On a planet that is already cloaked in mystery, long-term observations by Venus Express orbiter show that winds in the upper atmosphere are racing faster than ever.
What gives with Earth's nearest planetary neighbor? Sure, Venus looks bland and featureless when viewed through a telescope. (See for yourself: it's the unmistakably bright "star" low in the west after sunset.) But this veiled planet has challenged planetary scientists for decades.
Fifty years ago, radar observations confirmed that Venus is rotating very, very slowly (once every 243 days) and backward (retrograde) with respect to Earth. Then, in the 1970s, passing spacecraft showed that the planet's mid-level atmosphere is racing ahead of the planet itself at hurricane-like speeds, swirling around in just about 4 days. The planet's clouds form distinctive chevron shapes, a combination of this superrotation and the slow migration of gas from the Sun-warmed equator to the cooler poles. No computer model has yet explained why the atmosphere moves so fast — let alone why Venus spins the way it does.
Now two independent research groups have found that the atmosphere's winds have been gradually speeding up for years. In 2006 the average velocity of the planet's cloudtops at low latitudes was about 180 miles per hour (80 m per second). But by 2012 it had risen to 250 mph (110 m/s) — a jump of more than a third in just 6 years.
This surprising discovery comes from long-term observation of cloud motions by a camera aboard the European Space Agency's Venus Express spacecraft, which maintains an elongated orbit that ranges in altitude from 150 miles (250 km) above the north pole to 41,000 miles (66,000 km) above the south pole. Over time its Venus Monitoring Camera has recorded thousands of images at ultraviolet and near-infrared wavelengths to reveal details in the topmost clouds roughly 44 and 38 miles (70 and 60 km) above the surface. The images were taken in clusters about every 100 days, whenever the spacecraft was positioned high above the planet's daylight hemisphere.
A team led by Igor V. Khatuntsev (Space Research Institute, Moscow) tracked 350,000 individual cloud features using a computerized identification program and another 45,000 by visual inspection. As they report in the planetary-science journal Icarus, speeds can vary a lot — even from one of the spacecraft's 24-hour-long orbits to the next. Also, the winds are typically faster over mid-latitudes than at the equator; the uppermost clouds take only 3 days to circle around in the mid-latitudes and up to 5 days above the equator.
But the winds' gradual strengthening over time was unexpected. "This is an enormous increase in the already high rotation rate of the atmosphere," Khatuntsev says in an ESA press release. "Such a large variation has never before been observed on Venus, and we do not yet understand why this occurred."
The second group, led by Toru Kouyama (National Institute of Advanced Industrial Science and Technology, Japan), used an automated cloud-tracking method for its analysis, which appeared earlier this year in Journal of Geophysical Research – Planets. The Kouyama team, which concentrated on images taken by Venus Express during its first 3½ years in orbit, finds that wind speeds vary by about 45 mph (20 m/s) in a cycle lasting about 250 days. This is longer than a Venus year (225 days), so it's probably not due to the varying intensity of sunlight during each orbit.
Notably, this isn't the first time the planet's wind velocities have wandered over time. Cloud tracking using Pioneer Venus Orbiter imagery revealed a slight slowdown from 1979 to 1982, followed by a quickening from 1982 to 1985.
But as the plot above shows, the top wind speeds derived from Venus Express are higher than those found at the same southern latitudes by any previous spacecraft — though they best Galileo's 1990 value only slightly. And since ESA's orbiter remains healthy and funded through 2014, it'll be interesting to see if the speeds continue to go up or come back down.