Possible Effects of Volcanic Eruptions on the Modern Atmosphere of Venus

This work reviews possible signatures and potential detectability of present-day volcanically emitted material in the atmosphere of Venus. We first discuss the expected composition of volcanic gases at present time, addressing how this is related to mantle composition and atmospheric pressure. Sulfur dioxide, often used as a marker of volcanic activity in Earth's atmosphere, has been observed since late 1970s to exhibit variability at the Venus' cloud tops at time scales from hours to decades; however, this variability may be associated with solely atmospheric processes. Water vapor is identified as a particularly valuable tracer for volcanic plumes because it can be mapped from orbit at three different tropospheric altitude ranges, and because of its apparent low background variability. We note that volcanic gas plumes could be either enhanced or depleted in water vapor compared to the background atmosphere, depending on magmatic volatile composition. Non-gaseous components of volcanic plumes, such as ash grains and/or cloud aerosol particles, are another investigation target of orbital and in situ measurements. We discuss expectations of in situ and remote measurements of volcanic plumes in the atmosphere with particular focus on the upcoming DAVINCI, EnVision and VERITAS missions, as well as possible future missions.

Surface changes observed on a Venusian volcano during the Magellan mission

Venus has a geologically young surface, but it is unknown whether it has ongoing active volcanism. From 1990 to 1992, the Magellan spacecraft imaged the planet's surface, using synthetic aperture radar. We examined volcanic areas on Venus that were imaged two or three times by Magellan and identified an ~2.2-square-kilometer volcanic vent that changed shape in the 8-month interval between two radar images. Additional volcanic flows downhill from the vent are visible in the second-epoch images, although we cannot rule out that they were present but invisible in the first epoch because of differences in imaging geometry. We interpret these results as evidence of ongoing volcanic activity on Venus.

Synergies Between Venus & Exoplanetary Observations: Venus and Its Extrasolar Siblings

Here we examine how our knowledge of present day Venus can inform terrestrial exoplanetary science and how exoplanetary science can inform our study of Venus. In a superficial way the contrasts in knowledge appear stark. We have been looking at Venus for millennia and studying it via telescopic observations for centuries. Spacecraft observations began with Mariner 2 in 1962 when we confirmed that Venus was a hothouse planet, rather than the tropical paradise science fiction pictured. As long as our level of exploration and understanding of Venus remains far below that of Mars, major questions will endure. On the other hand, exoplanetary science has grown leaps and bounds since the discovery of Pegasus 51b in 1995, not too long after the golden years of Venus spacecraft missions came to an end with the Magellan Mission in 1994. Multi-million to billion dollar/euro exoplanet focused spacecraft missions such as JWST, and its successors will be flown in the coming decades. At the same time, excitement about Venus exploration is blooming again with a number of confirmed and proposed missions in the coming decades from India, Russia, Japan, the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). Here we review what is known and what we may discover tomorrow in complementary studies of Venus and its exoplanetary cousins.

Venus, an Astrobiology Target

We present a case for the exploration of Venus as an astrobiology target-(1) investigations focused on the likelihood that liquid water existed on the surface in the past, leading to the potential for the origin and evolution of life, (2) investigations into the potential for habitable zones within Venus' present-day clouds and Venus-like exo atmospheres, (3) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus' clouds and Venus-like atmospheres, and (4) application of these investigative approaches toward better understanding the atmospheric dynamics and habitability of exoplanets. The proximity of Venus to Earth, guidance for exoplanet habitability investigations, and access to the potential cloud habitable layer and surface for prolonged in situ extended measurements together make the planet a very attractive target for near term astrobiological exploration.

Day-night cloud asymmetry prevents early oceans on Venus but not on Earth

Earth has had oceans for nearly four billion years¹ and Mars had lakes and rivers 3.5-3.8 billion years ago². However, it is still unknown whether water has ever condensed on the surface of Venus^3,4 because the planet-now completely dry⁵-has undergone global resurfacing events that obscure most of its history^6,7. The conditions required for water to have initially condensed on the surface of Solar System terrestrial planets are highly uncertain, as they have so far only been studied with one-dimensional numerical climate models³ that cannot account for the effects of atmospheric circulation and clouds, which are key climate stabilizers. Here we show using three-dimensional global climate model simulations of early Venus and Earth that water clouds-which preferentially form on the nightside, owing to the strong subsolar water vapour absorption-have a strong net warming effect that inhibits surface water condensation even at modest insolations (down to 325 watts per square metre, that is, 0.95 times the Earth solar constant). This shows that water never condensed and that, consequently, oceans never formed on the surface of Venus. Furthermore, this shows that the formation of Earth's oceans required much lower insolation than today, which was made possible by the faint young Sun. This also implies the existence of another stability state for present-day Earth: the 'steam Earth', with all the water from the oceans evaporated into the atmosphere.

Geologic Map of the Niobe Planitia Region (I-2467), Venus

We present a 1:10M scale geologic map of the Niobe Planitia region of Venus (0°N-57°N/60°E-180°E). We herein refer to this area as the Niobe Map Area (NMA). Geologic mapping employed NASA Magellan synthetic aperture radar and altimetry data. The NMA geologic map and its companion Aphrodite Map Area (AMA) cover ~25% of Venus' surface, providing an important and unique perspective to study global and regional geologic processes. Both areas display a regional coherence of preserved geologic patterns that record three sequential geologic eras: the ancient era, the Artemis superstructure era, and the youngest fracture zone era. The NMA preserves a limited record of the fracture zone era, contrary to the AMA. However, the NMA hosts a diverse and rich assemblage of material and structures of the ancient era and structures that define the Artemis superstructure era. These two eras likely overlap in time and account for the formation of basement materials and lower plain units. Impact craters formed throughout the NMA recorded history. Approximately 40% of the impact craters show interior flood deposits, indicating that a significant number of NMA impact craters experienced notable geological events after impact crater formation. This and other geologic relations record a geohistory inconsistent with postulated global catastrophic resurfacing. Together, the NMA and the AMA record a rich geologic history of the surface of Venus that provide a framework to formulate new working hypotheses of Venus evolution and to plan future studies of the planet.

