Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus‐Like Exoplanets

Abstract

One popular view of Venus' climate history describes a world that has spent much of its life with surface liquid water, plate tectonics, and a stable temperate climate. Part of the basis for this optimistic scenario is the high deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted to imply Venus had a shallow ocean's worth of water throughout much of its history. Another view is that Venus had a long-lived (∼100 million years) primordial magma ocean with a CO 2 and steam atmosphere. Venus' long-lived steam atmosphere would sufficient time to dissociate most of the water vapor, allow significant hydrogen escape, and oxidize the magma ocean. A third scenario is that Venus had surface water and habitable conditions early in its history for a short period of time (<1 Gyr), but that a moist/runaway greenhouse took effect because of a gradually warming Sun, leaving the planet desiccated ever since. Using a general circulation model, we demonstrate the viability of the first scenario using the few observational constraints available. We further speculate that large igneous provinces and the global resurfacing hundreds of millions of years ago played key roles in ending the clement period in its history and presenting the Venus we see today. The results have implications for what astronomers term "the habitable zone," and if Venus-like exoplanets exist with clement conditions akin to modern Earth, we propose to place them in what we term the "optimistic Venus zone." Plain Language SummaryWe have little data on our neighbor Venus to help us understand its climate history. Yet Earth and Venus are sister worlds: They initially formed close to one another and have nearly the same mass and radius. Despite the differences in their current atmospheres and surface temperatures, they likely have similar bulk compositions, making comparison between them extremely valuable for illuminating their distinct climate histories. We analyze our present data on Venus alongside knowledge about Earth's climate history to make a number of exciting claims. Evaluating several snapshots in time over the past 4+ billion years, we show that Venus could have sustained liquid water and moderate temperatures for most of this period. Cloud feedbacks from a slowly rotating world with surface liquid water reservoirs were the keys to keeping the planet clement. Contrast this with its current surface temperature of 450 • and an atmosphere dominated by carbon dioxide and nitrogen. Our results demonstrate that it was not the gradual warming of the Sun over the eons that contributed to Venus present hothouse state. Rather, we speculate that large igneous provinces and the global resurfacing hundreds of millions of years ago played key roles in ending the clement period in its history.

Transits of Known Planets Orbiting a Naked-eye Star

“…With the host star being similar to the Sun, the known HD136352 planets lie far interior to the inner boundaries of the Habitable Zone (Kasting et al 1993;Kopparapu et al 2013Kopparapu et al , 2014Kane et al 2016a), but they do lie within the Venus Zone . This is mostly relevant to planet b and other terrestrial planets that may be present within the system, because the exploration of planetary habitability and comparative planetology aims to study the major factors that drive the bifurcation of habitable versus uninhabitable environments (Hamano et al 2013;Kane et al 2019;Way & Del Genio 2020). Terrestrial planets orbiting close to a bright host star, such as those discussed here, provide the best opportunities to conduct the needed atmospheric studies to inform the diversification processes (Ostberg & Kane 2019).…”

The Fundamental Connections between the Solar System and Exoplanetary Science

“…However, dynamical studies have shown that the different terrestrial planets likely received different amounts of material from different reservoirs of volatiles-at least in the case of the exogenous delivery of those volatiles (e.g., ; T. Owen & Bar-Nun, 1996; Raymond et al, 2004), a result that has been replicated in studies of planet formation around other stars (e.g., Ciesla et al, 2015). This finding has implications, for example, for the volatile inventory of Venus and its similarity (or not) with that of Earth (e.g., Way & Del Genio, 2020).…”

“…6. Did Venus have a habitable period (Way & Del Genio, 2020;Way et al, 2016)? That is, did Venus ever cool after formation (Hamano et al, 2013)?…”

“…Moreover, the relative lack of knowledge regarding the bulk composition of Venus makes it difficult to infer the mineralogy of exoplanets based on stellar abundances . Most importantly, the evolution of Venus potentially represents a pathway from habitable to uninhabitable conditions, a pathway whose nature may be common for terrestrial planets (Foley, 2015;Foley & Driscoll, 2016;Way & Del Genio, 2020). Thus, the study of planetary habitability will benefit from understand which of the myriad of differences between Venus and Earth dominated the divergence in their planetary evolutions (Kane et al, 2019).…”

Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond

“…The leading theory on Venus' climatic history is that it left surface habitability early in its history during a runaway greenhouse period [120][121][122]. However, some recent climate models suggest that Venus may have harbored liquid water on its surface for up to three billion years after the formation of the Solar System [123,124]. In these scenarios, the onset of a strong greenhouse effect was delayed by Venus' slow rotational period [123].…”