In December 2014, the European Space Agency’s (ESA) Venus Express probe was purposefully sent to its death in the atmosphere of Venus at the end of its mission. Now, scientists have revealed some of the science returned from that fatal plunge – and the results are truly fascinating.
The dip into the Venusian atmosphere began in June 2014. The spacecraft, which launched in November 2005, was nearing the end of its mission. With limited fuel available, scientists decided to make the most of a unique opportunity to probe unexplored regions of Venus, namely its upper atmosphere and its poles. The findings from this finale have now been published in the journal Nature Physics.
"None of Venus Express' instruments were actually designed to make such in-situ atmosphere observations,” said lead author Ingo Müller-Wodarg of Imperial College London, UK, in a statement. “We only realized in 2006 – after launch! – that we could use the Venus Express spacecraft as a whole to do more science."
What these results reveal is that parts of Venus are much, much colder than expected. The average temperature on Venus makes it the hottest world in the Solar System, with its thick atmosphere trapping heat and giving rise to scorching temperatures of 460°C (860°F) on the surface.
But measurements taken by Venus Express at an altitude of 130 to 140 kilometers (81 to 87 miles) above the surface have revealed the atmosphere near the poles has temperatures far below that on Earth. In fact, the polar atmosphere on Venus drops to -157°C (-251°F), which is 70 degrees colder than expected. It is also 22 to 40 percent less dense than thought.
"These lower densities could be at least partly due to Venus' polar vortices, which are strong wind systems sitting near the planet's poles,” said Müller-Wodarg. “Atmospheric winds may be making the density structure both more complicated and more interesting!"
Venus Express used the atmosphere of Venus to slow its velocity, known as aerobraking. ESA/C. Carreau
Another interesting finding was that the polar region is dominated by something known as atmospheric gravity waves. Don’t be fooled by the name, though; these are nothing to do with the more widely-known gravitational waves. Instead, atmospheric gravity waves are ripples in the atmosphere that travel vertically, from low to high altitudes, as the density decreases. We actually have some of these on Earth.
Venus Express found that there were atmospheric waves originating from the upper cloud layer on Venus, about 90 kilometers (56 miles) above the surface. In addition, larger-scale waves caused by the planet’s spin, known as planetary waves, were also found to be present.
To make these findings, Venus Express was required to perform aerobraking maneuvers, using the Venusian atmosphere to slow the spacecraft’s velocity. And this maneuver could have implications for the European Trace Gas Orbiter, currently on its way to Mars, which will use a similar technique to measure the composition of various gases in the Martian atmosphere.
Venus Express may have died a fiery death more than a year ago, but results from the mission are still turning up some surprises. And with Japan’s Akatsuki mission recently beginning its own science mission around Venus, it's clear there is still much to learn about the second planet from the Sun.