The latest decadal survey identified the Uranus system as the highest-priority new target for a NASA Flagship mission. Ariel and Miranda are potential ocean worlds with evidence of resurfacing potentially due to past elevated heat flow. Learning about the geologic histories of these icy moons is important for understanding the potential for life in the outer solar system. Using limited data acquired by the Voyager 2 spacecraft, we explore open questions about the surfaces of Uranian satellites to gain a better understanding of their evolutionary histories. In this work, we update the estimates of Ariel and Miranda’s simple-to-complex transition diameters, which have not yet been measured using modern GIS techniques and reprocessed data. The simple-to-complex transition diameter is a value used on many worlds to infer the composition of the surface. For the Uranian satellites, this value was last estimated shortly after the Voyager 2 flyby with a dataset of 18 craters. We use reprocessed topography from more than 100 craters to estimate a simple-to-complex transition diameter on Ariel of ~26 km, consistent with an icy surface composition. We place a lower limit of ~49 km on the transition diameter for Miranda, where we cannot identify any complex craters. We also estimate the relative and absolute ages of terrains on Ariel and Miranda. Our results agree with recent studies showing that they likely experienced relatively recent resurfacing. Finally, we suggest imaging requirements for the future mission to Uranus to answer outstanding questions about Ariel and Miranda.

Lightning in the atmosphere of Venus is either ubiquitous, rare, or non-existent, depending on how one interprets diverse observations. Quantifying when and where, or even if lightning occurs would provide novel information about Venus’s atmospheric dynamics and chemistry. Lightning is also a potential risk to future missions, which could float in the cloud layers (~50–70 km above the surface) for up to an Earth-year. Over decades, spacecraft and ground-based telescopes have searched for lightning at Venus using many instruments, including magnetometers, radios, and optical cameras. Two optical surveys (from the Akatsuki orbiter and the 61-inch telescope on Mt. Bigelow, Arizona) observed several flashes at 777 nm (the unresolved triplet emission lines of excited atomic oxygen) that have been attributed to lightning. This conclusion is based, in part, on the statistical unlikelihood of so many meteors producing such energetic flashes, based in turn on the presumption that a low fraction (< 1%) of a meteor’s optical energy is emitted at 777 nm. We use observations of terrestrial meteors and analogue experiments to show that a much higher conversion factor (~5–10%) should be expected. Therefore, we calculate that smaller, more numerous meteors could have caused the observed flashes. Lightning is likely too rare to pose a hazard to missions that pass through or dwell in the clouds of Venus. Likewise, small meteors burn up at altitudes of ~100 km, roughly twice as high above the surface as the clouds, and also would not pose a hazard.

Venus’ steep-sided domes are circular volcanoes ~10s of km wide and ~1 km tall, which are known for their characteristic flat tops and steep sides. However, their composition remains mysterious. These “pancake” domes are likely formed by a high-viscosity lava, and other studies have predicted a range of compositions, from rhyolite to basalt. In this study, we build on previous work modeling pancake domes as spreading viscous gravity currents. However, previous models of dome formation assumed that they form over a rigid lithosphere. We confirmed that lithospheric flexure occurs at pancake domes, and therefore build a new model of dome formation over a bending elastic lithosphere. We find that flexure during formation can influence the shape of the resulting pancake dome. Our results also support the idea that pancake domes continue to spread for a long time after their emplacement. In comparing our model to the topography of a real pancake dome, we find a range of high, though variable, lava viscosities. Our range of lava viscosities is related to the size of the observed dome, and our results for a large dome agree with those of other studies. We test different lava densities and find that a lava density of ~2400 – 2700 kg/m3 best reproduces the flexural signatures observed at pancake domes. Low-density lava (~1500 kg/m3) does not reproduce the flexural signatures, implying that dome-forming lava is not highly vesiculated.

Topographic flexure in response to vertical loads reveals key lithospheric properties, including elastic thickness and the heat flow from the interior. Flexural stresses may also control volcano morphology. One previous study predicted that steep-sided domes on Venus usually form where the elastic thickness is ~15-40 km. We surveyed flexural signatures around steep-sided domes and confirmed this hypothesis. We determined elastic thickness from topographic profiles with a curve-fitting algorithm and a plate bending model in Cartesian and axisymmetric geometry. We used a yield stress envelope to convert elastic thickness and plate curvature into mechanical thickness and surface heat flow. The average elastic thickness for domes not near coronae is ~30 km, corresponding to a heat flow of ~60 mW/m2. Coronae on Venus are typically associated with elastic thicknesses of <10-15 km. Domes near coronae yielded elastic thicknesses in this range, and higher heat flows than domes not near coronae.

Venus is the planet in the Solar System most similar to Earth in terms of size and (probably) bulk composition. Until the mid-20th century, scientists thought that Venus was a verdant world—inspiring science-fictional stories of heroes battling megafauna in sprawling jungles. At the start of the Space Age, people learned that Venus actually has a hellish surface, baked by the greenhouse effect under a thick, CO2-rich atmosphere. In popular culture, Venus was demoted from a jungly playground to (at best) a metaphor for the redemptive potential of extreme adversity. However, whether Venus was much different in the past than it is today remains unknown. In this review, we show how now-popular models for the evolution of Venus mirror how the scientific understanding of modern Venus has changed over time. Billions of years ago, Venus could have had a clement surface with water oceans. Venus perhaps then underwent at least one dramatic transition in atmospheric, surface, and interior conditions before present day. This review kicks off a topical collection about all aspects of Venus’s evolution and how understanding Venus can teach us about other planets, including exoplanets. Here we provide the general background and motivation required to delve into the other manuscripts in this collection. Finally, we discuss how our ignorance about the evolution of Venus motivated the prioritization of new spacecraft missions that will essentially rediscover Earth’s nearest planetary neighbor—beginning a new age of Venus exploration.