You've seen the pictures, now listen to the soundtrack! As Phoenix descended onto the surface of Mars, its telemetry signal was picked up by the European Mars Express orbiter. The ESA has just released a version of that signal converted to audio, so you can hear the signal. At first I was confused, but now I understand: the pitch of the signal goes up because Mars Express was approaching Phoenix as it orbited Mars. That caused a Doppler shift in the signal, increasing the frequency (or shortening the wavelength if you prefer). Just like an approaching car or train whistle will apparently go up in pitch (change to a higher note) as it approaches, so did the signal from Phoenix as Mars Express sailed toward it. For the public, there is nothing really to learn from this, and nothing of any great import to deliver. Still, why not? Not everything has to be earth-shattering. Sometimes things are just cool.