sign in sign up
<<
Home , Atmosphere, Atmosphere of Venus, Ishtar Terra, Lada Terra, Mapping of Venus, Mesopause

Voyage 2050 White Paper Wilson, Widemann et al. August 2019

Venus: Key to understanding the of terrestrial

A response to ESA’s Call for White Papers for the Voyage 2050 long- term plan in the ESA Science Programme.

?

Contact Scientist: Colin Wilson Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, UK. E-mail: [email protected] Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

Executive summary

In this Voyage 2050 White Paper, we emphasize the importance of a Venus exploration programme for the wider goal of understanding the diversity and evolution of habitable planets.

Why are the terrestrial planets so different from each other? Venus, our nearest neighbour, should be the most -like of all our planetary siblings. Its size, bulk composition and solar input are very similar to those of the Earth. Its original was probably similar to that of early Earth, with large atmospheric abundances of dioxide and . Furthermore, the young ’s fainter output may have permitted a water on the surface. While on Earth a moderate ensued, Venus experienced runaway greenhouse warming, which led to its current hostile climate. How and why did it all go wrong for Venus? What lessons can we learn about the story of terrestrial planets/ in general, whether in our or in others?

Comparing the interior, surface and atmosphere evolution of Earth, and Venus is essential to understanding what processes determined habitability of our own and Earth-like planets everywhere. This is particularly true in an era where we expect thousands, and then millions, of terrestrial exoplanets to be discovered. Earth and Mars have already dedicated exploration programmes, but our understanding of Venus, particularly of its and its history, lags behind.

ESA’s mission was tremendously successful, answering many questions about Earth’s sibling planet and establishing European leadership in Venus research. However, further understanding of Venus and its history requires several further lines of investigation, from orbiters, probes, balloons and landers.

The most urgent priority for orbiters, arguably, is geophysical investigations including a 21st century and complementary observation to establish Venus’ current volcanic and tectonic activity; such investigations are currently under consideration both at ESA (M5 EnVision orbiter) and by NASA (VERITAS / VOX Discovery & New Frontiers proposals) and may have launched by 2035. Such an orbiter will no doubt reveal a complex, active world next door meriting follow-up observations with a high- resolution targeted radar orbiter (in the same way that Mars orbiters have carried ever more capable imagers). On the timescale of Voyage 2050, further investigations should also be conducted to study the Venus atmosphere’s super-rotation, extreme and complex chemistry, and the processes which govern volatile escape to from its . Direct measurement from orbit of , using heterodyne sub-mm observations would be particularly valuable for understanding Venus’ atmospheric super-rotation and mesospheric chemistry.

In situ atmospheric measurements from descent probes are needed to unravel Venus’ complex chemical cycles. A simple -style entry probe, equipped with modern instrumentation, would provide invaluable measurements of noble isotopic ratios which preserve a record of the formation and evolution of the planet, and vertical profiles of active . NASA’s DAVINCI and VICI Discovery & New Frontiers proposals and ’s Venera-D are such missions, currently in development, so it is likely that these missions will be conducted before ESA develops its own version of such probes.

Longer-duration measurements in the layer could be achieved by using balloons, whether these are constant- or variable-altitude balloons. Balloons offer a lifetime of weeks in arguably the most habitable environment found outside Earth, where is around 20 degrees C, is Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

0.5 bar, and there is ample and liquid water in the (albeit mixed with sulphuric acid). Such a mission would explore the coupled chemical, dynamical and radiative processes at work in this critical part of the Venus atmosphere. The EVE M3 proposal to ESA gives a blueprint for such a mission.

In situ measurements at the surface are needed. All previous probes have landed in basaltic plains; a new in the highlands, thought to represent the oldest on Venus, is necessary to understand Venus’ geological history, in particular its tectonic style. Such composition measurements can be achieved with a Venera-type lander, relying on insulation and thermal inertia to give a lifetime of an hour or two. and seismometry, on the other hand, require measurements over months or years. Rather than try to use silicon electronics with associated cooling and power systems, such long-duration lander measurements can be implemented using high- temperature electronics of silicon carbide, gallium nitride or other wide band-gap materials. A review of the science case and state of the art of such lander technologies is given in [Wilson et al. 2016]. As well as their own scientific investigations, such long-term landers would serve as technology demonstrators developing technology in preparation for mobile surface exploration, which is a post- 2050 Venus exploration goal.

Between now and 2050, then, we recommend that ESA launch at least two M-class missions to Venus (in order of priority):

• a -focussed orbiter (the currently proposed EnVision orbiter); and • an in situ atmospheric mission (such as the EVE balloon mission). • We recommend that ESA aim to two such missions whether or not other agencies also launch Venus missions. With 460 million km2 of diverse, complex surface to explore, there is a huge number of scientific questions for a geophysics orbiter to study, with ever more capable instruments. The same argument applies to in situ missions: there is a huge range of measurements needed from such missions, leaving scope for more than one agency’s efforts.

Only after the above missions have been secured, the next priorities are:

• a further orbiter, with follow-up high-resolution surface target imaging, atmospheric and/or ionospheric investigations. • a small lander with high-temperature electronics, as pathfinder for long-life surface missions.

An in situ and orbital mission could be combined in a single L-class mission, as was argued in responses to the call for L2/L3 themes [see Wilson et al., 2013; Marcq et al., 2013; Limaye et al., 2013]. Combining an orbital and an in situ element would lead to strong synergies; for example, an orbiter would provide both high-rate data relay and context observations for an in situ lander or balloon platform. However, it could also be achieved using M-class mission category, with or without partner agencies such as NASA or . Venus exploration presents clear opportunities for international co-ordination: it is scientifically fascinating yet relatively accessible, being close to earth.

In summary: study of Venus is central to the study of comparative planetology and solar system evolution, crucial to understanding both our own planet and earth-sized exoplanets everywhere.

