The past month's big pictures from space include a giant "Bootprint on Mars," provided courtesy of the European Space Agency's Mars Express orbiter.

This elongated, 240-mile-long crater, known as Orcus Patera, is arguably even more enigmatic than the famous "Face on Mars": The face is basically an eroded mesa that just happens to look like a face with the right lighting and focus. But scientists really don't know how Orcus Patera came about.

As explained in today's ESA image release, the term "patera" is usually used to describe deep, complex or irregularly shaped volcanic craters. Orcus Patera is situated between two huge Martian shield volcanoes, Olympus Mons and Elysium Mons, but it's not clear whether volcanic activity is responsible for the bootlike shape. The depression could have started out as a round crater that was subsequently deformed by geological movements. It could have developed from the erosion of separate craters that were lined up next to each other.

"However, the most likely explanation is that it was made by an oblique impact, when a small body struck the surface at a very shallow angle, perhaps less than five degrees from the horizontal," the Mars Express team says.

Mars grazed by a meteor? Sounds sensible to me. It'd take one giant leap of logic to believe that giants once walked the Red Planet. Almost as big a leap as it'd take to believe that someone left a human-sized bootprint on the Martian surface.

You'll find more big pictures in our newly published Month in Space slideshow. Click on these links for bigger versions of the images, plus the stories behind the pictures:

• Colorful clash: Antennae galaxies, as seen by NASA's Great Observatories.
• Wonders of Earth and sky: Perseid post-mortem, plus the big picture.
• Readying the Robonaut: Kennedy Space Center gets r0obot set for liftoff.
• Live fast, die young: The star WR 22, from European Space Observatory.
• Ice on the loose: EO-1 spots Petermann Glacier's breakup in Greenland.
• Tsunami on the sun: Magnetic storm seen by Solar Dynamics Observatory.
• All dressed up: Get the big picture for Dmitri Kondratyev's dunking in Russia.
• A mudslide's mortal blow: Chinese landslide, as seen by WorldView 2.
• Land of the crescent: The moon looks even bigger in this big picture.
• Rovers ready to roll: Learn about Desert RATS and see NASA's big picture.
• Before and after the flood: Pakistan tragedy documented by Landsat 5.
• Fixing the space station: More about repair spacewalks plus the big picture.
• Glorious galaxy: No slideshow is complete without a Hubble galaxy image.
• Great view: Check astronaut Doug Wheelock's Twitpic account.
• Ghostly mystery: Hubble provides a spooky view of a dark nebula.