Flying a satellite in atmosphere

The Mars Atmospheric and Volatile Evolution  ("Maven")  mission, getting ready to launch for Mars, is planning to do just that.  To investigate the Martian upper atmosphere, the satellite will be placed in an elliptical orbit around Mars, which dips into the Martian upper atmosphere to take readings.  The orbit will go within 150 kilometers of the surface.
   That's about the altitude that the old Mercury capsules orbited at.   On Earth, there is enough air left above that altitude to form the ionosphere, important to HF radio transmission.  Clearly the experimenters believe there is a Martian ionosphere, thick enough for Maven's instruments to take a reading on.
   They didn't say how long this can go on before atmospheric drag pulls the satellite down. 
   They mentioned that Mars used to have a strong magnetic field which disappeared some time in the distant past.  That's a new one on me.  It is thought that the magnetic field used to shield Mars from the solar wind.  When the field went away, they think the solar wind stripped away most of the Martian atmosphere.  Just how the ancient Martian magnetic field was discovered, and what might make it go away wasn't discussed. 
   Anyhow they hope to use Maven to measure what in the upper atmosphere might be related to, or causes of, Mars' lack of air and water.