Wri tten by Colin Wilson (Oxford University, UK) and Thomas Widemann (Paris Observatory & Université de Versailles – St. Quentin, France), with input from EnVision, EVE & VL2SP science teams. Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

1 Introduction

1.1 The importance of Venus for comparative planetology

One of the central goals of planetary research In an era where we will soon have detected is to find our place in the . thousands of Earth-sized exoplanets, planetary Investigation the evolution of solar systems, science must seek to characterise these planets, and of life itself, is at the heart of this planets and to explain the diverse outcomes quest. The three terrestrial planets in our solar which may befall them. Are these exoplanets system - Earth, Mars, and Venus - show a wide habitable? Many efforts have been made to range of evolutionary pathways, and so define the edges of the ‘habitable zone’, i.e. represent a “key” to our understanding of the range of distances from a parent at planets and exoplanets. which a planet can sustain liquid water on its surface. The inner edge of the habitable zone Earth and Venus were born as twins – formed has been estimated to lie anywhere from 0.5 at around the same time, with apparently to 0.99 AU – this latter figure, from Kopparapu similar bulk composition and the same size. et al., 2013, should be a cause of concern for However, they have evolved very differently: us Earth-dwellers! Detailed study of Venus is the enormous contrast between these planets indispensable if we are to understand what today challenge our understanding of how processes determine the inner edge of the terrestrial planets work. The atmosphere is habitable zone. surprising in many ways – its 400 km/h winds on a slowly rotating planet; its enormous The habitable zone’s boundaries will evolve surface temperature, even though it absorbs over a planet’s lifetime due to the evolution of less sunlight than does the Earth; its extreme the star’s output as well as changes in the aridity, with sulphuric acid as its main planet and its atmosphere. There are only condensable species instead of water. The three terrestrial planets at which we can study planet, too is mysterious: its apparent geophysical and evolutionary processes: lack of geodynamo and , the Venus, Earth, and Mars. Exploration of the uncertainty of its current volcanic state, the latter two is firmly established, while in apparent young age of much of its surface. contrast there are no confirmed Venus How and why does a planet so similar to Earth missions after . end up so different?

1.2 Context: The state of Venus science after Venus Express and Akatsuki

Venus Express was a very successful mission. different altitudes; it has measured the spatial During its science operations, from 2006 – distributions of key chemical species, and 2014, it made a wealth of discoveries relating discovered new ones; it has improved our to the atmosphere at all altitudes from the understanding of how unmagnetised Earthlike surface up to the . It has mapped planets lose water. Despite its atmospheric cloud motions to reveal velocities at focus: some of its most intriguing legacy may

Page 4 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 be the hints it has provided of current-day propose a new set of investigations that focus volcanic activity. on understanding the evolution of Venus through a combination of surface and JAXA’s Akatsuki orbiter – previously known as atmospheric investigations. the Venus Climate Orbiter – has been orbiting Venus since 2015. Its scientific focus is on Thanks to Venus Express, Europe is at the atmospheric dynamics; it carries a set of forefront of Venus research – arguably, this is cameras using different to image a situation unmatched in the rest of the solar atmospheric motions at different altitudes. Its system exploration programme. Research observations have revealed new atmospheric groups across Europe have participated in the waves, providing ever more detailed insights construction of and analysis from the scientific into Venus’ . payload; dozens of researchers have completed doctoral theses based on Venus In short, Venus Express and Akatsuki have Express research. Europe is thus well-placed to provided much-needed data for atmospheric lead a future Venus mission. ESA can now build dynamics, chemistry, and radiative transfer, on this position, capitalising on investments and for understanding of its ionosphere & made in Earth Observation programme and induced . These new data are advanced satellite technologies, to address invaluable for constraining models of how fundamental questions about the evolution of Venus works today. However, the history of terrestrial planets and the appearance of life. Venus still remains enigmatic. In this paper we

2 Science Themes

2.1 Geology (Interior, tectonism, , mineralogy, )

The major unknowns in Venus geological include catastrophic global lithospheric science are associated with its resurfacing overturn which take place every 500 to 700 My history and establishing whether it is currently [Turcotte et al., 1999], equilibrium resurfacing geologically active. models more similar to those found on Earth [Stofan et al. 2005], as well as many models in Resurfacing history between.

The age of Venus’ surface is poorly known. The community has used the existing Unlike , the , and Mars, Venus radar data to attempt to resolve these has a thick atmosphere that represents a fundamental conflicts by applying mapping powerful filter to small impactors. As a result, techniques to establish stratigraphic its crater population is limited to a few large relationships among surface units and craters; there are very few craters with structures. NASA’s Magellan orbiter, launched diameters <20 km, and there are fewer than in 1989, obtained global radar maps with a 1000 craters in total. The observed crater spatial resolution of 100 – 200 m. An important population offers poor constraints on surface limitation to using radar images for geological ages, allowing a number of different mapping is that geological mapping requires production and resurfacing scenarios. These the ability to identify distinct rock units, whose

Page 5 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 formation represent geological processes (e.g., by crustal conduction, or does volcanic activity distinct flows or sedimentary units). play an important role? Is the loss rate Magellan imagery provides the to sufficient to maintain an equilibrium or is heat identify some units; for example, it is possible building up in the interior, potentially leading to map lava flow boundaries to high precision to an episodic resurfacing scenario? in some terrains. But in many, many other Understanding how Venus loses its internal cases, the materials being mapped have been heat is important for understanding both affected by later tectonic structures; Earth’s earliest history and for understanding moreover, the highly deformed tessera , those exoplanets larger than Earth, both of thought to be the oldest on Venus, is which share its problem of a buoyant characterized by overlapping structures whose lithosphere; these mechanisms at work may relationships are ambiguous in currently profoundly affect the atmosphere and climate, available observations. Different methods for and prove catastrophic for life. accommodating this complication have led to There are some hints of recent geological widely divergent mapping styles, which in turn have resulted in a range of surface evolution activity particularly from Venus Express data models, mirroring the range of interior (Smrekar et al., 2010, Marcq et al., 2012), but these analyses are indirect. A new radar evolution models (see e.g. review by Guest & Stofan, 1999). Many remaining debates over, dataset would enable not only better for instance, the sequence and relative timing understanding of current surface and alteration processes, which is needed in of tectonic deformation in the complex tesserae cannot be resolved using currently order to calculate ages for geologically recent available radar data, because of the limitations changes such as lava flows and dune movements, but would also enable direct represented by the spatial resolution and single polarization of those data. searching for surface change.

Current geological activity Case for next-generation radar:

Venus is thought to have similar internal heat Because of the extreme surface conditions and production to Earth, but it is not clear how the opaque clouds on Venus, geological investigation requires orbital , internal heat is lost to space. Is heat lost solely

Images of the same region of Mars illustrate the revolutions in understanding which are enabled by increasing spatial resolution by an order of magnitude, particular for surface processes. Quoted pixel resolution is that of the displayed image used rather than the full resolution of the original. Image credits: NASA/JPL.

Page 6 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 with techniques including interferometric synthetic aperture radar (InSAR), gravitometry, altimetry and observation using nightside infrared windows. The value of radar mapping at Venus was demonstrated by NASA’s Magellan orbiter, launched in 1989, which obtained global radar maps with a horizontal resolution of 100 – 200 m and altimetry with a vertical resolution of 100 m. Advances in technology, data acquisition and processing, and satellite control and tracking, Differential InSAR revealed altimetry changes in this in mean that the spatial resolutions in the 1 – 10 Kenya which previously had been thought to be dormant. m range are now possible. [Sparks et al., 2012; observations from ERS-1 & ERS-2].

