Our Solar System: Venus
I was already in the Science Team room at NASA’s Jet Propulsion Laboratory when the second cycle of radar images of the surface of Venus were beamed down in May of 1991. I waited impatiently for the five-inch print roll to start spooling out. The first cycle had gone well and most of Venus’s surface was mapped for the first time. We were the first people in history to see the unveiled face of Venus. What we saw were impact craters, huge volcanoes, and lava flows everywhere. The reason I was so early to the Science Team room was that those lava flows looked super fresh, implying that they may have been flowing even as we imaged them. That would make Venus one of only three planets with active volcanoes. If the second cycle showed changes to any of the lava flows, it would be big news.
I had already pulled out of the archive the previous coverage of the area that was newly re-imaged. It was many feet long and printed on thermal paper: while the images were digital, computers in the ‘90s weren’t quite up to the task of displaying a five-inch by many feet long image swath, as captured by the Magellan spacecraft radar system. The new image had been acquired the day before, but raw synthetic aperture radar data requires a lot of computer processing to create the image. I laid the new image over the old one and flipped back and forth to see if there were any changes in the margins of the lava flows. No changes popped out; the question of Venus volcanism is still undecided.
Earth’s Evil Twin
Because its size is almost identical to the Earth’s, Venus has always been called Earth’s twin. But if it is, it’s Earth’s evil twin, as just about everything else about Venus is completely alien to Earth: it’s covered in thick clouds that produce a surface pressure of 100 times that of Earth – more like the bottom of our ocean! That thick atmosphere is almost all carbon dioxide, which produces a runaway greenhouse effect making the planet, at 700-900°F, hotter than Mercury, the innermost planet. The thick atmosphere also diffuses the heat, so everywhere is pretty much the same temperature. Interestingly, at about 100,000 feet up, the pressure and temperature of the atmosphere are about the same as Earth’s.
Venus’s rotation is also different from Earth’s and every other planet in the solar system: it rotates in the opposite direction and very slowly, possibly the result of a collision with some huge object early in its history. Its “day” is 243 Earth days long, longer than its year (225 Earth days).
Because of its thick, hot atmosphere, landing a probe on its surface has proved very challenging. Oh, and did I mention in addition to carbon dioxide, there is a large amount of sulfuric acid floating around? But way back in the 1970s and ‘80s, the Soviet Union managed to land several probes, which lasted up to two hours. They took gas samples on the way down and photos of a rocky, volcanic surface. But NASA’s Magellan mission was the first to map the whole planet with enough resolution to interpret its geologic history. With its radar penetrating the thick clouds, Magellan saw the two California-sized highlands that had been discovered with Earth-based radar systems, including the giant Arecibo dish in Puerto Rico that recently collapsed. Those two areas were called, with drab scientific succinctness, Alpha and Beta Regio. As Magellan mapped more and more features, we named them after prominent women, fittingly for planet Venus.
Interesting features abounded in addition to the 1,600 volcanoes. There were huge volcanic plains similar to the ancient lava plains of eastern Washington and Oregon. We discovered that many volcanoes were topped with a substance that reflected radar waves much like metal. The best guess was it was a product of sulfuric-acid weathering. As expected, there were few impact craters due to the thick atmosphere, but some of the meteors that made it to the surface produced parabolic shock-wave patterns in the surface deposits, something we’ve never seen on another planet. We also discovered sand dunes in the ejecta of one crater, proving that there was a source of sand and wind enough to move it.
More Than Meets the Eye
Venus has continued to serve up surprises since Magellan was sent crashing into its surface at the end of its mission in 1994. Venus Express, a European Space Agency mission, orbited between 2005 and 2015. It detected lightning in the atmosphere below and a strange thermal infrared anomaly at one volcano, indicating the possibility that Venus is indeed active. And earlier this year there was an unexpected announcement that phosphine, a gas associated with life, was discovered through Earth-based observations of Venus’ atmosphere. Since then, more analysis indicates that it may be a calibration error, but work is continuing. NASA is considering a new mission called DAVINCI to precisely measure the composition of the atmosphere as a follow-up on these observations as well as to understand better how the atmosphere evolved and determine whether Venus ever had an ocean.
NASA is considering a new mission called DAVINCI to precisely measure the composition of the atmosphere as a follow-up on these observations as well as to understand better how the atmosphere evolved and determine whether Venus ever had an ocean
A big mystery still revolves around the population of impact craters on Venus’s surface. Even accounting for the thick atmosphere, there are fewer craters on its surface than would be expected if its surface had sat around for the roughly four billion years of its history. In fact, models indicate that the crust we see is only about 750 million years old. It could be that eruptions forming lava plains have been enough to cover the older craters, but some researchers have proposed that there was enough heat trapped within Venus to completely melt the crust, causing it to founder into a magma ocean, resetting the “clock.” An orbital mission proposed to NASA called VERITAS would produce better geologic and topographic maps to confirm whether Venus ever had plate tectonics, or if the crustal foundering process may have occurred instead. Future ideas include balloons that could float in the more hospitable upper atmosphere, dropping down occasionally to sample the surface.
Meanwhile, Venus will be our Morning Star for a few more months, later slipping behind the Sun and returning as the Evening Star later in 2021.