Both Mars and Venus have been objects of scientific and popular speculation since at least the beginning of the 20th century, and since the 1960s spacefaring nations have been sending robotic probes to explore both worlds. Mars has gotten far more attention, however. Since 2002 no fewer than two Mars probes have been actively gathering data during any given year. Last year there were seven.
This is understandable. Mars is far more hospitable than Venus, where surface temperatures reach nearly 480 degrees Celsius, surface pressure is 92 times that of Earth and the planet is permanently shrouded by thick clouds of sulfuric acid. We have direct evidence that water once flowed and pooled on Mars. It cannot be ruled out that life once existed there and may conceivably exist still.
Venus is far more Earth-like than Mars in its size (it is only 5 percent smaller than Earth), composition and surface gravity, but its harsh environment leaves little hope that the planet could ever host life. Yet it is still worth studying Venus to learn why it is the way it is—and how Earth could avoid a similar fate.
Venus could also help us understand newly discovered extrasolar planets. A surprising number of these planets lie very close to their stars, with revolution periods as short as a few days. So far most of these are massive “hot Jupiters” or “hot Neptunes,” but improving instruments should one day allow astronomers to find “hot Venuses.” If that happens, our sister planet would serve as an invaluable reference point for interpreting observations of distant worlds.
Venus is also an intriguing world in its own right. Although it is Earth-like in size and composition, there is no evidence of the kind of plate tectonics that continuously recycles our planet's crust. Nevertheless, Venus's surface is rich in volcanoes, lava flows and other geologic evidence of past tectonic activity. If tectonic activity is still going on, which might well be the case, studying it could give us important information about the planet's inner structure and dynamics.
The dynamics of the Venusian atmosphere are equally fascinating. The planet rotates on its axis once every 243 days—in a direction opposite to Venus's motion around the sun, unlike every other planet.* But its clouds take just four days to circulate, in a phenomenon known as superrotation, and this superrotation involves virtually the entire atmosphere, up to an altitude of 80 to 90 kilometers. The only exception is the poles, where spectacular, continuously changing vortices develop. Venus's atmospheric motion thus resembles a great, planetary-scale hurricane with two “eyes” residing on both poles. Scientists hope that studying Venus's atmospheric dynamics could help them understand how to predict terrestrial hurricanes and even control them.
For the general public, the search for extraterrestrial life is probably the most important reason to do planetary exploration. Does Venus's hellish climate mean that any type of biology is strictly impossible there? Surprisingly, some experts say no. They argue that the abundant aerosol particles in Venus's atmosphere could in principle host some form of life. All the necessary components are there: a moderate thermal regime at 50 to 70 kilometers above the surface, liquid water and rich chemistry. Only future studies will show if this hypothesis, which seems fantastic, is true or not.
Yet despite all this scientific promise and Venus's proximity to Earth, the planet has been relatively poorly studied. Indeed, when the Venus Express mission was launched in 2005, it had been 20 years since the previous Venus probe. Only a tiny handful of probes have been launched since then. For all these reasons, planetary scientists around the world believe that we need a new campaign to investigate Venus, complete with orbiters, landing probes and flying platforms. We hope that funding agencies will agree.
*Editor's Note (7/6/16): This sentence from the print article was edited after it was posted online. The original misstated the number of Earth days it takes for Venus to complete one rotation on its axis.