Not being able to think up my own April Fool’s joke, I’ll pass on somebody else’s: the HiRISE camera team at the University of Arizona. Be sure to click on the Grey at bottom right.
For my money (and I guess it partly is my money), HiRISE has returned some of the most beautiful images in the history of planetary exploration since it first began orbiting Mars in 2006. Take, for example, this recent picture of “spiders”—odd fan-shaped patterns that appear near the Martian south pole every spring (here's the hi-res version). As the ground heats up, ice beneath the surface turns to gas, then flows through channels until it vents to the thin air (dramatically, too). Dust carried by the venting gas turns the channels dark.
As with many HiRISE images, there’s something familiar and archetypal about this pattern. Some people, including the late Arthur C. Clarke, once convinced themselves they were seeing the seasonal bloom of Martian plants. The spiders turned out to be something far more alien—and almost as marvelous.