This high-resolution radar The revolutions in radar performance go would revolutionise geological understanding. beyond just spatial resolution. Differential Generations of Mars orbital imagery have seen InSAR allows surface change detection at successive order-of-magnitude improvements, centimetre scale. This technique has been used as illustrated above. As imagers progressed to show surface deformations after from the 50 m resolution of Viking towards the and volcanic eruptions on Earth 5m resolution of MGS/MOC and the higher (see figure); similar results on Venus could resolutions of MEx/HRSC and MRO/HiRise, our provide dramatic evidence of current volcanic conception of Mars as a frozen, inactive planet or tectonic activity. was followed by hypotheses that geologically The maturity of InSAR studies on Earth is such recent flow had occurred, to actual detection that data returned from Venus can confidently of current surface changes (e.g. gullies & dune be understood within a solid theoretical movement). For Venus, metre-scale imagery framework developed from coupled terrestrial will enable study of Aeolian features and dunes InSAR data and ground-truth observations, (only two dune have been given the absence of ground-truth data on unambiguously identified to date on Venus); Venus. Note that the radar resolutions may be will enable more accurate stratigraphy and high enough to permit identification of Venera of layering; will constrain the and Pioneer Venus landers, which serves as morphology of tesserae enough that their some ground truth even before a new history and structural properties can be generation of landers is taken into account. constrained; will enabled detailed study of Radar mapping with different polarisation styles of volcanism by enabling detailed states constrains surface roughness and mapping of volcanic vents and lava flows; and dielectric properties; mapping at different look will enable direct search for surface changes angles and different wavelengths will provide due to volcanic activity and Aeolian activity. further new constraints on surface properties. Metre-scale resolution would even enable search for changes in rotation rate due to The proximity of Venus to Earth, the relatively surface-atmosphere exchange, calm (if extreme) surface conditions and lack of which could constrain internal structure water, the absence of a large satellite and its (Karatekin et al., 2011). moderately well-known geoid and topography, all help to ease the technical demands on the mission. Europe has developed a number of

Page 7 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 world-leading radar systems and could, for minor elements, even within the engineering example, adapt a GMES Sentinel-1 or constraints of Venera-like landers. NovaSAR-S modular array antenna for use at Venus, providing higher-resolution imagery, A new generation of geologic instrumentation should be brought to the surface of Venus, topography, geoid and interferometric change data that will revolutionise our understanding including Raman/LIBS and XRF/XRD; this would of surface and interior processes. would allow mineralogical, as well as merely elemental, composition. Such precise analyses In addition to radar techniques, the surface can would be welcome for of Venus’ also be observed by exploiting near-infrared lowland plains, but would be especially spectral window regions at wavelengths of 0.8 desirable for the highland tesserae and for – 2.5 µm. On the nightside of Venus, thermal . The tesserae are thought to emission from the surface escapes to space in represent ancient crust that predates the most some of these spectral windows, allowing recent volcanic resurfacing event and so mapping of surface thermal emissivity, as provide a geochemical look into Venus’ distant demonstrated by VEx/VIRTIS [Mueller et al., past. Ishtar Terra, too, may be composed (at 2008]. A new instrument optimised for this least in part) of granitic rocks like Earth’s observation could map mineralogy and also continental crust, which required abundant monitor the surface for volcanic activity [Ghail water to form. Coronae samples will reveal et al., 2012]. how magmatic systems evolve on Venus in the absence of water but possibly in the presence Case for Venus in situ geological of CO2, SO2 or other . Surface investigations: geological analysis would benefit from high The Venera and Vega missions returned data temperature drilling/coring and sample about the composition of Venus surface processing capabilities, although further materials, but their accuracy is not sufficient to investigation will be needed to assess the permit confident interpretation. The Venera extent to which this can be within the scope of and VEGA analyses of major elements (by XRF) even an L-class mission opportunity. did not return abundances of Na, and their Long-lived stations would provide essential data on Mg and Al are little more than seismological and meteorological data. The detections at the 2σ level. Their analyses for K, technological barriers to such missions are U, and Th (by rays) are imprecise, twofold: (1) the high temperature except for one () with extremely high environment of the Venus surface, too hot for K contents (~4% K2 O) and one () with silicon electronics, and (2) the lack of sunlight a non-chondritic U/Th abundance ratio. The at the surface, making solar power unviable. As landers did not return data on other critical has been discussed by Wilson et al. 2016 and trace and minor elements, like Cr and Ni. In Kremic et al, 2018, recent advances in high- addition, the Venera and VEGA landers temperature electronics have made long- sampled only materials from the Venus duration uncooled landers a possibility which lowlands – they did not target sites in any of could be explored in the coming decades; this the highland areas, the coronae, tesserae, nor requires continued investment in the the unique plateau construct of Ishtar Terra. development of the electronics, in their Currently available instruments could provide packaging, and in their environmental much more precise analyses for major and qualification in Venus conditions. Power for long-duration landers on the surface could

Page 8 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 come from primary molten salt batteries, for a abundance of , including the first-generation of landers. Second-generation carbonate (Fegley & Treimann 1992) or pyrite- long-life landers could be powered by RTGs or magnetite (Hashimoto & Abe, 1997) buffer even wind power – both of which would hypotheses. Measurements of near-surface require technology development. abundances and vertical gradients of trace

gases, in particular SO2, H2O, CO and OCS, Descent imaging has not yet been performed would enable discrimination between by any Venus lander. Descent imaging of any different hypotheses. Correlating these data landing site would be useful, particularly so for with lander and orbiter data will reveal how the tessera highlands where it would reveal important and widespread the sources and the morphology of the highland surfaces and sinks of these species might be. yield clues as to what weathering processes have been at work in these regions. Multi- Measurement of the temperature, pressure imaging in near-infrared and N2 abundance in the lowest is wavelengths would yield compositional also essential. Only one Venus descent probe, information to provide further constraints on in 1984, reported temperature and surface processes. In particular, descent pressure all the way down to the surface imaging can establish whether near-surface profiles; and the gradient of this temperature weathering or real compositional differences profile in the lowest few km appears far more are the root cause for near-IR emissivity convectively unstable than is thought to be variations seen from orbit, (Helbert et al. 2008) possible. At the altitudes where this occurs, providing important ground truth for these carbon dioxide is in a regime, orbital observations. and it has been suggested that carbon dioxide and are separating due to exotic Profiles of atmospheric composition in the supercritical processes [Lebonnois & Schubert, would reveal which near-surface atmosphere Nat Geosci 2017]. This effect would have major chemical cycles are responsible for maintaining implications for our understanding of high the enormously high carbon dioxide pressure everywhere, from the concentrations in the atmosphere, and would gas giants to exoplanets. High-accuracy also reveal details about surface-atmosphere measurements of pressure, temperature and exchanges of volatiles. Several mechanisms N2 abundance down to the surface would have been invoked for buffering the observed resolve this intriguing question.

