Probing a planet’s ionosphere can be all about the timing. Confusion about how many layers of charged particles exist in the Martian atmosphere might be due to what time of day measurements were made.
Several spacecraft have, over the years, detected two ionosphere layers on the Red Planet. But both Viking landers noticed only one layer during their 1976 descents. The other spacecraft probed the atmosphere at sunrise and sunset when conditions favor the formation of two layers, space physicist Majd Mayyasi and astronomer Michael Mendillo report online September 10 in Geophysical Research Letters. The Vikings, however, landed on Mars around 10 a.m. and 4 p.m. (local time), when the midday sun blends the layers together.