Quirks & Quarks

Venus is a hellscape now, but might once have been blue like Earth

For billions of years Venus might have been temperate and pleasant — and even habitable

For billions of years Venus might have been temperate and pleasant — and even habitable

Venus, as it is today (left) and how it may have been (right) for billions of years. (NASA)

New modelling by NASA researchers seems to suggest that for much of its history the second planet from the Sun —Earth's 'sister' planet Venus — may have been habitable.

It certainly isn't today. Venus has a carbon dioxide atmosphere that is 90 times as thick as that of the Earth, which exerts crushing pressure on the surface. It has sulphuric acid clouds, and a runaway greenhouse effect combined with its proximity to the Sun means it has a surface temperature of about 462 C, which is more than hot enough to melt lead.

But a billion years ago Venus might have been quite different Michael Way, a researcher with NASA's Goddard Space Flight Center in New York told Quirks & Quarks host Bob McDonald. His new studies suggest that Venus once had liquid water on the surface and mean temperatures of between 15 and 30 C. "It's not bad. I mean it's it's within what we would call temperate so it's reasonable temperatures."

Venus may have had water and been habitable similar to its sister planet, Earth. (NASA)

Oceans on Venus?

Forty years ago NASA's Pioneer Venus mission found hints that the planet may have had an ocean of water up to 300 metres in depth. This is consistent with theories of planetary formation that predict that Venus would have formed in much the same way, and of the same materials, as Earth did. Thus the early Venus would have been much like early Earth, with a mostly carbon dioxide atmosphere, and a relative abundance of water.

The problem, of course, is that Venus is so much closer to the Sun that intense solar radiation would have evaporated its early ocean and even caused the water vapour in the atmosphere to break up into hydrogen and oxygen, with the light hydrogen molecules then escaping to space.

The key discovery Way made in his climate models of early Venus was a way the planet might have been able to keep itself cool.  And that, in turn, depended on the planet's uniquely slow rotation rate — a Venusian day is 117 Earth days.

Artist's impression of the Pioneer Venus Orbiter. Forty years ago this mission found hints that Venus may have had a shallow ocean for billions of years. (NASA)

Way said "All of our models rely on the idea that an early Venus had surface liquid water available. And if you have surface liquid water available on such a slowly spinning world, you can create super large anvil-type clouds that blocked most of the radiation." That means the water could have remained on the surface for billions of years without burning off.

The team ran their climate models on Venus covering a period of billions of years of the planet's geological evolution.  Their assumption was that the same changes we saw on Earth happened on Venus, including the gradual increase of nitrogen in the atmosphere. For two to three billions of years, said Way, things looked pretty benign.  "The temperatures we produce in the model are moderate temperatures."

One of the exciting implications of this is that these conditions might well have been suitable for the evolution of life, said Way. "We know that life has been here on Earth for billions of years and has filled every ecological niche. It would have done the same on Venus if Venus had these habitable conditions over these billions of years." 

This radar map from the Magellan spacecraft's visit to Venus in 1990 shows a Venusian mountain and lava fields. Conditions on the planet are incredibly hostile today, but a billion years ago might have been quite pleasant. (NASA)

When things went bad for Venus

The available evidence suggests that about 700 million years ago Venus experienced a dramatic transformation. Spacecraft that have surveyed the planet suggest, based on the number of craters from impacts that are visible, that at that time Venus experienced a major "resurfacing event."

This might have been a period of extended vulcanism that reshaped the surface but also injected a large amount of carbon dioxide into the atmosphere that had previously been stored in rock.

If this event had been intense enough it might have broken the benign cycle that had kept Venus' ocean from evaporating. The greenhouse effect might have evaporated the oceans. Without water, tectonic activity would have shut down, reducing the planet's ability to convert atmospheric CO2 into mineral form.

"You have nowhere to put the carbon dioxide, it has nowhere to go it has no way to react with the surface and therefore it stays in the atmosphere and that's what we see on Venus today," said Way.