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Understanding the divergent evolution of Venus and Earth is a fundamental problem in planetary science. Although Venus today has a hot, dry atmosphere, recent modeling suggests that Venus may have had a clement surface with liquid water until less than 1 billion years ago [1]. Venus today has a nearly stagnant lithosphere. However, Ishtar Terra’s folded mountain belts, 8-11 km high, morphologically resemble Tibet and the Himalaya mountains on Earth and apparently require several thousand kilometers of surface motion at some time in Venus’s past. Loss of liquid surface water increases the coefficient of friction in fault zones, favoring a transition from an early mobile lithosphere to a present-day stagnant lithosphere [2]. Solar-driven climate evolution could contribute to a prolonged epoch of water loss on Venus and may be the ultimate cause of the divergent evolution of both the climate history and the interior dynamics of Venus and Earth. In addition to being a problem of first-order importance for Solar System evolution, understanding the divergent evolution of Venus and Earth is also important for understanding the temporal and spatial distribution of habitable environments in the Solar System. Understanding the evolution of Venus is also a key test for models that interpret Earth-sized exoplanets. Testing evolutionary hypotheses requires interpreting clues that were left behind in both the isotopic composition of the Venus atmosphere and in the rock record of the Venus surface. Although several mission concepts are currently competing for possible flights to Venus, only the Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus (DAVINCI+) mission [3] can examine both the atmospheric isotopic record and the rock record of Venus. DAVINCI+ is therefore a compelling choice for selection in the current NASA Discovery Program Phase A competition. DAVINCI+ includes an atmospheric entry probe and a carrier spacecraft (Figure 1). The probe measures atmospheric composition using a mass spectrometer and tunable laser spectrometer, performs descent imaging, and measures atmospheric structure. Following completion of the probe mission, the carrier spacecraft enters Venus orbit and images the Venus surface in the 1 micron atmospheric window. This payload is ideally suited for testing models of Venus evolution. Figure 1: The DAVINCI+ entry probe studies the atmosphere while imaging the Alpha Regio landing site. Afterward, the carrier probe performs infrared imaging of selected targets from orbit. The History of Water: Isotopic Record Pioneer Venus measured the D/H value of an H2SO4 cloud droplet at ~55 km as 157±30 times the terrestrial value, which was interpreted as the signature of escape of water from the Venus atmosphere [4]. Terrestrial spectroscopy produced a similar range, whereas Venus Express measured a value up to three times larger between 70-90 km [5]. The large value relative to Earth shows that Venus lost a substantial amount of water, but the large uncertainty and the lack of data below the clouds makes it difficult to quantitatively model the history of water loss [6]. DAVINCI+ will measure D/H to high precision from above the clouds down to the surface, greatly improving our ability to interpret the history of water loss on Venus. In addition, the abundance and isotopic ratios of Xe and Kr, together with the Kr/Ar and Xe/Ar ratios, which DAVINCI+ will measure with high precision, will be instrumental in revealing whether Venus and Earth formed in the same way and how their climates diverged. The History of Water: Rock Record A key but poorly answered question is the extent to which Venus has produced granitic or felsic (SiO2-rich) volcanism. Small amounts of felsic magma can be generated by lithospheric processes [7], but large amounts of felsic material requires the presence of water in the melting zone [8], as in terrestrial subduction zones. Tessera, which are regions of old, thick, highly tectonized crust, are widely accepted as the most likely location for felsic material on Venus. Venus Express observations suggest that tessera in Alpha Regio has a felsic composition [9]. DAVINCI+ will explore the presence and distribution of felsic rock in two ways. Comparison of the reflectivity at 1 micron and in panchromatic descent images at the Alpha Regio landing site will test the presence of felsic rock at patch sizes much smaller than can be observed from orbit. Descent imaging will also explore the landing zone geomorphology, and stereo topography will enable quantitative modelling of faulting and folding. Orbital imaging in the 1 micron atmospheric window will test for the presence of felsic rock in other tessera, including Tellus Regio, Fortuna Tessera, Maxwell Montes, Ovda Regio, and Thetis Regio. Our approach is similar to VIRTIS on Venus Express [9] but focuses on regions in the northern hemisphere and near the equator that were not imaged by VIRTIS. The History of Volcanism: Isotopic Record Volcanic outgassing releases radioactive decay products such as 40Ar and 4He to the atmosphere. DAVINCI+ measurements of their atmospheric abundance can be used to estimate volcanic outgassing over time. 40Ar provides an integrated record of volcanism over Venus history. Because 4He escapes from the atmosphere to space, its atmospheric abundance constrains geologically recent (last billion years) volcanism. Existing measurements of 40Ar and 4He are too imprecise to strongly constrain the volcanic history of Venus [10] but will be measured with much greater accuracy by DAVINCI+. Because DAVINCI+ will constrain the history of both water and volcanism on Venus, it will provide new insights into the feedbacks that shaped the divergent evolution of Venus and Earth. References [1] Way and Del Genio, JGR 125, e2019JE006276, 2020. [2] Weller and Kiefer, JGR 125, e2019JE005960, 2020. [3] Garvin et al., LPSC 51, abstract 2599, 2020. [4] Donahue et al., Science 216, 630-633, 1982. [5] Bertaux et al., Nature 450, 646-649, 2007. [6] Donahue et al., Venus II, 385-414, 1997. [7] Elkins-Tanton et al., JGR 112, E04S06, 2007. [8] Campbell and Taylor, GRL 10, 1061-1064, 1983. [9] Gilmore et al., Icarus 254, 350-361, 2015. [10] Namiki and Solomon, JGR 103, 3655-3677, 1998.

