No planet has teased the imagination as much as Mars. In ancient Greece and Rome, the planet's fiery red hue made skywatchers think of their god of war. In the 19th century, astronomer Percival Lowell thought he saw canals on Mars; his observations triggered a flood of novels and science fiction stories about strange and wonderful beings thought to inhabit the Red Planet.

Today, we know the canals were mostly an illusion: Mars is a very dry world. But the presence of some trenches and gullies has hinted of past ages when water flowed across the Martian landscape. Satellite images released in late 2006 have even raised the possibility of liquid activity on the planet's surface within the past decade.

The horizon of Mars, part of the first color image from the rover Spirit. (NASA/JPL)

True, no signs of life have been discovered on Mars, but scientists can't rule out the possibility that microbial life did, at one time, evolve there. With its giant volcanoes, gaping canyons and vast deserts, Mars remains as intriguing as ever.

"The fascination with Mars is that it's a place that you could go to," explained Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology.

"It's got geology, it's got an atmosphere, it's got clouds, it's got seasons, it's got polar caps, it snows.… A lot of the things we know and love about the Earth, in terms of the complexity of the interactions of all the systems, you see on Mars."

Mars 101

Mars is the fourth planet from the sun and has often been called a sister planet to Earth. A "day" on Mars lasts 24½ hours, just 30 minutes longer than here on Earth. The planet is tilted on its axis by 25 degrees, just two degrees more than the Earth's tilt. Because of its greater distance from the sun, however, Mars takes substantially longer to complete each orbit — a "year" on Mars lasts 668 Earth days, nearly twice as long as a year here.

Mars (Courtesy of NASA)
Click picture to see a planet-wide dust storm on Mars

Because of its small size, the pull of gravity on the planet's surface is just 38 per cent as strong as on Earth (a 200-pound man would weigh just 76 pounds on Mars). Because of its weak gravity, Mars has retained only a thin atmosphere — about 100 times thinner than Earth's. Its main component (95 per cent) is carbon dioxide.

It can be windy, though, with giant dust storms sometimes engulfing the entire planet. Because of its thin atmosphere and its distance from the sun, Mars is a very cold world. Although midsummer temperatures can reach 26 C, it can also drop to a numbing –111 C.

Two distinct polar ice caps can be seen even through small telescopes. The north cap (the larger of the two) is mainly normal ice (frozen water), while the southern cap seems to be mostly frozen carbon dioxide. Why the two caps are so different is a mystery.

Besides its vast, rocky deserts, Mars also has enormous canyons such as the Valles Marineris, which stretches some 5,000 kilometres along the planet's equatorial region, as well as giant extinct volcanoes like the 27-kilometre-high Olympus Mons, the largest volcano in the solar system. The planet also has intriguing channels that look as though they were carved by flowing water, which suggests that Mars may once have been both wetter and warmer than it is today.

Mars has two small, irregularly-shaped moons, Phobos and Deimos, each less than 30 kilometres across.

Water

This satellite file photo shows gullies on the walls of a meteor crater in the Newton Basin on Mars. Photos of other gullies on the planet, suggest water may have flowed through them in the 1990s. (NASA, Jet Propulsion Laboratories/Associated Press)

Because of the planet's cold temperature and thin atmosphere, liquid water is not stable on the Martian surface. Any water would either evaporate into the atmosphere or freeze and be absorbed into the soil.

However, there's quite a bit of evidence suggesting that water once flowed across the Red Planet. The most recent and exciting development came in December of 2006, when a new study of photographs taken from orbit suggested water flowed on Mars as recently as a few years ago.

Photos taken by the Global Surveyor satellite in 1999 of gullies and trenches on the planet's surface showed movement of deposits over time in these gullies. The scientists concluded the deposits — possibly mud, salt or frost — were left behind when water recently travelled through channels.

The sub-zero surface temperatures and low atmospheric pressure of Mars preclude water from remaining a liquid for long. Once it reaches the surface, it would either freeze into ice or disperse in the atmosphere as gas.

But some studies have suggested the possibility that liquid water could be briefly appearing on the surface from an underground water source.

Scientists had already found signs of water in the planet's past. Evidence has suggested an ocean may have once covered the planet's northern hemisphere. As well, images from Surveyor and other spacecraft show the channels that may well have been carved by flowing water billions of years ago.

"You clearly had a period, very early on, where you had liquid water flowing on the surface," said Zuber of MIT.

So where did all the water go? Scientists have come up with a number of models, but none of them is quite satisfactory. It's possible that most of the water evaporated, and then the water molecules — continuously bombarded by the solar radiation — may have broken down into their components (hydrogen and oxygen). These gases may then have been lost into space.

But current models suggest that this should have taken many billions of years. In other words, there hasn't been enough time for Mars to lose so much of its water so quickly.

What makes the possibility of an underground source so compelling is it would also suggest a stable heat source, which, along with water, is a key condition for supporting an environment favourable to life.

Life

The question of life on Mars, either past or present, remains a great motivator in the exploration of the Red Planet.

"No definitive evidence of past life has been identified," said Zuber, although she added that it is still possible microbial life has survived below the surface.

The lack of clear evidence of water makes many scientists pessimistic. None of the Mars landers or rovers has discovered water or any other signs of life. If life at one point flourished on Mars, it may have died off as the planet dried out.

Another intriguing hypothesis raised in January 2007 is that life on Mars could be found in a water-hydrogen peroxide mix as opposed to the salt-water mix that is the internal liquid of Earth-based living cells.

Dirk Schulze-Makuch, a geology professor at Washington State University, said in a paper a water-hydrogen peroxide mix stays liquid at low temperatures (–55 C), allowing it to thrive more easily on Mars.

Schulze-Makuch said NASA space probes on the Viking mission in 1977 may have stumbled upon alien microbes on the Red Planet — and inadvertently killed them.

In one Viking experiment, water was poured on the soil. That would have essentially drowned hydrogen peroxide-based life, Schulze-Makuch said. A different experiment heated the soil to see if something would happen, but that would have baked Martian microbes, he said.

A number of scientists have also raised the intriguing possibility that life on Earth may have originated on Mars. After all, we know that material from Mars has reached Earth in the form of meteorites.

If there were microbes embedded in those rocks, scientists speculate it's possible that they could survive their interplanetary journey and eventually taken root on Earth.

Of course, it could also have happened the other way around: if microbial life is eventually discovered on Mars, one could argue that it evolved on Earth first, and arrived on Mars via a wayward meteorite.

Seeing Mars

If you know where to look, you can see Mars quite easily from your backyard. Even if there are other planets or bright stars nearby, Mars can be identified by its distinct reddish-orange colour.

A telescope will show you the planet's disc, although a typical "beginner" telescope may be too small to reveal much detail on the planet's surface. According to the Canadian astronomy magazine SkyNews, a basic telescope with a diameter of 15 centimetres or more will enable sky-watchers to see peach-coloured deserts, white polar caps and dark markings.

Mars is at its brightest when it's at "opposition" — that is, when it's located opposite from the sun in our sky. At those times, the planet is closest to Earth — sometimes coming within 60 million kilometres.

On Aug. 27, 2003, the Earth was closer to Mars than it has been in more than 60,000 years. At 5:52 a.m. ET, the two planets came within 55,758,006 kilometres of one another. Astronomers say the last close encounter of this kind happened in 57,537 BC and they won't be as close again until the year 25,695.

(Source: The Observer's Handbook, published by the royal Astronomical Society of Canada; also Sky & Telescope magazine.)