There are also many volcanoes visible on the surface of Venus-in fact, several of these carbon-dioxide belching mountains appear to be active today although there's no definitive proof.
Between the Mariner II and Magellan probes to Venus, spanning the years 1962 to 1994, researchers such as astronomer David Grinspoon of the Denver Museum of Nature and Science reasoned that some bizarre global catastrophe resurfaced the planet every few million years-this all due to the large number of young volcanic peaks and the small number of old asteroid impact structures (fewer than 1,000 impact craters have been found).
It was clear that impacts had occurred on Venus, but some planetwide event apparently erased the ancient impact craters within a short span of geologic time.
Thus, a planetwide volcanic event, on an apocalyptic scale never seen before in our solar system, was proposed as the reason for Venus' "fresh" resurfacing.
Expanding this theory, astronomers then concluded that nothing much happened on Venus after the planetary upheaval.
Now, planetary geologists Timothy Bond and Mike Warner of Imperial College in the U.K. have shown that the global volcanic catastrophe on Venus probably never took place.
Bond and Warner looked at NASA's Magellan spacecraft data from the 1990s and found that that the distribution of old impact craters on Venus was random; the ancient craters were nearly pristine, untouched by anything resembling a planet-scale cataclysm. Bond and Warner rejected the accepted theory as simply too fantastic to take seriously.
"No planets that we knew of had experienced volcanic eruptions of this scale and speed," Bond said in a BBC-TV interview.
So, a single, titanic volcanic event may not have been responsible for smoothing out the Venusian surface. Instead, a series of volcanic eruptions-more like Earth's supervolcanoes of the Yellowstone kind, even smaller-were more likely to have resurfaced Venus 500 million years ago. But what triggered these more modest series of volcanic eruptions that reshaped a planet the size of Earth? Ah-now there you have a real mysery.
What's in the Sky: The first quarter Moon is south of Saturn this weekend. Look for the beautiful ringed planet and the Moon in the southwest about one hour after sunset.
Lou Varricchio, M.Sc., is a former senior science writer at the NASA Ames Research Center in California. He is a current member of the NASA-JPL Solar System Ambassador program in Vermont. You can contact him at: firstname.lastname@example.org.