Francesco Bianchini, librarian to Pope Alexander VIII, drew the first map of Venus in 1726.
Too bad he got it all wrong. Using a 150-foot-long telescope, he outlined oceans and continents. But he was seeing things. The thick Venusian atmosphere obscures the surface, which is covered with plains, mountains, volcanoes and craters.
Modern scientists can observe Venus` surface with radar, which penetrates the clouds. The National Aeronautics and Space Administration has sent two radar mapping missions to Venus: the Pioneer-Venus Orbiter and Magellan.
Pioneer reached the planet in December 1978. Magellan joined it in August 1990 and began mapping the following month. Magellan makes images 10 times more detailed than ever, and it has mapped more than 97 percent of Venus.
Although a broken transmitter has put a stop to mapping, NASA is trying to solve the problem and plans to map another 1.5 percent of the planet within a few weeks.
Last month, planetary scientists met to discuss the radar images and to debate various theories of Venusian geology at the International Colloquium on Venus in Pasadena.
The Pioneer and Magellan radar images present a truly planetary conundrum. They show a quiet planet, with little geologic activity-unlike the Earth, where continents roam the globe and chunks of crust crash together in the process known as plate tectonics.
”The basic findings are that there is not plate tectonics on Venus,”
said James Head, a planetary geologist at Brown University in Providence, R.I. But Venus shouldn`t be quiet. Like Earth, Venus contains radioactive elements that decay deep inside the planet.
The radioactive decay produces heat, and that heat should cause geologic activity.
Magellan may help scientists solve the heat-loss conundrum. At the Pasadena meeting, some scientists proposed that violent geologic activity occurs every 500 million years or so, releasing a bunch of heat all at once.
Others claimed that Venus has an extremely thin crust that lets the heat leak out. Still others think that spurts of geologic activity happen from time to time around the planet.
Planets get rid of heat by making crust, a layer of rock on the outside of the planet.
On Earth, the crust forms the upper portion of the lithosphere, a layer of solid rock that floats on the liquid magma of the lower mantle. Earth`s crust is broken up into plates that move around on that molten level.
Heat produced deep in the Earth moves the plates. Magma rising at mid-ocean ridges actually makes crust when it solidifies into rock on the ocean floor.
If the Earth were a balloon, it would just inflate as crust oozed out. But the Earth can`t inflate, so the new crust pushes the old crust along on Earth`s surface.
Eventually the old crust of one plate collides with another plate and plunges into the Earth and melts, using more heat in the process.
”Plate tectonics on Earth is fairly simple because the Earth`s crust is broken up into 10 plates,” said Magellan project scientist Stephen Saunders. The boundaries of those plates are marked by signs of tectonic activity such as volcanoes and earthquakes. Plate boundaries wrap around Earth in a connected net that is ”like the seams on a baseball,” said Gerald Schubert of the University of California at Los Angeles.
When scientists saw Magellan`s images of Venus, they knew right away that it couldn`t have plate tectonics because it didn`t have those seams.
Its volcanoes aren`t lined up along boundaries. They`re distributed randomly across the planet.
”The volcanoes are virtually scattered all over the place. There really isn`t any obvious plate scenario,” said Saunders of the Jet Propulsion Laboratory in Pasadena.
Without plates, Venus can`t make crust the same way Earth does, at mid-ocean ridges. Head thinks that magma rising from deep inside Venus simply solidifies on the bottom of the outer crust, like ice freezing on a lake.
”In contrast to the Earth, it appears to be creating its crust vertically rather than horizontally,” he said.
But unlike ice on a lake, the Venusian crust is denser than the material supporting it. Eventually the crust grows too heavy to support, and it collapses.
”As it gets thicker and thicker, it`s no longer stable,” Head said.
Every 500 million years or so, the thick crust breaks up and sinks into the mantle, said Donald Turcotte of Cornell University in Ithaca, N.Y. Magma gushes up through the cracks in the crust. When the magma solidifies, Venus has a new surface.
