Was Venus once habitable?

[lpetrich]

Alas, poor Amtor
reporting on
Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus‐Like Exoplanets - Way - 2020 - Journal of Geophysical Research: Planets - Wiley Online Library

Amtor? That's what Venus's inhabitants called the planet in Edgar Rice Burroughs's Venus stories of a century ago. He is best-known for his Tarzan and Mars ones. But the planet turned out to be a hothouse with 90 bar of mostly CO2 at 450 C at its surface, and clouds of concentrated sulfuric acid. Most of its water is now gone, as indicated by a concentration of deuterium that is about 100 times our planet's concentration.

This recent modeling of the Venerian atmosphere suggests that there may have been a long period of relative coolth in the planet’s history. The runaway greenhouse effect wouldn’t have occurred until a period of intense volcanic activity that produced LIPs, Large Igneous Provinces, released even more CO2, and then the temperatures soared.

That surge occurred less than a billion years ago, so it’s easy to imagine warm (mean temperature of around ~20°C, compared to Earth’s current ~15°C) oceans in which life could have evolved before global warming slammed the hammer down and burnt the soup.

During that habitable time, Venus's atmosphere would have been mostly nitrogen. The planet's recent volcanism would have released a lot of CO2, causing a runaway greenhouse effect. It would in turn have baked the planet's crust, forcing out more CO2. That would have boiled the oceans, and solar UV dissociating upper-atmosphere H2O would be followed by loss of the resulting H2 to outer space.

 

[lpetrich]

The evolution of Venus: Present state of knowledge and future exploration - ScienceDirect

... While Earth and to a lesser extent Mars (thanks to the analysis of SNC meteorites) are extensively studied in a comparative planetology context, the history of the most Earth-like planet of the Solar System, Venus, is still poorly understood. For how long has Venus been in its current extreme climate state? When and how did it diverge from a (possible) early Earth-like state? Has Venus been a potentially habitable planet at some time of its early history? Did a “cool early Venus” stage occur between the end of accretion and the late heavy bombardment, like suspected for Earth? What are the implications of the Venus/Earth comparison for the nature and evolution of habitable terrestrial planets throughout the universe? ...

Was Venus the first habitable world of our solar system? - Way - 2016 - Geophysical Research Letters - Wiley Online Library

Present‐day Venus is an inhospitable place with surface temperatures approaching 750 K and an atmosphere 90 times as thick as Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3‐D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 Gya, present‐day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a prograde rotation period slower than ~16 Earth days, despite an incident solar flux 46–70% higher than Earth receives. At its current rotation period, Venus's climate could have remained habitable until at least 0.715 Gya. These results demonstrate the role rotation and topography play in understanding the climatic history of Venus‐like exoplanets discovered in the present epoch.

Obliquity Variability of a Potentially Habitable Early Venus | Astrobiology

Venus currently rotates slowly, with its spin controlled by solid-body and atmospheric thermal tides. However, conditions may have been far different 4 billion years ago, when the Sun was fainter and most of the carbon within Venus could have been in solid form, implying a low-mass atmosphere. We investigate how the obliquity would have varied for a hypothetical rapidly rotating Early Venus. The obliquity variation structure of an ensemble of hypothetical Early Venuses is simpler than that Earth would have if it lacked its large moon (Lissauer et al., 2012), having just one primary chaotic regime at high prograde obliquities. We note an unexpected long-term variability of up to ±7° for retrograde Venuses. Low-obliquity Venuses show very low total obliquity variability over billion-year timescales—comparable to that of the real Moon-influenced Earth.

The OP paper's 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 CO2 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.”

Simple-language summary:

We 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.