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Essay title: Mars

In the minds of men, Mars has been changing throughout the ages, as the capability of scientific instruments advanced. When ground-based telescopes gave way to visiting spacecraft, the myth of intelligent beings on Mars disappeared, and new mysteries evolved.

As the most Earth-like planet, Mars has held special interest, in that it is the most likely place in the Solar System that could have supported at least primitive life. So leaving aside little green men, or H.G. Wells' depressing creatures in The War of the Worlds, the question is posed: "Was there life on Mars?"

Even with the discovery over the past decade of the flourishing of life in extreme environments on Earth, the presence of liquid water remains a prerequisite. So to answer this most profound question, the current space missions have been designed to search for evidence of water on Mars.

There have been many hints, and more recently, evidence that is almost irrefutable, that water existed on the surface of Mars. But, until now, it was unclear whether liquid water was there billions, millions, or tens of thousands of years ago, or even in recent decades.

On Dec. 6, scientists described a discovery made through an intensive effort to compare images of Mars over time taken by the Mars Global Surveyor orbiter. The results led them to conclude that there is liquid water underground today on Mars, which periodically spurts up to the surface.

The high-powered Mars Reconnaissance Orbiter, which has just begun its science mission at Mars, along with the in-service European Mars Express, are using radar to search for reservoirs of ice and/or liquid water under the surface of the planet. More surprises can be expected.

The Long Road to Mars

Two centuries ago, using ground-based telescopes, astronomers could see shades of light and dark material, somewhat fancifully interpreted as structures made by intelligent life forms. Periodic changes in the surface features of Mars led to the idea that the differences in coloration were due to vegetation, the growth of which waxes and wanes with the planet's seasons.

When the Space Age first brought men's sensory equipment closer to Mars, during the 1960s Mariner fly-bys, scientists were disappointed to find that not only was there no evidence of intelligent beings, but that Mars looked hauntingly like the dry, dead, pock-marked Moon.

The first Mars orbiter, Mariner 9, provided a closer look at the planet, revealing a landscape with dazzling geological features, including very large impact craters, the largest canyon in the Solar System (Valles Marineris), and the largest known volcano, Olympus Mons. It was clear that even were Mars today a relatively unchanging world, it had undergone an evolution similar to that of the Earth.

The mid-1970s Viking mission sent two orbiters and two landers to Mars, providing global measurements along with the first-ever "ground truth." A new Mars was revealed. The orbiters could see features ten times smaller than Mariner 9.

The Viking orbiters obtained 52,000 images of the surface of Mars, and helped characterize the planet's atmosphere. Water vapor, it was found, is highly variable, depending upon local time, elevation, latitude, and the season. Mars, like Earth, has changing weather. Photographs taken by the Viking landers showed periodic layers of morning frost on surface rocks on Mars, demonstrating the movement of water ice and vapor around the planet. The Viking orbiters confirmed earlier evidence that there is a cache of water in the permanent ice cap at the north pole of Mars.

Some of the most dramatic evidence of previously existing significant amounts of liquid water on the surface, was seen in the photographs of Valles Marineris, with ancient river channels, connected valleys, and layers of material in the sides of the canyons. The most likely explanation for these features was an earlier warmer, wet Mars, with flowing water on its surface. The question became: Did liquid water exist on Mars long enough for life to have been able to flourish there?

Over the past decade, and to the present day, the next-generation, post-Viking spacecraft have all found more evidence that liquid water once existed on the surface. These include NASA's Mars Global Surveyor, Mars Odyssey, and Mars Exploration Rovers, and Europe's Mars Express spacecraft. But if liquid water did exist, how long ago was that?

In the year 2000, members of the Mars Global Surveyor (MGS) imaging team released spectacular photographs that showed gullies that had been formed inside the slopes of craters. These were similar to such formations on Earth, and suggested that they had been formed by liquid water running

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