If Venus were a newly discovered exoplanet, it would be one of the most Earth-like yet identified. Its similarity in size, bulk density and cloud top temperatures give no clue to the hellish temperatures at its surface. Venus hosts an array of geological features as complex as Earth, but without its organisation, and sustains a chemically reactive atmosphere, but without life. Proposed in response to ESA’s M5 call, with enabling support from NASA, EnVision is currently in Phase A study for the next medium-class mission opportunity proceeding towards a final mission selection in summer 2021 for a launch in 2032. Here we discuss the science questions motivating the mission in more detail. Building on discoveries from Magellan, Venus Express and Akatsuki, EnVision will focus on three overarching questions: Is Venus geodynamically active? How did Venus arrive at its current state? How does the Venus climate work and how do the interior, surface and atmosphere interact? 1 – Is Venus geodynamically active? Venus should be geologically active today and its globally young surface implies extensive volcanic resurfacing, but whether this happened in episodic global events or in a continuous process of small-scale resurfacing is uncertain. Constraining the rate of volcanic and tectonic activity can reveal whether Venus occupies one of these end members, or lies somewhere on the spectrum between. Magellan radar image mosaic of a 550 km wide region of Lada Terra (47°S, 25°E) showing a system of radar-bright and dark lava flows breaching a ridge belt and ponding in a large radar-bright deposit covering 100,000 km². Ammavaru, the source caldera, lies 300 km to the west (JPL/Caltech). Understanding the tectonic regime driving resurfacing and heat loss is as important as knowing the rate of activity. The surface appears partitioned into areas of low strain bounded by narrow high strain margins. The abundance of steep slopes and landslides implies active uplift in these high strain areas, but existing data provide no constraint on the frequency of landslides or rates of tectonic movement. Are the low strain regions actively created and destroyed, like Earth’s oceanic plates, or simply mobilised locally? What is the significance of the global network of elevated rift systems and linear lowland wrinkle-ridged plains? Unique to Venus are coronae, quasi-circular volcano-tectonic features, typically 100–500 km across. Are they the surface expression of plumes, or magmatic intrusions, or subduction zones? The processes of weathering, mass wasting and aeolian transport are critical for understanding both the geological history and climatic evolution of Venus. Venus Express found anomalously high IR emissivities near suspected active volcanoes, interpreted as fresh, unweathered lava flows, but both their mineralogy and the weathering processes involved are unknown. Magellan detected several, probably impact-related, dune fields at the limit of its resolution, and found indirect evidence for globally distributed smale-scale ripples. 2 – How Did Venus Arrive at its Current State? The cratering record reveals that the Venus surface is, on average, under 1 Ga old. While some areas are likely active today, other regions, e.g. the tessera highlands, may be considerably older. Magellan data imply a variety of age relationships and long-term activity, with a non-random association between geological features and elevation, e.g. the uplands are consistently more deformed than the lowlands. Impact craters themselves show alteration to dark floors, perhaps from airfall deposits or magmatic resurfacing. Densely fractured plains (right) abutting tessera (bottom); both are embayed by younger plains (dark areas). A steep-sided volcanic dome (upper left) may be the source of the plains or unrelated to them, and older or younger than the tesserae. Magellan image at 46°N, 360°E, after Basilevsky and McGill (2007). Constraining the history is critical to understanding not only when and how resurfaced, but whether that activity has changed systematically through time. Were the plains formed in a short period by massive outpourings or by many thousands of small flows over their entire history? Or did many different mechanisms – including sedimentary processes – operate at different times and places? Globally, Venus exhibits perhaps even more tectonic deformation than Earth: what is its role in resurfacing the planet and have the regimes of tectonic deformation changed over time? How did tesserae, especially, accumulate their extraordinary degree of deformation? The interior of Venus is probably Earth-like, but not the same; its core size is poorly constrained by Magellan gravity data. These data are consistent with an organised pattern of mantle convection but lack the resolution necessary to connect it with geological-scale features. 3 - How does the Venus climate work and how do the Interior, Surface and Atmosphere Interact? How and why the atmospheres of two Earth-like planets evolved so differently is one of the many compelling reasons to study Venus, directly addressing the question of how our own world became habitable. Understanding its evolution depends on knowing the exchanges between its interior, surface and space. The lower/middle clouds of Venus as imaged by Akatsuki IR2 camera. Dark areas represent thicker clouds, which may represent volcanic plumes of ash or sulphate particulates. (JAXA/ISAS/DART/Damia Bouic) Models suggest that the clouds are maintained by a constant input of H2O and SO2, both of which vary considerably, perhaps because of volcanic emissions. Elevated D/H ratios indicate the loss of early oceans, but the ‘starting’ mantle D/H ratio is unknown. Identification of granites in the ancient highlands would support this inference, but the direct detection and characterisation of volcanic volatiles is key to inferring past climate evolution. Beyond the specific investigations outlined above, however, the most important unknown factor in determining the present day state of Venus and its atmosphere, and how it arrived at that state, is the interaction between the interior, surface and atmosphere: the whole being more than the sum of the parts. References Basilevsky and McGill (2007), Surface evolution of Venus, in Exploring Venus as a Terrestrial Planet, Geophysical Monograph Series 176, edited by L. W. Esposito, E. R. Stofan and T. E. Cravens, pp. 23-43, American Geophysical Union, Washington, DC