Comparison of this sequence of events with those revealed by continued geologic mapping of Venus will permit further testing and assessment so that a consistent and comprehensive documentation of the geological record can be presented and used to test models of the geodynamic evolution of Venus.

Model of formation of Ishtar Terra, Venus

For more than a decade, the radar mapping of Venus' surface has revealed that it results from a complex volcanic and tectonic history, especially in the northern latitudes. Ishtar Terra (0°E–62°E) consists of a high plateau, Lakshmi Planum, surrounded by highlands, Freyja Montes to the north and Maxwell Montes to the east. The latter is the highest relief of Venus, standing more than 10 km in elevation. The high resolution of Magellan radar images (120–300 m) allows us to interpret them in terms of tectonics and propose a model of formation for the central part of Ishtar Terra. The detailed tectonic interpretations are based on detailed structural and geologic cartography. The geologic history of Ishtar Terra resulted from two distinct, opposite tectonic stages with an important, transitional volcanic activity. First, Lakshmi Planum, the oldest part of Ishtar Terra is an extensive and complexly fractured plateau that can be compared to a terrestrial craton. Then the plateau is partially covered by fluid lava flows that may be similar to Deccan traps, in India. Second, after the extensional deformation of Lakshmi Planum and its volcanic activity, Freyja and Maxwell Montes formed by WSW-ENE horizontal crustal shortening. The latter produced a series of NNW-SSE parallel, sinuous, folds and imbricated structures that overlapped Lakshmi Planum westward. So these mountain belts have the same structural characteristics as terrestrial fold-and-thrust belts. These mountain belts also display evidence of a late volcanic stage and a subsequent period of relaxation that created grabens parallel to the highland trend, especially in Maxwell Montes.

A migratory mantle plume on Venus: Implications for Earth?

A spatially fixed or at least internally rigid hotspot reference frame has been assumed for determining relative plate motions on Earth. Recent 1:5,000,000 scale mapping of Venus, a planet without terrestrial‐style plate tectonics and ocean cover, reveals a systematic age and dimensional progression of corona‐like arachnoids occurring in an uncinate chain. The nonrandom associations between arachnoids indicate they likely formed from a deep‐seated mantle plume in a manner similar to terrestrial hotspot features. However, absence of expected convergent “plate” margin deformation suggests that the arachnoids are the surface expression of a migratory mantle plume beneath a stationary surface. If mantle plumes are not stationary on Venus, what if any are the implications for Earth?

Ocean science leaders seek congressional support

“Isn't it interesting that Congress will fund studies aimed at living in space or on the surfaces of the Moon and Mars but does not fund studies to see what technologies will be needed for us to live in the sea around us?…Ironically, we have better topographic maps of Venus than of Earth; we know more about the physiography of small volcanoes on the far side of the Moon than of similar features in our own exclusive economic zone off our shores.”So spoke Robert Ballard, director of the Center of Marine Exploration at the Woods Hole Oceanographic Institution, as he led off a day of testimony to three subcommittees of the U.S. Congress. At a late‐January hearing on the subject of “Leveraging National Oceanographic Capabilities,” leaders of the science community explained to U.S. Representatives that the federal government possesses tremendous ocean science resources and capabilities, but that without proper leadership and awareness from Congress, these assets could be squandered.

