Unlike the Earth, where the sky is blue and sunsets are reddish, Mars has a pinkish sky (due to light reflected from red dust in the atmosphere) and a bluish sunset (caused by the relatively large dust particles blocking longer wavelengths of light more effectively than shorter wavelengths, whereas the tiny molecules in the Earth's atmosphere scatter shorter wavelengths more effectively than longer wavelengths). Note that as the Sun nears the horizon it does not not appear to flatten out as it usually appears to do on the Earth, because the differential refraction of its image in our thick atmosphere is the cause of its "flattening", and the two hundred times thinner Martian atmosphere has a two hundred times smaller differential refraction.

     During the Martian winter temperatures are so low that the atmosphere freezes out as dry ice (frozen carbon dioxide) snow. At mid-latitudes such as the Viking 2 site, the dry ice quickly sublimes (evaporates) back into the atmosphere, but a layer of water ice and dust about a thousandth of an inch thick, which freezes out of the atmosphere with the carbon dioxide, remains on the surface for several months. (JPL, NASA, Planetary Photojournal)

The Pathfinder landing site, with the Sojourner rover next to a rock nicknamed "Yogi", near the center. Click on the image for a larger view. (IMP Team, JPL, NASA, apod000514)

Bright soil uncovered by the Spirit rover while trundling across the Martian landscape contains a high proportion of iron sulfates and other salts, probably created by the evaporation of mineral-rich waters at some time in the past. This is one of many lines of evidence confirming the one-time presence of water on Mars. Unfortunately, there is no way to determine from present evidence whether water was present at this location for long periods of time, or only as a result of hot springs associated with volcanic activity; and in any event water cannot have been present in any significant amounts at most places on Mars at any time in the last 4 billion years, or the numerous ancient craters on the southern half of the planet would have been completely worn away, or at least show extensive signs of weathering and erosion. Still, the presence of water in any form at any time encourages those hoping to find (probably microscopic) fossil lifeforms. (Along those lines, exploration of the region near Mount Sharp by the Curiosity rover has confirmed the presence of a relatively long-lived lake partially filling the crater containing that peak, though again, we are talking about times well over 3 and perhaps closer to 4 billion years ago.) (Mars Exploration Rover Mission, Cornell, JPL, NASA, apod060406)

Unusually pitted boulders litter this Martian hillside. The surface of the boulders is so similar to similar pits or vesicles in gas-rich volcanic rocks on the Earth that there is little doubt that the boulders were ejected from a volcano at some time in the past. The large boulder close to bottom center is a little less than a foot and a half high, and was about six yards from the Spirit Rover when this image was taken, on April 13, 2006. (The boulder of course remains in the same place no matter the date, but as the rover moved around, its distance from the boulder changed.) As is true of most of Mars, wind-blown sand covers most of the surface, and fine dust colors the sky. (Mars Exploration Rover Mission, Cornell, JPL, NASA, apod060515)

On the other side of Mars the Opportunity rover had this view looking away from Victoria Crater shortly after arriving there in October 2006. In this exaggerated color image a sea of gently undulating sand lies beneath a sky filled with wispy clouds. Depending upon the temperature at the clouds' altitude, they would be composed of either water ice or carbon dioxide (dry) ice. (Mars Exploration Rover Mission, Cornell, JPL, NASA, (Image processing: Michael Howard, Tayfun Öner and Damien Bouic for unmannedspaceflight.com), apod061017)