Venus is not a placid paradise — that much we know. In addition to searing surface temperatures, wind in the upper atmosphere howls at up to 250 mph, carrying clouds around the planet once every four days.
Yet Venus itself spins very slowly: one rotation every 243 Earth days — in the wrong direction, no less, opposite to almost every other body in the solar system.
On the whole, the atmosphere on Earth rotates about the same speed as the planet. So why does the air on slow-spinning Venus speed around so much faster than the planet itself?
The Japanese space probe Akatsuki, now in orbit around Venus, seeks to solve the mystery of so-called super-rotation. Scientists working on the mission are presenting some of their early findings at a meeting this week of the American Astronomical Society's Division for Planetary Sciences in Pasadena, Calif.
That is not just an idle trivia question for planetary scientists. Computer models of our own weather fail when applied to Venus, and knowledge of the planet's workings could better our understanding of Earth's.
"We don't know what is the missing point in meteorology," said Masato Nakamura, Akatsuki's project manager. "If we know what makes such a super-rotation, we will have a much deeper understanding of the atmospheric dynamics, not just on Venus but also on Earth. We will learn much more about the Earth climate."
In recent years, Venus has been a backwater of planetary exploration, even though it is much closer in size to Earth than is Mars. For a long time, scientists imagined there could be a habitable tropical paradise beneath Venus' thick clouds.
In the late 1950s, intense thermal emissions, measured by a radio telescope on Earth, told a different story. Venus broils.
The average surface temperature is more than 850 degrees Fahrenheit — an extreme demonstration of the heat-trapping prowess of carbon dioxide, the primary constituent of the Venusian atmosphere. Clouds of sulfuric acid make it an even less appealing place to visit.
In the 1990s, NASA's Magellan spacecraft precisely mapped the topography of Venus through radar. Except for a few flybys by spacecraft on the way to somewhere else, NASA has not returned to Venus, although the agency is considering two modest proposals.
A European mission, Venus Express, studied the planet from 2006 to 2014, discovering among other things a frigid layer of atmosphere, minus 280 degrees Fahrenheit at an altitude of 75 miles, sandwiched between two warmer layers.
But now Akatsuki, which entered orbit last December, has begun its work. Takehiko Satoh, one of the mission scientists, said that one of "the most exciting, most surprising results" so far came almost immediately after the spacecraft arrived.
The camera that captures long-wavelength infrared light from the cloud tops discovered an arc-shaped white streak that stretched some 6,000 miles from nearly the south pole to nearly the north pole.
Curiously, this giant atmospheric feature does not move with the atmosphere. "It seems to be fixed to the ground," Satoh said. The arc sits above Aphrodite Terra, a highland region about the size of Africa that rises up to three miles from the surface. Scientists working on data from the Venus Express reported a similar finding in July.
One possibility is that as the wind blows over Aphrodite Terra, clouds are pushed higher and the temperature of the cloud tops falls. "Our interpretation is there is some disturbance from the high mountain," Nakamura said.
Satoh said there were primarily two competing ideas for where the energy for the Venus wind comes from. One is that energy coming from the sun accelerates the wind. The second is that atmosphere is so thick that it gradually slows down the spinning of the planet, and that angular momentum is transferred to the air.
According to this theory, even though breezes on the surface are slight — a couple of miles per hour — the speeds increase at higher altitudes as the air thins.
The small spacecraft — the main body is a box a bit bigger than a refrigerator — carries five cameras, collecting light at different wavelengths to monitor the Venusian atmosphere at different altitudes.
In another experiment, scientists will observe how the radio signal from the spacecraft to Earth is distorted when it passes through the atmosphere. That will reveal temperature, abundance of sulfuric acid vapor and other properties. By observing the atmosphere at different altitudes, they can detect wavelike features that rise and fall, like blobs in a lava lamp.
"If the solar heating or thermal tide hypothesis is correct," Satoh said, "we may see different propagation of the wave, from cloud top to the lower level." If the viscosity theory is correct, the waves should propagate in the opposite direction, from the ground to the clouds.
Perhaps the answers will become clear in a year or maybe four. "We need to analyze a lot of big data," Nakamura said.
That Akatsuki, which means "dawn" in Japanese, is there at all is the result of ingenuity and perseverance.
It launched in May 2010 and arrived at Venus seven months later. But when its main engine failed to fire properly, it sailed right past the planet.
"It was a very sad moment," Satoh said.
Within a day, Satoh said, calculations indicated that in six years, Akatsuki, in orbit around the sun instead of Venus, could meet up with Venus again. But it was not clear the spacecraft still would be able to slow down and enter orbit.
An investigation found that a valve in the engine had leaked, leading to the formation of salts that fused it shut. The engine, as it fired, had overheated beyond repair.
Akatsuki still had the maneuvering thrusters that were to be used after it entered orbit. They were not as powerful as the broken engine, but they could apply enough force to slow it down enough so that Venus' gravity could capture it.
Because of worries that the longer stay in space, with the bombardment of solar radiation and cosmic rays, would degrade the instruments, the craft was maneuvered so the second rendezvous would occur a year earlier, in November 2015.
Then calculations suggested that orbit might not be stable, and the spacecraft might crash into Venus shortly afterward. Another adjustment pushed the arrival back a couple of weeks to Dec. 7, exactly five years after the original arrival date.
This time, everything worked.
The Akatsuki's orbit is different from the one originally envisioned. Instead of being synchronized to the spinning atmosphere, which would have allowed scientists to better track small changes, the spacecraft now loops around Venus in a large elliptical orbit.
That provides different benefits. Instead of intently staring at one spot, seeing the smallest changes, scientists are now able to see what happens on a global scale, although they will miss some of the details.
Akatsuki is to continue operating until at least April 2018, depending on how much fuel it has left. "We know at least we have one kilogram of fuel," said Nakamura, likening the uncertainty to an imprecise fuel gauge in a car.
If it turns out that Akatsuki has more, the spacecraft could continue operating for perhaps up to six years, he said.