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Did Venus Have Oceans? Earth-Like Past Revealed As 'Hot Planet' This Week Becomes An 'Evening Star'

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Venus today reaches what astronomers call “superior conjunction,” when it appears to go behind the Sun from our point of view on Earth. For much of the year Venus has been a morning object–often referred to as the “Morning Star”–but later this week it will begin to appear in the western sky after sunset to become a super-bright “Evening Star.”

Was Venus once like Earth? New research out this week indicates that the hottest planet in our solar system may have once had liquid oceans, but doesn’t now because it rotates too slowly.

In "Consequences of Tidal Dissipation in a Putative Venusian Ocean," published recently in The Astrophysical Journal Letters, researchers at Bangor University in Wales argue that tides in an ancient Venusian ocean would have been large enough to slow the planet’s rotation rate.

Here are five things about Venus that will help you understand their findings:

  • Venus is the second closest planet to our Sun.
  • The average temperature on Venus is 864°F/462°C. That’s hot enough to melt lead.
  • Venus takes 225 days to orbit the Sun.
  • Venus rotates so slowly. A Venusian day is equivalent to 243 days on Earth.
  • Venus rotates clockwise. All other planets in the solar system go anti-clockwise.

Geological evidence and computer model simulations suggest that Venus may have been a very different place billions years ago. It may have been much cooler and had an ocean. In short, it may have been more Earth-like.

It’s all down to the rotation of Venus. Now it’s very slow, and its backwards–relatively speaking–but in the past it may have been a lot faster. Working in conjunction with the NASA Goddard Institute for Space Studies, Uppsala University in Sweden, and the University of Washington, lead author Dr. Mattias Green argues that tides are critical in making a planet’s rotation change, and therefore whether it’s habitable, or not.

What difference do tides make on Earth?

On our planet, oceans cover 70% of the surface. The tides cause friction between the tidal currents and the sea floor. They act as a brake on Earth’s rotation, lengthening an Earth day by about 20 seconds per million years. Tides on Earth are most obviously caused by the moon, but there is also a small tidal bulge due to the Sun. On Venus, only a "solar tide" would be in place since the planet has no moons.

What difference do tides make on Venus?

In the paper, researchers show that tides in an ancient Venusian ocean, though small compared to Earth, would have been large enough to slow the rotation rate of Venus by tens of Earth days per million years if Venus was spinning at a similar rate to how Earth does today. It’s possible that tides could have changed the Venusian day length by up to 72 days per million years depending on rotation rate. The work suggests that Venus may have had oceans whose “tidal brake” took 10-50 million years to slow down Venus to its current rotation state. Variables include the depth of the ocean and how fast Venus was rotating to start with. “Venusian tides may have had a profound impact on the rotational evolution of Venus,” reads the paper.

Why is this important?

As well as farthing our understanding of how the solar system formed, this work could also help those seeking to understand exoplanets. In short, the rotation rate could be critical to understanding climate dynamics–and therefore the possibility of extraterrestrial life. Ocean tides on Earth have a profound impact on the planet’s rotation, and could be a driver for evolution and mass extinction events. Such effects could even be much stronger on other worlds. “This work shows how important tides can be to remodel the rotation of a planet, even if that ocean only exists for a few 100 million years, and how key the tides are for making a planet habitable,” says Green. “These calculations have implications for the rotational periods of similarly close-in exoplanetary worlds and the location of the inner edge of the liquid water habitable zone,” reads the paper.

The study doesn’t implicitly state the Venus had oceans or oceans, but that if it did, it could help explain its current slow rotation rate. And, therefore, why it's such a hellish place. However, for stargazers a super-bright Venus lowdown in the west in the evening sky is something to look forward to–and it's coming really soon.

Wishing you wide eyes and clear skies.

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