Just about every two years, the planet Mars makes its closest approach to Earth... around 36 million miles.

That's when we pack our robotic emissaries off to the Red Planet, timing their launches to spend the least effort to get there.

Some fly around it... snapping pictures...

Others land ... to sample its surface....

...a few to crawl around its canyons and craters.

These probes may pave the way for human explorers... and, perhaps permanent settlers... who'll dig deeper still... in search of answers to our most pressing question:

Did Mars develop far enough -- and stay that way long enough -- for life to arise?

And, if so, does anything live now within Mars' dusty plains... beneath its ice caps... or maybe somewhere underground?

Mars does not give up its secrets easily ... it's almost as if the little planet is embarrassed.

Over a century ago, a few observers thought they saw clues that Mars is alive.

In 1877, the Italian astronomer Giovanni Schiaparelli noted markings... which he saw as a latticework of lines. He called them "canali" in Italian... meaning nothing more than "shallow channels" in English.

American astronomer, Percival Lowell, found the lure of these features irresistible.

He saw Schiaparelli's channels as artificial canals. He speculated that they carried melting snow from the poles to the dry interior.

After all, on Earth, the Suez Canal had recently opened to ship traffic. The Panama Canal was beginning to be dug.

The Martian canals, Lowell said, were built by a sophisticated society confronting an environmental catastrophe on the grandest of scales.

Those Martians, he thought, must face urgent choice: move water across vast arid regions, or perish on an increasingly dry planet.

As the 19th Century gave way to the 20th, Lowell took his case to the public, in a series of three best-selling books.

And the public responded with... questions.

Who were these Martians, who had the means to remake an entire planet?

Some offered schemes for making contact. Giant mirrors would flash greetings... Light beams... Mental telepathy.

Many astronomers grew deeply skeptical... but Lowell's vision of a harsh, yet Earth-like planet endured in the public's imagination..

That vision was dealt harsh blow in 1964. The Mariner Four spacecraft ventured in for a closer look... And what it saw looked like the Moon.

Three more Mariners followed.

They found huge dormant volcanoes... the deepest and longest canyon in the solar system...but not a trace of life, present or past.

In the mid-1970's, two lander-orbiter robot teams, named Viking, took up residence at Mars.

Maybe the Martians were just hiding, so theVikings tested the soil for signs of life.

But all the evidence from Viking told us... Mars is not only barren... but in fact hostile to life.

It's no wonder. Martian air temperatures range from --20 degrees Fahrenheit to down below --200.

It's also very, very dry. The Sahara Desert on Earth is a rainforest, by comparison. If all of the water vapor in Mars' thin atmosphere fell as snow, it would make a layer of frost not thicker than your fingernail.

On Earth, impact craters erode over time from wind and water... and even volcanic activity.

On Mars, they can linger for billions of years.

But so can the imprint of riverbeds, lake bottoms and ocean shorelines... And the Viking orbiters saw a lot of them.

It's not hard to believe that a great deal of water once flowed here.

But where did all the water go?

To find out, scientists needed to do real field-geology on Mars. They needed rovers... travelling robots with tools and instruments.