Blobs in Photos of Mars Lander Stir a Debate: Are They Water?

By KENNETH CHANG

Published: March 16, 2009

Several photographs taken by NASA’s Phoenix Mars spacecraft show what look like water droplets clinging to one of its landing struts.

Some of the scientists working on the mission are asserting that that is exactly what they were. They contend that there are pockets of liquid water just under the Martian surface even though the temperatures in the northern plains never warmed above minus 15 degrees Fahrenheit during the six months of Phoenix’s operations last year.

The scientists believe that salts may have lowered the freezing temperature of the Martian water droplets to perhaps minus 90 degrees, or more than 120 degrees colder than the usual freezing temperature of 32 degrees for pure water.

Nilton O. Renno, a professor of atmospheric, oceanic and space sciences at the University of Michigan who proposed the hypothesis, was careful to say, “This is not a proof.”

But he added: “I think the evidence is overwhelming. It’s not circumstantial evidence.”

Dr. Renno will present his data and arguments this month in a talk at the Lunar and Planetary Science Conference in The Woodlands, Texas, and he is the lead author among 22 authors of a scientific paper submitted to The Journal of Geophysical Research.

Others are completely unconvinced. “There are simpler explanations,” said Michael H. Hecht, a scientist at NASA’s Jet Propulsion Laboratory and a co-investigator of the Phoenix’s wet chemistry instrument. Dr. Hecht, who described himself as the “designated curmudgeon,” said he believed that the process proposed by Dr. Renno to describe the formation and movement of water droplets was “flat-out wrong for these materials.”

Peter H. Smith, the mission’s principal investigator, is left to mediate the disagreement. He wonders whether the material, splashed up and possibly transformed by the heat and spray of chemicals from the thrusters when Phoenix landed, tells much or anything at all about the conditions on Mars.

Because of the lack of consensus, the Phoenix science team never brought up the liquid droplet hypothesis during any of the NASA news conferences.

The core facts are not in dispute. There were blobs on the strut. The blobs changed and moved over time before disappearing later in the mission.

The scientists also agree that the fundamental physics of Dr. Renno’s hypothesis is sound. And it dovetails with the major undisputed finding of the Phoenix mission: the unexpected discovery of chemicals known as perchlorates in the soil.

Perchlorates are salts, and if they were dissolved in high enough concentrations in water, the resulting brine would be a liquid at Martian surface temperatures.

Dr. Renno believes that Phoenix’s thrusters splashed a pocket of brine from just below the surface to the landing strut. He said the salts would have absorbed water vapor from the air, explaining how they appeared to grow in size during the first 44 Martian days before slowly evaporating as the temperatures dropped.

But Dr. Hecht believes that the droplet shape was in part a trick of low-resolution images and lighting. His simpler explanation is that these were just small clumps of frost.

The central scientific disagreement is whether the landing strut was warmer or colder than the ground. Exposed ice seen below the lander was clearly disappearing over time, vanishing into water vapor. Dr. Renno maintains that because of the heating systems on the spacecraft, the landing struts were warmer than the ground, and it is thermodynamically impossible for simple water frost to move from the cold ground to the warmer leg. But perchlorate salt could act as a sponge to absorb water vapor.

Dr. Hecht said that the strut was in the shadow of the lander and that the ground was in sunshine, so the leg was colder.

The Phoenix Mars lander, as expected, froze to death in November with the oncoming winter. But Dr. Renno and collaborators in Spain have been conducting experiments to see if they can replicate the behavior on Earth.

“The initial results,” Dr. Renno said, “are consistent with what we see as liquid water.”