Artist's impression of Venus Express aerobraking. Credit: C. Carreau
The last pieces of data sent back to Earth by the Venus Express spacecraft show the planet’s atmosphere to be colder and less dense than expected, and rippling with strong atmospheric waves.
Venus Express arrived at Venus in 2006 and spent eight years analysing the planet from orbit.
It then began to descend into Venus’s atmosphere before losing contact with Earth in November 2014.
As the spacecraft sank into the planet’s atmosphere, it continued to send back information to Earth. Venus Express experienced considerable drag on its way through the atmosphere and was able to measure the deceleration in a process known as aerobraking.
"Aerobraking uses atmospheric drag to slow down a spacecraft, so we were able to use the accelerometer measurements to explore the density of Venus's atmosphere," says lead author of the study Ingo Müller-Wodarg.
"None of Venus Express' instruments were actually designed to make such in-situ atmosphere observations.
We only realised in 2006 – after launch! – that we could use the Venus Express spacecraft as a whole to do more science."
The results show that Venus’s polar atmosphere is as much as 70°C colder than expected, with an average of -157°C.
Also, the polar atmosphere is not as dense as expected. At 130km and 140km in altitude, it is 22% and 40% less dense than predicted respectively.
"This is in line with our temperature findings, and shows that the existing model paints an overly simplistic picture of Venus' upper atmosphere," says Müller-Wodarg.
"These lower densities could be at least partly due to Venus' polar vortices, which are strong wind systems sitting near the planet's poles.
Atmospheric winds may be making the density structure both more complicated and more interesting!"
Further data from Venus Express showed the polar region to contain strong atmospheric gravity waves and planetary waves.
The former are a ripple in the density of a planet’s atmosphere.
These waves travel from lower to higher altitudes and become stronger as they rise through less dense atmosphere.
The latter are larger waves associated with the planet’s spin on its axis. Both types of waves transfer energy across regions of the atmosphere, and so could reveal much about how the atmosphere is shaped.
"We found atmospheric gravity waves to be dominant in Venus' polar atmosphere," says co-author Sean Bruinsma.
"Venus Express experienced them as a kind of turbulence, a bit like the vibrations you feel when an aeroplane flies through a rough patch.
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."
The findings show how similar techniques could be adopted by ESA’s ExoMars Trace Gas Orbiter, which launched this year, to reveal more about the atmosphere of the Red Planet.
"For Mars, the aerobraking phase would last longer than on Venus, for about a year, so we'd get a full dataset of Mars's atmospheric densities and how they vary with season and distance from the Sun," says Håkan Svedhem, project scientist for ESA's ExoMars 2016 and Venus Express missions.
"This information isn't just relevant to scientists; it's crucial for engineering purposes as well.
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."