Geologic Map of Aphrodite Map Area (AMA; I-2476), Venus

We present a 1:10-M-scale geologic map of the Aphrodite Map Area (AMA) of Venus (0°N-57°S/60-80°E). Geologic mapping employed NASA Magellan synthetic aperture radar and altimetry data. The AMA geologic map, with detailed structural elements and geologic units covering over one eighth of Venus' surface, affords an important and unique perspective to test models of global-scale geologic processes through time. Geologic relations record a history inconsistent with global catastrophic resurfacing. The AMA displays a regional coherence of preserved geologic patterns that record three sequential geologic eras: the ancient era, the Artemis superstructure era, and the youngest fracture zone era. The ancient era and Artemis superstructure, with a footprint covering more than 25% of the surface, are recorded in the Niobe Map Area to the north. The latter two eras likely overlap in time. The fracture zone domain, part of a globally extensive province, marks the most spatially focused tectonomagmatic domain within the AMA. Impact craters are both cut by and overprint fracture zone structures. Twelve percent of AMA impact craters that occur within the fracture zone domain predate or formed during fracture zone development. This observation indicates the relative youth of the fracture zone era and is consistent with the possibility that this domain remains geologically active. The AMA records a rich geologic history of large tract of the surface of Venus and provides an important framework to formulate new working hypotheses of Venus evolution and contribute to planning future studies of the surface of planets.

Global tectonic evolution of Venus, from exogenic to endogenic over time, and implications for early Earth processes

Venus provides a rich arena in which to stretch one's tectonic imagination with respect to non-plate tectonic processes of heat transfer on an Earth-like planet. Venus is similar to Earth in density, size, inferred composition and heat budget. However, Venus' lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non-plate tectonomagmatic processes. In this paper, I explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time, divided respectively into temporal eras. Impactors played a prominent role in the ancient era, characterized by thin global lithosphere. The Artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume. The fracture zone complex era, marked by broad zones of tectonomagmatic activity, witnessed coupled spreading and underthrusting, since arrested. These three tectonic regimes provide possible analogue models for terrestrial Archaean craton formation, continent formation without plate tectonics, and mechanisms underlying the emergence of plate tectonics. A bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of Archaean crust, and recycling of significant Archaean crust to the mantle.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Volcanism and tectonism across the inner solar system: an overview

Volcanism and tectonism are the dominant endogenic means by which planetary surfaces change. This book, in general, and this overview, in particular, aim to encompass the broad range in character of volcanism, tectonism, faulting and associated interactions observed on planetary bodies across the inner solar system – a region that includes Mercury, Venus, Earth, the Moon, Mars and asteroids. The diversity and breadth of landforms produced by volcanic and tectonic processes are enormous, and vary across the inventory of inner solar system bodies. As a result, the selection of prevailing landforms and their underlying formational processes that are described and highlighted in this review are but a primer to the expansive field of planetary volcanism and tectonism. In addition to this extended introductory contribution, this Special Publication features 21 dedicated research articles about volcanic and tectonic processes manifest across the inner solar system. Those articles are summarized at the end of this review.

Gravity field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) mission

Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

Recent hotspot volcanism on Venus from VIRTIS emissivity data

The questions of whether Venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change. Nine "hotspots"--areas analogous to Hawaii, with volcanism, broad topographic rises, and large positive gravity anomalies suggesting mantle plumes at depth--have been identified as possibly active. This study used variations in the thermal emissivity of the surface observed by the Visible and Infrared Thermal Imaging Spectrometer on the European Space Agency's Venus Express spacecraft to identify compositional differences in lava flows at three hotspots. The anomalies are interpreted as a lack of surface weathering. We estimate the flows to be younger than 2.5 million years and probably much younger, about 250,000 years or less, indicating that Venus is actively resurfacing.

Volatile accretion history of the terrestrial planets and dynamic implications

Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of the Moon-by a giant impact on the proto-Earth-and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U-Pb and I-Xe chronologies, which show that water delivery peaked approximately 100 million years after the isolation of the Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the emergence of life. This mechanism may also have worked for the young Venus, but seems to have failed for Mars.

Understanding oblique impacts from experiments, observations, and modeling

Natural impacts in which the projectile strikes the target vertically are virtually nonexistent. Nevertheless, our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical. Recent theoretical, observational, and experimental work is improving this situation, but even with the current wealth of studies on impact cratering, the effect of impact angle on the final crater is not well understood. Although craters' rims may appear circular down to low impact angles, the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts. Experimental studies established that crater dimensions depend only on the vertical component of the impact velocity. The shock wave generated by the impact weakens with decreasing impact angle. As a result, melting and vaporization depend on impact angle; however, these processes do not seem to depend on the vertical component of the velocity alone. Finally, obliquity influences the fate of the projectile: in particular, the amount and velocity of ricochet are a strong function of impact angle.