2.2 Planetary evolution as revealed by

Geology is a powerful witness to history, but it planet. 40Ar, produced from the decay of long- does not provide answers about evolution in lived 40K, has been continuously accumulated the time before the formation of the oldest over >4 billion years and so its current rocks, which on Venus were formed only about abundance constrains the degree of a billion years ago. For constraints on earlier volcanic/tectonic resurfacing throughout evolution, we must turn to isotope history. 129Xe and 130Xe are produced from the geochemistry. now extinct 129I and 244Pu respectively within the first 100Ma of the solar system’s history. Radiogenic noble gas provide The depletion of these isotopes in the information about the degassing history of the

Page 9 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 atmospheres of Mars and Earth reveal that Non-radiogenic noble gas isotopes provide these two planets underwent a vigorous early information about acquisition and loss of degassing and blow-off, although the planet-forming material and volatiles. Venus is mechanisms of this blow-off and of less depleted in and isotopes than subsequent deliveries of materials from are Earth and Mars, but its Xe and Kr isotopic and vary according to abundances are still unknown (Xe isotopic different scenarios. Neither the bulk Xe abundances have not yet been measured, and abundance nor the abundances of its eight past measurements of Kr abundance vary by isotopes have ever been measured at Venus. an order of magnitude, providing little useful Measurements of these abundances would constraint). The significant fractionation of provide entirely new constraints on early xenon on Earth and Mars can be attributed to degassing history. 4He, produced in the massive blowoffs of the initial atmospheres in from long-lived U and Th decay, has an the period after the radiogenic creation of atmospheric lifetime of only a few hundred xenon from its parent elements, ~50-80 Myr million years before it is lost by escape to after planet formation. However, it could also space. Therefore its current atmospheric be that the fractionation is reflecting that of a abundance provides constraints on recent source material delivered late in planetary and escape rates within the last 108 formation, perhaps from very comets. If – 109 years. Venus has the same xenon fractionation

Isotopic ratios provide keys to constrain planetary origins (Ne isotopes), early atmospheric loss processes (non-radiogenic Xe, Kr, Ar), late impact scenarios, recent mantle outgassing and late resurfacing, and escape of water. Modified from Baines et al., 2007.

Page 10 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 pattern as Earth and Mars, this would support meteorites all exhibiting isotopic ratios the idea that a common source of fractionated different from the terrestrial values; it also is xenon material was delivered to all three affected by preferential escape of 14N. planets, and weaken the case that these reflect Measurement of the nitrogen isotopic ratio large blowoffs. If we see a pattern with less Xe therefore helps establish whether the gas fractionation then that would support the source was primarily meteoritic or cometary, blow-off theory for Earth and Mars. and constrains history of escape rates. Considered together with other non- radiogenic isotopic abundances, this allows Taken together, measurements of these determination of the relative importance of isotopic abundances on Venus, Earth and Mars EUV, impact-related or other early loss are needed to provide a consistent picture of processes. These measurements would also the formation and evolution of these planets allow tighter constraints on the how much of and their atmospheres, and in particular the the gas inventory originated from the original history of water on Venus. Early Venus would disk, how much came from the solar have had an atmosphere rich in carbon dioxide wind, and how much came later from and water vapour, like that of Earth. and comets. Late impacts such from this early steam as the Earth’s moon-forming impact also have atmosphere would have been rapid – but an effect on noble gas isotopic ratios so can would it have been rapid enough to lose all of also be constrained through these measure- Venus’ water before the planet had cooled ments. enough to allow water to condense? If Venus did have a liquid water ocean, how long did this Light element isotopic ratios, namely H, C, O era persist before the runaway greenhouse N etc, provide further insights into the origin warming ‘ran away’, with the and subsequent histories of planetary evaporating and the resulting water vapour atmospheres. Measurement of 20Ne/21Ne/22Ne being lost to space? The of early escape and/or of 16O/17O/18O would enable processes is as yet too poorly constrained to determination of whether Earth, Venus and answer these questions. An early Venus with a Mars came from the same or from different liquid water ocean would have arguably have parts of the protoplanetary nebula, i.e. are been more Earthlike than was early Mars and they are truly sibling planets of common origin. could have taken steps towards development Venus’ enhanced to ratio, of life. A habitable phase for early Venus would 150 times greater than that found on Earth, have important consequences for our suggests that hydrogen escape has played an understanding of and the important role in removing water from the habitable zones of exoplanets. , removing more than a terrestrial ocean’s worth of water during the More detailed treatments of Venus isotope first few hundred million years of the planet’s geochemistry goals and interpretation can be found in Chassefière et al., 2012 and Baines et evolution (Gillmann et al., 2009). 14N/15N ratios have been found to vary considerably in the al., 2007. solar system, with the , comets, and

Page 11 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

2.3 (Dynamics, Chemistry & clouds, structure & radiative balance)

