Microbes on Mars?
In
the search for alien life, some clues are here on Earth.
Have you ever wondered about life on other planets? Christopher
McKay thinks about it a lot.
He started wondering about life on Mars and elsewhere in the universe when the Viking spacecraft landed on Mars in 1976.
“What I thought would be interesting to examine for several weeks turned into something I’m still studying twenty years later,” he said.
Dr. McKay is a planetary scientist with the National Aeronautics and Space Administration (NASA) at the Ames Research Center in California. He is interested in exobiology, which is the study of whether life exists on other planets or moons, or elsewhere in outer space.
Dr. McKay studies life that survives in the harshest climates known on our planet. Then he determines where evidence of life might exist in hostile environments beyond Earth, particularly on Mars. NASA uses this information to decide where to search for evidence of life on Mars and elsewhere.
Dr. McKay has scuba dived under the ice cover of Lake Hoare in Antarctica, observing tiny life forms called algae that live on the bottom of the lake. In Siberia, he has drilled deep into the permanently frozen soil, which is called permafrost, to study frozen organisms that have survived there for millions of years.
He and his team often travel a week before arriving at remote locations, such as the Gobi Desert in Mongolia. They camp and explore for a month before helicopters come to take them home.
Antarctica
Dr. McKay’s first exobiology mission was to the dry
valleys of Antarctica. He wanted to see which forms of life
could live in that harsh part of the world. If Mars ever
had life, its last survivors might have been similar to
the tiny life forms in these valleys of Earth.
Although most of the continent is ice-covered, these freezing-cold
valleys are not. They are Mars-like in appearance and climate,
with dry soil and no plant life. The valleys have ice-covered
lakes, like those that might have been on Mars long ago.
From some of NASA’s pictures of Mars, scientists think
the planet’s surface must have been shaped by flowing
water. To have had flowing water, the planet must have been
warmer than it is today—maybe warm enough to support
life. But now the planet, like these Antarctic valleys,
is cold and dry.
To study some of Antarctica’s toughest survivors, Dr. McKay built instruments to measure the growth of tiny life forms that live inside rocks. They are called cryptoendolithic organisms.
“Crypto means hidden, endo means in, and lithic means rock—organisms hidden in the rock,” he said. “If you look at the rock’s surface, you can’t see them because they are living beneath the surface.”
Using the findings from his instruments, Dr. McKay then determined how well these Antarctic organisms might grow in even colder temperatures, like those on Mars. Since these organisms live in very dry climates, he has placed similar instruments in deserts around the world.
Where
to Search?
Dr. McKay believes that Mars once had the water and sunlight
needed to support life. But where do we search for evidence
that life existed? Dr. McKay thinks the 
best
places are the bottoms of dried Martian lakes.
“The main thing we learned in Antarctica was that
life survives under the ice covers of lakes in the dry valleys,”
he said. “A similar situation could have existed on
Mars. That’s probably the kind of place where life
survived long after the surface of the planet died. Even
if the surface never had life, there could have been life
underneath ice covers like these.”
NASA is planning a mission to Mars to collect rock samples
and bring them back to Earth. Dr. McKay hopes the samples
will come from a site that once was a Martian lake.
What will Dr. McKay look for in those first samples from Mars? He hopes to find fossils—the imprints of life. But what excites him even more is the idea that dead remains of tiny life forms might be frozen in Mars’s permafrost.
On
Earth, Dr. McKay has studied frozen organisms by drilling
more than seventy-five feet into the Siberian permafrost.
These organisms are about 3.5 million years old.
In the same way, the extreme cold at Mars’s south pole might have preserved similar tiny life forms. “They may be frozen, they may be corpses, but their actual remains could still be there,” Dr. McKay said.
What would these remains reveal? Would we find the same life forms that exist here on Earth? Or would we find something totally different?
“Imagine finding another life form that’s built out of different building blocks than life on Earth, something that we can't fathom,” Dr. McKay said. “That could really be revolutionary!”
