A vortex of air swirls over the south pole of Venus, discovered by Venus Express. Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. Oxford

Jan 27, 2015 Epitaph for a Venusian probe

Sen—On Sunday January 18, 2015, the Venus Express probe gave what is almost certainly its last shout to planet Earth.

The European Space Agency mission launched in November 2005 and arrived at the second planet from the Sun just 5 months later (hence the “express” part of it name). It settled into a 24-hour orbit, and began routine operations. Its mission: study the Venusian atmosphere over a long period of time, to see how it changes, how it interacts with the solar wind, map it in high-resolution, and determine its chemical composition.

Until quite recently, our “sister” planet has been quite a mystery; its surface hidden by thick clouds and an extremely dense atmosphere. The air on Venus is almost entirely carbon dioxide, probably due to a catastrophic runaway greenhouse effect early in its history that raised the temperature on the surface to over 450° C. The surface pressure is a crushing 90 times that at Earth’s sea level as well. Venus may be our sister, but she’s nowhere near our identical twin.

Previous missions to Venus had taught us a lot about the planet, and the exploration by Venus Express was also a huge success. Among its findings:

The NASA Magellan probe had found some interesting evidence of recent volcanism on the planet, and Venus Express corroborated that, finding bright spots on the surface associated with recent lava flows. They may be only a few millennia old.

Bizarrely, and quite unexpectedly, it found Venus is slowing its spin. The planet’s rotation had slowed by a staggering 6 minutes between the visits of Magellan and Venus Express. This may be due to complex interactions of the ridiculously thick atmosphere with the surface, but the details behind the world-spanning brake system are still unknown.

Oddly enough, while the solid body of the planet is slowing, the upper atmosphere circulation has sped up. The air circulates around Venus about every 100 hours, while the planet spins once every 243 Earth days! The mechanism behind this so-called “superrotation” is still unknown. One idea is that upper atmosphere circulation is amplified by interaction with the solar wind, and this speed is transferred down to lower layers, speeding them up as well. But the actual cause is still a mystery… and Venus Express made things even more complicated when it found that, over the years, the superrotation has been accelerating. Why? No one knows, but the observations made by Venus Express will no doubt prove crucial to eventually understanding it.

My favorite discovery by the mission, though, is the existence of a complex and spectacular vortex of air spinning at the planet’s south pole. A vortex is simply a fluid that spins, like a hurricane or a tornado. Many worlds have permanent vortices at their poles, including Earth, Saturn, and Saturn’s moon Titan. A vortex had already been seen at the north pole of Venus, but Venus Express discovered one at the south pole as well, and it was complex and ever-changing—in fact, it appears to be a double vortex, two separate but interacting spinning systems. The photo at the top of this article shows a snapshot of it taken on April 7, 2007; just hours later it had changed in shape significantly.

The overall vortex is caused by warmer air at the planet’s equator rising and moving toward the poles. It cools and sinks there, causing the air to spin. Seen in infrared in the Venus Express images, the vortex sits 60-70 kilometers above the surface.

These discoveries, and many more, will be the legacy of the mission. After eight years at Venus, Venus Express is out of fuel needed to maneuver. Last year, in 2014, the orbit of the probe was changed to let it dip much lower into the atmosphere for higher-resolution observations, then fuel was used to raise it back up. But that fuel is gone, and the orbit is slowly dropping due to atmospheric drag.

Contact was lost with Venus Express in December 2014, and the mission was declared over. In January, though, the main antenna briefly pointed at Earth, and the carrier signal suddenly jumped in strength. The contact only lasted for an hour or so, and then was once again lost. It’s almost certainly the last we’ll hear from the spacecraft.

But we’ll visit Venus once again. It’s so much like Earth, yet so different, that studying it up close is an essential piece of understanding the story of our own home. And for the better part of a decade, Venus Express told us that tale.