The terrestrial planets today have very cloud features returns information about different . Study of the fundamental winds at altitudes from 48 to 70 km altitude, processes at work on these three planets will depending on the wavelength used. However, lead to a deeper understanding of how it is not clear at what altitude to assume that atmospheres work, and of climates evolve. the derived cloud vectors apply; the formation mechanism for the observed contrasts is not Dynamics and thermal structure known so it cannot be ascertained to what One of the crucial factors determining extent the derived velocity vectors represent is the redistribution of true air motion rather than the product of, for heat around the planet. The solid planet of example, wave activity. Furthermore, cloud Venus rotates only once every 243 days but the tracking on the day- and night-sides of Venus is atmosphere above exhibits strong super- accomplished using different wavelengths, rotation, circling the planet some 40-50 times referring to different altitudes, so global faster than the solid planet below. General averages of wind fields are not achievable by Circulation Models are now able to reproduce wind tracking, frustrating attempts to super-rotation, but are very sensitive not only understand global circulation. to model parameters but also to the details of Direct measurement of mesospheric wind how those models operate. Efforts to improve velocities (or at least their line-of-sight modelling of the Venus atmosphere have led components) from orbit can be achieved by to improvements in how Earth handle details using Doppler sub-millimetre observations, like conservation of angular momentum and Doppler or other such instruments, and temperature dependent specific heat this would help to constrain circulation capacities [Bengtsson et al., 2013]. Exchanges models. Tracking of descent probes will yield of momentum between surface and direct measurement of the vertical profile of atmosphere, an important boundary condition horizontal winds in the deep atmosphere, for the atmospheric circulation, depend which will provide important constraints on sensitively on the thermal structure in the the mechanisms of super-rotation. Balloon lowest 10 km of the atmosphere. Many probes elements are ideal for measuring vertical wind experienced instrument failure at these high speeds but also provide information about so the atmospheric structure in wave and tidal activity at constant altitude. the lowest parts of the atmosphere is not well known. The atmospheric circulation is driven Chemistry and clouds by solar absorption in the upper cloud; most of this energy absorption is by an as yet Venus has an enormous atmosphere with unidentified UV absorber, which is spatially many complex chemical processes at work. Processes occurring near the surface, where and temporally variable. Efforts to identify this UV absorber by remote sounding have failed, carbon dioxide becomes supercritical and so in situ identification will be necessary. many metals would melt, are very different from those at the where Knowledge of the wind fields on Venus is temperatures can be below -150°C, colder currently achieved by tracking cloud features than any found on Earth. A diversity of or by tracking descent probes. Tracking by observations is clearly required to understand

Page 12 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 this diversity of environments. While chemical to achieve. However, as mentioned above, processes in the and lower near surface abundances and their vertical are now being studied by Venus profiles will permit determination of whether Express, processes in the clouds and below are there are active surface-atmosphere very difficult to sound from orbit and require in exchanges taking place and of what surface situ investigation. reactions are buffering the atmospheric composition. The dominant chemical cycles at work in Venus’s clouds are those linking the sulphuric Thermal structure and radiative fluxes acid and sulphur dioxide: Sulphuric acid is photochemically produced at cloud-tops, has a The cloud layer of Venus is highly reflective so net downwards transport through the clouds, Venus presently absorbs less power from the sun than does the Earth. Its high surface and then evaporates and then thermal temperature is instead caused by its enormous dissociates below the clouds; this is then balanced by net upwards transport through greenhouse warming effect, caused by carbon the clouds of its stable chemical precursors dioxide, water vapour and other gases. Its clouds, too, have a net warming effect because (SO2 and H2O). Infrared remote sensing observations of Venus can be matched by they prevent thermal fluxes from escaping the assuming a cloud composition entirely of deep atmosphere. sulphuric acid mixed with water, but -D radiative and radiative-convective models descent probe XRF measurements found also for the determination of climate are suddenly tantalising evidence of P, Cl and even Fe in the widespread as researchers worldwide attempt cloud particles [Andreichikov et al., 1987]. If to determine the likely climate of exoplanets. confirmed these measurements would provide Venus offers a proving ground for these important clues as to exchanges with the models much closer to home, one where the surface: are these elements associated with conditions are much better known than on volcanic, Aeolian or other processes? An in situ exoplanets. Radiative transfer calculations on chemical laboratory floating in the clouds, or Venus are difficult: uncertainties in the multiple descent probes, would be needed to radiative transfer properties of carbon dioxide address these measurement goals. at high temperatures and are the As to lower atmosphere chemistry, it is poorly main unknown, particularly in the middle- and understood because the rapidly falling descent far- infrared where there are no spectral window regions to allow empirical correction. probes did not have time to ingest and fully As on Earth, clouds play an important role, analyse many atmospheric samples during their brief descent. Sub-cloud hazes were reflecting away sunlight but also trapping detected by several probes but their upwelling infrared . The state-of-the art Venus radiative balance are still mainly 1-D composition is unknown. Ground- and space- based observations in near-infrared window models representing an average over the regions permit remote sounding of only a few whole planet. However, we now know that the clouds are very variable; the vertically major gases, but many minor species which may play important catalytic or intermediate integrated optical thickness (as measured at roles in chemical cycles cannot be probed 0.63 µm) can vary by up to 100% [Barstow et al 2012] and the vertical structure of clouds remotely. Spatial variation of volcanic gases would be a direct way of finding active varies strongly with latitude. In-situ but would be very difficult measurements of cloud properties with co-

Page 13 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 located radiative flux measurements are needed to determine the diversity of cloud effects on the global radiative balance.

3 Mission elements

A satellite in low, near-circular orbit is hazard is the concentrated sulphuric acid required for radar mapping, and for LIDAR and which makes up the cloud particles; however, Doppler sub-mm measurement of wind effects can be mitigated by choosing speeds. Wide-angle cameras like the appropriate materials for external surfaces. MRO/MARCI camera can be used to obtain Balloons at this altitude can take advantage of continuous imaging coverage of UV cloud-top the fast super-rotating winds which will carry features. Recent examples of proposed low the balloon all the way around the planet in a circular Venus orbiters include the Envision week or less (depending on latitude and radar mapper [Ghail et al., 2016], the RAVEN altitude). Horizontal propulsion (with motors) radar mapper [Sharpton et al., AGU 2009], is not advised because of power requirements VERITAS radar mapper [Hensley et al., 2015], and the difficulty of countering the fast (250 MuSAR radar mapper [Blumberg, Mackwell et km/h) zonal windspeed. A cloud-level balloon al., URSI 2011], and the Vesper sub-mm is an ideal platform for studying interlinked sounding orbiter [Allen et al., DPS 1998]. dynamical chemical and radiative cloud-level processes. It also offers a thermally stable A satellite in a highly elliptical orbit can long-lived platform from which measurements provide synoptic views of an entire of noble gas abundances and isotopic ratios hemisphere at once; One example of this is can be carefully carried out and repeated if Venus Express, whose polar apocentre allows necessary (in contrast to a descent probe, it to study the circulation of the South which offers one chance for making this polar region; a second example is the nearly measurement, in a rapidly changing thermal equatorial 40-hour orbit of Japan’s Akatsuki environment). orbiter, which allows it to dwell over low- latitude cloud features for tens of hours at a Balloons can be used to explore a range of time. The large range of altitudes covered is altitudes. Operation in the convectively stable also useful for in situ studies of thermosphere upper clouds, above 63 km, would be optimal and ionosphere and thus for studies of solar for identification of the UV absorber, but the wind interaction and escape. low atmospheric density to a relatively small mass fraction for scientific payload. Balloons are ideally suited for exploring Venus Operation below the main cloud deck at 40 km, because they can operate at altitudes where has been proposed by Japanese researchers, pressures and temperatures are far more with a primary goal of establishing wind fields benign than at the surface. Deployment of two below the clouds. Balloons can also be used to small balloons at 55 km altitude, in the heart of image the surface, if they are within the lowest the main convective cloud layer, was 1-10 km of the atmosphere, but high successfully demonstrated by the Soviet VeGa temperatures here require exotic designs such mission in 1984. At this altitude, the ambient as metallic bellows which are beyond the temperature is a comfortable 20° C and the scope of this paper. An intriguing possibility for pressure is 0.5 atm. The main environmental

