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Scientists Convinced Mars Lander Found Ice Under Soil

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The Washington Post

Published: June 21, 2008

Scientists with the Phoenix Mars Mission declared Friday for certain there is ice on Mars, putting them an essential step closer to answering the question that has driven three decades of Mars exploration and centuries of Earthbound speculation: Could there have been life there?

Pictures beamed 170 million miles to Earth from the Phoenix lander atop Mars' polar plain erased any doubt about the issue, they said, although in a roundabout way.

Last Sunday, several dice-sized solids were observed at the bottom of a trench that had just been dug by Phoenix's robotic arm. On Thursday, they were gone.

The only reasonable explanation, scientists said, is that the objects were pieces of ice that evaporated into the dry Martian atmosphere through a process called "sublimation." And the presence of ice means that Mars might once have had liquid water, which is essential for life - at least as it is known on Earth.

It was too soon to know whether all astrophysicists will accept the disappearing objects reported Friday as proof, but the Phoenix researchers said they did not need any more convincing.

The rocket thrusters that slowed Phoenix to a soft landing revealed a white, hard substance in the ground beneath it - and tantalizingly out of reach - when the lander touched down on May 25. Similar white material was visible when the arm began to dig below the top few inches of Martian soil.

One possibility was that it was salt of some sort, but ice was always the more likely explanation.

"Salt does not behave like that," said Mark Lemmon, a scientist at Texas A&M University in charge of Phoenix's stereo surface imager. "We found what we were looking for. This tells us we have water ice within reach of the arm."

Although Mars is much too cold now to have liquid water on its surface, scientists think that may not always have been the case. Images from as far back as the Viking missions in the 1970s revealed channels and gullies that appear to have been carved by flowing liquid at some point in history.

The Mars Odyssey orbiter, using a device called a gamma-ray spectrometer, proved in 2002 that huge quantities of hydrogen existed under the Martian topsoil. Although many compounds are high in hydrogen (including petroleum), the scientists think the only one that could be there in such quantity is water ice, which has two hydrogen and one oxygen atoms

"I don't know how you could have so much hydrogen under the surface, and something that disappears at just the temperature of ice, and have it not be ice," said Peter Smith, a physicist at the University of Arizona who is the principal investigator for the Phoenix mission.

The researchers chose Mars's northern polar plain as the landing site specifically because they thought it would give them the best chance of finding ice.

Water is necessary for life but is not sufficient. Scientists will also have to find a fair amount of carbon before they are willing to say the planet might have been habitable.

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