Venus is not just Earth's immediate neighbour in the solar system, but also has almost the same size, mass and density. But still, while the Earth has conditions where life has prospered, Venus is a furnace at 457 C - hot and dry with scarcely any water vapour, and an atmosphere made of carbon dioxide weighing down with pressure 92 times that on Earth. It looks like a planet needs a lot more than just being 'earth-like', when seen from hundreds of light years away, to be anywhere near life supporting!

Venus Express mission

Venus Express is the European Space Agency's exploration spacecraft, in orbit around Venus and collecting scientific data since April 2006. Venus Express is an adaptation of the earlier Mars Express, which reached Mars in 2005 and has carried out extensive studies of the Jovian atmosphere and surface. But there are differences. For instance, since Venus is half the distance from Sun, as compared to Mars, the radiant heating of the craft is four times greater. The ionisation radiation is also harsher. The construction thus needs to be more robust. On the other hand, being nearer to Sun, the illumination of the solar panels is better and the craft has more electric power for its manoeuvres.

Mars Express was well equipped for investigating the planet's surface, even to send down Beagle-2, a lander probe to work on-the-spot. But Venus Express works entirely from orbit and has special devices to study the Venus atmosphere and clouds in detail. For instance, it has the ASPERA-4, or "Analyser of Space Plasmas and Energetic Atoms" which studies the impact of the solar wind on the atmosphere. Or the MAG, the magnetometer, to study how the solar wind affects the planet's magnetic field.

The major objective is to see why Venus is so unlike Earth? On Earth, ¾ of the surface is covered by oceans and the total water is 300 times more massive than the atmosphere. But on Venus, there are no oceans and the water vapour is only a trace in the atmosphere. Both planets have roughly the same quantities of nitrogen and carbon dioxide.

The CO2 on the Earth is in the form of carbonates in limestone, etc., but on Venus, most of it is in the atmosphere. That makes the atmosphere pretty dense, and the pressure is 92 times that on Earth.
Where has the water disappeared? What is the atmosphere like? Why is the weather so different from that on Earth? Probably because of Venus’ proximity to Sun.

Water must have evaporated into the atmosphere. Water vapour is a greenhouse gas and this could have led to more warming and more evaporation, leading to more warming and so on, till all the water resides in the atmosphere.

Further, strong sunlight must have split the water molecules into hydrogen and oxygen. As lighter atoms in a gas move faster, hydrogen atoms gradually escaped into space, while oxygen remained to oxidise the planet's crust.

Handy indicator

A handy indicator of this mechanism becomes available because of the nature of hydrogen atoms. There are two forms of hydrogen atom, one with only one proton and the other, called heavy hydrogen, which has a proton and a neutron. As heavy hydrogen atoms would move much slower than normal hydrogen, and not escape as fast, it is expected that in time, the hydrogen in the atmosphere would have more heavy hydrogen than before.

That the hydrogen left in Venus' atmosphere is indeed richer in heavy hydrogen was roughly verified by NASA's pioneer mission in 1978. But instruments aboard Venus Express have now confirmed this accurately.

Venus Express has also sent down other surprising results about the atmosphere. As the atmosphere is more massive than the Earth's, it should store and distribute heat efficiently. Day-night temperature differences should then be lower. This had been confirmed by the Pioneer findings, but Venus Express' radio wave detectors show large temperature differences within Venus' clouds.

The dense material of the atmosphere and the slow rotation of Venus (once in 177 earth days) throw up more surprises in the way winds and currents behave. Here again, Venus Express has found things not expected and difficult to explain.

One such is lightning - there should not be lightning on Venus and indeed, no flashes have been seen. Still, Venus Express has detected low frequency electromagnetic waves that could arise from lightning activity!

(The writer can be contacted at simplescience@gmail.com)