Chris Palomares tweeted this:

Can someone explain to me what the default #iOS9 default wallpaper is supposed to be? Looks like a sand dune, maybe?

Jason Snell responded:

Yes, it’s a sand dune—on Mars! Many iOS 9 wallpapers are actually images from the HiRISE instrument on the Mars Reconnaissance Orbiter.

Pretty cool. Here’s a link to Jason laying this out via Twitter back and forth, and here’s a link to the HiRISE catalog.