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Scientists probed the Martian fog using NASA's Phoenix Mars Lander. The craft touched down on Mars in May 2008—during the planet's late northern summer—and collected data for about five months before succumbing to frigid winter conditions.
Pictures from Phoenix revealed, for example, that there's water ice in the shallow Martian soil, but scientists aren't yet sure whether that ice is a relic of ancient times or is formed by ongoing processes. During four nights near the middle of the Phoenix mission, scientists aimed a green laser beam affixed to the lander into the Martian skies and used a pair of cameras to record the result.
The new analysis of this data shows that Martian fog contains about 1.7 milligrams of water ice per cubic meter. Most of the fog is made of particles a little smaller than a thousandth of an inch wide, with an occasional larger particle settling out and falling toward the surface. "If you went outside on a dusty or foggy night with a laser pointer and pointed it straight up, you'd see what we saw on Mars," said study co-author Mark Lemmon, a planetary scientist at Texas A&M University. "You'd see the green beam and points of light every once in a while as a particle drifted through that beam."
From a seat next to the Phoenix lander, an observer during those four nights would have enjoyed a spectacular sunset as the late-summer day drew to a close. But soon after the bluish colors faded from the horizon, the observer would see the skies just a few meters overhead begin to choke with icy fog.
Some of the water vapor reenters the atmosphere, but some likely penetrates the soil and becomes part of the subsurface ground ice, the scientists surmise. Either way would suggest some dynamic hydrological processes still at work on Mars.