In one of its final experiments before it plunged to its death, the European Space Organisation's Venus Express spacecraft coughed up a valuable piece of Venus' puzzle: our neighbour's polar atmosphere is much colder and thinner than we thought.
A series of low-orbit flybys between 24 June and 11 July 2014 dipped into the planet's atmosphere at high latitudes and measured temperatures 70 ºC lower, and density of about half, than expected.
Although Venus is our nearest planetary neighbour and has similar size, shape, composition and mass to Earth, it's an inconceivably inhospitable place.
As it orbits closer to the Sun than Earth, its surface temperature averages around 460 ºC with atmospheric pressure more than 92 times that on Earth. It's cloaked in a thick layer of sulfuric acid clouds. But our knowledge of its polar atmosphere has relied on extrapolating observations taking near the equator by NASA's Pioneer probe in the 1970s.
So one of the final missions of Venus Express, which launched in November 2005 and slotted into Venus orbit on 11 April 2006, was to explore and measure the temperature and density of the polar atmosphere. A decade after it started orbiting – to the day – those results were published in Nature Physics.
With Venus Express zooming around at an altitude of 140 kilometres, the polar atmosphere was 40% less dense than predicted – perhaps due to strong wind systems whirling around the poles. And planetary scientists thought the temperature would be around -90 ºC, but the craft's thermometer dropped to a mind-boggling -157 ºC.
The polar region was also dominated by atmospheric waves, which are thought to be instrumental in shaping a planet's atmosphere. Two types were measured on Venus: atmospheric gravity waves and planetary waves.
Atmospheric gravity waves are like pond ripples in the atmosphere, but they travel vertically instead of horizontally. As an atmosphere's density decreases with altitude, the waves become stronger. They can interfere with weather patches and cause turbulence.
Planetary waves are linked to a planet's spin. They're much larger and can affect entire weather and pressure systems.
"Venus Express experienced [atmospheric gravity waves] as a kind of turbulence, a bit like the vibrations you feel when an aeroplane flies through a rough patch," says study co-author Sean Bruinsma.
"If we flew through Venus' atmosphere at those heights we wouldn't feel them because the atmosphere just isn't dense enough, but Venus Express' instruments were sensitive enough to detect them."
Measuring Venus' atmospheric density wasn't one of Express' original experiments. It was only after it launched that planetary scientists realised they could use an "aerobraking" manoeuvre to measure drag as it pushed through the atmosphere and determine its density.
The spacecraft was designed to operate for 500 days, but with eight years under its belt, finally ran out of fuel towards the end of 2014. It finally lost contact in November that year and slowly spiralled to its demise over the next few weeks.
But the ExoMars mission, which launched in March, will use similar aerobraking techniques to completely map the atmosphere of our next door neighbour on the other side.
"This information isn't just relevant to scientists; it's crucial for engineering purposes as well," says co-author Hakan Svedhem.
"The Venus study was a highly successful test of a technique that could now be applied to Mars on a larger scale – and to future missions after that."
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