What is the simplest measurement a satellite can make of the atmosphere of any planet? If you can get a determination of its position, the answer is the density of that atmosphere.
Anything travelling through a gas, however tenuous, will have to push against the particles of that gas to get anywhere. The denser the gas, the more it pushes back and slows the thing. This is called aerodrag. Venus Express, the European mission to the second rock from the Sun, has been conducting an aerodrag experiment to measure the upper reaches of the atmosphere of that planet. Rather than simply recording how much it is slowed by friction – which would require some very sensitive calculations, it has angled its solar panels in such a way as to use them as sails. Their large surface area means they are the least aerodynamic part of the spacecraft and by having one of them positioned horizontally and the other pointing upward, the craft can be made to start to spin round by the thin atmosphere at its orbital altitude. The spin is tiny, but the satellite’s automatic trajectory correction system can detect it and correct it and it is this information that gets received back on the ground. From the correction can be found the induced spin and hence the atmospheric density.
More details on this experiment can be found here.