The latest forecast on Mars calls for morning fog and swift-moving clouds -- along with light snow.
The surprising weather report was part of the latest scientific findings from NASA's Phoenix lander, which has been taking measurements at the Martian north pole since May 25.
At a press briefing Monday at NASA headquarters in Washington and the Jet Propulsion Laboratory in La Canada Flintridge, scientists said the discovery of snow on Mars was made by an instrument that shined a laser into clouds about two miles above the ground, revealing the presence of ice crystals.
"Nothing like this has ever been seen on Mars," said Jim Whiteway, the lead scientist for the Canadian-designed Meteorological Station aboard the Phoenix lander. In coming weeks, he said, scientists will be searching for evidence that the snow actually falls on the ground.
Even before Phoenix landed, scientists knew water-ice, along with ice made of carbon dioxide, accumulated on the ground in the northern latitudes during the harsh winter, when temperatures plunge to minus 184 degrees Fahrenheit. But the discovery of snow in the atmosphere above the pole was a surprise.
Also on Monday, the scientific team revealed that chemical measurements have detected calcium carbonates in the soil, the major ingredient of chalk, and clay-like materials that on Earth form only in the presence of water.
Much of the soil shows evidence of past interaction with water, yet it is far too cold, even in the summer months, for water to remain in liquid form. The scientific team is racing to understand such anomalies before Phoenix succumbs to the winter, probably in mid to late November, according to Phoenix project manager Barry Goldstein.
When Phoenix landed, it was late spring on Mars. The sun remained up almost all day, allowing Phoenix's solar panels to collect plenty of energy to run its various instruments, which include a robotic digging arm and two chemistry labs. Now, with winter approaching, the sun is below the horizon about four hours of each Martian day, which is about 24 hours, 40 minutes long.
As temperatures plummet, Phoenix also must use more and more energy to operate the heaters that keep the instruments warm.
Goldstein said a "Lazarus" capability was designed into Phoenix. So it is theoretically possible the lander could rise from the dead next year, when spring returns. But he was doubtful. "It's going to get a lot, lot colder pretty soon," he said.
NASA, as well as the scientists operating the mission, have pronounced the $428-million mission a success. Besides analyzing the soil, it has dug down to the ice layer lying only inches below the lander. The discovery proved past measurements from orbiting NASA spacecraft showing vast quantities of ice underground at the pole were right on the money.
Still, scientists have not accomplished one of the most crucial goals of the mission, finding complex organic molecules that could indicate the planet was once, or still might be, habitable for some simple life forms.
Complicating the task is the failure to place ice samples into one of the eight ovens attached to the Thermal and Evolved Gas Analyzer, or TEGA, which cooks samples up to 1,800 degrees and sniffs the gas given off to unmask the chemical composition.
The tiny ovens have proved too small to accept large samples, while pouring in smaller samples has been made difficult by the clumping of the soil and the tendency of the ice to stick to the scoop.
William Boynton, the lead scientist for the TEGA instrument, said four of the eight ovens have been used so far. If there are any organic chemicals in the soil, "it is not much," Boynton said.
"The best I can say is we are still looking," he said.
The mission, originally designed for 90 days, was extended once by 30 days. On Monday, NASA announced that it was further extending the mission indefinitely, which means until the lander dies.
Peter Smith of the University of Arizona, the lead scientist for the mission, said the team is going to try something new in the last weeks of Phoenix's life.
The lander carried a microphone, which was designed to listen to the roar of the descent engines as the craft settled onto the Martian surface.
The microphone was not used then. Now, Smith said, the scientific team intends to turn on the microphone "and listen to Mars for the first time."