Page 14 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

High priority Venus questions can be addressed by a broad range of mission concepts using surface, aerial, and orbital platforms. While many missions can be implemented in the near-term using existing capabilities, investments in new technologies will enable long term surface science as well as missions that take advantage of mobility in the surface, near-surface, and atmospheric environments. Venus exploration will be undertaken by a range of international agencies; for the period 2030 - 2050, we recommend that ESA focus on launching at least one geophysics orbiter and one cloud-level balloon. revealing winds in the lower atmosphere is to measurements at cloud level then a parachute use passive balloons, reflective to radio waves, may be deployed during the initial part of the which could be tracked by radar – this entry phase in order to slow the rate of descent possibility should be studied further if a radar during the clouds. This was carried out, for + entry probes architecture were to be studied example, by the Pioneer Venus Large Probe. further. Alternatively, if the main focus of the probe is measurements in the lower atmosphere or Descent probes provide vertical profiles of surface then the probe may dispense with a composition, radiation, chemical composition parachute completely. Descent imagery of as a function of altitude, and enable access to impact/landing site at visible wavelengths will the surface. Science goals for a descent probe be invaluable for contextualising surface include: cloud-level composition and results; Rayleigh scattering limits the altitudes microphysical processes; near-surface from which useful surface imagery can be composition, winds, and temperature obtained to ~1 km if imaging in visible structure; surface composition and imaging; wavelengths, or to 10 km if imaging at 1 µm and noble gas abundances and isotopic ratios. wavelength [Moroz, PSS 2002]. If the scientific focus of the probe is

Page 15 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

Landers must cope with the harsh conditions Gamma- and XRF spectroscopy have been of ~450 °C at the surface of Venus. Venera- performed on Venera and Vega landers, but style landers use only thermal inertia and modern equivalents of these instruments thermal insulation to keep a central electronics would provide much improved accuracy; also, compartment cool, allowing operation times of repeating the composition analyses at a only hours [see e.g. Venera-D mission, tessera region (not before sampled) would Vorontsov et al., Solar System Research 2011]. reveal whether these tessera regions are The principal science payload of a such a lander chemically differentiated from the lava plains would include surface imagers and non- where previous analyses have been contact mineralogical sensors such as Gamma conducted. spectrometer with Neutron activation, capable As discussed above, long-lived landers would of measuring elemental abundances of U, Th, K, Si, Fe, Al, Ca, Mg, Mn, Cl (Li, Mitrofanov et provide not only essential meteorological and al., EPSC 2010) and/or Raman/LIBS (Clegg et seismic measurements – but also would serve as essential precursors for post-2050 surface al., LPSC 2011). Inclusion of surface sample ingestion via a drill/grinder/scoop would allow missions including more capable seismometry further analysis techniques (e.g. mass stations (more like Insight in their capability) but also for eventual surface rovers – which we spectroscopy; X-ray fluorescence (XRF) spectroscopy) but would require significant envisage as post-2050 developments. technology development and verification.

4 A strawman L - class mission architecture

A Large mission to Venus should include both situ elements, but also to place those in situ orbital and in situ science measurements. One measurements into atmospheric and possible strawman mission concept which geological context. The in situ measurements would could address this theme would be a are required to measure parameters, like combination of an orbiter, a cloud-level noble gas abundances and surface mineralogy, balloon platform, and (optionally) a Russian which cannot be determined from orbit. The descent probe. As a strawman payload, we whole mission is greater than the sum of its suggest the balloon element be modelled on parts. This has been amply demonstrated by the 2010 EVE M3 proposal [Wilson et al., the of missions at Mars, and by 2011]. The radar orbiter may be based on a the Cassini/ collaboration at . reuse of ESA’s GMES Sentinel-1 InSAR technology, whose application at Venus was Not all of these mission elements need be first described in the 2010 Envision M3 provided by ESA; There is ample scope for international co-operation in creating this proposal [Ghail et al., 2012]. Finally, the landing probe envisaged is based on the lander mission architecture. In particular, Russia has component of the Venera-D mission unequalled heritage in providing Venus descent probes from its Venera and Vega [Vorontsov et al., 2011] descent probes, and will gain new heritage It is very important to have multiple mission from its Venera-D lander, planned for the elements working together simultaneously at coming decade. A range of mission proposals Venus. An orbiter is necessary both to increase have been developed in the USA for orbiters, vastly the volume of data returned from the in balloons and descent probes, from Discovery-

Page 16 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 class to Flagship-class, many of which could separate launchers, for example in order to form parts of a joint NASA-ESA exploration insert the orbiter into a low circular orbit programme should a high-level agreement be before the arrival of the in situ elements for reached. Japan has an active Venus research optimal data relay and context remote community, with its Akatsuki (Venus Climate sounding for the in situ measurements. Orbiter) spacecraft still in , and has been developing prototypes for a Venus sub-cloud This scenario, Orbiter + balloon + Descent Probe, was proposed to ESA in 2007 in balloon [Fujita et al., IPPW 2012]. After its successful moon and Mars missions, ISRO has response to the M1/M2 mission Call for Ideas been developing a Venus orbiter which may as a joint European Russian mission, with a European-led orbiter and balloon, and a launch as soon as 2023 – but details are not Russian descent probe. For EVE 2007, the known. Israel’s TECSAR satellites enable 1-m scale radar mapping with a 300 kg satellite, in entire mission was to be launched on a single the frame of future ESA-Israel agreements, Soyuz launch, however subsequent studies revealed that this scenario was not consistent collaborations on Venus radar could be fruitful. In this time frame collaborations with China with a single Soyuz launcher, and would be are also feasible. more consistent with an L-class rather than an M-class opportunity. These could be launched as a stack on a single launcher, or it may prove convenient to use

