An Icy Discovery on Mars, but Where’s the Water?

Photo: A layer of water frost is seen near the Phoenix Mars lander, but the frost was gone after 6 a.m. (NASA/JPL-Caltech)

After a much ballyhooed discovery two months ago of water ice in the northern plains of Mars, scientists are now perplexed by the water that NASA’s Phoenix Mars lander has not found.A few years ago, the Mars Odyssey spacecraft found, from orbit, signs of vast quantities of water ice a few inches below the planet’s surface. In July, mission scientists confirmed that patches of white seen in the soil near the lander were indeed ice. Phoenix’s weather station has also detected wisps of water vapor in the thin Martian air, and scientists expected that as the nighttime temperature plunged to minus-110 degrees Fahrenheit from minus-20 — and with it the amount of moisture that the Martian air can hold — minuscule specks of moisture would glom onto dust particles at the surface. The presence of water would show up in electrical measurements by a probe stuck into the soil. Except Mars has not cooperated.“We’re seeing nothing,” said Aaron Zent of the NASA Ames Research Center at Moffett Field, Calif., the lead scientist for Phoenix’s thermal and electroconductivity probe. “Big fat nothing.”Actually, the first measurement did yield the expected readings. “A lovely signal,” Dr. Zent said. “But we never saw it again.”Every subsequent measurement, taken at almost all hours of the day, indicated dry soil.

On Earth, dropping moisture level in the air leads to the condensation of morning dew; on Mars, because the water layer on the dust particles would be only a couple of molecules thick, it would not freeze into the crystal structure of ice, but instead remain more liquidlike, with molecules able to move along the surface of the grains.The moisture in the air during the day has to go somewhere at night, and that somewhere seems almost certain to be the soil. “It has to,” Dr. Zent said. “There’s no other place for it to go. The soil is sucking it up at night. We certainly expect that we should be able to see some of this.”

Dr. Zent said that perhaps the signal was more subtle than expected. It is possible that the water layer is somehow thick enough to freeze into ice, which would not show the expected electrical behavior. (Photographs of the landscape do show frost on the ground in the morning.) Or the water layer is so thin that the molecules bind tightly to the dirt; that, too, would suppress the electrical signal.The next step is for Phoenix to jam its electroconductivity probe deeper into the soil, closer to the ice layer. Maybe then, Phoenix will once again discover water.