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Was Venus once habitable?

Artist’s concept of lightning on Venus

If Venus had more water in its distant past, could it have been a habitable planet like Earth?

  • Venus might once had have more water
  • Water split by sunlight; hydrogen/oxygen escaped to space
  • If it was wetter, could it have had life?

The Venus Express spacecraft is helping planetary scientists investigate whether Venus once had oceans. If it did, it may even have begun its existence as a habitable planet similar to Earth.

These days, Earth and Venus seem completely different. Earth is a lush, clement world teeming with life, whilst Venus is hellish, its surface roasting at temperatures of a furnace.

Venus in the ultraviolet

Sunlight breaks up water molecules in Venus' clouds, letting hydrogen and oxygen atoms to escape into space.

But underneath it all the two planets share a number of striking similarities. They are nearly identical in size and now, thanks to the European Space Agency’s (ESA) Venus Express orbiter, planetary scientists are seeing other similarities too.

“The basic composition of Venus and Earth is very similar,” says Håkan Svedhem, ESA Venus Express Project Scientist.

One difference stands out—the planet has very little water. Were the contents of Earth’s oceans to be spread evenly across Venus, they would create a layer 3km deep. If you were to condense the current amount of water vapour in Venus’ atmosphere onto its surface, it would create a global puddle just 3cm deep.

Water lost into space

Yet there is another similarity here. Billions of years ago, Venus probably had much more water. Venus Express has confirmed that the planet has lost a large quantity of water into space.

This happens because ultraviolet radiation from the Sun streams into Venus’ atmosphere and breaks the water molecules into their atoms—two of hydrogen and one of oxygen. These then escape to space.

Venus Express has measured the rate of this escape and confirmed that roughly twice as much hydrogen is escaping as oxygen. It’s therefore thought that water is the source of these escaping atoms.

It has also shown that a heavy form of hydrogen, called deuterium, is enriched in the upper echelons of Venus’s atmosphere, because the heavier hydrogen finds it harder to escape the planet’s grip.

Artist's impression of the Venus Express spacecraft

The Venus Express spacecraft is helping scientists study the water history of Venus.

“Everything points to there being large amounts of water on Venus in the past,” says Colin Wilson, Oxford University, UK. But that doesn’t necessarily mean there were oceans on the planet’s surface.

No oceans, but life anyway?

Eric Chassefière, Université Paris-Sud, France, has developed a computer model that suggests the water was largely atmospheric and existed only during the very earliest times, when the surface of the planet was completely molten.

As the water molecules were broken into atoms by sunlight and escaped into space, the subsequent drop in temperature probably triggered the solidification of the surface. In other words, no oceans.

Although it is difficult to test this hypothesis, it does raise a key question. If Venus ever did possess surface water, could planet have had an early habitable period?

Even if true, Chassefière’s model does not preclude the chance that colliding comets might have brought additional water to Venus after its surface solidified, and these could have created bodies of standing water in which life may have been able to form.

Adapted from information issued by ESA / MPS / DLR / IDA / J. Whatmore.

Surfing an alien atmosphere

Artist's impression of Venus Express in orbit around Venus

Venus Express has two solar power wings with two panels per wing. As Venus is closer to the Sun, there is twice as much sunlight in Venusian orbit as there is in Earth orbit.

The European Space Agency’s Venus Express spacecraft has completed an ‘aerodrag’ campaign that used its solar “wings” as sails to catch faint wisps of the planet’s atmosphere.

The test used the orbiter as an exquisitely accurate sensor to measure atmospheric density barely 180 km above the hot planet.

During the five aerodrag measurements, Venus Express’ solar arrays and control systems were operated as one big flying sensor, with the solar arrays rotated at various angles to the direction of flight.

The special configuration exposed the wings to the faint wisps of atmosphere that reach to the boundary of space around the planet, generating a tiny but measurable aerodynamic torque, or rotation, on the satellite.

This torque can be measured very accurately based on the amount of correction that must be applied by reaction wheels, which counter-rotate inside the spacecraft to maintain its orientation in space.

The solar panels rotated through five daily-changing sets of orientations. While one panel remained perpendicular to the direction of flight, the other rotated in steps, gradually increasing the torque to be counter-balanced by the reaction wheels.

On the last day, the solar arrays were rotated at plus and minus 45° to the atmospheric flow, mimicking the vanes of a windmill, in order to gather additional information on the behaviour of the molecules of the atmosphere bouncing off the solar wings.

Venus Express seen during pre-launch testing

Solar arrays on Venus Express seen during pre-launch testing in September 2005.

More tests planned

“The aerodrag campaign went without problem, and conclusively demonstrated that Venus Express can be securely and accurately used to sense the density of the planet’s atmosphere. Venus Express has shown once again that it is a very capable satellite,” said Spacecraft Operations Manager Octavio Camino.

The mission operations team will study the results to develop an optimised configuration for more aerodrag campaigns in October 2010 and in 2011. Aerodrag testing was also conducted in 2008, 2009 and February 2010.

Continued positive results may enable Venus Express to conduct more sophisticated investigations deeper in the atmosphere, which would be of immense interest to planetary scientists.

The solar array on Venus Express comprises two symmetrical wings supporting gallium-arsenide solar cells. Their combined 5.7 sq m can generate up to 1400 W of power in Venus orbit.

Adapted from information issued by ESA.