Mars is the only planet which surface we can see with small telescopes. This characteristic quickly made it one of the most studied planets. Dark spots on a colored, reddish-sand background can be seen, and two small white patches: ice caps. For a long time it was thought that these meant that there should also be liquid water, but today we know that the martian climate does not allow this. However, there was water in the past. What happened with it is a mystery.
By the late nineteenth century it was already known that the size of Mars was half that of Earth, that its color is actually due to the fact that Mars is a big desert, that its rotation period is very similar to Earth's, almost 24 h, that its rotation axis is tilted, causing winters and summers and making the ice caps grow and retreat. Even clouds were seen, very sparse, moving with the wind. The mystery nonetheless were the dark spots, and what happened when the ice melted.
THE "CANALS" OF MARS
In 1894 American millionaire Percival Lowell founded an important observatory in the Arizona desert to study Mars. At the time the best way to record observations was simply drawing by hand what was seen, and so eventually Lowell obtained very detailed maps of the martian surface. One characteristic that all his maps had was that, between the spots, Lowell drew straight lines. They were sometimes double, sometimes crossed, sometimes joined at a point like spokes of a wheel.
The explanation found by Lowell for his lines shocked the world: he hypothesized they were too straight and therefore had to be of artificial origin. He developed a whole theory to explain the origin and purpose of these: they might be the portentous works of an advanced civilization, that facing the climate change and desertification of its planet, was performing a desperate effort to build canals for transporting water from the melting ice caps to the cities, located farther out near the equator.
For many years the controversy about whether or not Mars was inhabited was a major one in Science. It stood up prominently in popular culture, inspiring famous works like "The War of the Worlds" by H.G. Wells, were the Martians give up their canals and solve their problem by invading the blue neighboring planet, without regard to its natives. The word "Martian" became synonymous with extraterrestrial beings.
By the early twentieth century, however, Lowell's theory began to show signs of not being solid. First, other equally competent astronomers with similar or better telescopes, like Edward Barnard, failed to see the straight lines claimed by Lowell. After 1909 Eugène Antoniadi proved that many of the lines were merely optical illusions. In the following decades, observing and accurately measuring different characteristics of the martian atmosphere, it was discovered that the planet's gas envelope is so thin that the pressure may not be enough to keep water molecules together in liquid form. Perhaps they could only be in the form of gas or ice. And a new instrument, the infrared sensor, showed that on most of the planet the temperature is very low.
But still, the mystery of the spots that changed in appearance over time continued. Many suggested that even though there were not advanced beings at least there could be some kind of primitive life, such as lichens, and in some way with the melting of the poles in summer they grew greener.
EXPLORATION WITH SPACECRAFT
It was not until the arrival of another technological revolution that means to solve these mysteries were at hand: spacecraft. In 1965, the unmanned U.S. spacecraft Mariner 4 successfully reached Mars after 8 months of traveling and during its rapid and brief flyby it was able to transmit the first 22 pictures at close range of the surface of our neighbor in space. To the chagrin of many they clearly showed numerous craters, like a lunar landscape. This indicates a lack of erosion on the planet, that is, at least for thousand of millions of years it has not rained on Mars. Thinking about life in these conditions may be too optimistic.
By 1971 it was possible to send a spacecraft, Mariner 9, with engines and enough fuel to match its pace to that of the planet's and enter into orbit around it. For a couple of years its telescopic cameras sent about 7000 photographs, revealing for the first time the real Mars. There is a huge canyon long as South America is wide and as deep as high are the highest mountains on Earth. And something not seen until then in another world: extinct volcanoes; the largest is 26 km, three times higher than Everest. But the biggest surprise was that, clearly distinguishable, there were dry riverbeds, many of them forming basins and deltas that lead to what would have been lakes and perhaps seas. Mars, after all, must have had water, although a very long time ago.
CURRENT KNOWLEDGE ABOUT MARS
In 1976 the Viking arrived: two satellites and two landers, the first successes on martian surface. They showed that Mars has no plate tectonics, but there are large geological landslides. Mountains eroded by the melting ice are seen. The sand would have been originated by water and there are large dune fields. The atmosphere is 95% CO2, with some nitrogen and argon, but its pressure is only 1% of Earth's air pressure. The temperature ranges from -70 degrees C at night to about 15 degrees C during the day. In this world the sky has a pink hue, which changes to blue at sunset. There are winds of up to hundreds of km/h and sandstorms that darken the sky for months, decreasing the temperature. It is this dust that alters the appearance of the spots. And there are tornadoes too.
Morning fogs and clouds at mountains tops are common. Clouds are of CO2 crystals (dry ice), although some are of water ice. The floor may be covered with frost and in certain seasons there are hurricanes and snowfall. The polar caps are formed when the air freezes, but below it lies a permanent residue of water ice. When they retreat they leave marks similar to growth rings in trees. The Viking sought lifeforms but without success.
THE MYSTERY OF THE WATER
Fifteen years ago, once again the United States of America began to send spacecraft to Mars, like Pathfinder with the Sojourner rover, which showed that large floods, perhaps of mud, altered expansive areas. The Mars Global Surveyor satellite photographed in great detail the surface, showing that there are still places where thawing can produce markings similar to small water flows. The spacecraft 2001 Mars Odyssey showed that in cold regions soil contains a high percentage of water ice. The European orbiter Mars Express detected methane in the atmosphere, which is often associated with living things, but any conclusion is premature. The rovers Spirit and Opportunity are on Mars since 2004 studying the soil, and have detected minerals characteristic of aqueous terrains. In 2008 the Phoenix lander excavated the permafrost in the northern polar region and confirmed the presence of large quantities of water ice there. In 2012 a complex mobile laboratory, Curiosity, was put on the planet to continue following the trail of water and other Life-benign ingredients.
All these lead us to ponder: what happened to the water? Clearly the atmosphere of Mars is so thin today, that while ices do melt, water cannot be contained in the liquid phase, but it must have been much thicker in the past for all those geological features to be explained. How a planet can suffer such a climate change, leaving it dry and dying, is now one of the greatest mysteries of the Solar System.
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Based on a lecture given at the USP, on 17 March 1999. Originally published in ABC Color, on 21 May 2006. A slightly retouched version of this article, joined to many other related articles from this website making a compilation titled "Do you believe in UFOs?", is available for sale in electronic book format at http://www.amazon.com/dp/B00GF0REFI. Photo: This spectacular photo mosaic by the Viking Orbiter 1 shows the arrival of winter in the southern hemisphere, so that the air freezes and falls as snow flakes, making the polar cap grow. In summer the process reverses, always bypassing the liquid phase. Photo credit: NASA / JPL-Caltech / U.S. Geological Survey.