5 Technology developments needed

Much of the technology required for Venus return from a radar orbiter. High speed entry missions in the Voyage 2050 period already modelling, thermal protection systems and exists at a high Technology Readiness Level, parachute development will all be useful for but further technology development both for Venus entry probes. Nuclear power systems spacecraft technologies and for science are not necessary for the strawman Venus payload technologies would be useful to mission elements described here, but would maximise science return and de-risk mission enable longer lifetimes and increased nightside aspects, and to pave the way for future surface operations for the balloon element of the missions. mission, and will be necessary for long-lived surface stations (in the even more distant Many of the mission-enabling technologies future) due to the scarcity of sunlight reaching required for Venus exploration are shared with the Venus surface and long night-time other targets. /aerocapture would duration. improve Δv and mass budgets for Venus orbiters. Further improvement in deep space Two technology areas specific to Venus communications, including development of Ka- exploration are balloon technology, and high- band or optical communications, would temperature components. The balloons provide increased data return from orbiters proposed in this strawman mission (based on which would be particularly useful for the large EVE M3 proposal) are superpressure volumes of data generated by high-resolution balloons, which are designed to float at radar instrumentation at Venus – 100 Mbps or constant altitude. Although ESA has little better would be enable the maximum scientific familiarity with this technique, thousands of

Page 17 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 helium superpressure balloons have been is particular important at Venus because launched on Earth, and two were successfuly Venus’ cloudy atmosphere is opaque to optical deployed on Venus in 1985 as part of the imagers. It can be argued that every Venus Russian VeGa programme, so this is a very orbiter should carry a radar imager, just as mature technology. Air-launching of balloons – almost every Mars orbiter has carried an deploying them from a probe descending optical imager, to enable follow up of under parachute – was achieved by the Russian geologically important targets, such as active VeGa balloons, was demonstrated by CNES in volcanic and tectonic regions, with ever more VeGa development programmes, and capable imagers. Development of more demonstrated recently by JPL engineers in capable and mass efficient , with ever their own Venus balloon test programme; increasing spatial and radiometric resolution, nevertheless, a new demonstration and capabilities such as polarimetry and programme in Europe would be required to DiffInSAR, will be invaluable at Venus. obtain recent European experience for this Leveraging the investment in Earth observing technology. Balloon envelope design and radar systems for application to Venus will be sulphuric acid resistance verification would advantageous. Investment in optical and sub- also be valuable to conduct in Europe. mm heterodyne receivers would hasten the Feasibility studies on other forms of aerial development of instruments capable of mobility, including phase change fluid measuring mesospheric wind velocities using balloons, <5 kg microprobes and fixed wing Doppler techniques. aircraft, would also be useful for expanding the possibilities of long-term future exploration For atmospheric in situ measurements, a key programmes. Compact X-band phased array instrument is a mass spectrometer with getters and cryotraps to isolate and precisely measure antennae are well suited for mobile atmospheric platforms like balloons (or indeed the noble gas and light element isotope rovers). Investment in all of these systems, in abundances. These technologies have been developed for Mars (e.g. MSL/SAM, collaboration with national agencies, would be valuable. ExoMars/PALOMA proposal), but further development to maximise the precision of the High temperature technologies are needed if measurements and to optimise the Europe is to provide mission elements or development for the thermal environment of payloads which need to operate in the lower Venus balloons and descent probes will be atmosphere, below 40 km altitude. For a long- needed. In situ GC+MS characterisation duration surface package, a full suite of is another mature field, subsystems will need to be developed from but further development in particular of an batteries and power distribution, to data aerosol collector system to allow detailed handling, to telecommunications. The characterisation of cloud particle composition development status of these technologies is would be valuable. reviewed elsewhere [Wilson et al., 2016, Kremic et al, 2018]. Specific payload developments for a landing probe should also include surface Further investment in scientific payload characterisation instruments – gamma ray and development is also needed to prepare for the neutron spectrometers, XRF/XRD, Raman Voyage 2050 timeframe. Continued instruments for mineralogical identification. development of planetary radar technologies, High-temperature drilling and sample though applicable to many planetary missions, ingestion systems are not currently proposed

Page 18 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019 for the Venera-D lander included in the In summary, most of the technologies needed strawman mission, but some feasibility studies for the proposed Voyage 2050 Venus mission in this area would be a useful investment. already have high levels of heritage either from Finally, high temperature chemical, Earth or from other planetary missions, but a meteorological and seismological sensors well-targeted development programme would using silicon carbide semiconductor de-risk mission elements and maximise the technology would usefully complement a science return. Many of the development lander’s payload. areas identified would also benefit other space missions and have spin-out potential on Earth.

6 Conclusions As we become aware of Earth’s changing climate, and as we discover terrestrial planets in other solar systems, we gain ever more reasons to study the Earth’s nearest neighbour and closest sibling, the only Earth-sized planet besides our own that can be reached by our spacecraft.

For the scientific and programmatic reasons outlined in this document, Venus is a compelling target for exploration. The science themes important for Venus research – comparative planetology and planetary evolution – are common to all of planetary and exoplanetary science Many of the payloads required – radar and atmospheric remote sensing, in situ mass spectrometers – are common to mission proposals for many other solar system targets, as are mission technologies like high rate deep-space telecommunications technologies. Venus-specific technology developments meriting special attention include high-temperature systems and balloons.

Venus is an excellent proving ground for fundamental understanding of geophysical processes of terrestrial planets; an excellent proving ground for techniques of analysis of exoplanets; an indispensable part of our quest to understand the evolution of Earthlike planets. For all these reasons, Venus will be an ever more compelling theme in the coming decades, and we therefore recommend its inclusion in the Voyage 2050 plan. We recommend that ESA aim to have launched at least two M-class Venus missions by 2050, including the EnVision M5 geophysics orbiter, and an in situ element such as a cloud-level balloon; or an L-class mission combining these elements.

