When I was a teenager in the Sixties, one of my favourite books was Exploring The Planets by Roy A. Gallant. Particularly intriguing seemed Venus.
The book presented two splendid pieces of artwork offering completely contrasting views of what might lie beneath Venus’s permanent cloud cover.
Across one two-page spread was a vision of a baking desert. Turn the page and you could view the alternative, a raging sea which I recall being particularly green and stormy.
For many years, thanks to visiting spaceprobes, we have known that the desert view is more correct (just as we have discovered that Mars has no canals and no vegetation). Venus is completely devoid of water.
Surprisingly, however, scientists now believe that Earth and Venus began as similar worlds and that there was a time when Venus might indeed have been covered with vast oceans of water. Conditions could even have been right for life.
But our two worlds took the wildly different evolutionary paths that have led to Venus being described as Earth’s “Evil Twin”. Scientists are beginning to understand why, thanks to the work of Europe’s Venus Express probe which has been in orbit around the planet for one and a half years.
A special report in the journal Nature suggests that Venus’s lack of a magnetic field left it vulnerable to the solar wind. This stream of charged particles stripped away the water by breaking molecules into oxygen and hydrogen atoms that then escaped into space.
Another surprise was the discovery that lightning is a regular feature on Venus, but unlike our lightning, it is produced from clouds of sulphuric acid and not water. The only other planet where lightning has been observed is giant Jupiter.
Announcing the latest findings in Paris, Professor Fred Taylor, of Oxford University, said, “It is now becoming clearer why the climate on Venus is so very different from that on Earth, despite the planets themselves being very similar in many ways.
“There are many common processes at work, such as a carbon-dioxide driven greenhouse effect, volcanic activity, and atmospheric erosion by solar particles and radiation. On Venus, these have worked to virtually eliminate water from the planet while maintaining high levels of carbon dioxide, while Earth has retained much of its water and lost most of its atmospheric carbon dioxide.”
Professor Taylor added: “In the light of the new data it is possible to construct a scenario in which the climates on Venus and Earth were very similar when they started out, and then evolved to the state we see now, like twins separated at birth. Billions of years ago there is even the possibility that Venus would have been habitable.”
Co-investigator Professor Andrew Coates of University College London, said: “One of the evolutionary differences that has made Venus the Earth’s ‘evil twin’ is that present-day Venus lacks a magnetic shield. This means that its atmosphere feels the onslaught of the solar wind and cosmic radiation, and has done for billions of years.
“We already know that Venus is a dry planet as the surface is so hot, but what we’ve found now is that Venus is still getting dryer. It loses hydrogen, helium and oxygen through its wake at a rate faster than similar escape from Mars.”
Picture: An ESA artist’s impression of a lightning storm above the baking surface of Venus.