Many scientists have a hard time believing in such a catastrophic event. But Turcotte insists that his scenario makes sense.
”Once it goes, it goes,” he said. ”It`s like when you punch a hole in the bottom of a ship. It sinks.”
Turcotte said the pattern of craters on Venus supports his theory. On geologically active planets like Earth, volcanoes, crust subduction, erosion and other forces erase craters. If Venus had geologic activity, its craters would be erased, too.
But Venus has 840 craters distributed randomly over its surface. Since there are craters everywhere on Venus, Turcotte said, no part of Venus is geologically active today.
Furthermore, he said, the Venus craters tell scientists when the last geologic activity happened.
Comparing the number of craters on Earth, Mars and the moon to the number on Venus demonstrates that the last geologic activity on Venus happened about 500 million or 600 million years ago.
Some scientists, including Roger Phillips of Washington University in St. Louis, disagree with Turcotte`s evaluation. Phillips acknowledges that craters are distributed randomly on the Venusian surface.
But he says that tectonic activity is distributed randomly too, so it doesn`t disturb the random pattern. Areas with few craters are probably geologically active and young compared with the rest of the planet, Phillips said.
”The surface of Venus cannot be of a single age, or even close to it,”
he said.
David Sandwell of the Scripps Institution of Oceanography in La Jolla, Calif., has found evidence of geologic activity in the coronae of Venus. Coronae are round, raised areas surrounded by a deep trench.
”It`s like someone cut a trough in the surface. That trough looks like the oceanic trenches” on Earth, said Schubert, a professor of geophysics and planetary physics at UCLA.
This observation led Sandwell and Schubert to investigate the possibility that Venusian crust is plunging into the mantle, or subducting.
On Earth, subduction occurs when a plate of oceanic crust slides beneath another plate. The trench occurs because the down-going plate buckles upward just in front of the overriding plate, and then plunges deep underneath it where the plates meet.
Sandwell began by calculating the thickness of the crust beneath several Venusian trenches. He could tell how thick the crust was by measuring how much it buckled upward in front of the trench. The thicker the crust is, the more it bends upward before it subducts.
The crust turned out to be much thicker than many scientists had thought. These scientists had figured that if Venus doesn`t get rid of its heat with plate tectonics, the heat must just leak out through a thin crust. But it doesn`t.
”The thickness varies quite a lot,” Sandwell said, ”but in general it`s much thicker than you would predict if you said that . . . the only heat transport mechanism on Venus is lithospheric heat conduction.”
Sandwell was not alone in dismissing the thin-crust theory. Turcotte pointed out that there are mountains on Venus that are six miles high. A thin crust could never support them.
”We`ve never been able to explain how you get such high topography”
with such a thin crust, he said.
After dismissing the idea of a thin crust, the scientists have to explain the apparent lack of geologic activity on Venus today.
”You have to invoke a catastrophic event” like Turcotte`s theory of occasional crustal turnover, Sandwell said.
He thinks that the trenches he sees around Venus` coronae could be the beginning of the next event, or the remnants of the last.
But first he has to prove that subduction is really happening.
Magellan may give Sandwell the evidence he needs. NASA plans to bring the craft into a lower orbit in September, where it will begin collecting gravitational data.
Gravitational data is easy to find-NASA just has to measure the speed of Magellan as it orbits Venus.
If Magellan passes over a particularly heavy object on or near the surface of Venus-such as a slab of subducting crust-it will speed up as it approaches the object and slow down after it passes over the object.
Geologists refer to the additional gravitational pull caused by heavy objects as a ”gravity anomaly.”
”If it`s subduction, you`ll see a big gravity anomaly over the trench,” said Catherine Johnson of the Scripps Institute of Oceanography.
Whether it`s subduction or not, Magellan should give planetary geologists plenty to talk about at their next colloquium.
”It`s going to be a while, I think, before we resolve this controversy,” Phillips said.
