Last summer I stood on the deck of a boat off Alaska watching the face of a glacier. We couldn't actually see the glacier moving, but moving it certainly was. Every few minutes a small piece of the white cliff would crumble, smash to rubble as it slid down in slow motion, and hit the ocean with a booming roar. In between times, the cliff face was pregnant with menace and suspense. We wondered if we were too close for safety. That ice was on the move.

The same thing seems to be happening, more or less, along a cliff face at the edge of Mars's north polar ice cap. Aboard the orbiting Mars Reconnaisance Orbiter, the powerful HiRise Camera (the best ever to orbit another world) took an enormous, high-res image that included a long polar cliff. Amazingly, the image caught four widely separated landslides in progress along the 2,300-foot-high cliff — all at once! That's one place on Mars I wouldn't want to be exploring on foot.

"It really surprised me," says discoverer Ingrid Daubar Spitale. "It's great to see something so dynamic on Mars. A lot of what we see there hasn't changed for millions of years."

Read all about it — and download incredible high-res images — at today's news posting from the HiRise Camera team. NASA has also put out a Science@NASA news story.