I could click on photos taken NASA’s Mars Reconnaissance Orbiter and never get to the end. The trusty spacecraft has taken nearly 50,000 images since it first entered orbit around the Red Planet in March 2006. The photos show us how similar Mars and Earth really are. While our planet has few well-preserved craters, we share river valleys, mountains, ice, dust storms, avalanches, clouds and sand dunes.
Both planets have polar caps, but Mars is so much colder that carbon dioxide in the atmosphere condenses out at high latitudes as frost that can reach depths anywhere from 3 to 32 feet. During the warmer spring and summer seasons, that frost sublimates directly from solid to gas, vaporizing back into the atmosphere to create a crazy looking pattern of exposed ground and lingering frost called “Swiss cheese” terrain.
In the featured photo, taken in late Martian summer not far from the South Pole, we see a layer of vaporizing CO2 that exposes a fascinatingly monotonous landscape of small hills resembling corduroy fabric. The crater may have been caused by an meteoroid impact or it might be a collapse pit that formed when subsurface ice melted, causing the ground above to collapse sinkhole-style. We can look down into the pit and see that at least some of the CO2 frost on its floor has vaporized away leaving only a nubbin of cheese.
During spring and summer, warmer weather causes much of the deposited dry ice frost to vaporize. It recycles back into the atmosphere increasing atmospheric pressure on the planet. Come winter, temperatures drop and CO2 freezes out of the atmosphere and back onto the surface, causing the pressure to drop by 25%. On Earth, the sea level pressure varies only about 2%.
You might wonder what Mars has been up to in the sky. Right now, it’s lost in the glare of twilight as it slowly transitions from the dusk to the dawn sky. It’s distant from Earth and faint. But if we’re patient, on July 27 next year, Mars will be making one of its closest approaches to Earth in years and prove the highlight of the summer sky.