Giant radiating dyke swarms on Earth and Venus

Concentrations of dykes of basic composition emplaced in the same igneous episode or along similar trends are known as mafic dyke swarms and they occur in a wide variety of environments and over a wide range of scales on Earth. Recent radar mapping of Venus has revealed families of linear features interpreted to be the surface expression of near-surface dyke swarms. The lack of significant erosion on Venus provides a view of the surface manifestation of dyke swarm emplacement, one which complements the terrestrial perspective of erosion to deeper levels. The goal of this review is to synthesize the information available on both planets in order to use the complementary and synergistic record of mafic dyke swarm emplacement to build toward a better understanding of this important phenomenon in planetary history. We focus on the formation and evolution of giant dyke swarms which cover tens to hundreds of thousands of square kilometres on both Earth and Venus. Mafic dyke swarms on Earth occur in a wide range of modes and are observed in environments ranging from volcanic edifices (e.g., Hawaii), to central complexes (e.g., Spanish Peaks Complex, USA; Ramon Swarm, Israel), spreading centres and ophiolite complexes, compressional plate boundaries in back-arc settings (Columbia River Basalts, USA) and in continent-continent collisions. One of the most impressive modes of occurrence is that linked to the formation and evolution of mantle plumes. Terrestrial examples include a giant radiating swarm covering 100° of azimuth (the Mackenzie swarm, Canada), a 360° giant radiating swarm (the Central Atlantic reconstructed swarm), deformed giant radiating swarms (the Matachewan swarm, Canada), rift-arm associated swarms (e.g., Grenville swarm, Canada; Yakutsk swarm, Siberia), and one consisting of widely separated dykes (e.g., the Abitibi swarm, Canada). We summarize the geometric, chemical and isotopic characteristics of terrestrial dyke swarms, including their size and geometry, ages, presence and absence of subswarms, and the relation between swarms of different ages. We also summarize the characteristics of individual dykes, examining dyke length and continuity, en echelon offsets, dyke bifurcation, dyke height, width and depth, dyke intrusion and cooling history, and evidence for flow directions. On Venus at least 163 large radiating lineament systems (radius generally > 100 km) composed of graben, fissure and fracture elements have been identified. On the basis of their structure, plan view geometry and volcanic associations, the radial elements of more than 70% of these are interpreted to have formed primarily through subsurface dyke swarm emplacement, with the remainder forming through uplift or some combination of these two mechanisms. These systems are essentially uneroded and provide a view of the surface characteristics of giant radial swarms prior to the erosion which commonly occurs on Earth. The individual graben, fissures and fractures of which the systems are composed are typically less than several kilometres in width and cluster near the centre, with fissures grading smoothly into fractures at greater distances to define the overall radial pattern. While the largest systems, like those on Earth, are thousands of kilometres in radius, the population average is about 325 km, and they generally do not extend to equal lengths in all directions. In their distal regions, however, the elements in 72% of the systems continue along a purely radial trend, while distal elements in the remaining 28% curve gradually into unidirectional, sub-parallel geometries, generally interpreted to be related to regional stress patterns. The radial systems have a strong association with volcanism; all but seven display some form of volcanic signature. A review of models of the emplacement of lateral dykes from magma chambers under constant (buffered) driving pressure conditions and declining (unbuffered) driving pressure conditions indicates that the two pressure scenarios lead to distinctly different styles of dyke emplacement. Emplacement of lateral dykes in the constant driving pressure (buffered) case, however, can produce dykes which have sizes and widths which are very large and independent of chamber size. On Earth, the characteristics of giant mafic dyke swarms such as the Mackenzie dyke swarm in Canada strongly suggest that they were emplaced in buffered conditions. On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments which have been dismembered and distorted by subsequent plate tectonic rifting events. The abundant intact giant radiating swarms on Venus provide criteria by which fragmented terrestrial swarms can be reconstructed.

The Magellan Venus mapping mission: Aerobraking operations

The orbit of the Magellan spacecraft was circularized during a 70-day aerobraking phase, which ended on 3 August 1993. Shrinking the orbit apoapsis from 8467 km down to 541 km was required to obtain meaningful gravity science data at high and moderate latitudes. Aerobraking was the only way to reach this nearly-circular orbit, since the amount of propellant on board Magellan was at least an order of magnitude too small to circularize propulsively. This paper will describe the steps taken by the Magellan flight team to successfully aerobrake the Magellan spacecraft into the nearly-circular orbit. Magellan is currently in a 541 by 197 km altitude orbit around the planet Venus. This paper will briefly describe the Magellan mission history and hardware, the goals of the continuing Magellan mission, the exciting aerobraking phase, and other science objectives beyond the primary continuing mission goal of producing a high-resolution global-gravity map of Venus.