What Our Current Understanding of Venus May Tell Us Of Earth-size Worlds Generally

Planet Venus is conventionally regarded as the poster child for a planet gone wrong. It’s about the same size as Earth (0.95 R_⨁ and 0.82 M_⨁), orbits the same star, formed at the same time, and is presumably composed of largely the same materials in around the same proportions. So how could its surface conditions — with a pressure equal to ~1 km water depth on Earth, and a temperature that of a self-cleaning oven — end up so different to those of Earth? But that may be the wrong question. Soon after the Mariner 2 Venus flyby in 1962, a view emerged that the second planet had experienced a runaway greenhouse effect at some point in its past. Perhaps because of a steadily brightening Sun, its surface temperature slowly but inexorably rose until any oceans present evaporated and its climate was irrevocably and fundamentally changed. But research published earlier in 2020 argues that, instead of rising insolation, Venus’ misfortune was self-inflicted: that the coincident timing of two or more major volcanic eruptions was responsible instead. These voluminous eruptions would have injected so much CO₂ into Venus’ atmosphere that, even if the planet had Earth-like plate tectonics to regulate its temperature, the outcome would have been inevitable. Perhaps Venus’ current state isn’t the price of being relatively close to its host star, then, but simply bad luck. Other recent work shows that numerous exposures of the poorly understood tesserae on Venus consist of layered rocks that have been folded and eroded. One explanation is that the tesserae are ancient stacks of basaltic flows, the very rock formed during the kinds of major eruptive event that may have rendered Venus hellish. But sedimentary rocks are also often layered, including sandstones, mudstones, and limestones—the depositional environments for which do not exist on Venus today. Could these tesserae have formed under an earlier climate regime, perhaps before the onset of the runaway greenhouse effect? As we continue to explore Venus, we should focus on those geological and atmospheric measurements that will tell us whether the second planet once had a more clement climate, and what process(es) set it down the path to the hellish landscape it is today. If such a climate catastrophe is not the inevitable outcome of proximity to one’s star, then our ability to interpret the geology, climate evolution, and even habitability of Earth-size worlds orbiting other stars becomes that much more difficult.