The dense clouds that have shielded the surface of Venus from human eyes have been lifted by a small robotic satellite sent to explore a hidden world, and the story of Venus is finally being told.
The mysteries that the Magellan spacecraft have unveiled are of more than pure scientific interest, because they address the fundamental question of why the Earth evolved into a planet that encouraged the blossoming of a diversity of life forms. Venus is Earth's sister planet, similar in size, mass and distance from the sun, yet so inhospitable to life that nothing can live there. Its dense, poisonous atmosphere would smother a human in a split second; its surface is so hot from an intense greenhouse effect that it would melt lead.
Until recently, scientists were frustrated in their efforts to study Venus because of the dense atmosphere that blinded earthbound telescopes. That began to change in 1962 with the flight of NASA's Mariner 2, the first spacecraft to visit the planet. Mariner told them something about Venus, including the surprising fact that it has no magnetic field, but the surface of the mysterious planet remained hidden.
During the following years, while the National Aeronautics and Space Administration concentrated on sending humans to the moon, Soviet scientists developed a series of spacecraft designed to penetrate the heavy atmosphere. That proved a challenge, but in 1970 they succeeded with Venera 7, and what a story it told.
The Soviet craft managed to briefly transmit data about the atmosphere, but no pictures of the surface, and the results were almost unbelievable. They showed that the atmosphere is indeed crushing. A square inch on the surface of Venus is under an atmospheric pressure of two-thirds of a ton--90 times greater than on Earth.
What kind of surface could lie beneath those oppressive clouds? Radiotelescopes on Earth gathered hints, and in the 1970s, scientists began to get their first answers to that question. NASA's Pioneer Venus Orbiters used radar to map 93% of the planet's surface, revealing what many had long suspected. Venus has--or at least had--volcanoes, accounting for an atmosphere rich in carbon dioxide and sulfuric acid. But those pictures were of low resolution, and many a scientist had difficulty trying to decipher the topography.
And then came Magellan.
NASA's Magellan spacecraft, which is managed by the Jet Propulsion Laboratory in Pasadena, used state-of-the-art radar imaging to strip away the clouds and reveal details on the surface of Venus with images of startling clarity. For the past year and a half the spacecraft has been passing over the cloud tops, pointing its sensitive instruments toward the ground. During the first 3,880 orbits, the craft mapped 95% of the surface, exceeding the goals of the program.
Those images are so sharp that scientists now have many tools they need to determine the history of how Venus evolved to its present state, and possibly where it will go from here. The photos on this page are among the best images, reflecting a diversity of geological structures on Venus.
It turns out that Venus has "a strong resemblance to Earth," said Edward Stone, director of JPL. Its surface is constantly being churned by active geologic processes, and its volcanoes have been so active that its entire surface "seems to be reasonably young, maybe only 400 years old," Stone said.
Yet the geology on the two planets is also quite different. On Earth, most geological processes are driven by the movement of giant chunks of the Earth's surface, called tectonic plates, that float around the molten mantle like rafts on the sea. In many areas of the globe, particularly around the Pacific Rim, these plates crush together, creating earthquakes and volcanoes.
Scientists had long wondered if Venus had a similar system. The Magellan data hints at an answer to that question, but it will take years of analysis before scientists are comfortable with their findings. Admitting he was venturing into scientific speculation, Stone said the images suggest that Venus may be about where the Earth was, geologically speaking, 3 billion years ago.
"Maybe, what we are seeing here, is incipient plate tectonics," Stone said. The surface of Venus, he said, is "remarkably fractured by faults," suggesting some form of plate tectonics.
But it does not have the great mountain ranges created on Earth by colliding tectonic plates.
So what does that suggest? Maybe, Stone said, Venus does have tectonic plates but they are so hot that they are very pliable, so when they rub together they do not shatter and create mountains like those on Earth.
"Maybe they are just too soft," Stone said. Instead of buckling into great mountains, maybe the plates just wrinkle, like a rug being pushed gently against a wall.
If in hundreds of millions of years the surface of Venus cools down, perhaps the plates will become more rigid and Venus will become more like Earth.
And other scientists have added their own warning: If the greenhouse effect caused by the burning of fossil fuels on this planet continues unabated, perhaps Earth might become more like Venus.
Meanwhile, Magellan will continue collecting data for another year that could strip away the mysteries of Venus even more.