Sea Change on Venus

Venus and Earth are a strange pair for two planets so similar in size and apparent composition. Our planet’s rapid spin sets the 24-hour day-night rhythm of light and life on Earth, gives us our cyclonic weather patterns, and shapes ocean currents. It’s a paradise next to Venus, which a runaway greenhouse desiccated and broiled long ago.

Venus rotates backward, compared to Earth and most other solar system planets, and so slowly that its yearly orbit outpaces its daily spin. You can’t see the stars on Venus, but if the sky was ever clear, their rising and setting wouldn’t be diurnal but annual.

You might expect the planet’s sunward side to be much hotter than its nightside, but Venus also possesses continuous planet-wide clouds of sulfuric acid and an atmosphere 92 times as thick as Earth’s. This redistributes heat so effectively that no temperature differences exist from day to night or from equator to pole. At ground level, it’s all a sweltering 460°C (860°F).

As hard as it is to imagine today, observations suggest that Venus might once have had water oceans. What effect might global seas have had on the planet’s unusual rotation rate? Oceanographer Mattias Green (Bangor University, UK), collaborating with planetary scientists Michael Way (NASA Goddard) and Rory Barnes (University of Washington), tackled this question in a recent study. They found that ocean tides could have caused drag forces on the planet, reining in its rotation by as much as 72 Earth days every million years. So if Venus started out with an Earth-like rotation rate and an ocean, it could have decelerated to its current day in less than 50 million years.

This would seem to relate to climate history as well, but how? In an illustration of just how complex and counterintuitive planetary science can be, several initial news stories about the team’s result got it backwards. As the rotation slowed, these stories implied, the planet’s oceans would have been more vulnerable to heating and evaporation by the young, warming Sun.

In reality, the effect would likely have been the opposite.When some colleagues and I, led by Way, modeled the ancient atmosphere of Venus with a general circulation model of the kind we use to model climate changes on Earth, we learned — to our surprise — that a slowly rotating Venus is more effective at holding onto an ocean than a rapidly rotating one.

This results from cloud behavior. On a slowly spinning oceanic Venus, we discovered that the clouds organize themselves so that the dayside is always cloudy and the nightside is always clear. This is perfect for keeping the planet cool, because clouds reflect sunlight but clear night skies allow for maximum cooling. So if Venus had a habitable ocean that put the brakes on its rotation speed, it might have helped keep the planet cool for up to 2 billion years.

Clearly, “habitable zone” is not a simple question of distance from a star. Through more missions and more modeling, we’ll need to understand the complexity of oceans, tides, and atmospheric motions to get a handle on just where a biosphere could thrive — on a primordial Venus or elsewhere.

This article originally appeared in print in the September 2019 of Sky & Telescope.