Page 19 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

Fujita et al., (2012), An overview of Japan’s References planetary probe mission planning. International Planetary Probe Workshop, Toulouse, France, Allen, M. et al. (1998), The VESPER Mission to 2012. Venus. American Astronomical Society, DPS meeting abstract #48.P08. Ghail, R.C. et al. (2012), EnVision: taking the pulse of our twin planet. Experimental , Andreichikov, B.M. et al. (1987), VEGA 1 and 2 X- doi:10.1007/s10686-011-9244-3. ray radiometer analysis of the Venus cloud aerosol. Kosmicheskie Issledovaniia (ISSN 0023- Ghail, R.C. et al (2016), EnVision: understanding 4206), vol. 25, Sept.-Oct. 1987, p. 721-736. why our most Earth-like neighbour is so different. ESA M5 mission proposal. Available at Anurup, M.S. et al. (2012), Indian Scientific Mission https://arxiv.org/abs/1703.09010. to Venus – Future Prospects and Experiments. National Space Science Symposium 2012, Tirupati, Hashimoto, G.L. & Abe, Y. (1997). Venus’ surface India. temperature controlled by a coupled mechanism of chemical and feedback. Bull. Amer. Baines, K. H. et al. (2007), Experiencing Venus: . Soc. 29, 1043. Clues to the origin, evolution, and chemistry of terrestrial planets via in-situ exploration of our Helbert, J. et al. (2008), Surface brightness sister world. In Exploring Venus as a Terrestrial variations seen by VIRTIS on Venus Express and Planet. Geophysical Monograph 176, American implications for the evolution of the Geophysical Union, Washington, DC. pp. 171-189. region, Venus. Geophysical Research Letters, doi: 10.1029/2008GL033609 Barstow, J.K. et al (2012), Models of the global cloud structure on Venus derived from Venus Hensley, S. et al. (2015), VISAR : A Next Generation Express Observations. Icarus, Interferometric Radar for Venus Exploration. doi:10.1016/j.icarus.2011.05.018 . Proceedings of the 2015 IEEE 5th Asia-Pacific Bengtsson, L. et al. (2013), Towards Understanding Conference on Synthetic Aperture Radar (APSAR), the Climate of Venus: Applications of Terrestrial doi: 10.1109/APSAR.2015.7306225 Models to Our Sister Planet. International Space Science Institute Scientific Report, Karatekin, Ö. et al. (2011), Atmospheric angular doi:10.1007/978-1-4614-5064-1. momentum variations of Earth, Mars and Venus at seasonal timescales. Planetary and Space Science, Blumberg, D.G. et al (2011), MuSAR: a novel SAR doi:10.1016/j.pss.2010.09.010 . mission to Venus. Int. Union of Radio Science General Assembly 2011, abstract J05.1. Kopparapu, R.K.. et al. (2013), Habitable zones around main-sequence : new estimates. Chassefière, E. et al. (2012), The evolution of Astrophys. J., doi: doi:10.1088/0004- Venus: Present state of knowledge and future 637X/765/2/131 exploration. Planetary and Space Science, doi:10.1016/j.pss.2011.04.007 . Kremic, T. et al., (2018) Seismic and Atmospheric Exploration of Venus (SAEVe) Final Report. Fegley B. Jr. & Treiman A.H. (1992) Chemistry of Available from NASA Venus EXploration Analysis Atmosphere-Surface Interactions on Venus and Group (VEXAG) website: Mars. in Venus and Mars: Atmospheres, https://www.lpi.usra.edu/vexag/reports/SAEVe-6- , and Solar Wind Interactions. AGU, 25-2018.pdf or see relevant abstracts. Geophysical Monograph No. 66, 7–71. Lebonnois, S. & Schubert, G. (2017), The deep Gillmann, C., et al. (2009), A consistent picture of atmosphere of Venus and the possible role of early hydrodynamic escape of Venus atmosphere density-driven separation of CO2 and N2. Nature explaining present Ne and Ar isotopic ratios and Geoscience 10, p 473–477 (2017). low atmospheric content. Earth Planet. Sci. doi:10.1038/NGEO2971. Lett. 286, 503–513. Limaye, S. et al (2013), Venus: a natural planetary Guest, J.E. & Stofan, E.R. (1999), A New View of laboratory. White Paper submitted in response to the Stratigraphic History of Venus. Icarus, ESA’s Call for the definition of science themes for doi:10.1006/icar.1999.6091. L2/L3 missions in the ESA Science Programme, May 2013. Can be downloaded from

Page 20 of 21 Venus Voyage 2050 White Paper Wilson, Widemann et al. August 2019

http://sci.esa.int/cosmic-vision/52030-white- Smrekar, S.E. et al. (2010), Recent Hotspot papers-submitted-in-response-to-esas-call-for- Volcanism on Venus from VIRTIS emissivity data, science-themes-for-the-l2-and-l3-missions/. Science, 328, p. 605-608, doi:10.1126/science.1186785 . Marcq, E. et al., Variations of sulphur dioxide at the cloud top of Venus’s dynamic atmosphere. Sparks, R.S.J. et al. (2012), Monitoring volcanoes. Nature Geoscience, doi:10.1038/NGEO1650 . Science, doi:10.1126/science.1219485 .

Marcq, E., (2013) et al. Europe returns to Venus. Stofan, E.R. & Smrekar, S.E., in Plates, Plumes, and White Paper submitted in response to ESA’s Call Paradigms, (Special Volume 388, Geological for the definition of science themes for L2/L3 Society of America, Denver, CO, 2005), pp. 861– missions in the ESA Science Programme, May 885. 2013. Can be downloaded from http://sci.esa.int/cosmic-vision/52030-white- Turcotte, D. L.et al. (1999), Catastrophic papers-submitted-in-response-to-esas-call-for- resurfacing and episodic subduction on Venus. science-themes-for-the-l2-and-l3-missions/ Icarus, doi:10.1006/icar.1999.6084 .

Mitrofanov et al., (2010), Neutron-Activated Vorontsov, V.A. et al. (2011), Prospective Gamma Ray Spectrometer (NAGRS) for the Venus Spacecraft for Venus Research: Venera-D Design. Surface and Atmosphere Geochemical Explorer Solar System Research, (SAGE) mission. European doi:10.1134/S0038094611070288. Congress 2010, abstract #264. Wilson, C.F. et al. (2011), The 2010 European Moroz, V.I. (2002), Estimates of visibility of the Venus Explorer (EVE) mission proposal. surface of Venus from descent probes and Experimental Astronomy, doi:10.1007/s10686-011- balloons. Planetary and Space Science, 9259-9. doi:10.1016/S0032-0633(01)00128-3. Wilson, C.F. et al (2013), Venus: Key to Mueller, N. et al. (2008), Venus surface thermal understanding the evolution of terrestrial planets. emission at 1 μm in VIRTIS imaging observations: White Paper submitted in response to ESA’s Call Evidence for variation of crust and mantle for the definition of science themes for L2/L3 differentiation conditions, Journal of Geophysical missions in the ESA Science Programme, May Research, doi: 10.1029/2008JE003118 2013. https://arxiv.org/abs/1703.10961

Sharpton, V.L. et al. (2009), RAVEN - High- Wilson, C.F. et al (2016), Venus Long-life Surface resolution Mapping of Venus within a Discovery Package. White paper submitted in response to Mission Budget. AGU Fall Meeting 2009 abstract ESA's Call for New Scientific Ideas, September #P31D-04. 2016. https://arxiv.org/abs/1611.03365

Page 21 of 21