An instrument on the lander that shone a laser into the clouds about two miles above the ground revealed the presence of ice crystals, though the snow disappeared before reaching the ground, according to the Los Angeles Times.
Scientists have known water ice, along with ice made of carbon dioxide, accumulated on the ground in the northern latitudes during Mars' winter, but the discovery of snow in the atmosphere above the pole was a surprise.
"Nothing like this view has ever been seen on Mars," said Jim Whiteway of York University in Toronto, lead scientist for the Canadian-supplied meteorological station on Phoenix, according to a NASA news release. "We'll be looking for signs that the snow may even reach the ground."
Soil samples dug from trenches by Phoenix's robotic arm also uncovered the presence of two minerals known to be formed in liquid water -- calcium carbonate, found in limestone and chalk, and sheet silicate. Most carbonates and clays on Earth form only in the presence of liquid water, according to NASA.
Scientists think there could have been standing water at the site in the past, or that the ice could have melted and interacted with the chemicals.
"Is this a habitable zone on Mars? I think we're approaching that hypothesis," said chief scientist Peter Smith of the University of Arizona, quoted the Associated Press. "We understand, though, that Mars has many surprises for us and we have not finished our investigation."
Phoenix has been taking measurements in Mars' northern latitudes since May 25, and NASA is extending the mission through the end of the year if the spacecraft can last that long. As temperatures drop, the lander must use more and more energy to heat its instruments.
In July, the Phoenix team reported definitive proof that water exists on Mars after the spacecraft scooped up ice and uncovered perchlorate, a chemical compound sometimes used by plants and microbes, Reuters reported.
The lander also sent back to Earth the first image of a speck of Martian dust taken through an atomic force microscope.
Phoenix will be out of touch with ground controllers briefly in November when the sun is between the Earth and Mars, blocking communications, according to the AP.